Recent Advances in the Use of High Pressure as an Effective Processing Technique in the Food Industr

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Recent Advances in the Use of High Pressure as an Effective

Processing Technique in the Food Industry

Toms Norton &Da-Wen Sun

Received: 27 May 2007 /Accepted: 17 July 2007 / Published online: 25 September 2007# Springer Science + Business Media, LLC 2007

Abstract High pressure processing is a food processing

method which has shown great potential in the foodindustry. Similar to heat treatment, high pressure processing

inactivates microorganisms, denatures proteins and extends

the shelf life of food products. But in the meantime, unlike

heat treatments, high pressure treatment can also maintain

the quality of fresh foods, with little effects on flavour and

nutritional value. Furthermore, the technique is independent

of the size, shape or composition of products. In this paper,

many aspects associated with applying high pressure as a

processing method in the food industry are reviewed,

including operating principles, effects on food quality and

safety and most recent commercial and research applica-

tions. It is hoped that this review will promote more

widespread applications of the technology to the food

industry.

Keywords High pressure . HPP. HPLT . Low temperature .

Inactivation . Enzyme . Microorganism . Shelf life .

Food quality. Food safety. Freezing . Thawing

Nomenclature

P pressure (Pa)

T temperature (C)

density (kg m3)

viscosity (Pa s)

Cp specific heat (W kg1 K1)

D characteristic length (m)k inactivation constant

thermal conductivity (W/m1 K)

t time

thermal expansion coefficient (K1)

A, B, C mass of each designated food component

CH compression heating (C)

Subscripts

M food medium

W water

p food product

pp food product packaging

hyd_me hydraulic mechanisms in processing medium

hyd_p hydraulic mechanisms in product

th_me thermal conduction in processing medium

th_p thermal conduction in food product

th_pp thermal conduction in product packaging

in inactivation

x, y, z designated food component

food composite food material

Introduction

Food processing involves synergism between different

physical processes to transform raw animal/plant materials

into consumer-ready products. Today, the food industry is

expected to prevent or reduce negative changes in food

quality over time to provide a wide variety of food rich in

colour, texture and flavour and to adapt and develop new

food processes to satisfactorily meet the requirements of a

wide demographic within different cultures. Without food

Food Bioprocess Technol (2008) 1:234

DOI 10.1007/s11947-007-0007-0

T. Norton : D.-W. Sun (*)

Food Refrigeration and Computerised Food Technology Group,

University College Dublin, National University of Ireland,

Earlsfort Terrace,

Dublin 2, Ireland

e-mail: [email protected]

url: www.ucd.ie/refrig; www.ucd.ie/sun
http://www.ucd.ie/refrighttp://www.ucd.ie/refrig


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processing, these goals could not be upheld, as food could

neither be transported over long distances nor stored from

time of plenty to time of need (Lund 2002).

In the present day, consumers judge food quality based

on its sensory and nutritional characteristics (e.g. texture,

flavour, aroma, shape and colour, calorie content, vitamins

etc.), and alongside shelf life, these now determine an

individuals preference for specific products. Consequently,retailers are reporting up to a 30% growth in fresh, chilled

and healthy food sales (Hogan et al.2005). US sales in pre-

cut salad mixes were $1.9 billion in 2001 and increased to

$2.11 billion in 2003 (Hodge 2003). However, the recent

upsurge in demand has presented challenges to the food

industry, mainly in implementing techniques to keep food

fresher for longer, whilst offering a reasonable shelf life and

convenience and assuring food safety. Owing to recent

consumer preferences, impetus has been given to the

development of concept-driven novel technologies that

provide the required processing through non- or mildly

thermal means (Welti-Chanes et al. 2005). Accordingly,much of the recent scientific research for the food industry

has focused on non-thermal processing techniques, with

high pressure processing (HPP) being amongst the few

experiencing great potential in commercial settings (Sun

2005).

Food safety and shelf life are often closely related to

microbial quality and other phenomena such as biochemical

reactions, enzymatic reactions and structural changes, and

thus, although often indirectly, can significantly influence

consumers perception of food quality (LeBail et al. 2003).

Physical (e.g. heating, freezing, dehydration and packaging)

and chemical (e.g. reduction of pH or use of preservatives)

preservation methods continue to be used extensively

(Manas and Pagan 2005). Conventional thermal sterilisa-

tion processes are the most commonly used methods of

food preservation and involve heat transfer from a process-

ing medium to the slowest heating zone of a product and

subsequent cooling. Thus, although being effective mech-

anisms for microbial inactivation, thermal processes can

permit changes in product quality and cause off-flavour

generation, textural softening and destruction of colours

and vitamins, the extent of which is dependent on the

product bein g treated and the temperatur e gradien ts

between food and process boundaries. Microbial inactiva-

tion provided by HPP mainly targets cell membranes of

treated cells, but in some cases, additional damaging events

such as extensive solute loss during pressurisation, protein

denaturation and key enzyme inactivation are also required

(Manas and Pagan 2005). The multi-target ability of high

pressure (HP) has meant that in situations where its sole

employment yields unsatisfactory results, a high level of

synergism can be obtained when combined with other

processing techniques. Effective preservation has been

reported from combinations of HP with pH (Raso and

Barbosa-Canovas 2003), HP with pulsed electric fields

(Ross et al. 2003) and HP with CO2 (Spilimbergo et al.

2002). Furthermore, when used in conjunction with mildly

thermal processes, HP has been found to significantly

increase the inactivation of bacterial spores (Raso and

Barbosa-Canovas2003).

High pressure processing is a technology that potentiallyaddresses many, if not all, of the most recent challenges

faced by the food industry. It can facilitate the production of

food products that have the quality of fresh foods but the

convenience and profitability associated with shelf life

extension (McClements et al. 2001). HPP has already

become a commercially implemented technology, spreading

from its origins in Japan, followed by USA and now

Europe, with worldwide take-up increasing almost expo-

nentially since 2000 (Fig. 1a); although as of yet, this has

not been homogenous throughout the food industry. HPP

can be applied to a range of different foods, including juices

and beverages, fruits and vegetables, meat-based products(cooked and dry ham, etc.), fish and pre-cooked dishes,

with meat and vegetables being the most popular applica-

tions (Fig. 1b). European companies presently employing

this technology include orange juice by UltiFruit; the

Pernod Ricard Company, France; and sliced ham by

Espua, Spain; fruit jams by Solofruita, Italy (Urrutia-

Benet 2005). Furthermore, as evident in Table 1, a wide

variety of companies provide HPP technology to the food

industry.

High pressure processing techniques have also gained

momentum in areas of food preservation outside of

sterilisation and pasteurisation. The range of possibilities

offered by combining high pressure with low temperatures

(HPLT) has allowed the basis of a new field of HP food

applications to be formed, such as pressure-supported

freezing, thawing and subzero storage. Much work has

been conducted in the development and optimisation of

HPLT processes, and new findings regarding the phase

transitions of water, with consequential benefits for the food

industry, have recently been revealed (Urrutia-Benet et al.

2004).

High pressure research and development in different

disciplines within the food industry has been recently

reviewed by some authors (Rastogi et al.2007; Torres and

Velazquez 2005; San Martin et al. 2002; San Martin-

Gonzalez et al.2006; Toepfl et al. 2006). A comprehensive

review was conducted by Rastogi et al. (2007) who, as well

as assessing many studies on the effect of HPP on enzymes

and proteins, also provided information on the successful

use of HPP, either solely or in combination with other

processing techniques. Other reviews have focused on the

effect of HPP on microorganisms and food constituents

(San Martin et al. 2002); the use of HPP in the dairy

Food Bioprocess Technol (2008) 1:234 3


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industry (OReilly et al. 2001; San Martin-Gonzalez et al.

2006; Huppertz et al. 2006); the commercial opportunities

and research challenges in HPP (Torres and Velazquez

2005); the energy efficiency of HPP (Toepfl et al. 2006)

and pressure-assisted freezing and thawing of foods

(Cheftel et al. 2002). However, no review has completed

a combined study of the modern engineering aspects of HP

technology alongside its conventional and novel uses in the

food industry. Moreover, the extensive progress made in

very recent years in non- and mildly thermal and low

temperature HPP merits a state-of-the-art review. Conse-

quently, this study addresses many of the aspects associated

with applying high pressure as a processing method in the

food industry, from the engineering principles involved,

through food quality and safety issues, to the most recent

commercial and research applications, all of which have

seen great development in recent times.

Engineering Concepts of HPP

The governing principles of HPP are based on the

assumption that foods which experience HP in a vessel

follow the isostatic rule regardless of the size or shape of

the food. The isostatic rule states that pressure is instanta-

neously and uniformly transmitted throughout a sample

whether the sample is in direct contact with the pressure

medium or hermetically sealed in a flexible package.

Therefore, in contrast to thermal processing, the time

necessary for HPP should be independent of the sample

size (Rastogi et al. 2007).

The effect of HP on food chemistry and microbiology is

governed by Le Chateliers principle. This principle states

that when a system at equilibrium is disturbed, the system

then responds in a way that tends to minimise the

disturbance (Pauling 1964). In other words, HP stimulates

Fig. 1 (Color online) The num-

ber of HP equipment installed in

Europe by Hyperbaric versus

a year of installment and b the

industrial sector for the install-

ment (Urrutia-Benet2005)

4 Food Bioprocess Technol (2008) 1:234


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some phenomena (e.g. phase transition, chemical reactivity,

change in molecular configuration, chemical reaction) that

are accompanied by a decrease in volume, but opposes

reactions that involve an increase in volume. The effects of

pressure on protein stabilisation are also governed by this

principle, i.e. the negative changes in volume with an

increase in pressure cause an equilibrium shift towards

bond formation. Alongside this, the breaking of ions is also

enhanced by HP, as this leads to a volume decrease due to

the electrostriction of water. Moreover, as hydrogen bonds

are stabilised by high pressure, as their formation involves a

volume decrease, pressure does not generally affect

covalent bonds. Consequently, HP can disrupt large

molecules of or microbial cell structures, such as enzymes,

proteins, lipids and cell membranes, and leave small

molecules such as vitamins and flavour components

unaffected (Linton and Patterson2000).

Due to the work of compression, HPP causes temper-

atures to rise inside the HP vessel. This is known as

adiabatic heating and should be given due consideration

during the preservation process. The value of the temper-

ature increments in the food and pressure transmitting

medium will be different, as they depend on food

composition as well as processing temperature and pressure

and the rate of pressurisation (Otero et al. 2007a). In food

sterilisation, adiabatic heating can be used advantageously

to provide heating without the presence of sharp thermal

gradients at the process boundaries (Toepfl et al. 2006).

Knowledge of the engineering concepts of HPP has been

broadened extensively in recent times. Therefore, relevant

engineering principles that promote the capabilities of HPP

are discussed in the following.

The Mechanisms of Cellular Inactivation

The effectiveness of a food preservation technique is

primarily evaluated on the basis of its ability to eradicate

the pathogenic microorganisms that are present. Cellular

inactivation is closely associated with morphological

changes that occur within individual microbial cells during

Table 1 Main suppliers of high pressure processing equipment and services

Company Company specialisation Services and/or products offered Pressure capacity

of standard

machines (MPa)

Resato International

http://www.resato.com

This company commercialises

laboratory and industrial high

pressure hydrostatic machines

The company pressure shift freezing systems. They

use single shot or reciprocating intensifiers which are

suitable for one or multiple autoclave systems

Up to 1,400

Avure Technologies Inc.,

http://www.avure.com

Manufactures batch presses that

pasteurize prepared ready-to-eat

foods, e.g. packaged meats

Have unique pumping systems that enhance product

throughput. Continuous systems are not currently

being developed

600

Elmhurst Research, Inc.,

http://www.

elmhurstresearch.com

Designs and manufactures batch

presses

The company has developed a system which

incorporates patent pending vessel technology. The

system that was developed exclusively for the food

processing industry from scratch

689

Engineered Pressure

Systems Inc.,http://www.

epsi-highpressure.com

Manufactures laboratory and

industrial high pressure equipment

for many industries

Manufacture hot, cold and warm isostatic presses 100900

Kobelco, http://www.

kobelco.co.jp

Manufactures laboratory and

industrial high pressure equipment

for many industries

Manufacture many hot and cold isostatic presses, wet

and dry-bag processes

98-686

Mitsubishi HeavyIndustries, http://www.

mhi.co.jp

Manufactures laboratory andindustrial high pressure equipment

for many industries

Manufacture isostatic pressing system with largeoperating temperature range as option

686

NC Hyperbaric, http://

www.nchyperbaric.com

European leader in manufacture of

industrial HPP equipment

Designed a system to work with different volumes,

guaranteeing the necessary versatility to process a

wide range of products of different sizes and shapes

600

Stansted Fluid Power

LTD.http://www.sfp-4-

hp.demon.co.uk

Offer a full range of advanced, high

pressure e quipment for research and

development applications

Single and multiple vessels with temperature control

from 20 C to +150 C. Multiple Telemetry option

and variable pressurisation times from 2s

Up to 1,400

Uhde Hockdrucktechnik,

http://www.uhde-hpt.com

Uhde develop and build high

pressure processes for industry and

research purposes

Help in the development of plant processes from

initial testing to full scale application

700

http://www.resato.com/http://www.avure.com/http://www.elmhurstresearch.com/http://www.elmhurstresearch.com/http://www.epsi-highpressure.com/http://www.epsi-highpressure.com/http://www.kobelco.co.jp/http://www.kobelco.co.jp/http://www.mhi.co.jp/http://www.mhi.co.jp/http://www.nchyperbaric.com/http://www.nchyperbaric.com/http://www.sfp-4-hp.demon.co.uk/http://www.sfp-4-hp.demon.co.uk/http://www.uhde-hpt.com/http://www.uhde-hpt.com/http://www.sfp-4-hp.demon.co.uk/http://www.sfp-4-hp.demon.co.uk/http://www.nchyperbaric.com/http://www.nchyperbaric.com/http://www.mhi.co.jp/http://www.mhi.co.jp/http://www.kobelco.co.jp/http://www.kobelco.co.jp/http://www.epsi-highpressure.com/http://www.epsi-highpressure.com/http://www.elmhurstresearch.com/http://www.elmhurstresearch.com/http://www.avure.com/http://www.resato.com/


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HPP; studies of which, as briefly reviewed by Hartmann etal. (2006), are summarised in Table2. From the small group

of investigations, which have thus far focussed on this area,

it is evident that cell disruption is highly specific to the

geometry of the bacteria, as opposed to its gram-type

(Ludwig and Schreck1997; Schreck et al. 1999), although

this is disputed (Yuste et al. 2001). Moreover, the presence

of a cell wall does not mean pressure resistance is

enhanced; in fact, quite the opposite has been hypothesised

by Ludwig et al. (2002) who suggested that pressure may

induce mechanical stresses on the microbial cell wall,

which, in turn, may interact with inactivation mechanisms.

Although the above studies show strong correlationsbetween the physiological state of the microorganisms and

degree of pressurisation, cell disruption during processing

remains poorly understood at the fundamental level of fluid

and cell interactions (Smith et al. 2000a). Up to quite

recently, this has been quantified via a cell-wall-strength

model which presumes disruption to occur when the fluid

stresses that are imparted on a cell wall exceed some

defined threshold. This has been successfully applied to

animal cells, as these have no proper cell wall (Thomas and

Zhang 1998). Progress, however, has been slower for

microbial cells whose well-structured cell walls add

considerable complexity. As a consequence, there is a lack

of understanding and characterisation of the mechanical

properties of microbial cell walls (Smith et al.2000a).

To appreciate the mechanical strength of microbial cells

and the factors that contribute to that strength, investiga-

tions of cell mechanical properties under periods of

pressurisation are necessary. As yeast cells are widely used

to produce intracellular bio-products of commercial interest,

experimental techniques have been employed to evaluate

their properties; for example, via micromanipulation, the

relationship between bursting force, diameter and therelationship between force and displacement of yeast cells

have been established (Mashmoushy et al. 1998). Fortu-

nately, yeast cell walls are structurally complex, so

experimentation may provide scope for understanding the

mechanisms of inactivation in complex microorganisms

such as Escherichia coli. In recent years, it has been found

that unless three dimensionless parameters, namely the

permeability constant, the initial thickness to radius ratio

and the initial radial stretch ratio, were found from experi-

ments, then non-unique properties for cell walls of

biological cells could be derived (Smith et al. 1998). To

determine the cell wall properties for yeast cells using thesedimensionless parameters, Smith et al. (2000a) conducted

compression experiments. They used osmotic theory to

interpret measurements of cell volume as a function of

external osmotic pressure. Then, they quantified the effect

of osmotic pressure and cell compression rates on the

bursting force, deformation at bursting and cell diameter. To

determine the intrinsic cell wall properties and cell wall

failure criteria, the force-deformation data obtained were

used in conjunction with a finite element (FE) mechanical

model (Smith et al. 2000b). Specifically, this model

determined the mean Youngs modulus (when used in

conjunction with simple membrane theory), mean maxi-

mum von Mises stress-at-failure and mean maximum von

Mises strain-at-failure. Unfortunately, internal organelles of

the yeast cell which are also susceptible to stress were not

considered, thereby reducing the models applicability in the

area of HPP.

Hartmann and Delgado (2004) addressed this issue by

using the above information in the development of a FE

mechanical model of a yeast cell during the compression

phase of HPP (as shown in Fig.2), which was experimen-

Table 2 Mechanisms of cellular inactivation

Target microorganism Findings Reference

E. coli S. aureus P.

aeruginosa

Morphological changes were only noticed for the rod shapedE. coli and P. aeruginosaof which

P. aeruginosawas more pressure sensitive, whereas the S. aureus (cocci) was the most resistant

to pressure.

Ludwig and

Schreck (1997)

M. pneumoniae The pressure sensitivity of M. pneumoniae, which has no cell wall, was high compared to cell

wall gram-positive bacteria. The cell wall wasnt found to protect the bacteria and no correlation

between gram-type and pressure sensitivity was observed. However, correlation existed

between cell shape and pressure sensitivity, similar to above

Schreck et al.

(1999)

L. monocytogenes S.

typhimurium

Cellular morphology of L. monocytogenes was not affected when exposed to pressures of

400 MPa and membranes were perforated in small part of the population.S. typhimuriumshows

morphological changes such as dimples and swellings.

Ritz et al. (2002)

S. cerevisiae Cell wall disruption occurs at 400 MPa to 500 MPA. The organelles of the cell are very sensitive

to pressure. The nuclear membrane begins to feel the affect at of 100 MPa, and at 400 MPA all

the organelles are disrupted

Shimada et al.

(1993)

S. fibuligera At 250 MPa the volume shrinkage of the cell was 15%, after compression. The volume of non-

viable cells was found to be 65% after the holding time of 15 min.

Perrier-Cornet et

al. (1995)



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tally validated with yeast cell volume reduction data from

Perrier-Cornet et al. (1995). Instead of using a volume loss

equation as was done in the study of Smith et al. ( 1998), a

reduced form of the Cauchy equation of motion represented

the mechanical behaviour of the yeast cell. Major organ-

elles were modelled to investigate the homogeneity of the

stress distribution in the cell as well as the cell deformation

characteristics. The authors found that at 400 Mpa, the

critical effective strain upon failure of the organelles

membranes of 80% (Shimada et al. 1993) was predicted,

correlating well to experimental studies of Shimada et al.

(1993). Most notably, Hartmann and Delgado (2004)

predicted a non-homogenous (as opposed to the widely

assumed homogenous) stress distribution in the cell. In

addition, through dimensional analysis, the authors found

that the compression rates did not influence cellular

inactivation. They found that a frequency of over

700 MHz would be required for any noticeable inactivation

to occur; this frequency exceeds the feasible range of

transient pressure protocols applicable in a pulsed-HPP

system. The possibility of independence between inactiva-

tion and compression rates has been shown experimentally

for other microbial species (Rademacher et al. 2002). Later,

Hartmann et al. (2006) derived a simple linear model to

explain the stress distribution on a spherical shell; although

the model assumed constant material properties, the model

still predicted the existence of heterogeneous mechanical

stresses under high hydrostatic pressure.

ThermalHydraulic Processes in HPP

As HPP often involves heat interactions and fluid flow,

thermal-hydraulic investigations, i.e. the study of thermo-

dynamic and fluid-dynamic phenomena, have shown to be

of high importance. The thermalhydraulic processes that

occur during the HPP of both fluid and solid food systems

can be highly influential on the efficiency and effectiveness

of the overall process (Hartmann2002; Rademacher et al.

2002). During compression/decompression phases, the

internal energy of the HP system changes, resulting in heat

transfer between the internal system and its boundaries. Thefirst experimental observations of fluid temperature in a HP

vessel were made by Pehl et al. (2000) who revealed a

heterogeneous temperature distribution via high-pressure

thermochromatic liquid crystals. Using the same experi-

mental rig at room temperatures, Rademacher et al. (2002)

noted periods of forced convection during the compression/

decompression phase followed by natural convection

during the pressure holding stage. The observed tempera-

ture gradients were found to be dependent on the pressure

ramp employed. These thermalhydraulic characteristics

were also confirmed through numerical simulations by

Hartmann (2002) who noted that if food particles or

microorganisms were to be suspended in the fluid they

would undergo periodic temperature treatment with a

variation of 6 K due to a vortex motion in the pressurised

cell. Owing to the ability of the numerical simulations to

provide non-intrusive flow, thermal and concentration field

predictions, such techniques were deemed necessary in

gaining thorough understanding of the phenomena inherent

in HPP, especially when the scale-up phenomena need to be

analysed (e.g. layout and design of high pressure devices,

packages, etc.; Hartmann2002).

An important contribution to the understanding of

thermalhydraulics in the HPP of a fluidfood system at

mild temperatures (i.e. 313 K) was made by Hartmann and

Delgado (2002). The authors used computational fluid

dynamics (CFD) and dimensional analyses to determine

the timescales of convection, conduction and bacterial

inactivation, the relative values of which contribute to the

efficiency and uniformity of conditions during HPP. These

timescales are summarised in Table 3 from which the

dependency of both convection and conduction timescales

on the geometry of the processing equipment and the

Fig. 2 Finite element model of yeast cell under compression

(Hartmann and Delgado2004)



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transport mechanisms of the fluid matrix, i.e. dynamic

viscosity and thermal conductivity, can be seen. During the

study, conductive and convective timescales were directly

compared to the inactivation timescale to provide a pictureof the thermalhydraulic states of HP vessel during

bacterial inactivation. Results of high industrial relevance

were provided as, for example; it was shown for pilot scale

systems that when processed fluids exhibit a larger

convection timescale than the inactivation timescale, inten-

sive fluid motion and convective heat transfer resulted in

homogenous bacterial inactivation. Conversely, non-unifor-

mities in the inactivation process were dominant when

the convection timescale was significantly smaller and the

conduction timescale was significantly larger than the

inactivation timescale. Furthermore, the simulations of

industrial-scaled systems showed greater efficiency in

bacterial inactivation as the compression heating subsisted

for greater time periods when compared to smaller

laboratory systems. As regards the HP vessel boundaries,

Otero et al. (2002a) and Hartmann et al. (2004) showed that

the thermal properties of the HP vessel boundaries have

considerable influence on the uniformity of the process, and

insulated material promoted the most effective conditions.

As well as this, the insulated vessel was found to increase

the efficiency of HPP by 40% (Hartmann et al. 2004). A

dimensionless analysis of the convective heat transfer

mechanisms in liquid foods systems under pressure was

also done by Kowalczyk and Delgado (2007a) who advised

that HP systems with a characteristic dimension of 1 m

alongside a low viscous medium should be used to avoid

heterogeneous processing of the product.

Other studies provided similar solutions to the thermal

hydraulic phenomena in HPP systems containing packaged

ultra-heat treatment (UHT) milk (Hartmann et al. 2003)

packaged enzyme mixture (Hartmann et al.2003) solid beef

fat (Ghani and Farid 2006) and solid food analogue

material (Otero et al. 2007a), e.g. tylose with similar

properties to meat and agar with similar properties to water,

were both used. In both of the investigations of Hartmann

et al., the most significant results, revealed by validated

CFD simulations, showed strong coupling between con-centrations of the surviving microorganisms and the spatial

distribution of low temperature zones within the food

package in the HP vessel. A low thermal conductive

package material was also found to improve the uniformity

of processing by preserving the elevated temperature level

within the package throughout the pressurisation phase; an

average difference of about two log reductions was found

per tenfold increase in the package thermal conductivity.

The two-dimensional CFD simulations of Otero et al.

(2007a) found that the filling ratio of the HP vessel played

a major role in process uniformity, with convective currents

having least effect on heat transfer when this ratio is large

(Fig. 3). They also showed that by anticipating the

temperature increase, which results from compression

heating, and by allowing the pressure transmitting medium

to supply the appropriate quantity of heat, the uniformity of

HPP was enhanced when both large and small sample ratios

were used (Fig.4). More recently, Ghani and Farid (2006)

used three-dimensional CFD simulations to illustrate both

convective and conductive heat transfer in a HPP system

loaded with pieces of solid beef fat. The simulation showed

a greater adiabatic heating in the beef fat than the pressure

transmitting medium owing to the greater compression

heating coefficient used in this case.

A notable feature of the above modelling studies was

that contrasting results were possible owing to (1) the HP

systems having different operational properties or (2)

numerical modelling limitations. Therefore, different

boundary conditions have been used, and, consequently,

results between studies cannot be directly compared. For

example, in the studies of Hartmann and Delgado (2003)

and Hartmann et al. (2004), a HP vessel which permitted

the transient pressure increase as a result of the mass

Table 3 HP-associated equations suitable for industrial application

Fluid-food systems Packed food systems Reason and

references

Convection

timescale

thydrD2

h thyd me

rD2k

h ; thyd p

rD2pk

h By calculating these timescales the

uniformity of HPP can be determined

(Hartmann and Delgado2002).

Conduction

timescale

tth rD2 cp

l

tth me rD2Cpk

l

; tth pp rppD

2ppCppp k

l

; tth p rpD

2pCppk

l

Inactivation

timescale

tin 1

ktin

k

k

Adiabatic

temperature

rise

dTdP

aTCPr

(1)

CHfood CHXACHYBCHZC

ABC(2)

dTdP

aTCPr

(1)

CHfood CHXACHYBCHZC

ABC(2)

Eq.1 should be used estimate the

temperature increase for water based

foods. Eq.2 should be used for fatty

foods (Rasanayagam et al. 2003).



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augmentation of the inflowing pressure medium and

deformation of the packaged food, also called the indirect

HP system, was modelled. Otero et al. (2007a) and Ghani

and Farid (2006) modelled a direct system, i.e. a plunger-

press which increased vessel pressure directly via the

displacement of a drive piston. Both a direct and an indirectsystem are illustrated in Fig. 5. In contrast to Otero et al.

(2007a), Ghani and Farid (2006) and Hartmann et al.

(2004), Hartmann and Delgado (2003) modelled pressure

buildup to occur instantaneously in HP vessel because of

modelling limitations. The authors noted that this assump-

tion was justified because the pressure holding time

exceeded the compression/decompression phase of HPP.

However, as investigations with a laboratory scale (0.8 l)

systems were cited in this justification, i.e. with small

convection and conduction timescales (Pehl et al. 2000),

whereas systems with much larger convection and conduc-tion timescales were modelled (6.3 l), this must be

considered cautiously. Overall, the difference between the

boundary conditions used in these HPP modelling studies

lies in the adjustment they provided to the relative

contributions of forced and natural convection and, as a

result, their effect on temperature distribution.

It was evident from the above studies that both

temperature and velocity fields are transient during the

phase of pressure holding, as the fluid velocity distribution

influences strongly the temperature distribution and vice

versa (Otero et al. 2007a). Therefore, to accurately study

the relative contributions of forced and natural convectionto the effectiveness of HPP, it would be most beneficial to

measure velocity as well as temperature and use both to

develop a comprehensive validation in future simulation

studies.

Thermophysical Properties

Designing safe, effective and efficient HPP systems

demands the modelling of conceptual designs throughout

the range of pressures and temperatures experienced in the

food industry. One of the main difficulties when developing

or optimising these systems is the lack of knowledge about

the important thermophysical properties of food while

under pressure. However, such knowledge is important as,

Fig. 3 Temperature distribution in an HP chamber with a large filling

ratio (Otero et al. 2006)

Fig. 4 Temperature evolution in a big Tylose sample calculated from the model: the initial temperature of both the Tylose sample and the pressure

medium is a 40 C and b 24 C, i.e. showing the benefits of anticipating the adiabatic temperature rise (Otero et al. 2006)



9/33

from an engineering point of view, theoretically based heat

and mass transfer models that allow the accurate prediction

of the physical history of food undergoing HP are desirable.

For example, considering the thermalhydraulic studies

reviewed above, it would not have been possible to

evaluate the relative importance of process parameters suchas the compression rate (Hartmann 2002), the size of the

HP vessel (Hartmann et al. 2003), the viscosity of the

pressure transmitting medium (Hartmann and Delgado

2002) and the process uniformity (Otero et al. 2007a) etc.

unless the physical properties of the systems fluids were

modelled as functions of pressure and temperature. For

these calculations, the thermophysical properties used

include density, viscosity, specific heat and thermal con-

ductivity of both the pressure-transmitting medium and the

food product being processed. Of course, not all properties

have been modelled precisely, especially when limited

experimental data were available concerning the propertysvariation over the desired pressure and temperature range,

and when omitting the precise details of its dependency

would not have a large bearing on the accuracy of

simulation results, e.g. as Hartmann et al. (2003) found

when prescribing constant values for thermal conductivity

in CFD simulations (note that the variation of thermal

conductivity with pressure and temperature above freezing

point is slight as can be seen in Fig. 6.). In addition, when

HPP involves a change of phase, the ice fraction, the

enthalpy and the initial freezing point also need to be

modelled (Otero et al. 2006). Models of these properties

during HPP can be derived from (1) additive models

considering the food properties under pressure (Otero et

al. 2006); (2) in the phase change domain data at

atmospheric pressure can be shifted according to thefreezing point depression, or an experimentally observed

change, associated with the applied pressure (Denys et al.

Fig. 5 Examples ofa a direct system and b an indirect systems (Urrutia-Benet 2005)

Fig. 6 The variation in thermal conductivity of a Tylose sample with

respect to temperature and pressure (Otero et al. 2006)



10/33

1997; Hartmann et al. 2003) and (3) the physical property

of water under pressure can be multiplied by a constant

which represents the ratio of the foods physical property to

that of water at atmospheric pressure (Hartmann et al.2003;

Ghani and Farid2006). Another method used by Chen et al.

(2007) and Kowalczyk et al. (2005) was to firstly run two-

dimensional CFD simulations for a food product undergo-

ing the HPLT process and then fit the resulting curves toexperimental data by varying the appropriate thermophys-

ical property. A similar technique was followed by Schluter

et al. (2004) who allowed coefficients in Weibull distribu-

tions of the physical properties to vary in accordance with

the prevailing experimental conditions. The variation of

some important thermophysical properties under pressure

are discussed in the following.

Viscosity

Fluids which undergo pressurisation become more viscousespecially at subzero temperatures. Forst et al. (2000) have

published experimental data on the viscosity of water at

various temperatures as a function of pressure. Effective use

of these data permits the results obtained from viscosity

temperature equations, such as that developed by Watson et

al. (1980), to be adjusted so that the pressure experienced in

the HP system can be represented (Hartmann et al. 2004).

Many other numerical representations for viscosity of fluid

systems as a function of temperature have been published

by Seeton (2006). For liquid food systems over limited

ranges of concentration, the effect of solids concentration

on viscosity of liquid food can be described by eitherexponential (Vitali and Rao 1984) or a power type of

relationship (Rao et al. 1986). The dynamic relationship

with viscosity and pressure, however, is not so well

documented. In HPP simulations of UHT milk, Hartmann

et al. (2003) considered milk to follow the same pressure

viscosity profile as that of water, represented by:

hM p; T hM T

hW T

ambp

hW p; T 1

However, owing to phenomena such as micelle disrup-

tion, the viscosity of milk during HPP cannot be explained

accurately in this way (Harte et al.2003).

Density

The equation of state developed by Saul and Wagner

(1989), which accounts for the compressibility of pure

water under high pressure, has been used to describe

density as a function of pressure and temperature during

studies when convection heat transfer during HPP is being

modelled (Ghani and Farid2006); other sources for density

data have also been used for water-like substances (Otero et

al. 2007a). As regards food, high pressure has been found

to increase the density of a food analogue by about 3.5% of

its original value for each 100-MPa increment in applied

pressure (Otero et al. 2006). Modelling compression\

decompression effects within a food sample during HPPrequires that the samples densitytemperaturepressure

relationship be taken into account. Denys et al. (2000)

measured this relationship in apple sauce and tomato paste

and regressed data to form a simple equation which they

then incorporated in their numerical heat transfer model.

When such measurements have not been possible, it was

necessary to allow the density of the food sample to vary as

a function of water density, assuming that no phase change

would occur during the HPP (Hartmann and Delgado2003;

Ghani and Farid2006), i.e.:

rM p; T rM T rW T

ambp

rW p; T 2

In the phase-change domain, food density also increases

with an increase in applied pressure. Otero et al. (2006)

have shown predictions from an additive density model

under pressure to be more accurate than shifting the

atmospheric pressure density data according to the freezing

point depression. This is because shifting the data did not

take into account the increment registered in liquid water

and ice densities under pressure (Otero et al. 2006).

Specific Heat

In many foods, water substantially influences specific heat.

In addition, for matters of reducing modelling complexity,

the specific heat of the solid food components of a food

matrix can be assumed independent of temperature and

pressure (Otero et al.2006). This means that the lower the

foods water content, the greater the difference between

predictions for the food and specific heat of water (Miles

1991). For pure water at temperatures over 0 C, increasing

the pressure causes the specific heat to decrease in an

almost linear fashion. For example, using the thermophys-

ical data corresponding to pure water (Lemmon et al. 2005),

its specific heat at 1 C was found to decrease gradually

from 4,216 J kg1 K1 at atmospheric pressure to 3,488 J

kg1 K1 at 600 MPa. A similar gradient in the specific heat

versus pressure curve exists for all water temperatures in

the range of 0 to 120 C (Otero et al. 2002b). By assuming

that this gradient is representative of a food sample, the

specific heat of the food can then be determined as a

function of pressure. For example, in the absence of

accurate data, Ghani and Farid (2006) represented the



11/33

dependency of specific heat on temperature and pressure as

follows, assuming that no phase change would occur during

the HPP:

Cp M p; T Cp M T

Cp W T

ambp

Cp W p; T 3

However, it must be noted that the food should have a

high water content for this type of modelling to be accurate.

It is well known that the latent heat of fusion is reduced

under pressure and must be carefully considered when

modelling high-pressure low-temperature processes. There-

fore, the apparent specific heat of foods, which includes the

contribution of the heat capacity and the latent heat of

fusion, is generally used in modelling studies. The reason

for this is that unlike the specific heat, the apparent specific

heat can be modified to account for the freezing point

depression and the reduction in latent heat of fusion via the

simple shifting approach. For more details, the reader is

referred to the articles of Otero et al. (2006) and Denys et

al. (2000).

Thermal Conductivity

In the modelling of HPP at moderate temperatures, Ghani

and Farid (2006) have followed the above methods in

describing the dependency of thermal conductivity on

temperature and pressure when no physical data for the

modelled food under pressure was available:

lM p; T lM T

lW T

ambp

lW p; T 4

In the main, thermal conductivity does not change

substantially under pressure in foods above their initial

freezing point and can even be considered constant in

modelling exercises (Hartmann et al. 2003). In the phase

change domain, both shifting the atmospheric data and

using the additive model to calculate the thermal conduc-

tivity give reasonably accurate results, as thermal conduc-

tivity shifts according to freezing point depression without

exhibiting anomalous behaviour (Otero et al.2006).

Phase Transitions

The level of pressure imposed on a system determines the

liquidsolid phase transitions in water and food. The most

important benefits of high pressure combined with low

temperatures can be observed in the phase change diagram

of water and include (1) freezing point depression (a

minimum of 22 C at 209 MPa), (2) reduced latent heat

of fusion (from 334 kJ/kg at atmospheric pressure to

193 kJ/kg at 209 MPa), (3) a reduced change in specific

volume and (4) possibilities for the crystallisation (from

209 MPa) of higher level ice polymorphs with greater

density than water (Schluter et al. 2004). All of these

conditions are evident in Fig. 7.

Phase changes are classified according to the thermody-

namic changes occurring at transition temperatures (Roos

2003). During food processing and storage, phase tran-

sitions govern the deviations in a foods physical state, with

the temperature and pressure at which they occur beingspecific to the food material. As discussed by LeBail et al.

(2003), Schluter (2003) and Roos (2003), two types of

phase transitions occur in food systems, namely those of the

first and second order. In first-order transitions, the first

derivatives of the thermodynamic functions exhibit a

discontinuity in heat capacity and thermal expansion

coefficient at transition temperature (i.e. solidliquidgas

transitions). The amorphous structures of a food system,

which are formed during freezing or other forms of

processing, will undergo second-order transitions involving

no such discontinuity as, unlike first-order transitions, no

latent heat is required during the phase change; instead,there is a step-change in the properties suffering disconti-

nuity in the first-order transition (Roos 2003). The

existence of second-order transitions in amorphous food

structures increases the complexity of physical and chem-

ical changes in foods (Slade and Levine 1991). The

freezing of foods gives rise to metastable, amorphous or

partially amorphous structures which exhibit time-depen-

dent changes as they approach an equilibrium state, i.e.

crystalline (Roos2003).

The concept of metastable states as regards the

formation of different ice types was introduced about

40 years ago (Urrutia-Benet 2005). The concept was also

recognised by Kalichevsky et al. (1995) who noted the

possibility of obtaining certain ice forms, such as ice III or

ice VI, outside their range of stability. Metastable states can

be defined as those states at which the free energy is at a

relative minimum (Schluter2003), i.e. they correspond to a

domain in which one phase exists where another phase

would have a lower free energy. Their very existence gives

exploitable advantages to the HPLT industry. For example,

in pressure shift freezing, the presence of a metastable

supercooled liquid phase in the domains of ice I or ice III

could allow larger thermal gradients to be employed,

thereby permitting reduced processing times, and greater

amounts of ice instantaneously formed upon depressurisa-

tion (Urrutia-Benet et al.2006). Moreover, HPLT microbial

inactivation was found to perform best in the range of

conditions corresponding to the metastable region in the

domain of ice III (Shen et al.2005). Schluter et al. (2004)

recently provided definitions of the various metastable

phases, which, in turn, have been illustrated on the phase

change diagram of water (Fig. 8) by Urrutia-Benet et al.

(2007). Schluter et al. (2004) also showed that freezing



12/33

within metastable states could be predicted by a one-

dimensional numerical heat transfer model, which used

initial freezing points obtained from an experimentally

determined phase diagram for a potato sample, illustrated in

Fig.9. The model itself was used as a tool to give back the

corresponding values for the thermophysical properties for

each experimental condition. Doing this allowed the

authors to gain a very close fit to experimental profiles,

even when solidsolid transitions (ice Iice II) occurred. In

the comprehensive study of Schluter (2003), the authors

made some important conclusions, namely (1) as volume

changes increase concomitantly with pressure from +9% at

0.1 MPa to +13 MPa at 209 MPa, it is desirable to

pressurise the sample to the domain of ice III and as close

as possible to the triple point so that ice III has a better

chance of being formed (volume changes are 3%), (2) the

total freezing time may not be reduced when freezing to ice

III, as precooling time may be higher, (3) the degree of

supercooling is enhanced when freezing to ice III, thereby

promoting uniformity in crystal size and distribution. As

evident from Fig. 8, depending on idealised freezing or

thawing path followed in the phase transition diagram,

numerous different freezing or thawing processes can be

achieved. In fact, according to the working group of the

European project SAFE-ICE, there are in total seven

governing processes, and within this total, 13 special cases

exist (Urrutia-Benet et al. 2004). In the same study, the

authors provided clear terminology for each of these 20

processes via schematic and experimental paths and

temperature and pressure profiles; the most commercially

important of these HPLT processes will be discussed at a

later stage of this review. Supercooling was also clearly

Fig. 7 The influence of pres-

sure on the enthalpy of fusion of

ice, the specific volume changes

and the phase transition temper-

atures (Urrutia-Benet2005)



13/33

defined as the sudden temperature increase from nucleationtemperature to the initial freezing point. Table4summarises

the standardised nomenclature in HPP research.

Of the governing high pressure freezing and thawing

processes, those that have been modelled include subzero

cooling at high pressure, pressure-shift freezing (PSF),

pressure-assisted freezing (PAF), pressure-assisted thawing

and pressure induced thawing (PIT). Numerical modelling

can provide a clearer picture of the complex heat and mass

transfer mechanisms that govern these processes, and so it

is quickly becoming a comprehensive optimising technique

in freezing applications. Denys et al. (1997) were one of thefirst to develop a numerical model of the conduction heat

transfer within an analogue food during PSF and PIT

processes. In their study, the thermophysical data were

shifted along the temperature melting curve according to

the prevalent pressure. Reasonable correspondence between

predictions and experimental measurements were achieved.

Later, the predictions were enhanced when the authors

correctly permitted the apparent specific heat to change as a

function of pressure (Denys et al.2000). Many of the other

pressure-supported phase-transition modelling studies, us-

Fig. 8 The metastable states

that exist on the phase diagram

of water (Urrutia-Benet2005)

Fig. 9 The phase diagram of apotato sample (Schluter2003)



14/33

ing conduction heat transfer models, were reviewed by

Denys et al. (2001) and Schluter et al. (2004) and will not

be covered here. Instead, the most recent contributions, all

of which include convective heat transfer from pressure

medium to the processed sample, will be reviewed.

Kowalczyk et al. (2004) were the first to model convective

heat transfer during the pressure-assisted freezing and

thawing of water. Conservation equations for phase changewere adapted to account for a compressible medium, and

alongside linearised source terms, they were solved with

CFD simulations. Contrasting heat transfer mechanisms

between freezing under atmospheric pressure and high

pressure were observed. Most notably, the authors stressed

the importance of future studies or applications to deter-

mine convection timescales for both the heating and

cooling processes and to provide correct heating parameters

during the heating phase of thawing to avoid recrystallisa-

tion. In a later study, Kowalczyk and Delgado (2007b)

found that gravity considerably influenced the shape of ice

formed under pressure, although volumetric ice formations

under low-gravity and normal conditions were not signif-

icantly different. Recently, convective and conductive heat

transfer through a tylose solution have been modelled with

the aim of determining optimum processing lengths for

semi-continuous HPLT unit, and the results indicated its

feasibility in a commercial setting (Otero et al. 2007b).

It is also worth noting the study of Ozmutlu et al. ( 2006)

who were the first to experimentally observe the phase

change of water under pressure. This study determined the

relative contributions of momentum and energy transfers

during the development of both ice I and ice III via particle

image velocitometry and thermography. Such encouraging

developments provide an excellent platform for the devel-

opment of comprehensively validated models to gain

understanding of the physical mechanisms that govern

HPLT processes.

Developments in HPP Equipment and Processes

The general process-flow for both batch and/or semi-

continuous HPP has been discussed by other authors and

will not be considered here in detail (see Hogan et al. 2005;

van den Berg et al. 2001; Mertens and Deplace 1993;

Torres and Velazquez 2005; Hjelmqwist 2005). Batch

processing is the more conventional of the two operations

and was relatively easy to implement when HPP was first

commercialised in the food industry, as hot and cold

isostatic pressing technologies could be directly adopted

from the ceramic and metal industries. For batch systems,

advances in mechanical engineering have allowed the

development of enhanced intensifier designs, advanced

opening and closing mechanisms that promote efficient

processing times and better prestressing techniques that

allow vessels to work under higher pressures with greater

fatigue resistance (van den Berg et al. 2001). A semi-

continuous (or in-line) system can act as an alternative to

batch operations only when a pumpable product is being

processed. Consequently, over the years, their development

Table 4 Summary of HPP/HPLT terminology

Term Definition Reference

HPP High pressure processing Commonly used

UHP Ultra high pressure Commonly used

HHP High hydrostatic pressure Commonly used

HP High pressure Commonly used

Come-up time Time taken to pressurise the HP vessel Commonly used

Hold-time Time taken to maintain pressure in the HP vessel at a predefined level Commonly used

HPLT High pressure low temperature Urrutia-Benet et

al. (2004)

PAF Pressure assisted freezing: an unfrozen sample is frozen after pressurization at a constant

pressure

Urrutia-Benet et

al. (2004)

PSF Pressure shift freezing: a sample is frozen due to a pressure release, leading to an instantaneous

crystallization of ice, homogeneously distributed throughout the sample

Urrutia-Benet et

al. (2004)

PIF Pressure induced freezing: a thawed sample can frozen by forcing to a phase transition by

pressure increase (not possible to get ice I)

Urrutia-Benet et

al. (2004)

PAT Pressure assisted thawing: a sample is thawed at a constant pressure, the difference between the

sample and the bath temperature being the driving force for this process

Urrutia-Benet et

al. (2004)

PIT Pressure induced thawing: a frozen sample can be forced to a phase transition from ice to liquid

water by applying pressure along the melting curve of ice I

Urrutia-Benet et

al. (2004)

Plateau time or phasetransition time

The time span between nucleation and reaching a sample temperature (center) 5C below thecorresponding initial freezing point

Urrutia-Benet etal. (2004)

Supercooling The sudden temperature increase from nucleation temperature to the initial freezing point Urrutia-Benet et

al. (2004)



15/33

has been specifically aimed at the food industry. Most

notably, a semi-continuous operation promoted by many

HP system developers couples a number of pressure

systems so that most of the energy stored in a pressurised

vessel can be then used to pressurise a second vessel, thus,

saving energy and process time (van den Berg et al. 2001).

Some of the recent engineering developments and innova-

tive concepts that have contributed to the efficiency of HPPoperations will be discussed in the following.

From a review of the patented technology, it is obvious

that scientific research has caused many of the HPP

developments in the food industry. For example, a

controlled temperature HP system has been developed

based on the adiabatic heating phenomenon (Ting and

Lonneborg 2002). The authors claimed that this system

would improve the efficacy of the pressure treatment

process by providing an insulated vessel into which the

food product could be placed. This simple concept came

about only 2 years after the research of Denys et al. (2000)

who proposed that a high level of HPP uniformity could beachieved if the temperature increase resulting from com-

pression was anticipated and an appropriate heat source at

the boundary of the product was then applied. As discussed

above, more recent contributions have confirmed this

hypothesis, adding more credence to the potential of this

invention (Otero et al. 2007a; Hartmann et al.2004).

Other inventions have also been patented contempora-

neously to scientific research. For example, recent studies

have observed textural changes in HP-treated vegetables to

be primarily associated with very rapid changes in

hydrostatic pressure (compression and/or decompression)

during processing, which promotes turgidity loss (Trejo-

Ayara et al.2007). Contemporaneously with these findings,

Ting and Anderson (2006) have developed a system and

method for decompressing a HP vessel in a controlled

manner over a selected period of time. In justifying this

invention, the authors claimed that by controlling decom-

pression, the texture of the processed product can in turn be

controlled, and as pressure is one of the primary thermo-

dynamic variables controlling complex biomolecular struc-

ture, controlling decompression may allow delicate

structures to remain near equilibrium. It was also suggested

that rapid decompression of a food material may cause

cellular damage due to rapid expansion of the gas that was

dissolved during pressurisation, and that slow decompres-

sion could allow gases to diffuse from structures without

cellular rupture. Although these suggestions are in line with

the scientific hypothesis of Trejo-Ayara et al. (2007), they

have yet to be proven within the scientific domain.

In batch HPP systems, the product is generally treated in

its final primary package; commonly, the food and its

package are treated together and so the entire pack remains

a secure unit until the consumer opens it. When

considering new technologies, which involve the treatment

of packaging materials, it is important to study the safety of

the material, the possible formation of compounds that

influence the odour and taste of the food and the effects of

pressure on mechanical and physical properties of the

packaging material, e.g. strength and barrier properties.

HPP requires airtight packages that can withstand a change

in volume corresponding to the compressibility of theproduct (Hugas et al.2002), as foods decrease in volume as

a function of the pressure applied, while an equal expansion

occurs on decompression. For this reason, the packaging

used for treated foods must be able to accommodate up to a

15% reduction in volume and return to its original volume

without loss of seal integrity or barrier properties. Packag-

ing materials, which are oxygen-impermeable and opaque

to light, have been developed for keeping fresh colour and

flavour of certain HP-treated foods (Hayashi1995). For HP

pasteurisation, a method and apparatus to store and

transport treated and untreated foods during HPP have

been develo ped by Hotek and Morrison (2006). Inproduction, the use of flexible pouches can achieve high

packing ratios; the use of semi-rigid trays is also possible,

and vacuum-packed products are ideally suited for HPP.

Miller and McLean (2006) have developed a flexible water-

resistant packaging to prevent water from coming in contact

with a food product during HPP. As the size and shape of

the product will have major effects on the stacking

effectiveness of the product carrier, they must be optimised

for the most cost-effective process. This allows further

development of innovative package shapes and printing

graphics (Ting and Marshall2002).

Recent Applications of HPP

Maintaining Food Quality Characteristics

Knowledge of the sensory and nutritional characteristics of

food products is essential for product development, quality

control, sensory evaluation and design and evaluation of

process equipment (Ahmed et al. 2003; Polydera et al.

2003). Thermal processing can often lead to quality

changes in foods such as the destruction of vitamins,

modifications to food texture and colour and the develop-

ment of off-flavours. It is generally considered that HP

operations can render harmful microorganisms inactive

without having a detrimental effect on food quality (Smelt

1998). Increasing treatment pressures will generally in-

crease microbial inactivation in shorter times, but higher

pressures may also cause greater levels of protein denatur-

ation and other potentially detrimental changes in food

quality when compared to the unprocessed product. Yet, as

no shear forces are generated by HPP, the physical structure



16/33

of most high-moisture product qualities remains minimally

changed after treatment. Food characteristics which dictate

the consumers perception of food quality, and consequent-

ly the ability of HPP as a processing technology that retains

these characteristics, are reviewed in the following. Some

of the effects of HPLT process on food quality are

summarised in Table5.

Fruit and Vegetables and Derived Products

As discussed by Cano and de Ancos (2005), the texture of

fruit and vegetable products are largely determined by the

structure of the cell wall and middle lamella. Under

pressure, the composition of these can change, as certain

cell wall enzymes are inactivated and/or structural changes

occur in the polysaccharide, lipid and protein fraction. On a

physical level, HP can disrupt the tissues morphology, cell

organelles and cell membranes (Hartmann et al. 2004).

Pressure has been shown to have a softening influence on

texture of fruits and vegetables, and tissue firmness may belost due to cell wall breakdown and loss of turgidity (De

Belie 2002). Trejo-Ayara et al. (2007) have found that

textural changes in raw carrots are primarily caused by loss

of turgidity induced by rapid compression and decompres-

sion. They noted that texture loss may be reduced by

turgidity manipulation of the cells or reduced by pectin

methylesterase (PME) activation during high pressure

processing given optimal conditions. In addition, they

observed loss in texture when carrots were treated with

pressures of above 300 MPa. Turgidity loss has also been

found in the cell structures of spinach, which were exposed

to a pressure level of 400 MPa for 30 min, owing to the soft

and elastic structures which characterise the cell walls; the

same was not found for tougher plant tissues such as

cauliflower (Prstamo and Arroyo 1998). Basak and

Ramaswamy (1998) found that pressure-induced textural

changes occurred in two phases, namely the textural change

due to instantaneous pressure application followed by a

gradual texture recovery or further loss during pressure

holding. In the same study, texture recovery was achieved

between 25 and 40 min for vegetable products under a

pressure of 100 MPa.

Biochemical changes also play an important role intexture modification during HPP. PME, which is found in

plants and bacteria, de-esterifies plant cell wall pectins,

resulting in methanol and pectin with a lower degree of

methylation. In some cases, PME may enhance the texture

of fruit and vegetable products (Villarreal-Alba et al.2004).

However, it is mostly known for inducing cloud separation

in fruit juices, making PME inactivation a prerequisite in

their processing. Moreover, the action of both PG and PME

results in the softening of plant tissues, a decrease in

viscosity, as well as cloud separation in fruit juices (Cano

and de Ancos 2005). In response to these attributes, HPP

has been used to improve or preserve the viscosity oftomato-based products by inactivating PG whilst maintain-

ing PME activity (Crelier et al. 2001; Fachin et al. 2002,

2004). As PME is reasonably tolerant to HP, complete

inactivation is only successful in real food samples at very

high pressures, i.e. pressures in the range 400 to 600 MPa

combined with mild heat (50 C) to accelerate PME

inactivation were advised by Nienaber and Shellhammer

(2001). Other influencing factors such as temperature, pH

and solids and protein concentrations must be considered

when pressure treating enzymes.

The colour of most fruit and vegetable products such

as jams, fruit juices and purees is generally preserved

once thresholds of temperature and/or pH are observed

(Ludikhuyze and Hendrickx 2001). For example, discol-

ouration of broccoli juice was found after exposure to pressures

Table 5 Summary of some food quality characteristics after HPP

Product type Treatment (MPa/C/min) Comparison to experimental control Reference

Orange juice 500/35/5 Improved shelf-life, better consistency, lower acid loss Polydera et al. (2003)

Sausages 500/65/5 and 15 Better texture, improved taste, more juicy, less firm, no loss in colour Mor-Mor and Yuste

(2003)Green Beans 500 /room temp./1 Retention of colour, good firmness and extended shelf-life, showed

residual peroxidase activity

Krebbers et al. (2002)

1000 /105/1.3 HPP showed similar reductions of vegetative cells and spores as in

heat-sterilized green beans.

Beef 150/60/30 Stimulate d proteolysis and ultra-structural changes, tougher meat,

less juicy

Bertram et al. (2004)

Salmon 200/20/10 Lighter colour, increased tissue firmness, shelf-life extended Lakshmanan et al.

(2003)

Cheese 400/20/20 Higher yield, highe r pH, reduced microbial content, less crumbly,

no colour change

Sandra et al. (2004)



17/33

at temperatures more than 50 C, owing to chlorophyll

degradation. However, below this temperature, pressures

of up to 800 MPa have been applied without having a

negative effect on chlorophyll (Van Loey et al. 1998). The

ability to preserve colour at high pressures is not evident in

some products, e.g. owing to polyphenol oxidase (PPO),

the colour of an onion becomes brown upon exposure to

pressure, turning browner contemporaneously with increas-ing pressure intensity (Butz et al. 1994). Krebbers et al.

(2003) observed an increase in colour of tomato juice when

treating the samples at 700 MPa, for 1 min at 8090 C, as

a result of compacting and homogenizing effects of the

high pressure treatment. Rodrigo et al. (2007) found that no

colour degradation of tomato appeared under combined

thermal and high pressure treatment (300700 MPa,

60 min, 65 C), and a maximum increase in colour of

8.8% was found for strawberry samples (pH 5). Thus,

recent results suggest that HPP promotes colour retention

once circumspect treatment is applied.

In many fruit and vegetable products, HPP has either noor minor influence on flavour. Lambadarios and Zabetakis

(2002) found that HP had very little effect on strawberry

flavour compounds. Highest flavour stability was observed

when samples were treated with pressures of 200400 MPa,

and the best flavour retention was observed at 400 MPa.

Fruit juices, jams and purees all show excellent retention of

fresh like flavours for a far greater time period than that

exhibited by conventional thermal treatment under optimal

storage conditions (Ludikhuyze and Hendrickx 2001). In

fact, quite recently Baxter et al. (2005) found that HPP of

orange juice could produce a product acceptable to most

consumers even after storage for 12 weeks at temperatures

up to 10 C. On the other hand, storage at 30 C causes

900% increase in the rate of flavour deterioration (Polydera

et al. 2004).

Meat and Derived Products

As pressure bears a considerable influence on the structure

and functionality of many proteins, it consequently affects

textural, sensory and nutritive properties of meat and meat-

derived products (Jung et al. 2000). For meat systems, the

effectiveness of HPP depends on the characteristics

associated with the specific meat product and the intensity,

holding-time and temperature of HPP operation. Other

influencing factors include whether a meat is in a pre- or

post-rigour state, the meats pH and ionic strength, etc.

(Cheftel and Culioli 1997). Although investigations of the

effects of HPP on meat quality are limited, studies have so

far found that HP treatments can influence texture and

colour in raw, cured and battered meat systems (Jung et al.

2000; Carballo et al. 2000).

From the studies of raw meat, HPP has been shown to

tenderise meat when applied pre-rigor, but does not have a

pronounced effect on post-rigor meat at low temperatures,

with some studies even showing that HP causes meat

hardening (Jung et al. 2000). Recently, Ma and Ledward

(2004) found a massive decrease in hardness, chewiness

and cohesiveness at 200 MPa and 70 C, which they

attributed to increased enzymic activity on protein struc-tures that have been drastically modified. At lower pressure

and temperature combinations, similar results to those

found in the literature were reported. Jung et al. (2003)

found that exposing raw meat to a high intensity of pressure

(520 MPa) for a short time (260 s) led to a decrease in the

evolution of total meat flora and a consequent delay of

growth of a week. It was then hypothesised that this delay

increases the meat maturation period, which, in turn, could

improve the meat tenderness. The authors also found meat

colour to be highly dependent on pressure intensity, as

pressures of 130 MPa improved redness, yet pressures

above 325 MPa resulted in strong discoloration, i.e. aheightening in brown colouration. Jung et al. (2003) related

this discolouration to the increase in metmyoglobin (Fe3+)

content in the meat after pressurisation.

High pressure technology has also been employed as

a stabilising and texturising technique for meat paste

(Apichartsrangkoon and Ledward2002; Apichartsrangkoon

2003; Jung et al. 2000). Pressure-induced changes in

protein and subsequent aggregation leads to the formation

of gels, which have better quality characteristics than those

procured through thermal means (Supavititpatana and

Apichartsrangkoon 2007). The influence of combined

pressure and heat treatment in gel formation may or may

not be synergistic, depending on the meat system under

investigation (Supavititpatana and Apichartsrangkoon

2007; Carballo et al.2000). Nevertheless, increasing either

pressure or temperature during treatment was found to

increase gel strength, leading to a useful means of

producing meat pastes with different eating qualities

(Supavititpatana and Apichartsrangkoon 2007). When

applied to cooked sausages, Mor-Mor and Yuste (2003)

reported that HPP increased cohesiveness and reduced

firmness when compared to heat-treated sausages. They

also reported that weight loss was significantly higher in

heat-treated sausages than in HP-treated control samples.

As for changes in colour, HP-treated meat pastes became

lighter, as both the intensity of pressure and temperature

increased, thereby reducing the saleability of meat products

after processing at higher intensities (Yuste et al. 1999;

Supavititpatana and Apichartsrangkoon2007).

For dry-cured meat products, their ability to retain

quality characteristics during HPP and throughout chilled

storage has been investigated by some authors (Rubio et al.

2007; Serra et al. 2007). Rubio et al. (2007) found that



18/33

deterioration in sensorial qualities of treated-cured ham

(500 MPa for 5 min) limited its storage time to 90 days,

although an adequate shelf life for microbial control was

found to be 210 days in the same storage conditions. Serra

et al. (2007) studied the textural and visual qualities of

pressure-treated frozen hams at different early stages in the

dry-cured process. They found that the pressurised hams

showed lower visual colour intensity than the control ones,but did not have any significant affect on sensorial

properties of the ham. They also observed HP to increase

the ham fibrousness, which, they hypothesised, could be

useful to improve the texture of dry-cured hams with

excessive softness.

Dairy Products

As noted by Huppertz et al. (2006), although milk was the

first food to undergo HP treatment by Hite (1899), up to

now, no milk products have been commercially treated with

HP, attributed accordingly to the complex changes that milkand derived products undergo during HP applications. The

effects of HP on milk constituents, milk properties and

bacteria that are present in milk have been comprehensively

reviewed by Huppertz et al. (2006). As well as this,

investigations into the functional improvements of milk

whey proteins promoted by HP treatment are discussed by

Lopez-Fandino (2006) and will not be covered here. Instead

of a detailed review of physiochemical and technological

changes that HP imposes on dairy products, some instances

of where the relevant functionality of dairy products, e.g.

milk and cheese, have been altered by the application of HP

technology will be discussed.

A recent finding of high importance was made by

Gervilla et al. (2001) who observed the level of free fatty

acids in ovine milk to either remain unchanged or be

reduced by HPP (500 MPa at 4, 25, 50 C), ameliorating

the effects of milk rancidity during storage. The effect on

milk fat globules was noted and seemed to be specific tothe temperature of the treatment. For example, smaller

globules were slightly increased at temperatures of both 25

and 50 C (which may have been due to the formation of

large casein aggregates; Huppertz et al. 2003), thereby

increasing milk stability, whereas at 4 C globules were

increased in size which in turn influenced the creaming

phenomenon. As seen in Fig. 10, the creaming phenome-

non in raw bovine milk was recently found to be highly

dependent on the level of pressure applied, with the volume

of percentage of cream peaking at 200 MPa and reducing to

a minimum at 600 MPa (Huppertz et al. 2003). The effect

of temperature on creaming was not examined. The authorstried to use Stokeslaw to explain this phenomenon, i.e. the

rate of rise of fat globules is inversely correlated with the

viscosity of the suspending medium. However, although an

increase in the milk viscosity was observed with increasing

pressure, this being attributed to the shape of the casein

micelle as well as the disruption caused to them during

treatment, the observed reduction in creaming was much

greater than that calculated from Stokeslaw. In addition, in

opposition to the results of Gervilla et al. (2001), no

significant HP-induced effect on milk fat globules was

Fig. 10 Change in the volumeof cream as a function of pres-

sure (Huppertz et al.2003)



19/33

noted. Consequently, HP-induced changes in the creaming

of milk were attributed to changes in the formation of

clusters of milk fat globules in the cold, i.e. cold

agglutination. From their findings, the authors concluded

that the use of HP presents some exciting opportunities in

the homogenisation of milk and in the development of new

milk products, as unlike traditional processing techniques,

flavour compounds are unharmed and microbial contentcan be contemporaneously reduced (Huppertz et al. 2003).

In cheese making, the attributes of HP treatment are

currently being studied extensively. The most interesting,

not to mention the most economically important, inves-

tigations include those highlighting the differences between

cheese made with treated and untreated milk, the acceler-

ation of cheese ripening and of course the reduction of

pathogenic or spoilage microbes. From studies undertaken

thus far, HP treatments at intensities greater than 200 MPa

have enhanced acid and rennet coagulation and curd

firmness times in cheese, with timescales being dependent

on the treatment temperature and pressure holding time(San Martin-Gonzalez et al. 2007; Huppertz et al. 2005).

The main problems with using HP-treated milk for cheese,

similar to heat-treated milk (of course depending on the

type of heat treatment), are associated with the deterioration

in composition that can arise; these can even violate the

prevailing standards for cheese and are owing to the

moisture retention abilities of HP-treated milk (San

Martin-Gonzalez et al. 2007). This increased moisture

retention was suggested to be due to the formation of a finer

structural network and to the water-binding properties of

denatured -lg incorporated into the protein matrix (Needs

et al. 2000) and has also been attributed to temperature

during HP treatment (San Martin-Gonzalez et al. 2007).

Overall, HP treatment has been found to affect rennet

coagulation and other cheese-making characteristics of milk

in a fairly positive manner, although treatments could be

economically costly due to relatively long treatment time

required on expensive equipment. HP treatment can also be

conducted during the cheese-making, e.g. it was also

reported that HP treatment of Mozzarella cheese signifi-

cantly accelerated the development of desirable functional

properties on melting (OReilly et al. 2002). However, the

application of HP as a pre-treatment of milk may limit the

cost of HPP (Huppertz et al.2005).

Inactivation of Microorganisms

A primary objective of a food preservation technique is to

prevent pathogenic microorganisms from affecting the

safeness of a product. Microorganisms are resistant to

selective chemical inhibitors due to their ability to exclude

such agents from the cell, mainly by the action of the cell

membrane. However, if the cell membrane becomes

damaged, e.g. due to HP treatment, this tolerance is lost,

and the cells are vulnerable.

A secondary objective is inactivation of spoilage micro-

organisms to improve the shelf life of the food. Growth of

microorganisms in foods can cause spoilage by producing

unacceptable changes in taste, odour, appearance and

texture. The stage of growth of the microorganism can

have an effect on its pressure resistance, with cells in thestationary phase being more resistant than those in the

exponential phase (McClements et al. 2001). HP treatment

is known to cause sublethal injury to microbes, which is a

particularly important consideration for any preservation

method.

Microbial inactivation by HP has been extensively

studied and has been concluded to be the result of a

combination of factors (Manas and Pagan 2005). The

primary site for pressure-induced microbial inactivation is

the cell membrane (e.g. modifications in permeability and

ion exchange; McClements et al. 2001). Microorganisms

are resistant to selective chemical inhibitors due to theirability to exclude such agents from the cell, mainly by the

action of the cell membrane; however, if the membrane

becomes damaged, this tolerance is lost.

The ability of HP to effectively inactivate microorgan-

isms is heavily reliant on the pressure range afforded by the

HP system, with current technology limiting commercial

HP applications to 700 MPa. Bacteria, fungi and viruses

can all be processed at pressures lower 800 MPa, the

growth and reproduction of which are severely hindered at

pressures up to 200300 MPa, with total inactivation

occurring at higher pressures. The mechanisms of microbial

inactivation including cell morphology as discussed above

and biochemical reactions and genetic mechanisms etc.,

these have been detailed by numerous authors and are not

discussed here (Hoover et al. 1989; Torres and Velazquez

2005). Instead, the following paragraphs will focus on the

influence of HP (

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