Heat Sterilization

Module- 40

Lec- 40

Dr. Shishir Sinha

Dept. of Chemical Engineering

IIT Roorkee

• Sterilization is the total elimination of all microorganisms including spores

Typically the last things to die are the highly heat- and chemical-resistant bacterial

endospores

Instruments used for invasive procedures must be sterilized prior to use

Moist heat or steam, radiation, chemicals (e.g., glutaraldehyde), and ethylene oxide (a

gas) are employed for sterilization

Sterilization by autoclaving, which uses moist heat, is used in most hospital and

microbiology laboratory settings

Heat Sterilization

• Applying heat to foods to decrease the concentration of the viable microorganisms to

such a level that would only allow growth of microorganisms and spores in the food

under defined storage conditions to an acceptable level (commercial sterility).

In heat processing, to achieve microbial stability and eating quality both:

1. The temperature of heating and

2. The duration of the thermal process are important. An optimum balance needs to be

found to avoid over- and underprocessing.

To design a heat process it is necessary to determine:

1. The heat resistance of the spoilage microorganisms (target microorganism)

2. The temperature history of the food at the slowest heating point. (thermal center)

Thermal destruction of bacteria

Bacteria have a logarithmic order of death when subjected to high temperatures.

Log of viable bacteria concentration vs. time of exposure is a straight line relationship

called a survivor or a thermal destruction curve.

F

co

log

The

log

D

D

D

te

T

ce

te

F

N

or the target

oncentration

g (N/Ni) = Sl

e slope of th

(N/Ni) = -t/D

D is called th

D(T)

D is the time

emperature.

The decimal r

ertain types

emperatures.

rom survivo

N = Ni x 10

-(t

Surviv

t microorgan

n at time t ca

ope (t-0)

he survivor cu

D

he decimal re

e period nee

reduction tim

s of food (

. It is strongl

or curve equa

t/D)

N0 on

vor or therm

nism, if the

an be estimat

urve is defin

eduction tim

eded to decre

me, D is dete

(growth me

ly dependent

ation:

nly if t

mal destruc

initial viabl

ted by:

ned as -1/D,

me which is c

ease viable

ermined for

dium, aw, p

t on tempera

ction curve

le cell conce

constant at a

cell concent

each type o

pH, compos

ature.

entration is N

a given temp

tration 10-fo

f target micr

sition etc.)

Ni, viable ce

perature. D

old at a give

roorganism i

for differen

ell

=

en

in

nt

A

Bas

P

ce

th

su

A

m

Effect of

During

Since D

T =

An infinite ti

sis for de

roduct will b

ells of the ta

hat the spoil

uggested she

A reduction e

m = log(Ni/N

f varying tem

g a thermal p

D = D(T), an

= T(t), D = D

ime will be

efining comm

be accepted

arget microo

lage hazard

elf life.

exponent is d

N0)

mperatures

process temp

n integration

D(t)

required fo

mercial steril

as commerc

organism is r

it presents

defined as:

s

perature vari

n w.r.t. time

logN

N

or the destru

lity.

cially sterile

reduced belo

is commerc

ies with time

is necessary

0

t

i

N dt

N D

uction of al

when the co

ow a certain

ially accepta

e at the therm

y:

l viable mic

oncentration

level N0 jus

able within

mal center of

croorganism

n of the viab

st low enoug

the period o

f the food.

ms.

le

gh

of

Ni,

tf :

Con

log

con

The

Ste

Nf : initial a

duration of t

ndition for c

g (Nf/N

i) lo

ndition for co

e processing

ps: Generate

and final viab

the thermal p

commercial s

og (N0/N

i) ,

ommercial s

g time tf can b

e T vs. t data

ble cell conc

process need

logN

N

sterility: Nf

since m = -

0

tfd

D

terility beco

be estimated

a find D v

centrations,

ded to achiev

0

tff

i

N dt

N D

N0

log (N0/N

i)

dtm

D

omes:

d by graphica

versus T data

ve commerc

al integration

a from litera

ial sterility.

n of 1/D ver

ature plot

rsus t

1/D versus t

t.

T

cor

If

The variation

rrelated as

f at temp. t

Mode

n in the lo

a linear fun

the decimal r

eling tempera

ogarithm of

nction of tem

reduction tim

ature depend

the decima

mperature.

me is D , th

dency of D

al reduction

hen at T, D w

n time D co

will be:

ould be weell

z-

te

Plu

lett

For c

L is d

F

-value is the

enfold. From

ugging into c

ting:

commercial s

defined as th

or each kind

e temperatur

m the equatio

condition for

sterility:

he lethal rate

d of microorg

re difference

on above:

r commercia

1L

(

0

110

tf

D

logD

D

1

D D

0

tf

Ld

ganism z-va

e required to

l sterility: 0

tf

( )/10 T z

( )/T zdt m

( )T

z

( )/110 T z

D

dt mD

alues can be f

o change th

fdt

mD

m

found in lite

e decimal re

erature.

eduction tim

me

(1

T

an

T

kn

fo

S

te

is

is called th

121.10

C), the

The value of

nd it is deno

The equivale

nown z-valu

or commerci

ince most ta

emperature

F

s used. For c

he reference

e max. temp

f the integral

oted by F.

ent time val

ues at a fixe

ial sterility is

arget microo

is usually t

= F0

(F121.1

)

commercial s

temperature

perature expe

l 0

tLdt

lues are est

ed reference

s denoted as

organisms h

taken as 121

)

sterility: F

e. For steriliz

erienced by t

is called the

timated for

temperature

zF

ave z-values

.10

C, for thi

mD

zation opera

the food in r

e equivalent

certain targ

e. Therefore

s close to 10

s specific ca

ations it is ta

retorts.

t time of the

get microorg

e, equivalent

0 and since

ase:

aken as 2500

e heat proces

ganisms wit

t time neede

the referenc

0

F

ss

th

ed

ce

Example

H

or

time n

the pr

e Problem:

Heat penetrat

rganism for

ecessary to

rocess ?

tion data on

this food is

achieve com

Time(m

0

2

4

6

8

11

14

17

20

n a vacuum

s C. Sporoge

mmercial ster

min) T (

packed cor

enes (D=0.

rility for thi

(Deg.F)

82

217

230

233

233

228

232

237

240

rn are given

8). What is

s food assum

L

0

0.0146

0.0775

0.114

0.114

0.06

0.1

0.215

0.278

in the Tabl

the minimu

ming instant

le. The targ

um processin

t cooling afte

et

ng

er

24 242 0.36

29 245 0.526

32 246 0.599

35 247 0.68

Formula method for thermal process evaluation

This method aims to perform the integral analytically to estimate the equivalent

time.

Let Tr be the constant temperature of the medium where the food is heated. A

dimensionless temperature V is defined as:

V = ( Tr – T) / ( T

r – T

0 )

T0 = initial temperature at the thermal center,

T = temperature at the thermal center at time t

at t = 0 ; V = 1.0 , as t , TTr , V0.0

A plot of logV vs. time can be approximated with a straight line.

0

tf

dt

mD

Thermal destruction of microorganisms occurs to the most part when the linear

asymptote forms a good approximation to the heating curve.

The linear asymptote is specified by defining two parameters; the lag factor j ( j=1.06-

1.40 for conduction-, j1.0 for convection heating) and the slope –1/f .

The equation for the asymptote is:

-1/f = (logV-logj) / (t-0) t/f = log(j/V)

log j ( Tr – T

0 ) / ( T

r – T ) = (1/f) t

dt = f M dT/(Tr-T) , M = loge = 0.4343

Inserting dt = f M dT/(Tr-T) into the integral for equivalent time

F= o

10(T- )/z

dt

F = o

10(T- )/z

fM dT/(Tr-T) this integral is

analytically solved in many steps to obtain:

F = M f exp(Tr-)/Mz -E

i(-g/Mz) + E

i -(T

r-T

0)/Mz

g = Tr-T at the end of the heating period (t=t

h)

Ei(-x)

is an exponential function, values of which are read from mathematical tables.

Since (Tr-T

0)/Mz has a high value, E

i -(T

r-T

0)/Mz is very small, this term is usually

neglected.

F = M f exp(Tr-)/Mz -E

i(-g/Mz)

This equation relates the equivalent time to the processing temperature (Tr) and

processing time (contained in g), for a given target microorganism of given z-value, for

a certain food (heat transfer characteristics, contained in g and f)

g = Tr-T

log j ( Tr – T

0 ) / ( T

r – T ) = (1/f) t

Summar

Microbio

D, z-valu

F, the equ

ry of heat pr

ological inpu

ues for the ta

uivalent tim

in

he

co

establishe

meet mic

and proce

rocess calcu

ut

arget microor

e necessary

processing

nitial temper

eating mediu

ooling mediu

ed process (p

crobiological

essing requir

ulations

Heat pen

rg. T vs

f, j

g conditions:

rature

um temp.

um temp.

processing t

l, heat penet

rements)

etration inpu

s. time data

j-values

:

time to

tration

ut

Steriliza

Mainly tw

1. S

2. S

Selection

on the pa

- tin

- gla

- film

Steriliza

M

ation method

wo methods

terilization i

terilization b

n of sterilizat

ackaging ma

(metallic) ca

ass jars

m pouches

ation in cont

Mostly carrie

ds

:

in containers

before placin

tion method

terial used:

ans

tainers

ed out by hea

s,

ng into the c

largely depe

ating the pac

ontainer

ends

ckaged foodss in saturatedd steam

S

pr

In retort

a) have

b) min

c) limi

1. c

2. u

con

3. p

Internal p

1. T

2. T

3. In

terilization o

ressurized v

t operations

e adequate v

nimize therm

it thermal an

control of he

use of press

ntainer.

processing ja

pressure incr

Thermal expa

Thermal expa

ncreased vap

of low acid

vessels (retor

s it is impor

venting of air

mal shock to t

nd pressure s

eat-up, cool-d

surized air d

ars immersed

rease ofconta

ansion of foo

ansion of hea

por pressure

foods is car

rts) are used.

rtant to:

r from the re

the food,

strain on the

down rates.

during coolin

d in water.

ainers:

od

adspace gas

of water

rried out at

.

etort and con

containers b

ng to balanc

temperature

ntainer surfa

by:

ce increased

es above 100

ces to avoid

d internal pr

00

C, therefor

d air pockets,

ressure in th

re

,

he

A vertica

Hydrost

al batch retor

tatic sterilize

rt

er

Sterilization of food outside container

High temperature processing (T1500

C) by means of high speed heat exchangers

reduces processing time substantially (to few seconds) and improves product quality.

Such processes are called high-short processes (HTST -applied to sterilization of

milk).

Improved product quality is due to the fact that destruction of nutrients and flavor

components in foods (vitamins, colors, antioxidants, enzymes, amino acids) are

similar to destruction of bacteria. But the z-values of nutient compounds are

considerably larger than that of the microorganisms.

Example: For a certain food F10

120 = 10 min is needed for commercial sterility. Two

alternative procedures:

1. Heat food instantaneously to 1200

C, hold at this temperature for 10 min and cool

instantaneously. F=10(T-)/z

tf= 10

(120-120)/10

x 10 = 10min.

2.

Heat food inst. To 1400

C, hold at this T for 0.1min and cool inst. F= 10(140-120)/10

x 0.1 = 10min.

Suppose this food contains a valuable enzyme with a z-value of 50C0

which requires 4

min at 1200

C for inactivation. At 1400

C time required for inactivation will be:

t = 4 x 10(120-140)/50

= 1.6 min.

Processing time needed Time needed for enzyme inactiv.

Procedure 1: 10 min 4 min

Procedure 2: 0.1 min 1.6 min

Aseptic processing

Sterilized food packed in sterile containers under aseptic conditions.

Advantages:

1. Product with higher organoleptic and nutritional quality,

2. Possibility to use large containers to pack the food,

3. Extended possibilities for using packaging materials of many package sizes, shapes

and materials,

4. Handling of containers during subsequent sterilization is avoided, recontamination

risk during cooling is minimized.

Limitations:

1. Large capital investment.

2. Pumping at high pressures, product must be relatively homogeneous.

3. Need for specific design of systems for a specific product.

4. Complex operation requiring careful control and sophisticated instrumentation, need

for highly trained personnel

5. Relatively limited filling rate (200 packages per min. versus 600 tin cans per min)

There are two methods of commercial sterilization:

• Heating the food after placing it in a container.

• Heating and cooling the food then aseptically packaging and sealing.

Heat

• Moist heat

– effective against all types of microorganisms

– degrades nucleic acids, denatures proteins, and disrupts membranes

– Boiling, autoclaving

• Dry heat sterilization

– less effective, requiring higher temperatures and longer exposure times

– oxidizes cell constituents and denatures proteins

– Flame, oven

• Endospores – greatest resistance

• Vegetative cells differ in sensitivity to heat

• Higher temperatures allow shorter exposure times

Measuri

• T

• D

Other m

• Z

• F

ing heat-kill

Thermal deat

– shorte

tempe

Decimal redu

– time r

tempe

measures…

Z value

– increa

value

– time i

spores

ling efficien

th time (TDT

est time need

erature and u

uction time (D

required to k

erature

ase in temper

n minutes at

s

ncy

T)

ded to kill a

under defined

D or D value

kill 90% of m

Fig

rature requir

t a specific t

all microorg

d conditions

e)

microorganis

gure 7.1

red to reduce

temperature

ganisms in a

s

sms or spore

e D by 1/10

needed to ki

a suspension

s in a sampl

ill a populati

n at a specif

e at a specif

ion of cells o

fic

fic

or

Moist he

• C

• 4

Boiling w

o 1

o B

o B

o M

• A

• u

• ki

• H

eat

Coagulation a

methods:

– Boilin

– Steam

– Pasteu

– Non-p

water:

00 °C

Boiling water

Bacterial end

Method less e

Autoclave

se saturated

ill endospore

Heat-resistant

and denatura

ng water

m under press

urisation

pressurised s

r kills vegeta

dospores - res

effective wit

steam under

es

t materials –

ation of prote

sure – autocl

steam

ative cells an

sistant to bo

th change in

r pressure to

– glassware,

eins – halts c

laving

nd spores of

iling water a

atmospheric

o reach temp

cloth, metal

cellular meta

eucaryotes w

and will not

c pressure

eratures abo

llic instrume

abolism

within 10 mi

be sterilized

ove boiling

nts, liquids,

inutes

d

• 121 o

C for 15 min - kills all endospores and vegetative organisms

• Wet steam generated under a pressure of 100 kPa/ 15 pounds per sq. inch to reach 121 o

C

• destroys nucleic acids, enzymes and proteins in the cell

• Endospores of Bacillus stearothermophilus or Clostridium used to determine

effectivity of heat sterilisation

Pasteurization of milk

• flash pasteurization:

– high temperature short-term – HTST: 72 °C for 15 seconds then rapid cooling

– batch pasteurization:

– Low temperature high term – LTHT: 63 °C for 30 min

• ultrahigh-temperature (UHT) sterilization:

– 140 to 150°C for 1 to 3 seconds

Tyndallisation

• Some products cannot withstand autoclaving temperatures

• Repeated heating at 90 – 100 °C for 30 min on 3 successive days and in between

incubation at 37 °C

• Allows endospores to germinate into less resistant vegetative cells

• Temperature not sufficient to kill spores – so multiple exposures required

• Eggs, carbohydrates, some canned foods

Dry heat sterilization

• Incineration – flame, incinerators

• Microbes reduced to ashes and gas

• Items are heated in an oven at 150 – 180 °C for 2 to 4 h

• Destruction of spores

• Glassware, powders, oil

Low temperatures

• freezing

– stops microbial reproduction due to lack of liquid water - bacteriostatic

– some microorganisms killed by ice crystal disruption of cell membranes

• refrigeration

– slows microbial growth and reproduction

• Psychrophiles

• -70 °C to -135 °C – preservation of cultures

Dessication and osmotic pressure

Dessication - dehydration of organisms – not microbicidal, but microbistatic Used in

food preservation

Freeze-drying – fast freezing, sublimation of water under a vacuum

Increasing the osmotic pressure of the external environment

High salt- or sugar concentrations

Hypertonic environment – cell lysis

Filtration

• reduces microbial population or sterilizes solutions of heat-sensitive materials by

removing microorganisms

• also used to reduce microbial populations in air

Filtering liquids

• Heat-sensitive liquids

• depth filters

– thick fibrous or granular filters that remove microorganisms by physical

screening, entrapment, and/or adsorption

• m

membrane fil

– porou

prima

lters

us membran

arily by phys

nes with de

sical screenin

Fig

efined pore

ng

gure 7.4a

sizes that remove mi

icroorganismms

Filtering

• su

• co

• hi

Fi

g air

urgical mask

otton plugs o

igh-efficienc

– used i

– Hospi

igure 7.5b po

ks

on culture ve

cy particulat

in laminar flo

ital theatres

olycarbonate

essels

te air (HEPA

ow biologica

e membrane

A) filters

al safety cab

with 0.4 μm

binets

m pores

Radiatio

• U

• T

• P

• L

m

• D

on

Ultraviolet lig

Thymine dim

enetrates gla

Low penetra

microorganism

Damaging to

ght (UV) - 2

mers – misrea

ass, water et

ating ability

ms

the skin and

Fig

260 nm

ading of gen

c poorly

y – used

d eyes

gure 7.6a

etic code – l

to decont

leads to cell

aminate su

death or alte

urfaces or

ered growth

for airborn

ne

• Io

• D

• E

• S

• O

• G

Sterilized

Advanta

1. P

2. P

3. E

an

onising irrad

Destroys endo

Effect on sug

terilisation o

Outstanding f

Gamma irrad

d food packe

ages:

roduct with

ossibility to

Extended pos

nd materials

diation: gamm

ospores and

gar-phosphat

of plastic, an

form of steri

diation with c

ed in sterile c

higher organ

use large co

ssibilities fo

s,

ma rays and

vegetative c

e backbone

ntibiotics, me

ilization and

cobalt 60 is u

containers u

noleptic and

ontainers to p

or using pack

x-rays

cells

of nucleic ac

edical produ

d penetrates t

used for cold

nder aseptic

d nutritional

pack t

kaging mate

cids

ucts and som

the specimen

d sterilizatio

c conditions.

quality,

the food,

erials of man

me foods

n.

on of antibiot

ny package

tics

sizes, shapees

4. Handling of containers during subsequent sterilization is avoided, recontamination

risk during cooling is minimized.

Limitations:

1. Large capital investment.

2. Pumping at high pressures, product must be relatively homogeneous.

3. Need for specific design of systems for a specific product.

4. Complex operation requiring careful control and sophisticated instrumentation, need

for highly trained personnel

5. Relatively limited filling rate (200 packages per min. versus 600 tin cans per min)

References

http://apps.who.int/phint/en/p/docf/

http://www.gibraltarlabsinc.com/gibraltarblog/difference-between-moist-heat-

sterilization-dry-heat-sterilization/

http://en.wikipedia.org/wiki/Sterilization_%28microbiology%29

http://en.wikipedia.org/wiki/Dry_heat_sterilization