Milk fat crystal networks formed under shear

Crystallisation of milk fat/sunflower oil blends: kinetics

and reological properties

Bert Vanhoutte, Imogen Foubert, André Huyghebaert and Koen DewettinkDepartment of food science and nutritionGhent University, Belgium

Microstructure

Fat crystal habit, size, distribution

Lipid-composition

Polymorphism, polytypism

Spatial distribution of fat crystals

Macroscopic properties

processing

Source: Marangoni&Hartel, Food Technology, 1998

supersaturating

nucleationcrystal growth

crystal size distribution

fraction solid fat

aggregation

gelation

strong network forming

post- and recrystallisationV

an d

er W

aals

fo

rces

sint

erin

g

pro c

essi

n gst

o ra g

e

stru

ctur

e

Source: PhD Thesis William Kloek

Crystallisation under shearAgitation rate 50, 100, 200 and 300rpmTemperature recordingSFC measurementsCrystallisation interrupted at 75% of equilibriumSamples for rheological tests and microscopic analysis

20mm

15mm

70mm

85mm

20mm

Temperaturerecording

20mm

15mm

70mm

85mm

20mm

Temperaturerecording

Rheological measurements

25mm2,5mm

25mm2,5mm

Polarised microscopyMicrostructure formation in tubs not under microscopic slides2D images of microstructure by cryotomographyParticle size measurements of primary crystal aggregates with a gridNO Quantitative analysis of spatial distribution

Processing conditionsTemperature of the coolant 21 and 26.5°CAgitation rate 50-100-200-300rpmFive blends High melting fraction milk fat (HMF) – Sunflower Oil (SFO) 60/40, 70/30, 80/20, 90/10 and 100/0

Multiple effect of agitationEffect on the cooling rateConvective heat transfer coefficient

Effect on the mass transferShear rate

).(... wcshcs

p TTFtTcm

Convective heat transfer coefficient (assumption temperature perfectly homogeneous in

vessel)

Shear rate (calculated at the tip of the impeller compared to the

vessel wall)

Processing

Crystallisation kinetics

Convective heat transfer h

Shear rate

emperature of the coolant

Lipid composition

Supercooling

Supersaturation

Induction time

Growth rate

?

Qualitative analysis

202530354045505560

0 10 20 30 40

Time (min)

T (°

C)

50100200300

2025

303540

4550

5560

0 10 20 30 40Time (min)

T (°

C)

50100200300

2025

30

3540

4550

5560

0 10 20 30 40Time (min)

T (°

C)

50100200300

20

25

303540

45

50

5560

0 10 20 30 40Time (min)

T (°

C)

50100200300

 

(60/40)21°C

(60/40)26.5°C

(100/0)21°C

(100/0)26.5°C

Anova on the induction time

Enter method

Stepwise method

Conclusion:The induction time is affected by agitation but mainly by an increase in heat transfer rather then an effect of mass transfer

Anova on the growth rateEnter method

Stepwise method

ConclusionThe growth rate is influenced by shear rate rather than by the convective heat transfer coefficient, which suggest the growth rate is more affected by the mass transfer than by the overall release of heat towards the coolant

Microstructure

0

100

200

300

400

500

600

50 100 200 300

agitation rate (rpm)

crys

tal a

ggre

gate

siz

e (µ

m)

21°C26,5°C

0

100

200

300

400

500

600

50 100 200 300

agitation rate (rpm)

crys

tal a

ggre

gate

siz

e (µ

m)

21°C26,5°C

0

100

200

300

400

500

600

50 100 200 300agitation rate (rpm)

crys

tal a

ggre

gate

siz

e (µ

m)

21°C26,5°C

0

100

200

300

400

500

600

50 100 200 300

agitation speed (rpm)

crys

tal a

ggre

gate

siz

e (µ

m)

21°C26,5°C

Anova on primary crystal aggregates

Size decreases with temperature of the coolant and more agitationNo effect on the lipid compositionEffect of agitation = effect on primary or secondary nucleation???

Effect on shear on crystalsHigh shear Low shear

+/- homogeneous size distribution

More heterogenous size distribution

Post crystallisationDepends on: The difference between crystallisation

temperature and the storage temperatureVan der Waals – Solid bridgesThe cooling rateThe specific surface area

Anova Rheology

Power-law modelsRelation between SFC and G’ can be described by power-law models

where A is the interaction parameter and µ is the scaling exponent

Fractal nature of fat crystal networksApplicable on this system?

µSFCAG '

DdSFCG 1

'

Regression analysisThe effect of agitation is larger when the degree of post-crystallisation is smallLonger storage leads to space filling of initial pores

-8

-6

-4

-2

0

2

4

6

8

0 50 100 150 200 250 300 350

agitation rate (rpm)

Log

A

Tw=21°CTw=26,5°C

The interaction term A

The scaling exponent µ

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300 350

agitation rate (rpm)

scal

ing

expo

nent

(µ)

Tw=21°CTw=26,5°C

Relation between process parameters, crystallisation

kinetics and rheological properties

T=low + shear=low T=high + shear=low

T=low + shear=high T=high + shear=high

Lipid composition

Crystallisation kinetics

Primary crystal aggregates

Final microstructure

Rheological properties

Temperature of the coolant

AgitationShear rate

Heat transfer

Storage temperature

AcknowledgementsIWT (Institute for the Promotion of Innovation by Science and Technology in Flanders)Aveve Dairy products, BelgiumSpecial thanks to Wouter Pillaert, Brecht Vanlerberghe, Leo Faes and Frank Duplacie