Heat and mass transfer during soaking process at low temperatures. Session 1: Fundamental aspects

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

1

Session 1:

Fundamental Aspects

Heat and mass transfer during soaking

process at low temperatures

CIRAD/CEMAGREF

Montpellier - France

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

2

Introduction

Immersion Chilling and Freezing (ICF)

ICF :

soaking foodstuffs in aqueous

refrigerating media.

Immersion media :

Binary brine solution

or ternary (ethanol, sugars)

Foodstuffs :

fish, shrimps, poultry, fruits

and vegetables

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

3

Introduction

Main advantages :

shorter processing times

energy savings

quality improvement

Drawbacks :

uncontrolled solutes intake

Recent studies open the way to :

reduce solutes intake

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SOAKING TREATMENTS/ OT and ICF

– Dehydration and formulmation,

– no phase change,

– « isothermal »

– osmotic phenomena,

shrinkage

– Chilling, freezing, and

formulation

– phase change

– non isothermal,

– shrinkage

« Osmotic treatments »

« Immmersion Chilling and

freezing »

Water (Liquid)

Solutes

Food solutes

Aqueous concentrated

solution Aqueous concentrated_solution

Food solutes

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Theoritical approach

Mathematical modelling

+

glass bead bed

Apple tissues

Salt flux in apple cylinder (T₀=+5°C) soaked in

NaCl liquids (4.55 mol/kg) at 2°Cand -17.8°C

(Lucas et al., 1998). X Z Y

T

_∞

= -17.8°C

0 10 20 30 40 50 60 0.00 0.04 0.08 0.12

T

_∞

= 2°C

Salt flux (kg·m

-2

·s

-1

)

time (min)

prédiction tools

☯physical state of the outer layer (frozen or thawed)

☯ freezing front location and impregnation level of thawed area

Coupling phenomena

☯solute concentration profile

☯ice weight fraction

87% ice in the outer layer 87% ice at core 90% ice at core X Z Y

ICF/ RESULTS

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0 0,2 0,4 0,6 0,8 1 1,2 1,4 0 10 20 30 40 50 60 time (min) sal t gai n (g /100 g i n it ia l gel )

Heat and mass transfer coupling

Question : links between heat (including phase change) and mass transfer ?

Study on model food (gelatin)

Salt gain evolution in gelatin

cylinders (T₀=+ 5°C) soaked in

NaCl solutions (Lucas et al., 1996).

A

B

C

A

B

C

A

B

C

-15.3°C X_NaCl=0.19 -17.8°C X_NaCl=0.21 -20.6°C X_NaCl=0.23 temperature

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

7 Table 1.

Comparison of different chilling and freezing techniques :

heat transfer coefficient, h

Freezing techniques

h (W.m

-2

.K

-1

)

Air-blast freezer

Tunnel

10-50

Fluidized bed

60 Plate freezer

100 Direct contact by immersion

Freon

500 liquid nitrogen

100

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Fair

CT 96 1118

Karlsruhe

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june

23

23 -

-

24th 2000

8 Immersion at 2°C

0 1 2 3 4 0 15 30 45 60 75 90

Time (min)

NaCl content (g/100g final

product)

0 5 10 15 20

Core temperature (°C)

T = +2°C

_∞

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

9 Immersion at -17.8°C

0 1 2 3 4 0 15 30 45 60 75 90

Time (min)

NaCl content (g/100g of final

product)

-20 -15 -10 -5 0

Core temperature (°C)

T = -17.8°C

_∞

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Fair

CT 96 1118

Karlsruhe

-

june

23

23 -

-

24th 2000

10 Long term storage at -17.4°C

0

1

2

3

4

0

12

24

36

48

60

72 Time (h)

NaCl content (g/100g of final