Freezing time, rate, models

Question 1

What is the most common definition of “freezing time?”

Answer 1

The time from INITIAL TEMP (open ended) to reach target temp of -10C (close-ended)

Question 2

Even if both ends of the freezing time curve are fixed, what could affect the time for freezing?

Answer 2

Higher initial temperature: need longer PRECOOLING TIME, freezing time will be shorter.

Question 3

How are speeds of freezing categorized? (based on ice propagation)

Answer 3

Fast: >5cm/hr
Medium: 1-5cm/hr
Slow: <1cm/hr

Question 4

Other than ice propagation, what other way can freezing rates be categorized?

Answer 4

time to cross zone of max ice formation
30 min or less = quick freezing
more than 30 min = slow freezing

Question 5

What is freezing rate based on? What are the 2 ways it is defined?

Answer 5

ICE PROPAGATION RATE
speed of progression of frozen layer from surface to centre

half thickness/time (freeze from both sides)
or
thickness/time (one side)

Question 6

What are some possible ways to define freeze time?

Answer 6

1. total time from start temp to final temp (variable)
2. time from initial temp to target temp of -10C or -18C
3. time from reaching 0 to reaching target temp -10 or -18C
4. time from SURFACE reaching 0, to CENTER reaching target temp -10 or -18C
5. time to cross ZONE OF MAX ICE CRYSTAL FORMATION (-1 to -5C)

Question 7

What is the original prediction model for freezing time calculation, and what does it estimate?

Answer 7

Planck’s model - estimate time for PHASE CHANGE

Question 8

Most time is required for the ____ period. What are the relative proportions of the other periods during freezing?

Answer 8

phase change (71%)

precooling (20%)
tempering (9%)

Question 9

What are the types of heat change that happen during each phase in the freezing process?

Answer 9

precooling - sensible heat change
phase change - latent heat change
tempering - sensible heat change

Question 10

In the planck model, when is freezing terminated?

Answer 10

When the ice front has progressed all the way.
reached end/bottom (if frozen from top)
reached center (if frozen from both sides

Question 11

What assumptions are made by the Planck model? (5)

Answer 11

1. Product is free water
2. FP is 0C
3. process starts with water at 0C and ends when all water is ice at 0C
4. all heat removed as LATENT heat
5. based on progression of ice front

Question 12

When the material is at its freezing point, what does not occur? (2)

Answer 12

sensible heat

tempering

Question 13

What are the 3 modes of heat transfer happening during the freezing process?

Answer 13

1. convection: from freezing medium to surface
2. conduction: heat propagating through frozen material
3. latent heat liberated from ice front

Question 14

The planck model requires determining: (3)

Answer 14

1. convection heat transfer at surface
2. conduction heat transfer in frozen region
3. latent heat absorption at the front ice

Question 15

How does shape of product affect using the planck model?

Answer 15

If regular geometric shape (cylinder, slab, sphere) use GEOMETRIC FACTOR (P) for that shape

If irregular - approximate to regular shape

Question 16

What is the modified version of planck’s method, and what are the changes made?

Answer 16

International Institute of Refrigeration (IIR) modification

account for precooling and tempering periods
change latent head (L) to TOTAL HEAT LOAD (delta H)

Question 17

How will using the planck’s model instead of the newer IIR version affect estimation of freeze time?

Answer 17

underestimation

Question 18

Freezing is ___ times more sensitive to changes in surface resistance than _____. What can we conclude from this?

Answer 18

4x; changes in internal resistance.

So: surface heat transfer plays big role in freezing time required

Question 19

In similar conditions, how does thawing compare to the freezing time? How can we decrease the time needed (thaw faster)

Answer 19

slower than freezing

microwave, direct contact, water immersion

Question 20

True/False: planck’s model can also be used for thawing.

Answer 20

True

Question 21

What are the differences between using planck’s model for freezing vs thawing?

Answer 21

thawing: use density and thermal conductivity of UNFROZEN Material
Ambient temp is higher than frozen food

Question 22

Thawing should be under ___ conditions, to prevent:

Answer 22

cold

microbial growth/spoilage