Commercial freezing equipment

Question 1

How can we achieve rapid freezing? (3)

Answer 1

increase heat transfer coefficient
increase surface area/decrease thickness
bigger temperature difference (colder medium)

Question 2

What is the most simple freezing method? what is the temperature range used?

Answer 2

air

-18 to -40C

Question 3

Freezing methods may be ___ or ___. What are the 4 methods?

Answer 3

direct; indirect (does not touch cooling medium)

1. air (direct)
2. plate (indirect)
3. liquid immersion (direct)
4. cryogenic (direct)

Question 4

How can we increase “h”?

Answer 4

use medium with high heat transfer coefficient (h)
rapidly moving medium
agitation
cryogenic conditions

Question 5

Air freezing can be:
natural circulation or \_\_\_\_\_\_
\_\_\_\_\_ or tunnel
batch or \_\_\_\_\_
mech. refrigeration or \_\_\_\_\_

Answer 5

forced circulation
room
continuous
cryogenic

Question 6

unpackaged goods in air freezing may suffer from:____

How can this problem be prevented in storage?

Answer 6

freezer burn (dessication) - dry cold air causes discoloration, texture change, especially for large products

can coat with layer of ice (GLAZING) to prevent moisture loss

Question 7

Describe “sharp freezing” (5)

Answer 7

type of air freezing
SLOW
slow/still air, room freezing
-5 to -20C
(uncommon in modern times)

Question 8

What type of air does air-blast freezing use? Is packaging required?

Answer 8

forced air circulation - very turbulent and high velocity (better h)
Can be packaged/unpackaged - in shelves, trays, cars, etc

Question 9

How does freezing with air blast compare to sharp freezing?

Answer 9

faster freeze rate due to better heat transfer

Question 10

What is a variation of the air blast freezing method that is continuous? What does it involve?

Answer 10

Tunnel freezing

conveyor belt thru insulated air blast tunnel

Question 11

Air blast freezing uses ___ freezers in a _____, in order to conserve energy.

Answer 11

blast; frozen storage chamber (product stored after frozen)

Question 12

How can tunnel freezing be altered for different product types?

Answer 12

different conveyor speed

Question 13

what is the usual flow of air vs product in tunnel freezing?

Answer 13

counter-current

can also be co-current, or bi-directional

Question 14

Some variations of tunnel freezing (3):

Answer 14

drag through
multi-pass
spiral

Question 15

A variation of air blast freezing good for lightweight products:

describe the process.

Answer 15

Fluidized bed freezing

particles placed on porous bed
powerful fans -> cold air lifts product
turbulence + agitation, product suspended in air
much faster freeze rates

Question 16

what is “through flow” freezing? what is another name for it?

Answer 16

variation of fluidized bed freezing
particles not completely suspended (air flow not strong enough) - but still lots of agitation/turbulence, so fast freeze rate

also called PACKED BED FREEZING

Question 17

Is fluidized bed freezing continuous or batch?

Answer 17

Can be either: POROUS BIN (batch) or POROUS BELT (continuous)

Question 18

What is the indirect method of freezing? Describe the process.

Answer 18

plate freezing

plates cooled by brine/refrigerant -> -30 to -50C
PACKAGED product sandwiched between plates
slight hydraulic pressure -> good contact -> good heat transfer

Question 19

What is plate freezing commonly used for?

Answer 19

high sea freezing - freeze fish into blocks

Question 20

Some advantages of plate freezing: (3)

Answer 20

good contact -> rapid freezing
reduced moisture loss
great for prepackaged/convenience foods

Question 21

What factors need to be considered when plate freezing?

Answer 21

1. thickness of packages
2. uniformity of thickness
3. prevent crushing - use spacers

Question 22

This type of direct freezing uses a non air medium:

What mediums are used? (5)

Answer 22

liquid immersion freezing

propylene glycol, glycerol, NaCl, CaCl2, sugar syrup

Question 23

When is liquid immersion freezing less suitable? Why?

Answer 23

unpackaged product

possible residues of cooling medium

Question 24

What type of freezing is done at the lowest temperature?

Answer 24

cryogenic

Question 25

How is heat transfer in cryogenic freezing accomplished? Why is this more effective?

Answer 25

boiling of cooling medium (liquid N or CO2 - very low boil/sublimation temp)

both sensible and latent heat are acting to cool

Question 26

What makes liquid CO2 and N2 good as cryogens? (2)

Answer 26

very low boiling/sublimation temperature

high latent heat

Question 27

What are other techniques used in freezing technology?

Answer 27

heat exchangers (tubular, SSHE for slushies)
freeze concentration - for juice or beer
freeze drying
dehydrofreezing

Question 28

cryogenic freezing has (high/low) capital costs, but (high/low) operating costs.

Answer 28

low; high