Lecture 11: Commercial Freezing Methods and Equipment

Question 1

what is the goal of freezing methods?

Answer 1

to achieve rapid freezing

Question 2

what are ways to achieve rapid freezing?

Answer 2

1. use a medium with high h
2. use a rapidly moving med
3. use very low temp
4. use thin profiles
5. agitate particles
6. use cryogenic conditions

Question 3

what are factors that are helpful for achieving rapid freezing?

Answer 3

high h
large delta T
large A
low d

Question 4

what are the two types of freezing, in terms of contact?

Answer 4

indirect and direct

Question 5

what are 4 types of freezing methods? are they direct or indirecT?

Answer 5

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

Question 6

describe the air freezing method

Answer 6

• most simple
• direct contact of cooling medium with product and package
• air temp: -18C to -40C
• natural circulation or forced circulation air
• room or tunnel
• batch or continuous
• mechnical refrigeration or cyrogenics

Question 7

what is freezer burn? what is it caused by?

Answer 7

• dessication/dehydration that causes discoloration and texture change
• problem during frozen storage
• especially for larger samples (eg. meat carcass) and un-packaged products

Question 8

what is glazing?

Answer 8

• coating of ice on surface of frozen food
• prevents moisture loss from product
• mostly done during frozen storage of the product rather than during freezing

Question 9

what is sharp freezing?

Answer 9

• slow and still air –> very slow freezing
• not a common method used today
• discovered in 1920’s by Lloyd Groff Copeman

Question 10

what is air blast freezing? what benefits does it have?

Answer 10

• forced air circulation with powerful fans

benefits:

• high velocity increases turbulence of air so the heat transfer rate increases (large h)
• conserves energy

Question 11

what is tunnel freezing?

Answer 11

• variation of air blasting freezing
• tunnel insulated
• has a conveyor through an air blast tunnel
• product has a counter-current flow (co-current or bi-directional is also possible)

Question 12

what is fluidized bed freezing?

how does the freezing rate compare to other methods?

what foods is this commonly used for?

Answer 12

• version of air blast freezing
• good for light weight products
  food is placed on a pourous bed. Powerful fans are used to force cold air to lift the product
• much faster freezing rate
• can be batch or continuous
• used for peas, beans, carrot cubes, etc…

Question 13

what is through flow freezing?

how does the freezing rate compare to other methods?

what foods is this commonly used for?

Answer 13

• variation of fluidized bed system
• also called packed bed freezing system
• high air velocity, but insufficient to lift particles
• fast freezing rates due to high air velocity and intimate contact
• used for fish fillets and nuggets

Question 14

what are the 5 techniques for air freezing?

Answer 14

1. sharp freezing
2. air blast freezing
3. tunnel
4. fluidized bed freezing
5. through flow freezing

Question 15

what is plate freezing?

what foods is it commonly used for?

Answer 15

• packaged product is in indirect contact with cooling medium
• good transfer rates due to good contact
• contact plates are cooled by cold brine or refrigerant
• temp: -30-50degC
• common for sea fish freezing

Question 16

what is liquid immersion freezing?

how does the cooling rate compare to the other methods?

how is heat transferred?

Answer 16

• immersion of product into liquid media
• intimate contact = provides fast and efficient cooling
• heat transfer goes from liquid to solid and air to solid

Question 17

why is liquid immersion freezingnot common for non-packaged product?

Answer 17

due to residue concerns

Question 18

what does the freezing media in liquid immersion freezing consist of?

Answer 18

propylene
glycol
glycerol
sodum chloride
calcium chloride
sugar syrup

Question 19

what is cyrogenic freezing?

Answer 19

• very low temp
• heat transfer from boiling of refrigerant
• combined latent heat and sensible heat potential
• direct contact between product and package

Question 20

describe cryogens used in cryogenic freezing

Answer 20

consists of liquid N2 and Co2

very low boiling/sublimintation temp

high latent heat