Unit 9C

Question 1

home freezing has moving or still air

Answer 1

still air

Question 2

commercial freezing methods include

Answer 2

contact with cold air- either still or moving
indirect contact with a cooling medium
immersion with a cooling medium

Question 3

1) still air or sharp freezing

Answer 3

-23 to -30 
slow common method 
much like fridge or freezer at home
large crystals are expected because slow freezing
use "rate insensitive" products

Question 4

2) blast freezing

Answer 4

use of high velocity low temp air

increase rate of freezing compared to still air

Question 5

3) Tunnel freezer

Answer 5

products placed on trays or mesh belts and run a counter current high velocity cold air
very rapid and efficient

Question 6

downside to tunnel freezing

Answer 6

freezer burn - need to maintain control over product dehydration
spray with water

Question 7

freezer burn

Answer 7

caused by ice sublimating or evaporating from a frozen food product

Question 8

4) indirect contact plate freezer

Answer 8

late packed products

Question 9

5) scraped surface heat exchanger

Answer 9

does not use air to freeze product
the cooling medium in this is not air
use ethylene glycol in the exterior jacket as a coolant
high surface area, forced convection, excellent heat transfer

Question 10

6) Immersion freezing

Answer 10

immersed in refrigerator medium such as ethylene glycol sugar ot salt solution resulting in good contact, good heat transfer and excellent for irregular shaped objects!!

Question 11

typical product frozen using immersion freezing

Answer 11

shrink wrapped chicken or turkey ( irregular shape )

Question 12

7) cryogenic freezing

Answer 12

lies between static and immersion freezing- cryogen is usually sprayed onto the product such as liquid nitrogen and liquid CO2 (-79 degrees )

removal of 386 kilojoules of heat/ kg of product

very fast

Question 13

liquid nitrogen (-198)

Answer 13

non toxic, non-ozone deteriorating , produces a very superior frozen product but highly expensive

Question 14

carbon dioxide

Answer 14

immediately turns to dry ice when sprayed
then dry ice sublimes to gaseous CO2 to cool the product
CO2 can absorb 2x as much energy as liquid nitrogen

Question 15

boiling point of CO2?

Answer 15

-79

Question 16

food dehydration advantages and disadvantages

Answer 16

earliest process
solar energy
low cost, simple, low tech

uncontrolled, contamination if just left out in sun, limited to only reducing water content to 15 %

Question 17

advances in food dehydration

Answer 17

achieve 1-5% water content

reduction of weight in food product , easier to store and faster to transport

Question 18

challenge with dehydrated food

Answer 18

reconstitution - get it back to its staring material

Question 19

drying principles

Answer 19

heat energy must be transferred into the product to convert the moisture into water vapour and the vapour has to be removed from the environment surronfding the product

Question 20

more surface area

Answer 20

more efficient drying

Question 21

temperature differential between the heating medium and the product

Answer 21

large

Question 22

relative humidity

Answer 22

the amount of moisture air can hold at a particular temperature
it is a function of time

Question 23

basic rule about RH and food and envirmonet

Answer 23

food will either gain or lose moisture as a function of the relative humidity of its environment unless food product and envirmoent have identical RH

Question 24

how many kj of heat/ kg of product is removed in total in cryogenic freezing?

Answer 24

386

Question 25

what is the latent heat of evaporation from liguid to gas phase in cryogenic freezing?

Answer 25

200kj/kg

Question 26

can you spray liquid nitrogen right onto product without any risks?

Answer 26

yes! non toxic and non ozone deteriorating

totally safe only thing is that it is expensive

Question 27

CO2 can absorb how much more energy than liquid nitrogen, causing it to be competitive freezing agent despite the temp difference of -196 and -79

Answer 27

2x

Question 28

dehydration gets water/moisture content down to what %

Answer 28

1-5%

Question 29

traditional method of drying could only reach a moisture content of what %?

Answer 29

15%