Week12C1

Question 1

Does freezing reduce the water activity of food?

Answer 1

Heating, freeze drying, freeze concentration, and osmotic concentration methods are used to reduce water activity of foods.

Question 2

T or F : in home freezers there is a good circulation of air.

Answer 2

F

Question 3

T or F : in commercial freezing the air is either still or moving

Answer 3

T

Question 4

What is the common freezing method used in home refrigerator

Answer 4

Sharp/still freezing

Question 5

What is the temperature of still air freezing

Answer 5

-23˚C to -30˚C

Question 6

What is the kind of product mostly suited for the still air freezing

Answer 6

rate insensitive products

Question 7

_____ uses high velocity air to speed the freezing rate

Answer 7

blast freezing

Question 8

T or F : blast freezing is a continuous method

Answer 8

False, but it can be with specialized equipment

Question 9

In the ____ method, product placed on trays or mesh belt and run counter current to high-velocity cold air - continuous process

Answer 9

tunnel freezer

Question 10

What are the problems associated with the tunnel freezer method

Answer 10

maintaining control over product dehydration (freezer burn).

Question 11

What is a freezer burn?

Answer 11

food injury due to the sublimation or evaporation of ice during freezing that leads to localized dehydration.

Question 12

Which method is best suited for the flat-packed products

Answer 12

indirect contact plate freezer

Question 13

Name the 7 freezing methods seen in class?

Answer 13

1. Sharp / still air
2. blast
3. Tunnel
4. Indirect contact plate freezer
5. Scraped surface heat exchanger
6. Immersion freezing
7. Cryogenic freezing

Question 14

T or F : in plate freezing, the cooling medium is air and it is in direct contact with the food

Answer 14

false, the contact is indirect

Question 15

What is the exterior jacket or coolant used in the scraped surface heat exchanger

Answer 15

ethylene glycol

Question 16

T or F : the scraped surface heat exchanger is a continuous method

Answer 16

true

Question 17

What is the method in which the air is not the cooling medium

Answer 17

scraped surface heat exchanger
immersion freezing
cryogenic freezing

Question 18

Name the continuous freezing methods seen in class

Answer 18

blast freezer (can be)
tunnel freezing
scraped surface heat exchanger

Question 19

Describe the immersion freezing

Answer 19

The packaged product are immersed in the refigerated medium such as ethylene glycol, sugar or salt solution.

Question 20

What is the method that is best suited for irregular shaped products?

Answer 20

Immersion freezing or cryogenics

Question 21

Which method is best suited for shrink wrapped turkey or chicken or fish

Answer 21

immersion freezing.

Question 22

Why do we use salt and sugar solution as the cooling medium

Answer 22

because they lower the freezing point

Question 23

What is the freezing point of ethylene glycol?

Answer 23

-67˚C

Question 24

What is the freezing point of liquid nitrogen and liquid CO2

Answer 24

nitro : -196˚C

CO2 : -79˚C

Question 25

What is the cooling medium in the case of cryogenic freezing

Answer 25

the cooling medium is the cryogenic liquid

Question 26

What is a cryogenic liquid?

Answer 26

a liquid with a boiling temperature that is under -180˚C

Question 27

How much heat is needed to be removed from products freezed with liquid nitrogen

Answer 27

386 kilojoules/kg of PRODUCT

Question 28

what is the latent heat of evaporation for the change od state from 195˚C to 196˚C in nitrogen?

Answer 28

200kj/kg OF PRODUCT

Question 29

Define temperature differential?

Answer 29

the difference in temperature from the food product and the cooling agent. IT is the temperature driving force in heat exchanger systems (freezing for example)

Question 30

What is the freezing point of carbon dioxide?

Answer 30

-79˚C and under

Question 31

What is the BP of carbon dioxide

Answer 31

-79˚C and over

Question 32

What is the BP of liquid nitrogen

Answer 32

-196˚C

Question 33

What is the freezing point of liquid nitrogen?

Answer 33

-196˚C

Question 34

T or F : the boiling point and freezing points of cryogenic liquids are nearly equal

Answer 34

true

Question 35

Why is dry CO2 considered a cryogenic liquid if its boiling point is 79˚C?

Answer 35

because it can absorb twice as much energy as liquid nitrogen, making up for the temperature differential.

Question 36

What is the temperature at which super freezers are operating?

Answer 36

-60˚C

Question 37

What are the different categories of processed foods preserved by reduced water activity?

Answer 37

1. dehydrated 2. concentrated 3. intermidiate moisture

Question 38

T or F : concentration was one of the earliest preservation method used by man

Answer 38

false, it is drying, and it was originally based on solar energy.

Question 39

What is the the lowest moisture content that can be reached with the simple solar drying?

Answer 39

15%

Question 40

What are the disadvantages of solar drying?

Answer 40

15% reduced water content uncontrolled (weather) contamination

Question 41

What is the moisture content that can be reached with the food dehydration?

Answer 41

1-5%

Question 42

Name one challenge associated with the foods made by dehydration

Answer 42

the reconstitution of the food product.

Question 43

What is the key to efficient dehydration?

Answer 43

maximizing the heat/mass transfer

Question 44

what are the parameters affecting the dehydration?

Answer 44

1. surface area 2. product profile 3. temperature differential 4. RH (influenced by the medium)

Question 45

There is an ____ relationship between the surface area and the particle size

Answer 45

exponential

Question 46

T or F : hot air is capable of retaining more moisture and so is better for the mass transfer in the case of drying

Answer 46

true.

Question 47

What will happen if the RH of the food product and the air is the same

Answer 47

the food product will not loose moisture