Week12C1 Flashcards
Does freezing reduce the water activity of food?
Heating, freeze drying, freeze concentration, and osmotic concentration methods are used to reduce water activity of foods.
T or F : in home freezers there is a good circulation of air.
F
T or F : in commercial freezing the air is either still or moving
T
What is the common freezing method used in home refrigerator
Sharp/still freezing
What is the temperature of still air freezing
-23˚C to -30˚C
What is the kind of product mostly suited for the still air freezing
rate insensitive products
_____ uses high velocity air to speed the freezing rate
blast freezing
T or F : blast freezing is a continuous method
False, but it can be with specialized equipment
In the ____ method, product placed on trays or mesh belt and run counter current to high-velocity cold air - continuous process
tunnel freezer
What are the problems associated with the tunnel freezer method
maintaining control over product dehydration (freezer burn).
What is a freezer burn?
food injury due to the sublimation or evaporation of ice during freezing that leads to localized dehydration.
Which method is best suited for the flat-packed products
indirect contact plate freezer
Name the 7 freezing methods seen in class?
- Sharp / still air
- blast
- Tunnel
- Indirect contact plate freezer
- Scraped surface heat exchanger
- Immersion freezing
- Cryogenic freezing
T or F : in plate freezing, the cooling medium is air and it is in direct contact with the food
false, the contact is indirect
What is the exterior jacket or coolant used in the scraped surface heat exchanger
ethylene glycol
T or F : the scraped surface heat exchanger is a continuous method
true
What is the method in which the air is not the cooling medium
scraped surface heat exchanger
immersion freezing
cryogenic freezing
Name the continuous freezing methods seen in class
blast freezer (can be) tunnel freezing scraped surface heat exchanger
Describe the immersion freezing
The packaged product are immersed in the refigerated medium such as ethylene glycol, sugar or salt solution.
What is the method that is best suited for irregular shaped products?
Immersion freezing or cryogenics
Which method is best suited for shrink wrapped turkey or chicken or fish
immersion freezing.
Why do we use salt and sugar solution as the cooling medium
because they lower the freezing point
What is the freezing point of ethylene glycol?
-67˚C
What is the freezing point of liquid nitrogen and liquid CO2
nitro : -196˚C
CO2 : -79˚C
What is the cooling medium in the case of cryogenic freezing
the cooling medium is the cryogenic liquid
What is a cryogenic liquid?
a liquid with a boiling temperature that is under -180˚C
How much heat is needed to be removed from products freezed with liquid nitrogen
386 kilojoules/kg of PRODUCT
what is the latent heat of evaporation for the change od state from 195˚C to 196˚C in nitrogen?
200kj/kg OF PRODUCT
Define temperature differential?
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)
What is the freezing point of carbon dioxide?
-79˚C and under
What is the BP of carbon dioxide
-79˚C and over
What is the BP of liquid nitrogen
-196˚C
What is the freezing point of liquid nitrogen?
-196˚C
T or F : the boiling point and freezing points of cryogenic liquids are nearly equal
true
Why is dry CO2 considered a cryogenic liquid if its boiling point is 79˚C?
because it can absorb twice as much energy as liquid nitrogen, making up for the temperature differential.
What is the temperature at which super freezers are operating?
-60˚C
What are the different categories of processed foods preserved by reduced water activity?
- dehydrated
- concentrated
- intermidiate moisture
T or F : concentration was one of the earliest preservation method used by man
false, it is drying, and it was originally based on solar energy.
What is the the lowest moisture content that can be reached with the simple solar drying?
15%
What are the disadvantages of solar drying?
15% reduced water content
uncontrolled (weather)
contamination
What is the moisture content that can be reached with the food dehydration?
1-5%
Name one challenge associated with the foods made by dehydration
the reconstitution of the food product.
What is the key to efficient dehydration?
maximizing the heat/mass transfer
what are the parameters affecting the dehydration?
- surface area
- product profile
- temperature differential
- RH (influenced by the medium)
There is an ____ relationship between the surface area and the particle size
exponential
T or F : hot air is capable of retaining more moisture and so is better for the mass transfer in the case of drying
true.
What will happen if the RH of the food product and the air is the same
the food product will not loose moisture
