Unit 9C
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home freezing has moving or still air
still air
commercial freezing methods include
contact with cold air- either still or moving indirect contact with a cooling medium immersion with a cooling medium
1) still air or sharp freezing
-23 to -30 slow common method much like fridge or freezer at home large crystals are expected because slow freezing use "rate insensitive" products
2) blast freezing
use of high velocity low temp air increase rate of freezing compared to still air
3) Tunnel freezer
products placed on trays or mesh belts and run a counter current high velocity cold air very rapid and efficient
downside to tunnel freezing
freezer burn - need to maintain control over product dehydration spray with water
freezer burn
caused by ice sublimating or evaporating from a frozen food product
4) indirect contact plate freezer
late packed products
5) scraped surface heat exchanger
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
6) Immersion freezing
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!!
typical product frozen using immersion freezing
shrink wrapped chicken or turkey ( irregular shape )
7) cryogenic freezing
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
liquid nitrogen (-198)
non toxic, non-ozone deteriorating , produces a very superior frozen product but highly expensive
carbon dioxide
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
boiling point of CO2?
-79
food dehydration advantages and disadvantages
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 %
advances in food dehydration
achieve 1-5% water content reduction of weight in food product , easier to store and faster to transport
challenge with dehydrated food
reconstitution - get it back to its staring material
drying principles
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
more surface area
more efficient drying
temperature differential between the heating medium and the product
large
relative humidity
the amount of moisture air can hold at a particular temperature it is a function of time
basic rule about RH and food and envirmonet
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
how many kj of heat/ kg of product is removed in total in cryogenic freezing?
386