Freezing Flashcards
why is super cooling needed?
ice crystals don’t form at exactly 0 degrees; need super cooling to arrange in lattice structure
freezing refers to storage of food between ___ and __ degrees C
-18; -30
3 stages of freezing curve?
removal of sensible heat (liquid), removal of latent heat (liquid and solid), removal of more sensible heat (solid)
why is there continual decline in freezing point for food materials?
presence of solute depresses freezing point–>pure water freezes out, unfrozen region gets very concentrated, FP v even more
what is freezing range?
between 0 and -5 degrees C
slow freezing leads to formation of ___ number of ___ ice crystals
small; large
effects of slow freezing?
1) damage cell structure, loss of structural integrity 2) pull water out of cell due to osmosis (cell dehydration)
3) cells can’t maintain rigidity when thawed
why is disruptive effect of size of ice crystals minimal in MFP?
elasticity of cell structure in muscles
why is there still some quality loss in muscle food when frozen?
denaturation of muscle proteins because during freezing, unfrozen region become [ ] with enzyme and acids which denature proteins
rate of freezing influenced by:
temp differential, heat transfer rate, SA of food, product thickness, agitation of liquid
commercial freezers at ___C, domestic at ___C
-25; -18
what happens to ice crystals when re-freeze?
size increases
decrease in humidity in freezer leads to ______ on food surface
sublimation of ice (freezer burn)
what is sublimation?
ice–>vapour (solid–>gas)
relative humidity inside freezer should be kept above ___%
90
food is ____ prior to freezing
glazed
what is glazing?
formation of water layer on food surface that turns into ice coat, protects food from moisture loss
types of heat transfer systems:
air cooling, plate cooling, cryogenic cooling (can be direct or indirect)
types of freezers
air-blast, tunnel, belt, fluidized bed, contact, scraped surface, cryogenic
freezer used for ice cream manufacture
scraped surface
cryogenic freezing uses liquid ___ or ___
nitrogen (-196C); ammonia (-33)
what is IQF?
Individual Quick Freezing
IQF used for:
frozen fruit processing
why do frozen concentrated juices exhibit low Vit C loss?
low pH, low O2, low water activity
how is ice wine made?
grapes left freeze naturally on vine, harvest below -8C and crushed, press frozen grapes to yield [ ] juice very sweet
why is juice in ice wine difficult ferment by yeast?
high sugar content
This is defined as the highest temp at which stable ice crystals appear
freezing point
