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NF2016N Food preservation 2

Presentation on low temperature treatments

Richard Marshall

on 25 June 2013

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Transcript of NF2016N Food preservation 2

NF2016N Food Preservation 2
Low Temperature Treatments
Why low temperature?
Prevent raw & processed food spoiling
Extend shelf life
Extend season - possibly all year?
Conserve food supply, food security
Allow long distance transportation
Provide time in shops
Help make food safer
Ways of preserving food
Application of heat - cooking, pasteurization
Cooling - chilling, freezing
Sterilizing - canning, bottling
Fermentation, pickling
Osmotic effects - sugar, salt
Additives - preservatives, antioxidants, stabilizers, emulsifiers
Irradiation, high pressure etc
Keeping foods fresh
Low temperatures keep food edible longer
Meat must be refrigerated
BUT some foods damaged by cold, eg
Melons (slow), cucumbers
Tomatoes (slow)
Sweet potatoes
Chilled food storage (Headley, 1985)
All fresh products generate heat, especially F & Vs
Chilling of fresh foods
Have to remove both sensible heat and heat of respiration
Once chilled, respiration is minimised
Examples of optimum storage temperatures
Chilled storage of processed foods
Primarily need to limit microbial growth
Has to be carried out under strict hygienic control - GMP, HACCP
Shelf life determined by:
Type of food
Degree of microbial destruction by processing
Degree of enzyme inactivation
Control of hygiene in process and packing
Barrier properties of packaging
Temperature in processing, storage & distribution
Fresh products
Processed products
Effect of temperature on metabolism & microorganisms
Low temperatures retard metabolism
Every 10 C lowering of temp. halves reaction rate
Reduces rate of spoilage

MOs have specific growth optima:
Thermophiles 55 - 65 C
Mesophiles 30 - 40 C
Psychrotrophs 20 - 30 C
Psychrophiles 12 - 18 C
Chilling of cooked foods
Cook/freeze system
Can be individual foods or complete meals
After processing must be packaged to protect
Particularly used in institutional catering, eg 'sous vide' (under vacuum)
Classification of cook/chill foods by risk
Chilling of meat
Increased storage time at <4 C
Rapid cooling essential in <20 h
38 C down to 0 - 1 C
Beef at <1 C stored 8 days before entering retail
Lamb 6 days
Consumer within 7 - 10 days

Longer storage requires freezing

Chilling of fish
Very perishable
Packed in ice on ship
At 0 C, keeps up to 3 weeks

Frozen fish keeps several months
Chilling of fruits and vegetables
Remain alive in cold storage (some damaged)
If alive, maintain freshness
Oxidise sugars, heat created
esp. apples, broccoli, lettuce, peas, spinach, sweet corn
Have to control humidity
Fruits - 85-90 % RH
Root crops, leafy veg - 90-95 % RH
Enzymic spoilage continues
peroxidase, catalase, lipase, pectinesterase, bromelain, sucrase etc
Fruits should be of right maturity

Cooling foods down
1-2 C
Chilling & freezing
Chilling - batch/continuous
First freezing to -1 to -2 C
Lower temp. to -18 C
Blast freezers
Cold air to -10 C or lower
Cryogenic freezing
Direct contact with refrigerant
Liquid N (-196 C)
CO snow (-78 C)



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Typical frozen foods
Fruits - whole, pureed, juice concentrates
Vegetables - peas, green beans etc
Fish fillets, seafoods, prepared fish
Meat - carcasses, joints, cubes, mince, products
Baked goods
Prepared foods, desserts, ready meals etc
Formation of ice crystals
Freezing point is temperature at which a liquid just begins to crystallise
Before freezing occurs, must have NUCLEATION

Two types:
Homogeneous nucleation
Chance formation of water molecule nuclei
Heterogeneous nucleation
Nucleation around particle, cell wall etc
Most likely type in foods
Happens during supercooling
Effect of cooling rate on nucleation
Rapid cooling gives large numbers of nuclei
Water molecules migrate to existing nuclei rather than form new ones
Rapid freezing produces large numbers of very small ice crystals
Slow freezing allows ice crystals to grow
Therefore less ice damage in fast freezing
Type of food has significant effect on crystal size
High water, high solute, cellular, non-cellular etc
Factors affecting heat transfer
Thermal conductivity of the food
Area available for heat transfer
Distance heat must travel
Temp. diff. between food and freezing medium
Insulating effect of boundary layer of air around food
Presence of packaging will impede heat flow
Water content and freezing point
Equipment used for freezing & chilling
Mechanical freezers - use refrigerant, cooled air, cooled surfaces
Cryogenic freezers - use solid or liquid CO or liquid N in contact with food

Slow freezers, sharp freezers (0.2 cm/h)
= still air freezers, cold stores
Quick air freezers (0.5 - 3 cm/h)
= air blast and plate freezers
Rapid freezers (5 - 10 cm/h)
= fluidized bed freezers
Ultra-rapid freezers (10 - 100 cm/h)
= cryogenic freezers
Cook/chill system
Storage life in frozen conditions, based on sensory evaluation (sig diff at p<0.01)
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Factors affecting shelf life of
chilled fresh foods
Type of food, variety or cultivar
Part of crop used as food
Condition at harvest
Temperature at harvest
Temperature in storage or distribution
Relative humidity on storage
Full transcript