Week 6: Low Temperature Preservation
Chilling: used in conjunction with other unit operations (e.g. MAP, pasteurisation, irradiation) for preservation. ✅
Chilling & Product Quality ✅
Freezing
Direct contact and Indirect contact system ✅
Recrystallization: From physical changes of ice crystals (shape, size or orientation) during the storage ✅
Freezing & Product Quality ✅
Freeze Drying
Factors affecting shelf life of chilled foods
Type/nature of food
Degree of microbial destruction or enzyme inactivation
Initial microbial load and type of microorganisms
Temperature/relative humidity during storage & distribution
Influences on product quality
Chill injury:
Physiological changes e.g. internal browning, skin blemishes
Cold shortening:
Undesirable changes caused by chilling of meat before rigor rmortis
Improves processing properties:
Ease of peeling/pitting of peaches, improve efficiency of meat cutting and bread siling
Control rate of growth & metabolism of desirable microorganism: Cool aging of wine
Direct contact ✅
Indirect contact ✅
Air-blast freezing: Cold air blown across the product
Batch: arrange in trays/trolleys or continuous: placed on moving conveyor belts
Cryogenic Freezing: Cryogen is in intimate contact with foods and rapidly remove the heat from food surface for rapid freezing,
Operating conditions: -30 to -40℃, air velocity 1.5 ~ 6.0m/s (fast freezing)
Application
Poultry, spring roll or any other foods products
Advantages:
Disadvantages:
Low equipment capital cost
Relatively economical to operate
Highly flexible (suitable for irregular product shape/size)
Compact equipment
Freeze burn(i.e. surface dehydration caused by sublimation of ice if no packaging)
Oxidative changes to unpackaged food/IQF foods
Necessitates for frequent defrosting of freezer
Operating conditions Air velocity 20 – 30 m/s
boiling point of liquid nitrogen -196℃, carbon dioxide -79℃.
Disadvantages:
Advantages
Relatively high operating cost (high cost of cryogens)
Product more susceptible to cracking/shattering
(Due to the formation of crust on food surface from rapid freezing to prevent further expansion. This creates an internal pressure to the product.)
Plate Freezing: Consists of vertical/horizontal stack up to 24 hollow stainless steel plates (e.g. 2.5 – 5.0 cm thick) and refrigerant is pumped through these hollow plates
Fastest freezing method
Superior product quality (rapid freezing so small ice crystals)
Low equipment capital cost
Highly flexible (Irregular product shape/size & delicate structure)
Minimize oxidative changes to unpackaged food (exclude oxygen during freezing)
Rapid start-up and no defrosting time of freezer
Operating conditions :
-30℃ to -50 ℃
Advantages
Disadvantages
Good space utilisation
Relatively low operating cost
High rate of heat transfer
Little dehydration of product/weight loss
Restriction of shape/thickness of foods
Relatively high capital cost
Scraped Surface Freezers: Heat exchangers specially designed for freezing liquid foods. Shell surrounding the product is filled with refrigerant, rotor acts as a mixing device while scraper blades enhance heat transfer
Available in batch or continuous system
Operating conditions: -4 to -7℃
Removal of 60-80% of latent heat from products
Migratory recrystallization is most common since it is mainly due to fluctuation of storage temperature
Temperature fluctuation can be prevented:
Tighten control of storage temperature (E.g. ± 1.5℃)
Air curtains or automatic door or airtight curtains for refrigerated trucks
Rapid movement during transportation within factory
Stock rotation and control
Degradation of pigments
Chlorophyll slowly degraded to brown pheophytin in frozen vegetables
Residual enzyme activity
E.g. polyphenol oxidase activity causes browning
E.g. lipoxygenase activity produces off-flavours/off-odours from lipids.
Oxidation of lipids
Reaction takes place slowly even at -18℃ thus causing off-odours and off-flavours. E.g. Fatty fish
Loss of vitamins
E.g. water soluble vitamins due to drip losses
Due to the low temperature usage and removal of liquid in foods, most microbiological and chemical deterioration reactions have stopped.
Freeze dried foods are usually nitrogen flushed and sealed in high barrier bag or containers. Their shelf life can ranged between 6 months to 1 year.
Disadvantages
Advantages
Highly complex thus relatively high capital cost
No heat recovery
Higher maintenance costs (e.g. scraper blades, seals, bearing)
Minimize fouling on tube wall since wall scraped continuously
Enhance product uniformity (e.g. flavour colour, aroma) due to constant blending action
Suitable for wide range of viscosity (i.e. including sticky products)
Suitable for fluids with particulate (size < 1cm)