Effect of freezing

Effect on microorganisms

• Microbial growth is stopped
• Kill spoilage microorganisms
• Kill 10% - 60% microbial in food

Effect on chemical changes and quality

• Colour loss (browning)
• Enzyme still active
• Slower rates of enzymatic & non-enzymatic reaction
• Fermentation activity decreases
• Acetaldehyde formation

Effect on physical changes and quality

• Affect the overall behavior of foods
• Free water frozen while bound water is unfreezable
• Formation of ice crystal in food
• Moisture loss (dehydration)

Freeze Burn

Frozen food require care for optimum quality. It is important to maintain the storage temperature and avoid any undesirable changes in food to occur.

Processing and Packaging

Pre-treatment

1. Blanching

• Immersion or steaming
• Providing good condition for freezing
• Improve eating quality
• Destruction of microorganisms
• Wilting vegetables
• Reduction of undesirable materials
• Inactivation of various enzymes
• Extension of shelf life

2. Heat treatment

• Low temperature long time
• Decrease cell damage and excessive softness
• Firmer texture

3. Dipping treatment

• Dipping / soaking in solution: brine, ascorbic acid, citric acid, sugar, etc
• Reduce browning
• Reduce enzymatic reaction
• Provide glazing that protect products from dehydration and oxidation

4. Bacterial 

ice nucleators

• Elevate nucleation temperature
• Reduce freezing time
• Increase freezing rate
• Improve quality

5. Antifreeze proteins (AFPs)

• Lower freezing temperature
• Bind to the ice crystal
• Retarding recrystallization

6. Osmotic concentration

• Soaking food items in concentrate solution such as sugar solution
• Partial removal of moisture from foods
• Improving product quality
• Reducing refrigeration load

7. Cryoprotection

• Protection of protein freeze denaturation
• Preserving functional properties of proteins
• Examples: Sorbitol, sucrose, polydextrose, maltodextrins
• Produce smooth textures in Surimi (crabstick)

8. Irradiation

• Treating with ionizing radiation generated by radioactive materials
• Retard spoilage microorganisms

Post-treatment

Thawing

• Essential to maximise quality retention
• Thawing through refrigerator, microwave oven, cooking
  

Transportation

1. Overland transport

Different refrigeration system is used:

• mechanical
• eutectic plates
• liquid nitrogen (total-loss refrigerant)   

Various vehicle types

• small van
• long container trucks
• train 

2. Sea transport

• good refrigeration system allowed fresh products to be 
• transported up to 40 days
• meat products can be kept up to 23 weeks at -2°C with 
• correct packaging
• containers used can be insulated or refrigerated
• mainly for semi-process / process food
• bulky amount of product

3. Air transport 

• useful for highly perishable products such as fruit and vegetables
• mark-up and profit rule as deciding factors
• products are not refrigerated during transport
• insulated containers are used
• dry ice can also be used 

Freezing Methods

There are different type freezing systems as there is no one-satisfy- all system available. Several freezing methods available nowadays are:

1. Contact plate freezing

• Product are sandwiched between metal plates
• Regular shaped products are preferable

The video below shows the application of contact plate freezing in food industry.

2. Immersion freezing

• Cooled high-concentration solution is used
• Solutes used must safe to food
• Heat transfer is quick and high concentration solution remain liquid at very low temperature

The soda pop is immersed into a cold brine solution (-35 °C) mixed with calcium as shown in video below.

3. Freezing by contact with cooled gas

3.1. Cabinet freezing

• Cold air is distributed into a cabinet

• High heat transfer an air velocity of at least 5 ms^-1

3.2. Air blast freezing

• Cold air is blown into freezing area

• Air velocity between 2.5 and 5  ms^-1

3.2. Cryogenic freezing

•  Liquefied gas such nitrogen and carbon dioxide are used
• Very fast heat transfer
• Create little damage to cell due to small crystals formed
• Use for high value products
• Applications through immersion, evaporite and blown to product or spraying on products

The diagram above shows the benefits using rapid cryogenic freezing method.

Frozen Food

Nowadays, frozen foods are popular and accessible in most developed countries, where refrigerators and freezers are standard home appliances. This had become the essential items in the convenience food stores, fast food chains, food services, grocery stores, retail food industries and vending machines.

What the advantages of frozen foods? 

Frozen foods have the advantages of being very close in taste and quality to fresh food as compared with other preserved or processed foods. The video below shows the benefit of frozen food when compared with fresh food.

Besides, frozen foods will have a shelf life of 6 months or more! (Generally longer shelf life than chilled foods) The table below will shows the differences between the frozen foods and chilled foods. 

In conclusion, freezing process reduce food deterioration rate and enhance product's shelf-life by lowering temperature and change water molecules from liquid to solid phase.