Effect of chilling

Effect of chilling on microorganism

• reduce the growth of microorganism but not stop the growth 
• the generation time of microorganism decrease as the temperature decrease
• decrease the rate of spoilage  
• changed the type of spoilage

Effect of chilling on enzymes and biochemical reaction

• Rate reduce as temperature is low 
• Enzyme contribute to perishability  
• Inhibit the thermal resistance enzyme such as catalase, peroxidase in fruits and vegetables

Effect of chilling on physical process

• Prevent the moisture loss
• Maintain the appearance of food such as colour of the food

Effect of chilling on physiological process

Reduce the metabolic process:

• respiration - reduce the moisture loss
• transpiration - reduce the moisture loss
• ripening - reduce rate of ripening and prolong shelf life of fruits

Principles of chilling

Chilling involves reducing food temperatures and remove the heat out from the food. As the heat removal occurs, it will

• decrease motion of molecules/atoms
• prevent reactions to take place

 

Reactions/processes that controlled by temperature in food items :

• microbial growth
• chemical/ enzymatic reaction
• physical process
• physiological processes

Chilled Food

Chilled foods are foods which are cooled to a temperature above their freezing point and which need to be maintained at that temperature to preserve quality. In 1990, the Institute of Food Science and Technology (IFST) has defined chilled food as "perishable foods which, to extend the time during which they remain wholesome, are kept within specified ranges of temperature above - 1 °C".

Chilled food can be designed to be ready to eat, to be reheated or to be cooked. The main categories of chilled foods are dairy products (70 % ), meat products (15 % - excluding raw meat, poultry and fish), ready meals (9 %), pizzas (5 %.) and prepared salads (1 %).

Benefits of chilled food:

• conveneice

• snacking 

• healthy eating

• variety and choice

•  taste

• competition 

• reduce protein denaturation

• extended quality of fish

• correct chilling temperature could extend shelf life of fruit and vegetables

It is reported that for 40 % of eating occasions, convenience is the most important factor.  Convenience is a specific need of the cash-rich time-poor consumer, working women, single-parent and single-person households with limited time available for home cooking.

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.