Refrigeration & food freezing

Introduction The job of a refrigeration plant is to cool articles or substances down to, and maintain them at a temperature lower than the ambient temperature. Refrigeration can be defined as a process that removes heat.

Freezing Foods :- Pre-freezing : Fruits and Vegetables Freezing : Frozen fruits Storage : Poultry and meats Thawing : Seafood Dairy Bakery Prepared foods

Principles of refrigeration As stated earlier, water thrown over a tent in the hot desert sun will cause cooling. Evaporation of a liquid needs heat energy. The latent heat required for the evaporation to take place is produced by the sensible heat from the tent interior which, in turn causes cooling of the tent. If you dip your fingers in a volatile liquid like gasoline or alcohol and then allowed the liquid to evaporate, you can feel a cooling effect. The same thing will happen with water but, will not be as noticeable, as the evaporation process is slower.

The cooling effect, again, comes from the heat being removed from your fingers to evaporate the liquid. Modern refrigeration units use the evaporation principle to produce the low temperatures necessary to do the job required. The process consists of a cycle of compression, cooling and condensation, then the expansion of the liquid, evaporation and re-compression of the vapour.

Safe refrigerator temperature Refrigerators should be set to maintain a temperature of 40 °F or below. Some refrigerators have built-in thermometers to measure their internal temperature. When the power goes back on, if the refrigerator is still 40 °F, the food is safe. Foods held at temperatures above 40 °F for more than 2 hours should not be consumed. Appliance thermometers are specifically designed to provide accuracy at cold temperatures. Be sure refrigerator/freezer doors are closed tightly at all time.

Fig. Temp of food for control of bacteria

Importance of Refrigeration Refrigeration slows bacterial growth. They are in the soil, air, water, and the foods we eat. When they have nutrients (food), moisture, and favorable temperatures, they grow rapidly, increasing in numbers to the point where some types of bacteria can cause illness. Bacteria grow most rapidly in the range of temperatures between 40 and 140 °F. A refrigerator set at 40 °F or below will protect most foods.

Factors Affecting Freezing Rate Absolute refrigeration required,ie. Ability of system to remove heat Temp. difference between product and medium. Air velocity Product thickness Contact between product and medium. Initial product.

A methods of food preservation :- The heat is removed (heat of fusion). Temperature of the food is reduced below its freezing point. A portion of water in food undergoes a change in state to form ice crystals.

Freezing Quality Foods The foods you freeze should be top quality, free from blemishes and suitable for freezing. The fresher the product, the more satisfactory the frozen product will be Freezing cannot improve the flavour or quality of food. Freezing temporarily stops the growth of spoilage organisms, but may not kill them. Once frozen foods thaw, surviving organisms grow. Fruits and vegetables must be washed before freezing to remove garden soil, which is a source of bacteria.

Storage Temperature Cool storage: 16 to -2 deg. C (60 to 28 deg. F) Refrigerators: 4.5 to 7 deg. C (40 to 45 deg. F) Frozen storage: 0 to -18 deg. C (32 to 0 deg. F)

Advantages of Food Freezing Freezing is the home food preservation method that best preserves nutrients, flavours and colours, although canning is not far behind. Used properly, home freezing has many advantages: • Saves time, work and money. It is an easy method of preserving food. • Can simplify meal preparation. • Provides available foods for quick service. • Supplies seasonal or favorite foods year-round. • Permits fewer shopping trips. • Allows stocking up on special sales when prices are low.

Psychrometry Is the study of the properties of gas –vapour mixtures. The mixtures most commanly encountered in food processing is air water vapour. The psychometric /humidity Chart is for air- water vapour mixtures at one atmospheric pressure.

Definations 1)Absolute or sp.humidity (H) :- Is the wt. of water present in the mixture, per unit wt. of dry air. Assuming perfect gas behavoir. H =MwPv / MA (P-PV) 2)Saturation humidity (HS):- Is the absolute humidity of saturated air. In saturated air the water vapour pressure equals the vapour pressure of water at air temperature. 3)Percentage abs.humidity :- is the ratio of the abs. humidity of a mixture at a given temp. to the saturation humidity at the same temp. 4)The dewpoint or saturation temp. :- Is the temp at which a given air-water vapour mixtures becomes saturated, if cooled at constant humidity.

5)Adiabatic saturation temp :- If a stream of moist air passes co-currently through water sprays at temp. under adiabatic conditions, so that the air leaves the system in equilibrium with the water and at temp. is known as adiabatic saturation temperature. 6)Wet bulb temperature:- Is the steady state temperature attained by a small quantity of water evaporating under adiabatic condition in an air stream.