Unit 1 Food Processing and Preservation by Low temperature.pptx

FOOD PROCESSING AND PRESERVATION BY LOW TEMPERATURE

1. Food Processing and Preservation Technology

2. Principle of Food Preservation The basic principle of food preservation includes: Preservation or delay of microbial decomposition: That is archived by : Keeping out microorganisms (asepsis) Removal of microorganisms,i.e ., by filtration Hindering the growth of activity of microorganisms ,i.e ., by low temperature, drying, anaerobic conditions, & Killing the microorganisms ,i.e ., by heat or radiations. Preservation or delay by self – decomposition of food . This is brought about by : Destruction or inactivation of food enzymes i.e , by blanching Preservation or delay of chemical reactions i.e , preservation of oxidation by means of an antioxidant. Preservation of damage caused by insects, animals & mechanical causes.

3. Bread Meat, fish, poultry, vegetables

4. Methods for preservation

5. Preservation of foods by low temperature Lowering the storage temperature of the food will reduce or prevent spoilage by microorganisms and/or chemical reactions.

6. Refrigeration slows down the biological, chemical, and physical reactions that shorten the shelf life of food. Temperatures typically between 45 - 32 ° F (7.2 - 0 ° C). Preferably below 38 ° F. All perishable foods should be refrigerated as soon as possible, preferably during transport, to prevent bacteria from multiplying.

7. Vapor compression cycle

8. Refrigeration cycle

9. TRADITIONAL REFRIGERATED FOODS Fresh foods (unprocessed), fruits and vegetables, (fresh meats, poultry, fish) Processed foods: doughs, minimally processed vegetables Refrigerated foods containing fruits and vegetables: entrees, dinners, salads. (Pasteurized dairy products cured meats)

10. Effect of refrigeration on food Refrigeration slows bacterial growth but not inhibits.   Loss in turgor pressure Intermixing of flavor of milk or sulphur containing foods Loss of moisture Wilting/yellowing of leaves Changes in textural and taste parameters

11. FREEZING Freezing is the unit operation in which the temperature of a food is reduced below its freezing point and a proportion of the water undergoes a change in state to form ice crystals. Principle of Freezing: When a substance changes its state from liquid to solid, the evolution of heat takes place from the body to its surroundings. The immobilization of water to ice and the resulting concentration of dissolved solutes in unfrozen water lower the water activity ( a w ) of the food < 32 o F (0 ° C)

12. Freezing curve AS: The food is cooled below its freezing point of 0º C. At point S, the water remains liquid. SB: The temperature rises rapidly to the freezing point, because ice crystals will form and at a higher speed release latent heat, which is extracted by freezing the food. BC: The heat is removed, eliminating the latent heat, formation of ice, temperature remaining constant. Increasing concentration of solutes in the unfrozen water fraction causes the decrease of freezing point, so the temperature decreases. This is the phase in which most of the ice forms. CD: The crystallization of water and solutes continues. The temperature falls for the mixture of water and ice.

13. Types of Freezing Rapid freezing prevents undesirable large ice crystals from forming throughout the product because the molecules don't have time to form into the characteristic six-sided snowflake. Slow freezing creates large, disruptive ice crystals. During thawing, they damage the cells and dissolve emulsions.

14. Methods of freezing The use of cold air blasts or other low temperature gases coming in contact with the food, e.g. blasts, tunnel, fluidized bed, spiral, belt freezers. Indirect contact freezing , e.g. plate freezers, where packaged foods or liquids are brought into contact with metal surfaces (plate, cylinders) cooled by circulating refrigerant (multi-plate freezers). Direct immersion of the food into a liquid refrigerant, or spraying liquid refrigerant over the food (e.g. liquid nitrogen, and freon, sugar or salt solutions).

15. AIR FREEZING - Products frozen by either "still" or  "blast" forced air. • cheapest (investment) • "still" slowest, more changes in product • "blast" faster, more commonly used 2. INDIRECT CONTACT - Food placed in direct contact with cooled metal surface. • relatively faster • more expensive

16. 3. DIRECT CONTACT - Food placed in direct contact with refrigerant (liquid nitrogen, "green" freon,  carbon dioxide snow) • faster • expensive • freeze individual food particles

17. Freezing equipment Mechanical Freezers - Evaporate and compress the refrigerant in a continuous cycle Cryogenic Systems - Use solid and liquid CO 2 , N 2 directly in contact with the food

18. Batch Freezer Source: Unit operations for food the food industries by: W.A. Gould Blast Type

19. Double Contact Plate Freezer Hydraulic Pump Top Pressure plate Connecting Linkage Corner Headers Refrigerant hoses Trays Contact plates Polyurethane and polystyrene insulated doors

20. Immersion freezing In immersion freezing, food is placed in a refrigerant prior to freezing. Brine is often used for fish, and a glycol, sugar solution for fruits. This provides a layer which protects the food from the dry atmosphere of the freezer.

21. Cryogenic freezers Freezers of this type are characterized by a change of state in the refrigerant (or cryogen) as heat is absorbed from the freezing food. The heat from the food therefore provides the latent heat of vaporization or sublimation of the cryogen. The cryogen is in intimate contact with the food and rapidly removes heat from all surfaces of the food to produce high heat transfer coefficients and rapid freezing. The two most common refrigerants are liquid nitrogen (-196 ℃ ) and solid or liquid carbon dioxide (-72 ℃ ).

22. Cryogenic Freezing Uses liquid nitrogen which is very cold (-196ºC) Food passes through a tunnel where nitrogen gas is sprayed downwards. A beefburger will be frozen in 1 minute at these extreme temperatures. This produces small crystals, and little moisture loss. This method is used when freezing prawns. The prawns are first dipped in liquid nitrogen to freeze the outside layer. This prevents the prawns sticking together and from sticking to the freezer belts.

23. Cryogenic freezer Source: Fellows (2000)

24. Ultra rapid: Direct Contact Liquid Nitrogen Tunnel Freezer Source: Unit operations for food the food industries by: W.A. Gould IQF Cryogenic freezer

25. Fluidized bed freezing (IQF: Individual quick freezing) Used to freeze particulate foods such as peas, cut  corn , diced carrots, and strawberries. The foods are placed on a mesh conveyor belt and moved through a freezing zone in which cold air is directed upward through the mesh belt and the food particulates begin to tumble and float. This tumbling exposes all sides of the food to the cold air and minimizes the resistance to  heat transfer  at the surface of the food.

26. Effect of Freezing on Food Low temperatures do not significantly affect the nutritional value of food, but thiamin and vitamin C may be destroyed when vegetables are blanched (briefly immersed in boiling water) before freezing. If fish is frozen too slowly, some of its cells may rupture and release nutrients into the liquid that drips from the fish when it thaws. Some flavours become weaker and some become stronger when food is frozen.

27. Advantages of Freezing Many foods can be frozen. Natural color, flavor, and nutritive value retained. Texture usually better than other methods of food preservation. Foods can be frozen in less time than they can be dried or canned.

28. How Freezing Affects Food Chemical changes Enzymes in vegetables Enzymes in fruit Rancidity Texture Changes Expansion of food Ice crystals

29. ISSUES with FROZEN FOODS (cont.) 2. Undesirable physical changes Fruits and vegetables lose crispness Drip loss in meats and colloidal type foods (starch, emulsions) Freeze product faster Control temperature fluctuations in storage. Modify starch, egg systems, etc.

30. 2. Undesirable physical changes (cont.) C. Freezer burn Package properly Control temperature fluctuations in storage. D. Oxidation Off-flavors Vitamin loss Browning E. Recrystallization

31. Dehydrofreezing Dehydrofreezing is a method of food preservation that combines the techniques of drying and freezing. The process of freezing partially dehydrated foods is known as dehydrofreezing . Fruits or vegetables dried at home have had 85 to 90% of their moisture removed to prevent mold growth. However, by removing only 70 percent of the moisture and storing the fruit or vegetable in the freezer, a tastier product results. The freezer's low temperature inhibits microbial growth. Fruits and vegetables processed this way have good flavor and color and reconstitute in about half the time it takes for dried foods.

32. Thawing Thawing is  the process of taking a frozen product from frozen to a temperature (usually above 0°C) where there is no residual ice , i.e. “defrosting”. Thawing is often considered as simply the reversal of the freezing process.

33. Methods of thawing

34. Damaging to the texture of the frozen fruit Increase in drip loss and a decrease in firmness Loss of cell wall materials in middle lamella . Loss of flavor Loss of nutrients Loss of turgor pressure Effect of thawing-freezing cycle