exp 9-food freezing (sew)

Exp 9-food freezing - Title Food Freezing Objectives 1 To know the pretreatment before freezing of fruits vegetables and poultry 2 To know the

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Title: Food Freezing Objectives: 1) To know the pretreatment before freezing of fruits, vegetables and poultry. 2) To know the effect of freezing on physical and sensory evaluation. 3) To learn the freezing technique using “blast freezer”. Introduction: Preservation of a food by freezing occurs by several mechanisms. The reduction of the product temperature to levels below 0 0 C causes reduction of growth rates for microorganisms and the deterioration of the product due to microbial activity. It will also influence the reaction of enzymatic and oxidation activity. In addition, the formation of ice crystals within the product changes the availability of water to participate in reactions. It causes minimal changes in the quality of food in terms of size, shape, texture, color, flavor and microbial load. This is assuming that the freezing process is carried out properly. A key factor during food freezing is how fast we freeze. It is known that when food is frozen slowly, the ice crystals formed will be large. Large ice crystals are undesirable as they will damage the cell structure of foods like meat and fish. This will result in poor texture and excessive dripping upon thawing. It is therefore important that freezing be carried out at a very fast rate. Frozen food must be maintained at the proper temperature at all times. Food processors freeze foods to an internal temperature of -18°C. The food must be maintained at this temperature or slightly lower during transport and storage. Many fruits and vegetables will retain good quality at the above temperature for up to 12 months or even longer. The expected frozen storage will vary with temperature. Although freezing as a preservation process generally results in a high quality product for consumption, the quality is influence by the freezing process and frozen- storage condition. The freezing rate or time allowed for the product temperature to be reduced from above to below the initial freezing temperature will influence product quality, but in a different manner depending on the food commodity. But generally, the freezing curve is as below.
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Pre-freezing - Stage where the body temperature will drop rapidly until reaches the temperature of freezing. Freezing – the temperature will not change but the water will change phase at this stage. Reduction to storage temperature – the temperature will drop rapidly to the temperature of freezing surroundings. To achieve freezing of a food product, the product must be exposed to a low temperature medium for sufficient time to remove sensible heat and latent heat of fusion from the product temperature as well as a conversion of the water from a liquid to a solid state (ice). In most cases, approximately 20% of the water remains in the liquid state at the storage temperature of the frozen food. To accomplish the freezing process in desired short times, the low temperature medium is much lower than the desired final temperature of the product and large convective heat-transfer coefficients are created. Following the freezing operation, frozen products are maintained in frozen
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This note was uploaded on 04/27/2010 for the course BSTM 3301 taught by Professor Ali during the Spring '10 term at Zhejiang University.

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Exp 9-food freezing - Title Food Freezing Objectives 1 To know the pretreatment before freezing of fruits vegetables and poultry 2 To know the

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