8/7/2019 Food_Science 183

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Cold Preservation an d Processing

Controlled Low Temperature

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Properly designed refrigerators, refrigerated storage rooms, and warehouses willprovide sufficient refrigeration capacity and insulation to maintain the room withinabout 1Cof th e selected refrigeration temperature. In orderto design a refrigeratedspace capable of maintaining this temperature, it is necessary to know, in addition toth e insulation requirements, all factors that may generate heat within this space orinfluence ease ofremoval of heat from the space. These factors include th e number ofheat-generating electric lights and electric motors that may be operating, th e numberof people that may be working in th e refrigerated space, how often doors to th e areawill be open to permit entranceof warm air, and th e kinds and amounts offood products

that will be stored in th e refrigerated area.This latter i tem is of importance for two major reasons: First, th e quantity of heatthat must be removed from any amount of foodto lower it from one temperature toanother is determined by the specific heat of the particular food; and second, duringand after cooling, such foods as fruits and vegetables respire and produce their ownheat at varying rates. Both th e specific heats and respiration rates of al l importantfoods ar e known or ca n be closely estimated. These values, in addition to th e itemsmentioned above, ar e necessary to calculate th e "refrigeration load," which is th equantity of heat that must be removed from the product and th e storage area in orderto gofrom an initial temperature to th e selected final temperature and then maintainthis temperature for a specified time.The heat evolved during respiration by representative fruits and vegetables is listed

in Table 9.2. The amount of heat produced varies with each product, and l ike allmetabolic activities decreases with storage temperature. Products with particularlyhigh respiration rates, such as snap beans, sweet corn, green peas, spinach, and strawberries, ar e particularly difficult to store. Such products, if closely packed in a bin,can ro t in th e center even when th e surrounding ai r is cool due to th e heat generatedby the product. The relationships between specific heats of foods and calculation ofrefrigeration load will be discussed in th e section on freezing and frozen storage.

Ai r Circulation and Humidi ty

Proper air circulation helps move heat away from th e vicinity offood surfacestowardrefrigerator cooling coils and plates. But the a ir that is circulated within a coldstorageroom must not be too moist or too dry. Air of high humidity can condense moistureon the surface of cold foods. I f this is excessive, molds will grow on these surfaces atcommon refrigeration temperatures. If th e ai r is too dry, i t will cause drying out offoods. All foods ar e different with respect to supporting mold growth and tendency todry out, and so for each, an optimum balance must be reached. The optimum relativehumidity (RID to be maintained in cool storage rooms for most foods is known. Table9.3 summarizes th e best storage temperatures and relative humidities for many fooditems and their approximate storage life. (This table also includes data necessary forcalculation ofrefrigeration loads.) Most foods store best at refrigeration temperatureswhen th e relative humidity of ai r is between about 80% and 95%. The optimal relativehumidity for a particu\ar foodis generallyrelated to it s moisture content and the easewith which i t dries outl-For example, celery and several other crisp vegetables requirea relat ive humidity of 90-95%, whereas nuts may do well at only 70%. On th e otherhand, dry and granular products such as powdered milk and eggs, which have extended