Product thickness. The thicker the product, the longeris the freezing time. For products less than 50 mm thick,doubling the thickness may more than double the freezing timewhereas doubling a thickness of 100 mm or more may increase thefreezing time fourfold. The rate of change of freezing time withthickness therefore, depends on the relative importance of theresistance of the fish to heat transfer.
Product contact area and density. In a plate freezer,poor contact between product and plate results in increasedfreezing time. Poor contact may be due to ice on the plates,packs of unequal thickness, partially filled packs or voids atthe surface of the block. Surface voids are often accompanied byinternal voids and this also results in poor heat transfer. Apartfrom increasing freezing time, internal voids also reduce thedensity of the block. The relationship between time, blockdensity and contact area for 100 mm blocks of white fish is shownin Table 6.
Product packaging. The method of wrapping and the typeand thickness of the wrapping material can greatly influence thefreezing time of a product. Air trapped between wrapper andproduct has often a greater influence on the freezing time thanthe resistance of the wrapping material itself. The followingexample illustrates the point. Smoked fish in a cardboard boxwith the lid on take 15h to freeze in an air blast freezer.Smoked fish in an aluminium box of the same shape and size andwith the lid on take 12h, but if the lid is taken off thecardboard box, the freezing time is only 8h because there is notrapped air acting as an insulation.
Species of fish. The higher the oil content of the fishthe lower is the water content. Most of the heat extracted duringfreezing is to change the water to ice; therefore, if there isless water, then less heat will require to be extracted to freezethe fish. Since the fat content of oily fish is subject toseasonal variations, it is safer to assume the same heat contentfigure used for lean fish in any calculation. This also ensuresthat the freezer capacity is adequate whatever the species offish being frozen.
Freezing times can be calculated, but there is usuallyinsufficient information available to make this calculationaccurate. Calculated freezing times can be fairly accurate foruniformly shaped products such as blocks of fillets but, forother products with irregular shapes, calculation can only give arough guide. The presence of wrappers and many other factors canmake calculation of the freezing time difficult and unreliable.
Formulae that have been used for quick calculations in thepast had to be simplified to make them practical. They alsoassume that the fish has been chilled before freezing and thatall of the heat is extracted at the initial freezing temperature.Calculated freezing times should therefore only be used to givean approximation of the true figure and should not be used fordesigning freezing equipment. Modern computer techniques have nowmade it possible to calculate freezing times more precisely.
Plank's formula for calculating the freezing time of fish hasbeen widely used in a variety of forms. It has proved to beparticularly valuable in extending the results from experimentalstudies to cover a wide range of variables. Thus, if anaccurately measured freezing time is known, others can becalculated if most of the freezing conditions are similar.
From the above formula, it can be seen that freezing time isinversely proportional to the temperature difference and,depending on other conditions, it may also be nearly proportionalto the square of the product thickness. This knowledge can beused to calculate other freezing times as shown in the examplesshown in Table 8.
Freezing time is directly proportional to the square of thethickness since in this case the surface heat transfercoefficient is high and the factor relating to the thickness ofthe block, PD/f, will be small. The new freezing time willtherefore be calculated as follows:
Water makes up over 90 percent of the weight of most fruits and vegetables. Water and other chemicals are held within the fairly rigid cell walls that give structure and texture to the fruit or vegetable. When you freeze fruits and vegetables you actually are freezing the water in the plant cells.
Celery and lettuce are not usually frozen because of this and we suggest that you serve frozen fruits before they have completely thawed. Partially thawed fruit is more appetizing when the effect of freezing on the fruit tissue is less noticeable.
Textural changes due to freezing are not as apparent in products that are cooked before eating because cooking also softens cell walls. These changes are also less noticeable in high starch vegetables, such as peas, corn and lima beans.
All freezer manuals give guidelines for the maximum number of cubic feet of unfrozen food that can be frozen at one time. This is usually 2 to 3 pounds of vegetables to each cubic foot of freezer space per 24 hours. Overloading the freezer with unfrozen products will result in a long, slow freeze and a poor-quality product.
Freezing, when properly done, can preserve more nutrients than other methods of food preservation. To maintain top nutritional quality in frozen fruits and vegetables it is essential to follow directions for pretreatment of the vegetables, to store the frozen product at zero degrees F and to use it within suggested storage times.
A: Vacuum packaging machines or vacuum sealers remove air and can extend the storage time of refrigerated, dried and frozen foods. Vacuum packaging is not a substitute for the heat processing of home-canned foods or for refrigerator or freezer storage. Vacuum packaging removes air from the contents of a package. In this oxygen-free environment, the spoilage bacteria don't multiply very fast, which helps maintain the quality of the food product.
A: Freeze only the amount that will freeze within 24 hours, which is usually 2-3 pounds of food per cubic foot of freezer space. For best quality set the freezer temperature at minus 10 degrees F at least 24 hours ahead of freezing quantities of fresh food. Once frozen, maintain a temperature at zero degrees F or less. Use an appliance thermometer to check the temperature of your freezer.
A: No. This is a quality versus a food safety issue. Recommended storage times ensure maximum quality. Food stored longer will be safe to eat but you may notice changes in flavor, color and texture. For best quality, use frozen fruits and vegetables within 8 to 12 months.
Background: Transbronchial cryobiopsy (TBCB), a novel way of obtaining a specimen of lung tissue using a flexible cryoprobe, can obtain large lung biopsies without crush artifacts. The freezing time of TBCB was empirically selected from 3 to 7 s in the previous studies. However, no consensus has yet been reached regarding the optimal freezing time used in TBCB.
Objectives: The primary endpoint was biopsy size in different freezing times. The secondary endpoints included sample histological quality, diagnostic confidence, and complications in different freezing times.
Methods: Patients who were suspected of DPLD requiring histopathological examination for further evaluation were enrolled in this study. Distinct biopsies were obtained by using different freezing times increased from 3 to 6 s sequentially. Samples were reviewed by 2 external expert pathologists.
Results: A total of 33 patients were enrolled, and 143 transbronchial cryobiopsies were taken in this trial. An average of 4.33 samples were taken from each patient. The mean biopsy size of different freezing times from 3 to 6 s was 9.10 Â 4.37, 13.23 Â 5.83, 16.26 Â 5.67, and 18.83 Â 7.50 mm2, respectively. A strong correlation between freezing time and biopsy size was observed (r = 0.99, p Conclusion: A longer freezing time was associated with a larger size of the biopsy sample but a higher risk of bleeding. The optimal transbronchial cryobiopsy freezing time is 3-4 s, which is easily achievable and provides an adequate biopsy size whilst creating a safety threshold from complications.
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