Food freezing. Freeze-drying

Various methods have evolved, depending on the available resources, the product concerned and the premium value it might earn in an improved frozen state. 

Where the product shape is irregular, the only way to extract its heat will be by using a cold fluid surrounding it. The most common of these is air. The air temperature will be of the order of - 40°C and the air speed over the product will be high, to get good heat transfer. 

Flat pieces of product, such as fish fillets, would suffer a change in shape in a free air blast and are better on a flat moving belt. Here, some of the heat goes direct to the cold air and some by conduction to the belt, which is usually of stainless steel. This tunnel 

Products in regular-shaped packages, such as ice-cream in flat cartons, are pressed between horizontal, flat, refrigerated plates. These can 

Trays of product to be frozen can be loaded onto trollies, which are taken through an air blast tunnel. The evaporator coils will usually be in the upper part of the tunnel, with air flow across the trays. 

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Shibata [4.20] described the freeze drying of boiled rice as a method to produce convert food (fast food). The paper is quoted as the advantages of food freeze drying are mmarized within it Structure and taste of the boiled food can be restored in approx. 3 in by adding hot water of +80 °C, and the dried product can be stored well. 

The solvents used for extracting isoflavones from soy foods were chosen according to the solubility of isoflavones and the food matrix involved. The diversity of soy isoflavones in polarity requires the use of a combination of organic solvent and water for extraction. The organic-to-water ratio (10 5) was established based on Murphy s study on solvent selection (Murphy et al., 2002). Water content in the solvent needs to be adjusted according to the moisture content in soy foods. Freeze-drying of samples before extraction simplifies the extraction process, but is not a prerequisite. 

Shelf Devices Equipment having heated and/or cooled shelves is available but is httle used for divided-sohds heat processing. Most extensive use of stationaiy shelves is freezing of packaged solids for food industries and for freeze drying by sublimation (see Sec. 22). 

Raw foods were freeze-dried and analyzed for carbon isotopes using mass spectrometry. Cooked foods were prepared following historic recipes, then were freeze-dried prior to analysis. For the trace element analysis, foods (both raw and cooked) were wet ashed using nitric acid in Teflon lined pressure vessels and digested in a CEM Microwave oven. Analysis of Sr, Zn, Fe, Ca and Mg was performed using Atomic Absorption Spectrometry in the Department of Geology, University of Calgary. 

TAPPEL A L (1956) Freeze-dried meat. 2. The mechanism of oxidative deterioration of freeze-dried beef, Food Res, 21, 195-206. 

Freeze-dried tomato powders obtained from whole tomato fruits and from their pulp after centrifugation, containing 474 and 5399 pg/g dry weight, respectively, were developed for use as additives for food fortification. Cis isomers of lycopene were determined in only a few smdies. The 5-cis-, 9-cis-, and 13-d5 --El5-d5 -lycopene were the isomers found in commercial tomato products. The structures of lycopene cis isomers are shown in Figure 4.2.1 and the structure of the dll-trans isomer is displayed in Figure 6.2.1 in Chapter 6. 

Tang, Y.C. and Chen, B.H., Pigment change of freeze-dried carotenoid powder during storage, Food Chem., 69, 11, 2000. 

Schauss, A.G. et al.. Phytochemical and nutrient composition of the freeze-dried Amazonian pahn berry, Euterpe oleraceae Mart. (a9ai), J. Agric. Food Chem., 54, 8598, 2006. 

Because most food matrices are water soluble, many efforts were directed to the formulation of lipophilic pigments (mainly carotenoids) into water-soluble formulations (powders or gels). For hydrophilic pigments like flavonoids, polar dried microcapsules are the most popular ways to stabilize their functionality. Extracts rich in P-carotene were encapsulated using three different encapsulation techniques (spray drying, drum drying, and freeze drying)." ... 

Kopjar, M., Influence of trehalose addition on anthocyanin retention of evaporated and freeze-dried strawberry pastes, in Proceedings of 4th International Congress on Pigments in Food, Hohenheim, Germany, Carle, R. etal., Eds., Shaker Verlag, Aachen, 2006, 223. 

Desobry, S.A., Netto, F.M., and Labuza, T.P., Comparison of spray-drying, drumdrying and freeze drying for P-carotene encapsulation and preservation, J. Food Sci., 62, 1158, 1997. 

Dried or freeze dried samples can be extracted with water-immiscible solvents such as EtOAc or diethyl ether. For quantitative extraction, dried samples are preferably rehydrated at different times for example, 5 to 10 min for dried mangoes, 30 min for lyophihzed red peppers and pasta. Rehydration is followed by extraction with acetone or MeOH. Bixin and norbixin from a mix dry powder of annatto and com can quantitatively be extracted with MeOH followed by acetone. In order to improve pigment recovery, extruded foods require pre-digestion with enzymes to liberate the pigment from the matrix. ... 

In paired comparison tests two different samples are presented and one asks which of the two samples has most of the sensory property of interest, e.g. which of two products has the sweetest taste (Fig. 38.3). The pairs are presented in random order to each assessor and preferably tested twice, reversing the presentation order on the second tasting session. Fairly large numbers (>30) of test subjects are required. If there are more than two samples to be tested, one may compare all possible pairs ( round robin ). Since the number of possible pairs grows rapidly with the number of different products this is only practical for sets of three to six products. By combining the information of all paired comparisons for all panellists one may determine a rank order of the products and determine significant differences. For example, in a paired comparison one compares three food products (A) the usual freeze-dried form, (B) a new freeze-dried product, (C) the new product, not freeze-dried. Each of the three pairs are tested twice by 13 panellists in two different presentation orders, A-B, B-A, A-C, C-A, B-C, C-B. The results are given in Table 38.3. 

Freeze Drying and Advanced Food Technology". Acadeaic Press, Orlando, FL., 1975. 

Asami, D.K., Flong Y.-J., Barret D.M. and Mitchell A.E. (2003). Comparison of the total phenolic and ascorbic acid content of freeze-dried and air-dried marionberry, strawberry, and com grown using conventional, organic, and sustainable agricultural practices , Journal of Agricultural Food Chemistry, 51, 1237-1241. 

Tsai HL, Chang SKC and Chang SJ. 2007. Antioxidant content and free radical scavenging ability of fresh red pummelo [Citrus grandis (L.) Osbeck] juice and freeze-dried products. J Agric Food Chem 55(8) 2867-2872. 

Schauss AG, Wu XL, Prior RL, Ou BX, Patel D, Huang DJ and Kababick JP. 2006. Phytochemical and nutrient composition of the freeze-dried Amazonian palm berry, Euterpe oleracea Mart. (Acai). J Agric Food Chem 54(22) 8598-8603. 

The addition of water or the use of an aqueous solvent mixture is important for the extraction of other organic analytes from dry foodstuffs or dehydrated foods. It is particularly necessary in aiding the permeation of solvent through freeze-dried samples. 

In general, unless a local reservoir (i.e., intermediate host that may or may not be affected by the virus) is established, pathogens are easily killed by unfavorable environmental factors such as fluctuations in temperature, humidity, food sources, or ultraviolet light. For this reason, their persistency is generally limited to days. However, freeze-dried pathogens can remain in a preserved state almost indefinitely and are reactivated when exposed to moisture. 

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