Temperature effects food systems

FIG. 13 Illustration of the effect of temperature (T) on aw for (A) a complex food system, (B) a small molecular weight solute, such as fructose, and (C) foods containing large amounts of solutes, such as raisins. In all case, Ti[Pg.26]

Freshly cut oranges or their juices may be exposed in an open glass for several hours without appreciable loss of I he vitamin because of the protective effect of the acids present and the practical absence of enzymes that catalyze its destruction. In potatoes, when baked or boiled, there is a slight loss of the vitamin, blit if they are whipped lip with air while hot, as in the production of mashed potatoes, a large fraction of the initial vitamin content usually will be lost. In freezing foods, it is common practice to dip them in boiling water or to treat them briefly with steam to inactivate enzymes, after which they arc frozen and stored at very low temperatures. In this state, the vitamin is reasonably stable. Vuamin C degradation in dehydrated food systems is described shortly. 

The composition of volatiles released from a food is different when it is sniffed (via orthonasal route) and when it is eaten (via retronasal route). This is partially due to conditions in the mouth that selectively affect volatility, thus altering the ratio of compounds that volatilize from a food system. Mouth temperature, salivation, mastication, and breath flow have all been shown to affect volatilization (de Roos and Wolswinkel, 1994 Roberts et al., 1994 Roberts and Acree, 1995 van Ruth et al., 1995c). The ideal gas law describes the effects of temperature. Saliva dilutes the sample, affects the pH, and may cause compositional changes through the action of the enzymes present (Burdach and Doty, 1987 Overbosch et al., 1991 Harrison, 1998). Mastication of solid foods affects volatility primarily by accelerating mass transfer out of the solid matrix. The gas flow sweeps over the food, creating a dynamic system. The rate of the gas flow determines the ratio of volatiles primarily based on individual volatilization rates and mass transfer. 

The glass transition as a reference state can be used to explain all transformation in time, temperature, and structure composition effects between different relaxation states for technologically practical food systems in their nonequilibrium nature. Among others, specific examples include reduced activity and shelf stability of freeze-dried... 

In addition, the effect of time and temperature on the behavior of the materials manifests a great influence on the rheological properties of the food systems. 

Reid, D.S. The freezing of food tissues. The Effects of Low Temperatures on Biological Systems, B.W.W. Grout and G.J. Morris, eds., Edward Arnold, London, 1987, chap. 15. 

The quantitative interpretation of the data for systems such as these needs some care372 as the theoretical models seem to be ambiguous at the moment and the treatment of systems in the rigid lattice condition needs attention to the details of the spin physics if the data are not to be misinterpreted.373 However, the use of the technique considerably extends the possibilities for the use of proton NMR in food systems and should allow a more detailed description of dynamical processes and an improvement in comparative studies. An important advantage is that the temperature dependence of the spectral density function becomes a measurable variable, thus allowing a more detailed investigation of temperature effects. 

During the last three decades water activity, has played a major role in many aspects of food preservation and processing. It is defined as the ratio of the vapor pressure of water P in the food to the vapor pressure of pure water Pq at the same temperature (fl = P/Pq). Next to temperature, it is now considered as probably the most important parameter having a strong effect on deteriorative reactions. The effect of water activity was studied not only to define the microbial stability of the product but also on the biochemical reactions in the food system and its relation to its stability. It has become a very useful tool in dealing with water relations of foods during processing. 

Awuah, G.B., Ramaswamy, H.S. and Piyasena, P. (2002) Radio frequency (RF) heating of starch solutions under continuous flow conditions effect of system and product parameters on temperature change across the applicator tube. Journal of Food Process Engineering, 25 (3), 201-223. 

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