Time-Temperature Relationship

Pasteurization may be carried out by batch- or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 63°C and held continuously at this temperature for at least 30 min. In the continuous process, milk is heated to at least 72°C for at least 15 s ia what is known as high temperature—short time (HTST) pasteurization, the primary method used for fluid milk. For milk products having a fat content above that of milk or that contain added sweeteners, 66°C is requited for the batch process and 75°C for the HTST process. For either method, foUowiag pasteurization the product should be cooled quickly to <7.2° C. Time—temperature relationships have been estabHshed for other products including ice cream mix, which is heated to 78°C for 15 s, and eggnog, which must be pasteurized at 69°C for 30 min or 80°C for 25 s. 

The Dg-value is a process-describing unit expressed in terms of minutes at 121.1 °C (originally 250°F) or a corresponding time-temperature relationship to produce the same complete spore-killing effect. 

The results are shown in Figure 3. The lifetime linearly changes inversely with the temperature. Some examples of the time-temperature relationship for typical treatment processes are also indicated in this Figure. It can be seen that the time-temperature relationship of PIQ is far greater than any encountered in LSI interconnection and assembly procsses. 

Time-temperature relationships have been established by various workers as being optimum for preventing or retarding the development of oxidized flavors in dairy products cream, 88 °C for 5 min condensed milk, 76.5°C for 8 min dry whole milk, preheated at 76.5°C for 20 min and frozen whole milk, 76.5°C for 1 min (Parks 1974). Few, if any, instances of a tallowy flavor have been reported in evaporated... 

Sub-glass transitions are generally determined by the molecular (local) scale structure. Their location in the (t, T) space undergoes only a second-order influence of the macromolecular (network) structure through internal antiplasticization effects. By contrast, glass transition is directly under the influence of the network structure (Chapter 10), so that it appears interesting to study the influence of crosslinking on the parameters of the time-temperature relationship (WLF equation) ... 

In any choice of material involving hydrolysis resistance, the time-temperature relationship with regard to hydrolysis must be known and appreciated. The newer types of esters offer better resistance than the traditional grades. This means that the newer material can operate in higher temperatures than the previous maximum of 50°C. MDI-ether-based materials are classed as the most hydrolysis-resistant polyurethanes. 

FIG. 35.—Time-temperature relationships using low and medium microwave heating for four starch-to-water ratios (from Ref. 288). 

One of the major problems associated with the flavor development of microwave products results from the fact that the reduced time/temperature relationship during baking is not conducive to the formation of a crisp, outer crust or many of the Maillard compounds associated with a conventionally baked flavor. These microwave baked products thus have a different flavor character from conventionally baked products and are typically judged "inferior by consumers. 

Figure 3-3 Time-Temperature Relationships for the Heat Denaturation of Whey Proteins in Skim Milk. Source From H.A. Harland, S.T. Coulter, and R. Jenness, The Effects of Various Steps in the Manufacture on the Extent of Serum Protein Denaturation in Nonfat Dry Milk Solids. / Dairy Sci. 35 363-368, 1952.

It is now easy to understand why the viscoelastic behavior of filled vulcanizates at large strains is so complex and why it is so difficult to investigate in definitive manner. None of the processes responsible for stress-softening occur instantaneously (16,182), nor can they generally be expected to obey the time-temperature relationships of linear viscoelasticity. Since time-temperature superposition is no longer applicable,... 

Autoclaving with steam involves time/temperature relationships, i.e. 134°C for 3 min or 121°C for 15 min or 115°C for 30 min or such combinations of temperature and time which ensure sterilisation. It is advisable to carry out autoclave tests on any plastic as part of the development programme. Now that autoclaves have balanced or overpressure facilities, experiments have to be carried out in order that the correct conditions can be selected. It should be noted that distortion or extension (initially due to internal pressure in the pack) is likely to occur during the cooling cycle and it is during this period that additional pressure is required to overcome the internal pressure. 

Temperatures of above 100°C can be achieved by moist heat under pressure in an autoclave and, as with dry heat, there is a time—temperature relationship ... 

Naphthalene and Derivatives. The time-temperature relationship is especially important in this series from the standpdnt of isomer distribution. Low temperatures (SO C and below) and/or diort reaction periods favor alpha substitution, while higher tena(peratures (about 160 C) and/or a longer time favor beta substitution. Even when the more stable beta isomer is the final product, the less stable (but more easily formed) alpha isomer is produced first, but imdergoes desulfonation and resulfonatimi to the beta sulfonate. The data for naphth ene are shown in Table 7-11. The same considerations hold in the disulfonation of naphthalene and in the preparation of the naphthd- and naphthylaminesuHonic acids. 

Rearrangement of equation (v) and subsequent substitution into (vi) gives the required time-temperature relationship. 

While correlation of fracture behavior in glassy polymers with the time-temperature relationships is receiving increasing attention [see, for example, Broutman and Kobayashi (1972), Johnson and Radon (1972), Radon (1972)], the case of rupture in elastomers is rather well developed, and is discussed briefly below. 

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