Reactions During Heating

However, temperature and pH strongly affect casein association and cause changes in micellular structure (cf. 10.1.2.1.2 and 10.1.2.1.3). An example of such a change is the pH-dependent heat coagulation of skim milk. The coagulation temperature drops with decreasing pH (Fig. 10.16 and 10.9). Salt concentration also has an influence, e. g., the heat stability of milk decreases with a rise in the content of free calcium. 

Heat treatment of milk activates thiol groups e. g., a thiol-disulfide exchange reaction occurs between /c-casein and P-lactoglobulin. This reaction reduces the vulnerability of K -casein to chymosin, resulting in a more or less strong retardation of the rennet coagulation of heated milk. 

Quality deterioration in the form of nutritional degradation, changes in color or development of off-flavor have also been predicted for other foods by application of a suitable mathematical model. 

In most cases the loss of quality fits a zero- or first-order rate law. Knowledge of the rate constant allows one to predict the extent of reaction for any time. 

The influence of temperature on the reaction rate follows the Arrhenius equation (cf. 2.5.4). Thus by studying a reaction and measuring the rate constants at two or three high temperatures, one could then extrapolate with a straight line to a lower temperature and predict the rate of the reaction at the desired lower temperature. However, these data allow only a prediction of the shelf life when the physical and chemical properties of the components of a food do not alter with temperature. For example, as temperature rises a solid fat goes into a liquid state. The reactants may be mobile in the liquid fat and not in the solid phase. Thus, shelf life will be underestimated for the lower temperature. 

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DTA-TG data for LiBH4 MgH2 0.3 1 is shown in figure 2. The data shows clearly the weight losses due to decomposition of the sample and heat flow due to endothermic and exothermic reactions during heating to 600°C. 

Effect of Phospholipids on Reaction Volatiles. As would be expected, the inclusion of phospholipids in the reaction mixtures produced many volatiles derived from lipid degradation these included hydrocarbons, alkylfurans, saturated and unsaturated alcohols, aldehydes and ketones. However, two other important observations were made. First, the concentrations of most of the hetero- cyclics, formed by the amino acid + ribose Maillard reaction, were reduced. For most of the major volatiles this reduction was of the order of 40 - 50%, but in the case of thiophenethiol and methyl- furanthiol the reduction was over 65%. This appears to support the findings that in meat and coconut, lipids exert a quenching effect on the amount of heterocyclic compounds formed in Maillard reactions during heat treatment (11,12). Second, and perhaps more important, the addition of phospholipid to the reaction mixtures resulted in the production of large amounts of compounds derived from the interaction of the lipid or its degradation products with Maillard reaction intermediates. 

A cooled solution of ethylene oxide in ether is added with stirring to a precooled solution of the Grignard compound. The mixture is then allowed to stand for a time or is heated before hydrolysis. Benzene is added as a diluent to prevent violent reaction during heating in the preparation of -hexyl alcohol (62%) from -butyl Grignard reagent. Some 2-hexanol is also formed in this preparation. " ... 

Garrido C. and Bodinier J.-L. (1999) Diversity of mafic rocks in the Ronda peridotite evidence for pervasive melt/rock reaction during heating of subcontinental lithosphere by upwelling asthenosphere. J. Petrol. 40, 729-754. 

The temperature at which the metastable perovskite decomposes (i.e., the low temperature reaction during heating) increased a mere 7°C when the CO2 concentration was reduced from 100% to 95%. Further reduction in CO2 concentration to 3.85% resulted in a further reaction temperature increase of only 33°C. The high-temperature reaction was also not significantly affected by the reduction in CO2 concentration from 100% to 95% and addition of 0.2% H2. There was a significant response to the further reduction in CO2 concentration to 3.85% and relative increase in H2-content (H2 CO2 increased from 1 475 to 1 1). The reaction temperature decreased by approximately 300°C. 

Some substances begin to decompose below their melting points. Thermally unstable substances may undergo elimination reactions or anhydride formation reactions during heating. The decomposition products formed represent impurities in the original sample, so the melting point of the substance may be lowered due to their presence. 

TPSR method can be used to study the kinetics and mechanism of surface reaction. The reactants pre-adsorption on the surface takes place reaction during heating process, and the location and the peak shape of the TPSR peak (Tr) can be determined by the kinetic parameters. Reaction on the surface is much more complex than the desorption process. TPSR kinetic equation can be obtained as... 

Hydroxy and carboxy end-groups, and oxydiethylene units are already present in the initial PEG and are obviously present in final copolyester. The vinyl ester function of the E series can be formed by a degradation reaction during heating (Equation 5.7) ... 

Fit a 1500 ml. bolt-head flask with a reflux condenser and a thermometer. Place a solution of 125 g. of chloral hydrate in 225 ml. of warm water (50-60°) in the flask, add successively 77 g. of precipitated calcium carbonate, 1 ml. of amyl alcohol (to decrease the amount of frothing), and a solution of 5 g. of commercial sodium cyanide in 12 ml. of water. An exothermic reaction occurs. Heat the warm reaction mixture with a small flame so that it reaches 75° in about 10 minutes and then remove the flame. The temperature will continue to rise to 80-85° during 5-10 minutes and then falls at this point heat the mixture to boiling and reflux for 20 minutes. Cool the mixture in ice to 0-5°, acidify with 107-5 ml. of concentrated hydrochloric acid. Extract the acid with five 50 ml. portions of ether. Dry the combined ethereal extracts with 10 g. of anhydrous sodium or magnesium sulphate, remove the ether on a water bath, and distil the residue under reduced pressure using a Claiseii flask with fractionating side arm. Collect the dichloroacetic acid at 105-107°/26 mm. The yield is 85 g. 

Polymerization. The polymerization of aziridines takes place ia the presence of catalytic amounts of acid at elevated temperatures. The molecular weight can be controlled by the monomer—catalyst ratio, the addition of amines as stoppers, or the use of bifimctional initiators. In order to prevent a vigorous reaction, the heat Hberated during the highly exothermic polymerization must be removed by various measures, ie, suitable dilution, controlled metering of the aziridine component, or external cooling after the reaction has started. 

In primary smelting, carbon (in the form of coal or fuel oil) is the reducing agent. During heat-up, carbon monoxide is formed by reaction with... 

Scoops of solid potassium tcrt-butoxide (purchased from E. Merck, Darmstadt, and specified to be at least 95% pure) were added over 20-30 minutes by temporarily removing the drying tube. At the beginning of the reaction much heat is evolved therefore the base should be added in small portions in order to keep the temperature below 10°. During the addition of the base, a precipitate is formed. 

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