Thermal degradation model systems

Because of its fundamental role as a precursor of vitamin A and the availability of P-carotene standard in crystalline form, the thermal degradation of P-carotene in model systems has been a subject of intense research. 

Rios, A.O., Borsarelli, C.D., and Mercadante, A.Z., Thermal degradation kinetics of bixin in an aqneons model system, J. Agric. Food Chem., 53, 2307, 2005. 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

Kanasawud and Crouzet have studied the mechanism for formation of volatile compounds by thermal degradation of p-carotene and lycopene in aqueous medium (Kanasawud and Crouzet 1990a,b). Such a model system is considered by the authors to be representative of the conditions found during the treatment of vegetable products. In the case of lycopene, two of the compounds identified, 2-methyl-2-hepten-6-one and citral, have already been found in the volatile fraction of tomato and tomato products. New compounds have been identified 5-hexen-2-one, hexane-2,5-dione, and 6-methyl-3,5-heptadien-2-one, possibly formed from transient pseudoionone and geranyl acetate. According to the kinetics of their formation, the authors concluded that most of these products are formed mainly from all-(E) -lycopene and not (Z)-isomers of lycopene, which are also found as minor products in the reaction mixture. 

Observed Rate Constant (Arobs) and Activation Energy ( a) Values Found for Carotenoid Thermal Degradation in Model Systems... 

Zepka, L. Q., C. D. Borsarelli, M. A. A. R da Silva et al. 2009. Thermal degradation kinetics of carotenoids in a cashew apple juice model and its impact on the system color. J. Agric. Food Chem. in press, doi 10. 1021/jf900558a. 

HDF has been identified as reaction product of a thermal treatment of sugars, especially the 6-desoxy sugar rhamnose [90], Analysis of the free sugars in the LMW-fraction revealed fructose-1,6-biphosphate (FBP) as the predominating carbohydrate in the LMW fraction (5.75 g/kg yeast), but no rhamnose was present [88]. To elucidate the contribution of FBP as precursors of HDF in the yeast fraction, the sugar phosphate was thermally degraded under the same conditions as used for the LMW fraction of yeast. The results revealed FBP, as effective precursor of HDF in aqueous model systems at lower reaction temperatures (100°C Table 19). It should be stressed that additions of the amino acids proline or alanine did not increase the concentrations of HDF from the carbohydrates listed in Table 19 (unpublished results). The data implied that FBP which was the predominant carbohydrate in yeast, is the... 

Among the other reported volatile TDP of B-carotene include B-cyclo-cltral, 5,6-epoxy-B-ionone and dihydroactinidiolide (25). These compounds were also found by Isoe et al. (30, 31), Wahlberg et al. (32) and Kawakami and Yamanishi (33) as photo-oxygenation products of B-carotene. Volatile thermal degradation of carotenoids has been extensively studied, mainly in nonfood systems. Hence, the objective of this study was to identify the volatile components of the TDP of B-carotene formed in a food model system. 

The heat processing of tea leads to many complex chemical changes in tea. Tea s taste and aroma is affected by heat in at least three ways by reducing the content of bitter soluble catechins, by the development of roast aromas and by the thermal degradation of 0-carotene. Studies pertaining to the heat-induced changes in tea and appropriate model systems are reviewed. 

In heated foods the main reactions by which flavors are formed are the Maillard reaction and the thermal degradation of lipids. These reactions follow complex pathways and produce reactive intermediates, both volatile and non-volatile. It has been demonstrated that lipids, in particular structural phospholipids, are essential for the characteristic flavor development in cooked meat and that the interaction of lipids with products of the Maillard reaction is an important route to flavor. When model systems containing amino acids and ribose were heated in aqueous buffer, the addition of phospholipids had a significant effect on the aroma and on the volatile products. In addition a number of heterocyclic compounds derived from lipid - Maillard interactions were found. The extent of the interaction depends on the lipid structure, with phospholipids reacting much more readily than triglycerides. 

The reactivity of the maleimide monomer was dependent on the substitution pattern of the phenyl ring, with the substituents in the ortho position tending to lower the molecular weight of the polymer formed. The THP substituent is readily removed either chemically or thermally to yield poly(iV-(hydroxyphenyl)maleimides). All polymers exhibited excellent thermal stability and showed no evidence of degradation below 360 °C. Reaction occurs between the phenolic ring of the polyimide and HMTA, to form benzoxazine-type derivatives. These reactions have been studied comprehensively using the monomeric model systems, Ai-(hydroxyphenyl)succinimides (Figure 8) . 

The kinetic aspect common to all the topics discussed in this chapter is the pyrolysis reactions. The same kinetic approach and similar lumping techniques are conveniently applied moving from the simpler system of ethane dehydrogenation to produce ethylene, up to the coke formation in delayed coking processes or to soot formation in combustion environments. The principles of reliable kinetic models are then presented to simulate pyrolysis of hydrocarbon mixtures in gas and condensed phase. The thermal degradation of plastics is a further example of these kinetic schemes. Furthermore, mechanistic models are also available for the formation and progressive evolution of both carbon deposits in pyrolysis units and soot particles in diffusion flames. 

It has been identified in the products of thermal degradation of cysteine and xylose in tributyrin (Ledl and Severin, 1973). It was found in a heated cysteine/glucose model system by Sheldon et al (1986) and in serine/threonine/sucrose model systems (as well as in coffee) by Baltes and Bochmann (1987a). Silwar and Tressl (1989) studied a model reaction involving heating cysteine and methionine with 2-furaldehyde (1.63) under roasting conditions and they found that an important part of the aldehyde is reduced to furfuryl alcohol. 

It is formed in the thermal degradation of glucose (Heyns et al., 1966a Fagerson, 1969). It has been found in a heated proline/glucose system by Brule et al. (1971), in a cysteine/glucose model system by Sheldon et al. (1986), and in threonine/serine/sucrose models by Baltes and Bochmann (1987a). 

Furoic acid is formed in the thermal degradation of glucose (Fagerson, 1969). It has been found in a heated cysteine/glucose model system (Sheldon et al., 1986). 

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