Reactant degradation, product formation

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed by several pathways during meat cookery, but one mechanism is Strecker degradation of cysteine in the presence of a diketone as established by Kobayashi and Fujimaki (29). The cysteine condenses with the diketone and the product in turn decarboxylates to amino carbonyl compounds that can be degraded to hydrogen sulfide, ammonia and acetaldehyde. These become very reactive volatiles for the formation of many flavor compounds in meat and other foods. 

Probably the most important reactant in the formation of volatile meat flavor compounds is hydrogen sulfide. It can be formed as a Strecker degradation product of cysteine in the presence of a diketone (37). 

When microwave emission is stopped, microwave absorption by reactants and further heating of the reaction mixture instantaneously ceases, minimizing by-product formation and product degradation... 

Cyclobttlttdiene. Cyclobutadicne itself is unknown but the complex cyclobuta-dicnciron tricarbonyl (I, 2,140 3, 101) is readily available. The diene can be liberated from the complex as a transient intermediate for use in synthesis by oxidative degradation with ceric ammonium nitrate. If the reactants or products arc sensitive to the acidic solutions of CAN, lead tetraacetate in pyridine can be used as oxidant. Thus the reaction of cyclobutadienciron tricarbonyl with p-benzoquinone in the presence of CAN leads to formation of e [Pg.72]

The most sensitive techiuques are based on the reversibility degree of the redox reactions of certain reactants present in solution. The reactions of TCNQ [60] and ferrocene derivatives [47,53] can be carried out quite reversibly on the freshly formed surfaces of encapsulated ceramic HTSC electrodes and high-quality films in dry acetonitrile. On addition of water, the reversibility is upset due to the formation of low-conductivity degradation products on the surface. The increase in the potential difference between anodic and cathodic peaks correlates with the degradation rate. According to [484] reversible responses to similar reactants can also be obtained on electrodes made of high-quality ceramics. 

In some instances, the reactive forms or the intermediates themselves, show polarographic waves which are separated from those of reactants and products. The formation of the thiol form of thiamine, < ) the bicyclic form of protopine< > and the enediol forms of some sugars< > and pyridoine< ) are examples of the formation and detection of reactive forms. A few examples of the detection of intermediates, are the diketone and enediol produced during the alkaline degradation of phenylglyoxal< > and of a, -unsaturated ketone as an intermediate in the cleavage< ) of... 

Fig. 3.8, TSs and TS s symbolically represent sets of three and four transition states respectively. The structures shown near TSs and TS s are those of the last transition states in each set. RC and PC stand for reactant complex and product complex respectively. M stands for the molecule lying near ferulic acid in Fig. 3.8. In the RC of Fig. 3.8, an OH radical is already added at the CIO site. A sequence of addition and hydrogen abstraction reactions (Fig. 3.8) lead to the formation of fera-lic acid and vanillin. Thus curcumin and its degradation products in total scavenge eight OH radicals.

The formation of brown pigments via the Maillard reaction, especially in model systems (e.g. glucose-glycine), usually follows zero-order kinetics, but the loss of reactants has been found to follow first- or second-order kinetics in foods and model systems. Activation energies of 109, 116 and 139 kJ mol-1 have been reported for the degradation of lysine, the formation of brown pigments and the production of hydroxymethylfurfural (HMF), respectively. 

Table 9.1 summarizes catalyst compositions and corresponding performances. The oxidation of ethane to acetic acid is now commercial an industrial plant is installed, with the technology developed by Saudi Basic. Elements that have contributed to the successful development of the process are (1) the discovery of a catalytically active compound, the multifunctional properties of which can be modified and tuned to be adapted to reaction conditions through incorporation of various elements (2) the stability of the main products, ethylene and acetic acid, which do not undergo extensive consecutive degradation reactions (3) the possibility of recycling the unconverted reactant and the major by-product, ethylene (4) the use of reaction conditions that minimize the formation of CO and (5) an acceptable overall process yield. 

On the other hand, the effective collision concept can explain the Arrhenius term on the basis of the fraction of molecules having sufficient kinetic energy to destroy one or more chemical bonds of the reactant. More accurately, the formation of an activated complex (i.e., of an unstable reaction intermediate that rapidly degrades to products) can be assumed. Theoretical expressions are available to compute the rate of reaction from thermodynamic properties of the activated complex nevertheless, these expression are of no practical use because the detailed structure of the activated complexes is unknown in most cases. Thus, in general the kinetic parameters (rate constants, activation energies, orders of reaction) must be considered as unknown parameters, whose values must be adjusted on the basis of the experimental data. 

Effect of Water Vapor on Photocatalytic Air Treatment. Several studies have reported on the effects of water vapor on the photocatalytic treatment of air (101-108). The effect of water vapor very much depends on the type of pollutant and, obviously, on the partial pressure of water against that of the pollutant. On one hand, water can compete with the adsorption of organic pollutants, especially those that are structurally related, such as alcohols. On the other hand, water can behave as a reactant in some of the successive steps of the degradation of organics and, in particular, can limit the formation of products that inhibit the photocatalytic activity. Water can be at the origin of the formation of hydroxyl radicals however, the importance of these radicals in gas-phase photocatalytic reactions is being debated on (109-111). The conclusion is that some humidity seems necessary for optimum photocatalytic activity. 

Koll and Metzger (1978) report on the use of supercritical acetone as the reaction medium for the thermal degradation of cellulose and chitin. Since the pyrolysis of these polysaccharides occurs at such high temperatures, it is necessary to remove the primary products from the reaction zone as soon as they are formed to avoid degradation of the products into coke. The high operating temperature also adversely affects both yield and product distribution. It is possible to reduce the carbon formation by carrying out the pyrolysis under vacuum but the reaction rate is also reduced because of the poor heat transfer to the reactants. 

Fast reactions are ideally suited for CFB risers. Processes in which CFB reactors are preferred include those in which high selectivities are essential, coke formation or some other poisoning mechanism is rapid, gas plug flow characteristics are desirable (i.e., where product inhibition or degradation is significant), or reactions where the solids are the primary reactant. 

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