Nonenzymatic browning due to the Maillard reaction between reducing sugars and glycine was studied in systems containing varying amounts of water, glycerol, and hydrophilic polymers. We observed that the browning rate decreased with increasing water content, except in systems in which mobility of reactants became substantially impeded at low water contents. The effect of water was complex and depended on the presence of various waterbinding agents among other factors. The inhibitory effect of high water contents could be due to the fact that water is a product of several condensation steps in the browning reaction.ugar-amino browning reactions in foods and model systems of low moisture content occur over a wide S range of water activities (Karel, 1960; Heiss, 1968).A maximum browning reaction occurs in most foods between water activities (a,) 0.3 and 0.7. The position of this maximum depends on the type of food. Therefore, water activity, though it reflects the effect of water being bound to specific polar groups in the food and other factors limiting the availability of water molecules for chemical reaction, cannot be used to predict optimum browning conditions. Furthermore, in some studies the browning rate increased with increasing water without a maximum (Jones, 1954(Jones, , 1956, and in others increased with decreasing moisture content (Rosen et al., 1953;Loncin et al., 1965). Water's influence on the rate of the sugar-amino browning reaction in a food system is unclear. At higher water activities the reaction rate decrease has generally been attributed to dilution of the reaction partners. The decreased reaction rate at low water activities when the amount of mobile water lowers has been ascribed to an increasing diffusion resistance which lowers the mobility of the reaction partners (Labuza et al., 1970).We performed this study to elucidate the influence of water content, water activity, dilution of the reactants, and of viscosity on the browning rate of reducing sugars and amino acids. By varying the glycerol content in a sugar-amino acid-glycerol-water system, we changed the water content of this system while maintaining a constant water activity and vice aersa. In this way we investigated the action of water content and water activity on the browning rate separately. We demonstrated the influence of dilution of the reactants by adding increasing amounts of glycerol to the above mentioned system. In order to show the influence of the diffusion resistance on the browning rate, we increased the viscosity of the system by adding water-soluble polymers and lowering the water activity. The viscosity was decreased by the plasticizing effect of glycerol. MODEL SYSTEMSThe model systems studied contained glucose (glucose and fructose) and glycine as the reactants, as well as additives allowing water activity and water content control. Materials. Microcrystalline cellulose (Avicel-PH-105), FMC Corp. Methylcellulose (Methocel), visc. 4000 cps, Type MC, Dow Chemical Co. Gum arabic (Hallmark), Stein, Hall & Co., Inc...

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Spice paprika (red pepper; Capsicum annuum) is the most cultivated spice worldwide and is used mainly for its color and pungency. However, current research is also focusing on the flavor as an important parameter. This paper deals with the kinetics of the formation of those volatiles that indicate a decrease in spice paprika quality due to Maillard reaction, hydrolytic reactions, and oxidative degradation reactions of lipids such as fatty acids and carotenoids. Spice paprika volatiles were quantitatively analyzed by means of headspace gas chromatography (HS-GC) and solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS). The kinetics of their formation were investigated, and the respective activation energies determined. Strecker aldehyde, acetone, and methanol formation followed a pseudo-zero-order reaction kinetic, and formation of dimethyl sulfide (DMS) was characterized by a first-order kinetic. The activation energies determined were between 86.3 and 101.8 for the Strecker aldehydes acetaldehyde (AA), 2-methylpropanal (2-MP), 3-methylbutanal (3-MB), and 2-methylbutanal (2-MB), 130.7 for acetone, 114.2 for methanol, and 109.7 kJ/mol for DMS. The amounts of Strecker aldehydes formed were correlated to the concentrations of the corresponding free amino acids present in the samples. The formation of hexanal and 6-methyl-5-hepten-2-one in Capsicum annuum during processing was confirmed, and the formation of beta-ionone was probably described for the first time. During heating, the concentration of hexanal increased rapidly. The formation of 6-methyl-5-hepten-2-one confirms that Capsicum annuum fruits contain lycopene.

The behavior of crude Sclerocarya birrea kernel oil (SCO) and Sorghum bug (Agonoscelis pubescens) oil (SBO) during deep-frying of par-fried potatoes was studied with regard to chemical, physical, and sensory parameters, such as content of FFA, tocopherols, polar compounds, oligomer TG, volatile compounds, oxidative stability, and total oxidation (TOTOX) value. Palm olein was used for comparison. Whereas potatoes fried in SCO that had been used for 24 h of deep-frying at 175°C were still suitable for human consumption, potatoes prepared in SBO that had been used for 6 to 12 h were not, considering the sensory evaluation. In looking at the chemical and physical parameters, SBO exceeded the limits, after no later than 18 h of use, for the amount of polar compounds, oligomer TG, and FFA recommended by the German Society of Fat Sciences (DGF) as criteria for the rejection of used frying oils. In contrast to SBO, SCO oil did not exceed the limits for the content of polar compounds and oligomer TG during the frying experiment. Only the amount of FFA was exceeded; this was because the amount of FFA at the beginning of the experiment was higher than for refined oils. The results showed that both oils were suitable for deep-frying of potatoes, but remarkable differences in the time during which both oils produced palatable products were found. SCHEME 1 FIG. 1. Development of the content of polar compounds in Sclerocarya birrea oil (I), Sorghum bug oil (N), and palm olein (L) during frying of prefried potatoes at 175°C.a Determinations were carried out in triplicate, and the mean value ± SD is reported. Means within a column followed by different superscripts are significantly different (P < 0.05).

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