A scheme of aspartame hydrochloride (Usal) decomposition in relation to the pH and temperature which takes into account the possibility of phenylalanyl-aspartic acid dipeptide formation is suggested and experimentally confirmed. Aspartyl-phenylalanine and diketopiperazine were found to be the main decomposition products. The concentration of diketopiperazine increases and that of aspartyl-phenylalanine decreases with increasing pH. At pH 2.9 less aspartyl-phenylalanine and more diketopiperazine is formed with increase in temperature; twice the concentration of phenylalanine methyl ester was found at 80" and 90°C when 50% of the Usal present in the medium was decomposed, as compared to that determined at 25" and 40°C. The distribution of the remaining products remained constant over the entire range measured. INTRODUCTIONIT IS WELL KNOWN that both the aspartame (l-methyl-N-L-a-aspartyl-L-phenylalanine) and its hydrochloride (prepared in Czechoslovakia under the trade mark Usal), i.e. dipeptidic sweeteners composed of two amino acids -methyl ester of phenylalanine and aspartic acid, exhibit only limited stabilities in aqueous solutions (Scott, 1974; Prude1 and Davidkova, 1981). The decomposition of this sweetener corresponds to a first order reaction and its stability is significantly affected by the pH value and temperature of the medium. The optimal pH range from the point of view of stability, ranges between 2.5 and 5.0; lower temperatures being more favourable.Aspartame decomposition products were first identified by Furda et al. (1975) who applied gas chromatography and proved that dipeptide aspartyl-phenylalanine resulted after splitting off methanol, aspartyl-phenylalanine then either hydrolyzes into individual amino acids or the 3-carboxymethyl-6-benzyl-2,5-dioxopiperazine (diketopiperazine) may form by cyclization. The hydrolysis of aspartame into aspartic acid and phenylalanine methyl ester had also been proved. Boehm and Bada (1984) investigated the racemization kinetics of aspartic acid and phenylalanine in solutions of aspartame heated at 100°C and pH 4 and 7.The purpose of this study was to investigate the kinetics of decomposition of aspartame hydrochloride in aqueous solutions in relation to pH and temperature.

Hexanal is rapidly autoxidized in mixture with nonlipidic substrates even at 25 degrees C. The formation of peroxides follows the kinetics of a first order reaction with respect to hexanal (k1 about 10(-5) min-1), and is higher in mixture with casein or lysine-impregnated cellulose than with cellulose. In mixtures containing pure hexanal, peroxides are decomposed more rapidly after second-order reaction with respect to peroxides (k2 about 10(-3) mmol X kg-1 X min-1) while only slowly by the first-order reaction (k3 about 10(-5) min-1). In presence of small amounts of hexanoic acid the rate of second-order peroxide decomposition remained unaffected while the rate of the first-order peroxide decomposition increased by 4 orders (k3 about 10(-1) min-1). The presence of peroxides was percepted in the odour by sensory analysis contrary to nonvolatile lipid hydroperoxides.

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