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.

The stability of USAL (I-L-aspartyl-L-phenylalanine methyl ester hydrochloride) was studied in solutions in relation to the pH value ( I . 10-6.95 at 20 "C), the temperature (20-80 "C at pH 2.2) and the ionic strength (0.16---2.0 at 20 'C and pH 4.0). The effects of pH and temperature of the medium were very significant.From the point of view of the stability the optimum pH value lies in the pH range 2.5-4.5 and at lower temperature. The ionic strengthdid not affect the stability in the tested range. Theeffectsofcitric acid (0-10 7;) and sucrose (0-75 "G) were also investigated. An addition of citric acid exhibited an effect on USAL stability corresponding to the shift of pH value that it caused. The Occurrence of sucrose in the solution had a favorable effect at higher concentrations.Artificial sweeteners based on peptides belong to the recently discovered sweet-tasting substances [l]. It seems that from the whole series of peptides known so far [2], the dipeptidic sweeteners a-L-aspartyl-L-phenylalanine methyl ester (ASPARTAM) and a-L-aspartyl-L-phenylalanine methyl ester hydrochloride (USAL) belong to the most promising for application as sweeteners. They are suitable both for low-energy foods and for diabetics diet. Some properties of ASPARTAM and the possibilities of its application have been already described in the literature [2--51. The stability ofthis sweetener is preimposed by its dipeptidic character. The temperature and the pH of the medium represent the main factors affecting the stability in aqueous solutions. However, in food systems an additional series of factors may exhibit their effects, i. e. the other food components and in some cases the microbiological influence might be expected [6].The stability experiments were performed in model systems, in citrate-phosphate buffers and in the solutions of citric acid and sucrose. The citrate-phosphate buffers very closely approximate by their pH values to the media characteristic for foods. The solutions of citric acid and sucrose form models of fruit sirups that have been used in the production of soft drinks. Experimental

Analytical evaluation of the artificial sweetener USAL (α‐L‐aspartyl‐L‐phenylalanine methylester hydrochloride) was carried out using a modified technique of amino acid analysis. The determination was performed using a functionally simplified semiautomatic amino acid analyser. The device was constructed using the accessories of the automatic amino acid analyser AAA 888 (Mikrotechna Praha).

A HPLC method for the determination of Usal (Aspartame hydrochloride, L-aspartyl-L-phenylalanine methyl ester hydrochloride) and its decomposition products was elaborated. Aspartic acid, phenylalanine, phenylalanine methyl ester, aspartyl-phenylalanine, phenylalanyl-aspartic acid, 5-benzyl-3,6-dioxo-2-piperazineacetic acid (DOP) and Usal were separated on Separon SI C-18. The mobile phase was: 0.5 M NaH2PO4 (pH 2.1) and methanol (85:15 v/v). The detection was carried out at 200 nm. The method for DOP determination was tested by the analysis of 10 types of soft drinks to which DOP was added. In two newly developed sorts of soft drinks sweetened with Usal the formation of DOP was followed during storage. The DOP increment after 34 days of storage reached 0.7 and 6 mg/l at 7 and 20 degrees C, resp. The method is also suitable for DOP determination in the sweetener itself.