Kinetics of Decomposition of Aspartame Hydrochloride (Usal) in Aqueous Solutions

Abstract

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.

Peptide Stability in Solids and Solutions

“…The impact of pH on aspartame degradation has been examined in liquid and solid systems. In solutions, aspartame is most stable between pH 3.5 and 5, with increased acid hydrolysis at lower pH values and increased base catalysis at higher pH values (Prudel and Davidkova, 1981; Prudel et al, 1986; Ozol, 1986; Bell and Labuza, 1991a,b; Tsoubeli and Labuza, 1991; Skwierczynski and Conners, 1993). The U‐shape pH−rate profile generated from aspartame solution data resembles the profiles of other pharmaceuticals (Conners et al, 1986), including ampicillin (Hou and Poole, 1969).…”

“…The degradation pathways of aspartame change as a function of pH. At pH < 5.2, the degradation products from aspartame include diketopiperazine (DKP), α‐aspartylphenylalanine (α‐AP), phenylalanine methyl ester, β‐aspartame, and β‐aspartylphenylalanine (Prudel et al, 1986; Stamp and Labuza, 1989, Bell and Labuza, 1991b). At pH > 5.2, aspartame degrades into diketopiperazine and α‐aspartylphenylalanine without the formation of the β‐isomers (Bell and Labuza, 1991b).…”

Comparative stability of aspartame and neotame in yoghurt

“…The pH of yoghurt varied from 4.5 to 3.8 during storage (Table ), which incidentally was the optimal pH range for APM stability (Prudel et al . ). It is therefore amply clear that APM exhibited excellent stability in yoghurt during 15 days of storage.…”

“…; Homler ; Prudel et al . ) as well as intermediate moisture systems (Bell and Labuza ). The degradation rate of APM increased with increasing water activity.…”

Kinetics of an Acid‐Base Catalyzed Reaction (Aspartame Degradation) as Affected by Polyol‐Induced Changes in Buffer pH and pKa Values

“…Of these parameters, the pH of the food is frequently controlled and monitored in an attempt to limit chemical reactivity. Some examples of chemical reactions influenced by pH include aspartame degradation (Homler 1984; Prudel and others 1986; Bell and Labuza 1991a), ascorbic acid and thiamin degradation (Dwivedi and Arnold 1972; Connors and others 1986; Mauri and others 1992), and sucrose hydrolysis (Kelly and Brown 1978). Another important pH‐dependent reaction is the Maillard reaction, which affects flavor, color, and nutritional quality of foods (Friedman 1996; Martins and others 2001).…”