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From the 8 May 1995 issue of Smart Drug News [v4n1]. Copyright (c) 1997, 2009. All rights reserved.

Aspartame in Solution

As soon as aspartame is dissolved in liquid is becomes unstable and begins to break down (hydrolyze) into its individual components (aspartate, phenylalanine and methanol). In addition, aspartame “cyclizes” into diketopiperazine (DKP) and methanol. The rate of breakdown and cyclization is dependent upon several factors, mainly temperature and pH [Stamp, 1989]. According to Searle’s original stability data submitted to the FDA as part of the approval process for aspartame’s use in carbonated beverages, 11-16% of the aspartame breaks down after eight weeks at 68°F, 38% breaks down at 86°F, and over 50% breaks down after nine weeks at 104°F.

More recent, independent tests by Tsang [1985] have shown how quickly aspartame can break down in carbonated beverages stored near room temperature. One of his tests was conducted on a diet cola sweetened with aspartame and stored at 71.6°F. The following are relevant figures for one liter (1.057 quart) bottles of diet cola.


At Time
of Bottling

6 Months
after Bottling

36 Months
after Bottling


550.0 mg

155.34 mg (28%)

19.70 mg (4%)

L-Phenylalanine methyl ester

0.0 mg

28.62 mg (9%)

13.01 mg (4%)

DKP (diketopiperazine)

0.0 mg

135.66 mg (28%)

173.28 mg (35%)


0.0 mg

158.31 mg (30%)

189.05 mg (36%)


0.0 mg

42.22 mg (14%)

101.27 mg (33%)

Other breakdown chemicals, such as free methanol, were not included in this test.