Further differential-pulse polarographic and visible spectrophotometric studies of the degradation of permitted synthetic food colouring matters with and without the addition of ascorbic acid: accelerated heat degradation studies

Abstract

Differential-pulse polarography (DPP) has been used to monitor solutions at pH 5.5 in acetate buffer of permitted synthetic food colouring matters containing azo groups during accelerated heat degradation studies carried out at 130 °C in sealed vials. Complete visible loss was effected in 0.5–28 d, depending on the food colour. The yields of simple amines (aniline, sulphanilic acid and naphthionic acid) were determined by visible spectrophotometry. The relatively high yields of aniline and sulphanilic acid obtained after complete visible loss of food colour were independent of further heating time; the stability of these two amines under the experimental conditions was verified separately. The ammonia formed in these experiments was determined by visible spectrophotometry and clearly was not formed from the simple amines, which are stable. The molar yields of simple amines and ammonia from the food colour solutions were concentration dependent; in general, the molar yields were lower at higher concentrations, presumably owing to the formation of stable nitrogen compounds of higher relative molecular mass. Red 10B was observed as an intermediate in the degradation of Red 2G at higher concentrations, and a stable yellow compound was formed in the degradation of Ponceau 4R also at higher concentrations.

The length of time required for complete visible loss of food colour was increased considerably by the addition of EDTA, whereas in the presence of 1 000 p.p.m. of ascorbic acid without the addition of EDTA degradation occurred within a few hours. In the latter instance relatively high yields of simple amines were again observed, but the amines were degraded further to ammonia by ascorbic acid. At higher concentrations of the food colours degradation of ascorbic acid was complete before that of the food colour and the simple amines formed remained undegraded. The ascorbic acid was determined by DPP and by this means the stoicheiometry of the reaction of ascrobic acid with the food colours was found to be 2.3, 2.3, 2.4, 3.0 and 3.9:1 for Sunset Yellow FCF, Ponceau 4R, Carmoisine, Brown FK and Chocolate Brown HT, respectively.