Évaluation de la stabilité de la couleur des œnocyanines

Probably the oldest natural food colorant was «enocyanin». It is produced from grape skins as a by-product of the wine industry in a number of countries, particularly Italy. The recovery of pigments from grape by-products has received considerable attention. Since wine grape skins are waste disposal problem, obviously the source material is available at very low cost. The pigments in the Vitis vinifera group are 3-monoglucosides of cyanidin, peonidin, malvidin, petunidin, plus the same compounds acylated with acetic, coumaric or caffeic acid. The hybrids may also contain the 3-5-diglucosides of the same aglycones and acylated compounds. Colorants from grapes may be expected to contain a number of other flavonoids, tannin, organic acids, etc. The anthocyanins show greatest stability under acidic conditions, but are generally unstable and degrade easily according to various reaction mechanisms. The degradation is influenced by oxygen, ascorbic acid, light, pH and temperature. Processing conditions used for the extraction and purification of pigments from wine grape skins can influence color stability. The present work was undertaken to study how pH (pH 2,3,4,5), time (15 days, 30 days, 60 days, 90 days) and temperature (4°C, 24°C, 36°C) influence the color stability of enocyanins concentrated by ultrafiltration, inverse osmosis and under vacuum. The color of the different samples has been measured both by spectrophotometry and by Hunter L*, a*, b* values. The spectrophotometric (A420 ; A520 ; T ; I) and colorimetric (L*, a*, b*) data resulted correlated significantly. Therefore colorimetric parameters which can be obtained by using a simple, rapid and easy method were used in order to study the color changes of the different enocyanins in the storage conditions. The obtained results have shown that the concentration techniques by ultrafiltration and inverse osmosis produced enocyanins characterized by a distinct red color, which was intense, but unstable in relation to pH variation. On the contrary, the enocyanins concentrated under vacuum resulted characterized by a poor red color, which was stable in relation to pH changes. In our experimental conditions, the results obtained showed that the degradation of the red color, in relation to time and temperature, proved to be independent of the concentration technique used. A ready visualization of the chromatic evolution of the different enocyanins, with reference to the stability parameters studied, was obtained using the color diagrams