Graduate Thesis Or Dissertation
 

Thermal and enzymatic degradation of raspberry anthocyanins

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  • The elucidation of the structure of the red pigments of the black raspberries. Monger variety, was achieved. The components of the pigment of the berries were (a) cyanidin-3-glucoside, (b) cyanidin-3, 5-diglucoside, (c) cyanidin-3-diglucoside and (d) cyanidin-3-rhamnoglucosido- 5-glucoside. The elucidation was carried out after isolation, purification, concentration and chromatographic separation of the components. Further analysis by paper chromatographic techniques and spectrophotometric methods were carried out on the pigments and their products after specific chemical degradations. The degradation of the major anthocyanin component, cyanidin- 3-diglucoside, was further studied in buffered model systems of various pH values at 50°C. As the pH of the medium decreased the anthocyanin stability increased. The same was true for total crude pigment and the anthocyanin in the juice. Nitrogen atmosphere enhanced the stability of cyanidin-3- diglucoside as compared to an oxygen atmosphere. This held for the crude pigment and juice as well. Cyanidin in buffered model systems at 50°C was much more unstable than cyanidin-3-diglucoside under the same conditions. Nitrogen atmosphere resulted in improvement of the pigment retention over that in atmospheric conditions. The thermal degradation of cyanidin-3-diglucoside in model systems followed first order kinetics. The rate constants of the reaction at various pH levels under air and nitrogen were determined. The effect of the presence of various sugars and their degradation products on the destruction of cyanidin-3-diglucoside was studied in buffered model systems of pH 3.25 at 50°C. All of these additives increased the rate of pigment destruction. No differences were revealed among the sugars glucose, fructose, xylose and sucrose, which were used. All reactions followed first order kinetics and the rate constants were determined. When these reactions were carried out in the presence of nitrogen instead of air, a marked decrease in the rate of the pigment destruction was detected. Ascorbic acid in model buffered systems of pH 3.25 at 50°C markedly accelerated the destruction of cyanidin-3-diglucoside. Metal ions and atmospheric oxygen acted synergistically with ascorbic acid in the destruction of this anthocyanin. When the action of either of these synergists was blocked, the stability of the pigment was increased. EDTA was found to improve the retention of cyanidin-3-diglucoside by means of its ability to chelate the metal ions present, thus indirectly inhibiting the effect of ascorbic acid. When nitrogen was used instead of air, an improvement of the stability of anthocyanin in this system resulted. The degradation of cyanidin-3-diglucoside and the disappearance of ascorbic acid followed the same pattern. The same observations were also true for the anthocyanins of the juice. Cyanidin-3-diglucoside in buffer at pH 6.5 was acted upon by tyrosinase. This activity was low but nevertheless demonstrable. When catechol was added to this system, a rapid decolorization of anthocyanin was produced. This effect was further investigated and a scheme of the enzymatic reaction was proposed. Protocatechuic acid and tyrosine were able to couple with cyanidin-3-diglucoside and enhance the destructive action of tyrosinase on anthocyanins. The rate of the decolorization of the anthocyanin was lower in these systems than in the coupling with catechol.
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