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Enzymic treatment of mature, anthocyanin-containing grape berry sub-epidermal tissues from DeChaunac grapes released intact protoplasts. Filtration of the protoplast suspension through glass wool under mild suction resulted in the release of vacuoles. The total and individual contents of anthocyanins, flavonol glycosides, hydroxycinnamic acid esters, sugars, organic acids, and cations were determined in both tissue and vacuole preparations. A method for pH determination in intact vacuoles is reported. Based on the qualitative and quantitative anthocyanin composition, the average pH of the vacuoles was determined to be 2.7 (sd +/- 0.17). The data suggest that anthocyanins are present in fruit subepidermal tissues in a noncomplexed form.
Moskowitz et al. (Tue,) studied this question.