Effects of concentration prior to cold-stabilization on anthocyanin stability in concord grape juice.

The color of Concord grape juice produced by concentration before cold-stabilization and detartration (direct-to-concentrate, DTC) was compared to juice produced via cold-stabilization prior to concentration (standard concentrate, SC) and evaluated by several metrics. Using the Boulton copigmentation assay, the majority of the absorbance at 520 nm in bottled SC juice (72%) was due to monomeric anthocyanins. Following reconstitution, DTC juice had a 63% greater absorbance at 520 nm than SC juice. A significant loss of anthocyanins was observed using a paired t test during cold-stabilization of single-strength juice during SC processing (mean loss: 79 mg/L as cyanidin-3-glucoside, 23% of total anthocyanins), while no significant loss of anthocyanins or change in other color metrics was observed during cold-stabilization of DTC concentrate. The concentration of anthocyanins in the SC bitartrate crystals was 0.80% w/w compared to 0.13% w/w in the DTC bitartrate crystals. Between DTC and SC, no difference in copigmentation was observed in cold-stabilized concentrate or reconstituted juice, indicating that the increased stability of anthocyanins could not be credited to greater copigmentation in DTC during detartration. HPLC analyses indicated that anthocyanin species with higher pKh and thus proportionally greater flavylium ion concentration at juice pH are preferentially lost during SC processing. The proportional changes in color metrics during shelf life stability testing (0-16 weeks, 2-30 °C) were not significantly different between SC and DTC juices.