Shelf-life evaluation of natural antimicrobials for Concord and Niagara grape juices.

This study was conducted to evaluate the effectiveness of natural antimicrobials for shelf-life extension of cold-filled still and carbonated Concord and Niagara grape juices, which have traditionally been preserved with chemical preservatives. Commercial juices were inoculated with a spoilage yeast cocktail of Dekkera, Kluveromyces, Brettanomyces, and Zygosaccharomyces at 10(2) and 10(4) CFU/ml. The following agents were added to still juices: no preservative (negative control), 0.05% potassium sorbate plus 0.05% sodium benzoate (positive control), 0.1 or 0.2% cultured dextrose, 250 ppm of dimethyldicarbonate (DMDC), 10 or 20 ppm of natamycin, and 250 ppm of DMDC plus 5 or 10 ppm of natamycin. Carbonated juice was treated with the negative control, positive control, and 250 ppm of DMDC plus 10 ppm of natamycin. Microbial stability of samples was assessed every 2 weeks during 6 months of storage at 21°C by yeast enumeration and measurement of turbidity, pH, and °Brix. Juices were deemed spoiled when yeast counts exceeded 10(6) CFU/ml. Cultured dextrose was not effective at levels tested in both types of juice. The most promising results were obtained with DMDC and natamycin combination treatments in still Niagara juice and in carbonated Concord and Niagara juices. In these treatments, shelf-life extension similar to that of the positive control (153 to 161 days) was achieved while maintaining similar turbidity, pH, and °Brix. Spoiled juices had lower pH and °Brix values and higher turbidity due to microbial activity and increased in microbial levels.

Impact of harvesting and processing conditions on green leaf volatile development and phenolics in Concord grape juice.

UNLABELLED: The disruption of plant cell walls during fruit juice processing results in the enzymatic formation of herbaceous-smelling green leaf volatiles (GLVs). Our objective was to assess the impact of thermal processing conditions on resulting levels of GLVs (hexanal, trans-2-hexenal, hexanol, cis-3-hexenol, and trans-2-hexenol), total phenols, monomeric anthocyanins, and percent polymeric color in Concord grape juice. The effects of fruit maturity and stage of juice processing on juice GLV content was also assessed. Of the GLVs studied, only trans-2-hexenal routinely exceeded its published sensory threshold in finished juice. We observed an inverse linear correlation between berry maturity (total soluble solids) and trans-2-hexenal levels in finished juice (P < 0.05, R(2)= 0.91). Trans-2-hexenal was at a maximum immediately following crushing (569 microg/kg, >30-fold over detection threshold [DT]), decreased to 100 microg/kg following depectinization, pressing, and pasteurization, and to 32 microg/kg following cold-stabilization. The loss of trans-2-hexenal could be explained primarily by its reduction to trans-2-hexenol, which increased from 53 microg/kg after crushing to 500 microg/kg after cold-stabilization. High temperature pretreatment of must immediately following crushing ("hot break") resulted in 5- to 6-fold higher concentrations of trans-2-hexenal in the final bottled juice as compared to conventional hot press. Contrary to expectations, no significant increase in phenolics and anthocyanins were observed in hot break conditions. These results indicate that hot break procedures may thermally inactivate enzymes responsible for transforming trans-2-hexenal under normal processing conditions and potentially alter the flavor qualities of the finished Concord juice. Different equivalent pasteurization regimes (82 to 93 degrees C) prior to bottling had no significant effect on GLV content of the finished Concord juices (P > 0.05). PRACTICAL APPLICATION: Introducing new processing techniques to fruit juice production can potentially result in undesirable changes to organoleptic properties. We have observed significantly higher levels of trans-2-hexenal, a potent herbaceous off-flavor, in Concord grape juice prepared with an initial high temperature heat treatment ("hot break"). Concord juice producers should be cautious in using hot break processing, especially with immature fruit, as it may result in persistence of green aromas in juice.