Impacts of depectinization of pear juice on alcoholic fermentation and indole formation

Recently, a producer of fermented ciders observed 'vinyl' off-odors formed during fermentation of pear juice previously depectinized at ≥ 49 °C but not if depectinized at lower temperatures. The objective of this study was to investigate the source of this spoilage and evaluate factors that affect formation.

Analysis of untainted and tainted samples obtained from the producer determined the causative agent to be indole, a compound sometimes produced by yeast (Saccharomyces cerevisiae) during fermentation. To mimic commercial depectinization conditions, pectinases were added to pear juices held at 35 °C for 45 min (Treatment A), 49 °C for 45 min (Treatment B), or 49 °C for 90 min (Treatment C). Juice processing conditions did not affect yeast growth nor progress of alcoholic fermentation. Although neither yeast strain (DV10 or MERIT) synthesized indole during fermentation of Treatment A juices, the compound was produced by MERIT in Treatments B (27.05 μg L^-1 ) and C (469.9 μg L^-1 ). Supplementation of Treatment C juice with pyridoxine (vitamin B₆ ) prior to fermentation resulted in no detectable indole formed. However, juices from Treatments A, B, or C contained similar concentrations of pyridoxine and non-detectable amounts of tryptophan, a potential precursor to indole. Furthermore, indole was not detected during fermentations of a synthetic pear juice medium without pyridoxine.

Supplementation of cider musts with pyridoxine prior to fermentation and choice of yeast strain can lower the risk of formation of off-odors caused by indole. However, other unidentified factors are present which affect its formation in perry. © 2019 Society of Chemical Industry.