| Popis: |
Indole is a yeast metabolite which causes 'plastic-like' or fecal off-odors and was recently encountered by a regional cider maker during pear juice fermentations. In hard ciders, the taint is thought to originate from the tryptophan biosynthesis pathway as a result of vitamin B6 deficiencies in juice. Since pear juices which previously developed the off-odor were depectinized at 49°C prior to fermentation, thermal processing was suspected to encourage the issue by causing degradation of vitamin B6. To investigate this theory, pear juices containing a blend of commercial pectinases were held at 35°C for 45 min (Treatment A), 49°C for 45 min (Treatment B), or 49°C for 90 min (Treatment C) and fermented with two strains of S. cerevisiae (MERIT or DV10). After thermal processing, the amino acid content and sum concentration of free and phosphorylated B6 vitamers were comparable in the resulting juices, with values ranging from 84.7 to 91.9 mg N/L and 0.25 to 0.34 mg/L, respectively. Furthermore, yeast growth and sugar consumption followed similar patterns during fermentations, and dryness was always achieved within eleven days of inoculation. Although DV10 never produced the taint, greater amounts of indole were formed by MERIT when juice was processed at higher temperatures for longer periods of time. Specifically, indole was not detected in Treatment A but concentrations of 27.05 and 469.9 µg/L were found at the end of fermentation in Treatments B and C, respectively. To determine the role of vitamin B6, pyridoxine was supplemented to juice which was most susceptible to developing the off-odor (Treatment C) prior to fermentation with MERIT. While control fermentations contained 89.2 µg/L, no indole was detected when 1 mg/L pyridoxine was added, suggesting that juice from Treatment C was deficient in vitamin B6. Additional fermentations with MERIT were conducted in a synthetic pear juice medium containing 0, 0.1, or 1 mg/L pyridoxine to confirm the requirement of vitamin B6 for avoiding the off-odor. However, indole was never detected even when vitamin B6 was absent, indicating that low availability of the vitamin was not the only factor which causes the off-odor. |
