Selection Of Sour Pomegranate Wine Yeast And Primary Research Of Its Resistance To Citric Acid

Punica granarum L. is rich in cultivar resources and widely distributed. Its fruit is featured by high nutritional value and health function. Pomegranate wine products could be fit for the development of liquor all over the world. It is important to select the wine yeast with high titratable acidity resistance and its characteristic of deacidification for the sour pomegranate juice fermentation. In this research, components of sour and sweet pomegranate juices, fermentation characteristics of 8 strains of yeasts in sour pomegranate juice, effect of sulfur dioxide (SO2) on organic acid metabolism by Saccharomyces cerevisiae, and resistance of Saccharomyces cerevisiae to citric acid were studied. The main research results were as follows:1. In the sour pomegranate juice, titratable acidity (TA) was 39.2g citric acid /L with pH value 1.92; whereas in the sweet juice, TA was 4.4g/L with pH value 2.45. Other components in sour juice, such as total sugar,α-AN and so on, were enough for wine yeasts growth except for the high acidity and low pH value.2. A RP-HPLC method for simultaneous determination of organic acids in pomegranate juices and wines was developed. Oxalic, lactic, citric, malic,α-ketoglutaric, acetic, succinic, fumaric, tartaric and gluconic acids were found in pomegranate juices. The main organic acids were lactic acid (51.2%), oxalic acid (26.8%), and citric acid (16.8%) in sweet pomegranate juice; while citric acid (84.4%) and lactic acid (14%) were the main organic acids in sour juice.3. Saccharomyces cerevisiae (WY-1, WY-2, WY-3) and Zygosaccharomyces bailii (WY-4) were able to endure 50mg/L SO2 in sour pomegranate juice. The wine yeast WY-1, WY-2, WY-3 had better fermentation performance with more rapid and steady fermentation speed and higher alcohol production; while WY-4 had a longer lag phase and slower fermentation speed. In all the finished wines, TA, total sugar, residual sugar and volatile acid content had no significant difference. Among the 4 wine yeast, no significant difference (P>0.05) was found in the ability of citric acid degradation, which was the main organic acid component in sour megranate juice. The total organic acid content in the finished wines decreased by 8.1~12.8% coinciding with citric acid decreasing by 8.7~13.3%.4. Saccharomyces paradoxus (WY-5), Klockera apiculata (WY-6, WY-7), Kluyveromyces marxianus (WY-8) couldn't ferment the sour pomegranate juice with adding 50mg/L SO2. The wine yeast WY-5, WY-6 had the best fermentation performance both with alcoholic content 5.96% in the finished wines, while WY-6 produced the most volatile acid that was 0.5g/L. The TA, pH value, total sugar, and dry extractive content had almost no difference in all finished wines. The ability of citric acid degradation for the 4 yeast had no significant difference (P>0.05) and the variation range of total organic acid content in finished wines was between 8.1% and 12.8%.5. Sensory evaluations showed that pomegranate wines fermented by 8 strains of yeasts were all luster-transparent but have a strong acid taste. The wine fermented by WY-6 was better in natural coulor and moderate flavor than that fermented by WY-1 and WY-5. Simultaneously, considering the fermentation characteristics and SO2 resistance, the Saccharomyces cerevisiae WY-1 was the optimal strain to ferment sour pomegranate juice.6. Adding 60mg/L SO2 could significantly enhance the succinic acid production of Saccharomyces cerevisiae WY-1 (P<0.05), while decrease the production of pyruvic acid (P<0.05). SO2 addition had no significant effect on the formation of tartaric, malic,α-ketoglutaric, citric, and fumaric acids during pomegranate wine fermentation (P>0.05). Moreover, effect of adding 60mg/L SO2 on oxalic, acetic and lactic acids was different between the sweet and sour fermented pomegranate juice.7. The Saccharomyces cerevisiae WY-1 could ferment the apple juice with citric acid addition of 0g/L to 61.5g/L, but the fermentation period was prolonged in turn. Total cell number, size, and mortality of the yeast and residual sugar, alcohol and volatile acid of the fermented apple juice were not influenced significantly (P>0.05) by citric acid at level of 0, 2.6, 5.4, 9.9, and 19.0g/L. When the citric acid added to the apple juice was up to 32.8g/L and 47.1g/L, the total cell number of the yeast was decreased significantly (P<0.05), the cell size of it was decreased extreme significantly (P<0.01), and the cell mortality of it, residual sugar content and production of volatile acid of the fermented apple juice were increased extreme significantly (P<0.01), however the production of alcohol of the yeast was not effected significantly (P>0.05). At addition of 61.5g/L citric acid, the total cell number and cell mortality were not remarkably different (P>0.05) compared to that of 47.1 g/L citric acid addition, while cell size was reduced significantly (P<0.05) and the alcohol fermentation could not finished thoroughly. When citric acid was added at level of 0, 2.6, 5.4, 9.9, 19.0, and 32.8g/L, titration acidity of the fermented apple juice was increased after fermentation, however it was decreased at level of 47.1g/L and 61.5g/L.