| Summary: | 碩士 === 國立中興大學 === 生物科技學研究所 === 107 === Yeast is one of the four ingredients in beer brewing. Interestingly, different yeasts can produce diverse flavors during fermentation. To obtain the local isolated yeasts different from those generally imported commercial yeasts, this study isolated 120 wild yeasts from 8 different fruits. The genomic DNA of the local isolated yeasts was extracted; and the specific DNA fragment was PCR amplified and sequenced. The sequence was compared with the database of NCBI. I obtained only 1 brewing yeast, Saccharomyces cerevisiae, designated S-1. Then S-1 was used to test the ethanol and glucose tolerance. The results indicated that S-1 could survive in a significant higher rate at all alcohol concentration than the commercial yeast US-05, but slightly lower than WB-06. Furthermore, S-1 was domesticated to adapt in high alcohol concentrations. The cell numbers in several concentrations after adaptation were shown an upward trend. Furthermore, S-1 was revealed to have better performance than S-04 and US-05 in 30% glucose concentration. Finally, the emitting aroma compounds were analyzed by GC-MS. The results showed that most of the detectable compounds were higher in S-1 than those in commercial yeasts. To obtain various strains derived from S-1, the mutagens EMS and UV were used. Interestingly, one of the UV-treated yeasts produced a new compound, anethole. Lastly, 3 different beer styles brewed with S-1 and commercial yeasts were proceeded with sensory evaluation. The score of S-1 in aroma from all 3 beer styles was clearly higher than that brewed by commercial yeasts. Overall, the wild yeast S-1 isolated from grape does have the potential to be used in beer brewing industry to improve flavor and enhance the aroma components.
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