The Isolation, Screening And Application Of Killer Yeasts In Cider

There has been an ever-growing apple production in recent years in China, resulting in desirable demands for the fine process of the fruit resources. Cider produced with yeast strains is a popular product in the domestic market The objectives of this study were to: (1) isolate yeast strains from naturally fermented apple-based juices of various varieties; (2) screen and characterize killer yeasts from the yeast strains able to have an inhibition toward wild yeasts; (3) use of the killer yeasts to ferment apple juice and optimize fermentation parameters necessary for cider production.14 killer yeasts out of 200 isolates obtained from fruit environments showed strong inhibition against sensitive strains. They were identified as Saccharomyces cerevisiae based on morphologic, physiological and biochemical characteristics. Among the 14 killer strains, strains ky45, ay61 and ys89 seemed to show better ability in cider fermentation and were used for further study.The optimal growth temperature of strains ky45, ay61 and ys89 was 25°C, and they grew better at pHof 5.0, 4.5 and 4.5, respectively. Temperature, initial inoculation and acidity level had a significantly effect on the growth and fermentation of these strains. The 3 killer yeasts showed good resistance to various stresses including acids, alkalis, osmotic pressure and high concentration of ethanol.The activity of the killer's toxin extracted by 80% ammonium sulfate was determined according to well-test method. The toxin activity of the killer yeasts reached the maximum level during the late exponential phase and the early stationary phase of cell growth. It was stable in the range of pH2.2 to 5.4. The killer toxin showed the optimal activity against sensitive strains when pHwas 4.2. The activity of the killer's toxin completely lost after treated at 40°C for 45 min. It was proved that the activity of the killer toxin was temperature-dependent and pH-specific.Inoculation of 6% killer strains into 11%～14% concentrated apple juice whose final sugar was adjusted to 25% to 30% produced the best quality of cider product in the present study if fermentation temperature of 15°C to 20°C was given and the end-product acidity of 2.9g/L to 3.3g/L controlled. The analysis of the orthogonal test showed the optimal fermentation conditions for cider was carbohydrate of 25%, temperature 20°C, inoculation of 6% and acidity of 2g/L. The ethanol level of the cider was about 12.0% produced with strains ky45, ay61and ys89 in the above fermentation conditions. The taste of the final cider products was very attractive and acceptable throughout both sensory assay and flavor compound's determination. The present data indicated that the 3 killer yeast strains would be promising candidates useful as starter culture for cider production.