Hydrogen Sulfide (H2S), is a negative off aroma produced during yeast fermentation and is common in cider and leads to consumer rejection. H2S has a very low odor detection threshold (ODT) and is often described as "rotten egg". H2S is produced when juice is deficient in yeast nutrients, such as amino acids and yeast assimilable nitrogen (YAN), which is a common problem in apples since they naturally low in nutrients. The purpose of this research was to investigate the effects of yeast nutrient addition to cider fermentation by adding four different nitrogen-rich supplements and evaluating the effects on H2S production, fermentation kinetics, and aroma quality during cider. Three yeast strains (M2, EC1118 and ICV OKAY), four yeast nutrients (Fermaid K, Fermaid O, Experimental Nutrient, and DAP) and single addition versus split addition of nutrient were tested. For single addition, all nutrient was added pre-fermentation and for split additions, the first addition was pre-fermentation and the second at one-third total soluble solid (TTS) depletion as measured by °Brix. Sensory evaluation was conducted on selected treatments. The greatest H2S was produced by M2 yeast strain (525 .63 ± 53.31 µg mL-1) while the least H2S on average was produced by EC1118 (118.26 ± 26.33 µg mL-1) and ICV OKAY produced an intermediate amount of H2S (209.26 ± 31.63 µg mL-1). Significant differences were observed between treatments and total H2S production within yeast strains. Yeast strain had the largest effect on H2S production. The second largest effect was yeast nutrient type. Classical text analysis of descriptions of cider aroma were evaluated and 25 attributes were chosen to describe the ciders. Check- all-that-apply (CATA), a rapid sensory technique that askes panelists, revealed that there was no clear pattern between variables tested. This work demonstrates that yeast nutrient type and yeast strain affect H2S production during cider fermentation. These findings provide a basis for improving the effectiveness of strategies used to prevent H2S production in cider fermentation. / Master of Science in Life Sciences / Cider, an alcoholic beverage made from fermenting apple juice, has grown in popularity and production in the United States in recent years. With increased in production and sales there is increase demand for high quality cider, but cider is prone to sensory faults. A common fault in cider aroma includes negative off aromas know as volatile sulfur compounds (VSCs). These aromas are often described as "rotten eggs", or "cabbage" and lead to consumer rejection of the product. One of the most recognized VSCs is hydrogen sulfide (H2S) which has a characteristic smell of "rotten eggs". These negative off aromas are thought to be produced during yeast fermentation under nutrient lacking conditions. Apples, depending on cultivar, ripeness, and other factors, naturally lack yeast assimilable nitrogen, vitamins, amino acids, and other nutrients needed for a successful yeast fermentation leading to off aromas. Yeast nutrients can be added to apple juice to increase nutrient availability, but little research has been focused on nutrient addition and timing of additions to prevent H2S production in cider. Most research focused on H2S production has been studied in wine must or grape juice. This knowledge may be limited when applying practices to apple juice due to differences in juice chemistry. Providing cider makers with specific scientific strategies to prevent off aromas, such as H2S, is important to the continued growth of the cider industry. This research is focused on exploring aroma quality and H2S prevention strategies in cider by evaluating how yeast nutrient addition via four exogenous nitrogen rich yeast nutrient and timing of yeast nutrient addition affect H2S production, fermentation kinetics, and consumer perception of aroma in cider fermentation.

On solid surfaces, laboratory strain Saccharomyces cerevisiae BY4742 forms horizontally and vertically stratified colonies. Central part of the colony differentiates into U and L cells occupying upper and lower colony regions, respectively. There are many morphological and physiological differences between these two cell types. However, molecular mechanisms of the vertical stratification are still largely unknown. In this study, I analyzed effects of the deletion and overexpression of Sch9p a Rim15p kinases with crucial role in nutrient sensing pathways, of transcription activators Msn2p and Msn4p involved i stress response and of post-diauxic shift transcription factor Gis1p in yeast colony differentiation and U and L cell survival. Microscopic analyses showed changes in morphology of cells in colonies of these strains and decreased ability of vertical stratification in colonies of the strain rim15Δ and partially in colonies of the strain PTEF-SCH9. Moreover, changed level of these proteins significantly affected long-term survival of U and L cells. Obtained data suggest an involvement of nutrient sensing pathways in colony formation and development.