2018 ASHS Annual Conference

Apple (Malus xdomestica Borkh.) juice typically contains low concentrations of yeast assimilible nitrogen (YAN). During fermentation, this can cause yeast (Saccharomyces cerevisiae) cells to produce the sulfur containing amino acids, methionine and cysteine, a reaction that reduces sulfate to hydrogen sulfide (H₂S). When H₂S is produced in excess, it is considered a sensory fault as this compound is associated with a “rotten egg” smell. The goal of this research was to determine the effect of YAN concentration, as adjusted with diammonium phosphate to low (86 mg·L^-1), intermediate (208 mg·L^-1), and high (433 mg·L^-1) concentrations, on yeast H₂S production and yeast gene expression during apple cider fermentation. Each YAN treatment was fermented in quadruplicate with UCD932 (a strain with a natural mutation in the MET10 gene and therefore produces no H₂S) and UCD522 (a commercially available strain known to produce relatively high H₂S concentrations). The same base apple juice was used for all treatments and all other fermentation conditions [e.g., temperature, physical agitation, and potassium metabisulphite (160 mg·L^-1) additions] were kept constant. All fermentations fully metabolized the available sugar. For both UCD932 and UCD522, the intermediate YAN concentration resulted in faster fermentation rates than the low or high YAN concentrations. Under the intermediate YAN concentration, the fermentation rate of UCD932 was 34% greater than that of UCD522. The fermentation rates showed the maximum difference at a YAN concentration of 208 mg/L. The fermentation rates showed the minimum difference at 86mg/L. The fermentation rates of UCD522 and UCD932 were similar. At these three different YAN concentrations, there were significant differences in H₂S production with UCD522. Under the medium YAN concentration, UCD522 produced 2 and 6 times greater H₂S than those under the low and high YAN concentration, respectively. Total RNA was extracted from yeast cells sampled at the initial phase of fermentation and at the peak of H₂S production using a hot phenol method. RNA samples were prepared as biological triplicates and for RNA-Seq Library sequencing using standard Illumina protocols. Further data analyses will include RNA-Seq, HPLC, and microarrays, as well as bioinformatics data analyses (Tophat v2.0.8b, PCA, MFA). Our results will identify the yeast genes associated with H₂S production in cider fermentation and ultimately reduce the production of ciders with a rotten egg smell.