Yeast Assimilable Nitrogen Concentrations Influence Yeast Gene Expression and Hydrogen Sulfide Production During Cider Fermentation

The fermentation of apple juice into hard cider is a complex biochemical process that transforms sugars into alcohols by yeast, of which Saccharomyces cerevisiae is the most widely used species. Among many factors, hydrogen sulfide (H₂S) production by yeast during cider fermentation is affected by yeast strain and yeast assimilable nitrogen (YAN) concentration in the apple juice. In this study, we investigated the regulatory mechanism of YAN concentration on S. cerevisiae H₂S formation. Two S. cerevisiae strains, UCD522 (a H₂S-producing strain) and UCD932 (a non-H₂S-producing strain), were used to ferment apple juice that had Low, Intermediate, and High diammonium phosphate (DAP) supplementation. Cider samples were collected 24 and 72 h after yeast inoculation. Using RNA-Seq, differentially expressed genes (DEGs) identification and annotation, Gene Ontology (GO), and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment, we found that gene expression was dependent on yeast strain, fermentation duration, H₂S formation, and the interaction of these three factors. For UCD522, under the three DAP treatments, a total of 30 specific GO terms were identified. Of the 18 identified KEGG pathways, “Sulfur metabolism,” “Glycine, serine and threonine metabolism,” and “Biosynthesis of amino acids” were significantly enriched. Both GO and KEGG analyses revealed that the “Sulfate Reduction Sequence (SRS) pathway” was significantly enriched. We also found a complex relationship between H₂S production and stress response genes. For UCD522, we confirm that there is a non-linear relationship between YAN and H₂S production, with the Low and Intermediate treatments having greater H₂S production than the High treatment. By integrating results obtained through the transcriptomic analysis with yeast physiological data, we present a mechanistic view into the H₂S production by yeast as a result of different concentrations of YAN during cider fermentation.

Sensory profiling of natural sweeteners and sucrose-sweetener binary mixtures

Overconsumption of added sugars is associated with higher incidences of obesity, type II diabetes, and cardiovascular disease. Alternative sweeteners have long been relied on as a strategy to reduce consumption of added sugars; however these alternatives differ notably from sucrose in their sensory properties. Recently, consumers have also been looking to reduce consumption of ingredients deemed "artificial," seeking natural alternatives despite poor definitions of these terms on the whole. Wider knowledge of the sensory properties of natural sweeteners would greatly aid in this goal. Descriptive analysis and time-intensity scaling were used to characterize the temporal profile and off-flavors of allulose, erythritol, rebaudioside (Reb) A, Reb D, Reb M, monk fruit, and thaumatin, as compared to sucrose. Further, each was blended to reduce sucrose by 50% and 75% in binary mixtures. Where significant off-flavors were frequently reported with sweeteners, when delivered in binary mixtures for partial sugar reduction, these were much mitigated, while also bringing temporal profiles closer to that of sucrose. This suggests that partial sugar reduction may be an effective way of reducing caloric intake while not compromising sensory experience. PRACTICAL APPLICATION: The use of both descriptive analysis and time-intensity scaling allows for a more complete understanding of the sensory properties of natural alternative sweeteners. Understanding their sensory deficits and ways to improve these sweeteners plays a critical role in creating healthier products that will be accepted by consumers.