Abstract
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.
Highlights
Hard cider, referred to as cider in this paper, is a fermented beverage made from apple (Malus × domestica Borkh.) juice
A transcriptomics-based approach was used to examine how yeast assimilable nitrogen (YAN) concentration impacted H2S production and subsequently mRNA levels and gene expression for S. cerevisiae yeast strains during a cider fermentation
By comparing a natural (UCD522) to a mutated (UCD932) yeast strain under increasing diammonium phosphate (DAP) concentrations, we characterized genes involved with H2S production within the sulfate reductase pathway
Summary
Referred to as cider in this paper, is a fermented beverage made from apple (Malus × domestica Borkh.) juice. Hydrogen sulfide, which smells like “rotten eggs” (sensory threshold >0.00041 mg L−1), is a common off-aroma compound formed by yeast (Saccharomyces cerevisiae) during cider fermentation. Many environmental factors, such as nitrogen, vitamin, and metal ion concentration, fermentation temperature, juice turbidity, soluble solids concentration, acidity, fungicides, and sulfite (SO2) additions, have been reported to affect H2S production (Spiropoulos et al, 2000; Peck et al, 2016; Boudreau et al, 2017b). The SRS pathway is multifaceted and responsive to numerous regulatory inputs Both cysteine and methionine are chemically reactive under reductive fermentation conditions, which induces H2S and other undesirable sulfur-containing compounds (Boudreau et al, 2017a). Previous reports suggest that cysteine and its derivatives, rather than methionine, are the main end products that regulate the pathway activity (Hansen and Johannesen, 2000)
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