Elucidating the interaction of carbon, nitrogen, and temperature on the biosynthesis of Aureobasidium pullulans antifungal volatiles

The combined biochemical impact of carbon, nitrogen and temperature on the biosynthesis of the antifungal volatile organic compounds (VOCs): ethanol, 2-methyl-1-propanol, 3-methyl-1-butanol and 2-phenylethanol produced by Aureobasidium pullulans A1 and A3 was investigated using a Box-Behnken experimental design and response surface methodology (RSM). Normalized peak areas derived from solid phase micro extraction-gas chromatography-mass spectrometry (SPME-GC-MS) analysis, indicated that initial carbon content had a significant influence on the biosynthesis of ethanol and alcohols with greater than three carbon atoms. This result suggests a dominant activity of the A. pullulans anabolic pathway to biosynthesize three higher alcohols via de novo biosynthesis of amino acids from sugar metabolism. Low concentrations of carbon (3-13 g l^-1 ) with nitrogen as both ammonium and amino acids in the growth medium resulted in a higher number of significant linear and quadratic relationships. Nitrogen availability and growth temperature had significant negative linear and quadratic correlations with VOCs biosynthesis in most instances. Isolate-dependant metabolic response was evident for all abiotic parameters tested on alcohol production. The findings of this study offer new perspectives to improve the production of key antifungal compounds by antagonists in biological control systems.

Effect of Immobilization Support and Fermentation Temperature on Beer and Fermented Milk Aroma Profiles

The food industry increasingly produces wastes like coconut and peanut shells. In addition, low temperature fermentation is always a challenge. Therefore, in the present study, a sustainable exploitation of these by-products is proposed through the production of carriers for immobilized cells of yeast and bacteria. The immobilized cells, after thermally drying, were evaluated for their efficiency in beer and milk fermentations respectively, in various fermentation temperatures and storage for up to three months. The beers and fermented milks were evaluated for their aroma and the results showed products of high quality. Coconut shells resulted in better products with increased fruity ester content in fermented milks and reduced dimethyl sulfite and vicinal diketones and increased ratio of esters to alcohol in beers. These results reveal the possibilities of immobilized cells in coconut and peanut shells for application in food industry, however, more research is needed to evaluate their effect on sensory characteristics and possible prebiotic and probiotic potential especially in the case of fermented milks.

Volatile Compound Screening Using HS-SPME-GC/MS on Saccharomyces eubayanus Strains under Low-Temperature Pilsner Wort Fermentation

The recent isolation of the yeast Saccharomyces eubayanus has opened new avenues in the brewing industry. Recent studies characterized the production of volatile compounds in a handful set of isolates, utilizing a limited set of internal standards, representing insufficient evidence into the ability of the species to produce new and diverse aromas in beer. Using Headspace solid-phase microextraction followed by gas chromatography-mass spectrometry (HS-SPME-GC-MS), we characterized for the first time the production of volatile compounds in 10 wild strains under fermentative brewing conditions and compared them to a commercial lager yeast. S. eubayanus produces a higher number of volatile compounds compared to lager yeast, including acetate and ethyl esters, together with higher alcohols and phenols. Many of the compounds identified in S. eubayanus are related to fruit and floral flavors, which were absent in the commercial lager yeast ferment. Interestingly, we found a significant strain × temperature interaction, in terms of the profiles of volatile compounds, where some strains produced significantly greater levels of esters and higher alcohols. In contrast, other isolates preferentially yielded phenols, depending on the fermentation temperature. This work demonstrates the profound fermentation product differences between different S. eubayanus strains, highlighting the enormous potential of this yeast to produce new styles of lager beers.

Effects of wort gravity and nitrogen level on fermentation performance of brewer's yeast and the formation of flavor volatiles

Normal gravity wort and high gravity wort with different nitrogen levels were used to examine their effects on the fermentation performance of brewer's yeast and the formation of flavor volatiles. Results showed that both the wort gravity and nitrogen level had significant impacts on the growth rate, viability, flocculation, and gene expression of brewer's yeast and the levels of flavor volatiles. The sugar (glucose, maltose, and maltotriose) consumption rates and net cell growth decreased when high gravity worts were used, while these increased with increasing nitrogen level. Moreover, high gravity resulted in lower expression levels of ATF1, BAP2, BAT1, HSP12, and TDH, whereas the higher nitrogen level caused higher expression levels for these genes. Furthermore, the lower nitrogen level resulted in increases in the levels of higher alcohols and esters at high wort gravity. All these results demonstrated that yeast physiology and flavor balance during beer brewing were significantly affected by the wort gravity and nitrogen level.