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Boosting yeast propagation via yeast extract supplementation
F. F. Jacob, M. Hutzler, M. Michel and F.-J. Methner

Yeast extracts are usually used as nutritional supplements to improve the fermentation performance of brewing yeast. In this work, the potential of yeast extract to boost yeast propagation performance was investigated. Therefore, mechanically (via ultrasonic sonotrode) and autolytically produced yeast extracts from brewer?s spent yeast, already characterised in recent studies, were supplemented in comparison to a pure nitrogen source (L-glutamine). The propagation process of the yeast strain Saccharomyces pastorianus TUM 34/70 was simulated in small-scale fully equipped bioreactors using standard all-malt (12??P) and high-gravity adjunct (D-(+)-glucose) wort (16??P, 18??P). Process data (apparent extract, individual sugars (fructose, glucose, maltose, maltotriose), free-alpha amino nitrogen, ethanol, cell count) were monitored and harvested yeast was checked for viability and vitality (test of fermentative capacity). As a result, supplementing a propagation process with yeast extract increased the yield and quality of propagated yeast. In detail, a maximum cell count of 260, 170 and 130 million/mL was propagated using 12, 16 and 18??P wort supplemented with yeast extract to an initial FAN concentration of 500 mg/L. The vitality of the propagated yeast was also improved despite the increased final ethanol concentration. No significant difference was found between the mechanically and autolytically produced yeast extract but both were more effective than the pure nitrogen source glutamine. Ensuring the practical benefit for commercial brewing, a pilot high-gravity (16??P) fermentation process was conducted, that was pitched with yeast harvested from a high-gravity (16??P) propagation process supplemented with yeast extract. The fermentation performance increased while the concentration of fermentation by-products (higher alcohols, esters), the sensory quality and the proteinase activity of the finished beer were not affected.

Descriptors: yeast propagation, yeast extract supplementation, high gravity wort, Saccharomyces pastorianus, vitality, yeast cell concentration

BrewingScience, 72 (September/October 2019), pp. 157-167