Premature yeast flocculation (PYF) is a sporadic problem in brewery fermentations that results in the incomplete utilisation of fermentable sugars. This work tests the hypothesis that barley or malt infected by xylanase-producing filamentous fungi contributes to premature yeast flocculation. Ten different fungi and yeasts were isolated and identified from malt and barley husk. These were inoculated onto barley husk and xylanase activity, arabinoxylan and the PYF value determined. The results from the fungal strains -Aureobasidium pullulans, Aspergillus flavus, Fusarium graminearum, and Alternaria tenuissimaindicated involvement in PYF. Small-scale malting and brewing was used to evaluate the PYF activity of fungal infected barley. The results showed that xylanase activity and arabinoxylan content were significantly (p < 0.05) related to PYF activity. With the exception of F. graminearum, xylanase activity was detected in the three other fungal isolates. The xylanases reduced the minimum concentration of arabinoxylan required for the induction of PYF. A small-scale brewing assay showed that xylanase secreted by the four filamentous fungi played a role in PYF. This research will contribute to the development of effective control strategies to prevent PYF factors in malt.

Premature yeast flocculation (PYF) is one of the pivotal problems affecting beer flavor and production. PYF is induced by certain non-starch polysaccharides produced by the degradation of malted barley husks upon the growth of contaminated microorganisms, such as Fusarium graminearum . In this research, the formation mechanism of PYF was uncovered by investigating the secretome of F. graminearum MH1 inoculated to the barley husk. The polysaccharide extract of degraded husk was ultrafiltrated into four fractions and characterized by the minimum PYF concentration, molecular mass distribution, monosaccharide composition, and zeta potential. Among the four fractions, the high-molecular-weight polysaccharide fraction had the highest content of uronic acid and the most negative zeta potential, which contributed to the most severe PYF phenomenon. In addition, the PYF yeast showed a more negative zeta potential than the control yeast during the small-scale brewing process. This is aligned to the negatively charged polysaccharides potentially bonded to the surface of yeast cells through the calcium cation in the same fermentation system, which results in rapid flocculation and precipitation. Approximately 12% of the 214 proteins identified in the Fusarium graminearum MH1 secretome were hemicellulases, which substantially interpreted the mechanism of polysaccharides inducing PYF yeast during beer brewing.

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Crystal malt, the most popular type of specialty malt used in beer brewing, plays a vital role in forming complex flavor and color. Nevertheless, crystal malt is only defined based on the malting process, and there is not any standard to evaluate its quality. In the current study, the volatile aroma constituents of commercial crystal malt samples were analyzed with headspace solid-phase microextraction combined with gas chromatography-mass spectrometry, in order to explore the characteristic aroma compounds of crystal malt. The average concentration of volatile aroma compounds in 10 crystal malt samples is 587 µg L −1 , ranging from 347 to 1265 µg L −1 . A total of 38 aroma compounds were identified, 47% of which were existed in all the 10 samples. Based on principal component analysis and odor activity value, isobutyraldehyde, 2-methylbutanal, furfural, 2-acetyl-1H-pyrrole, oct-1-en-3-ol, 4-methyl-2-phenyl-2-pentenal, and (2E)-2-isopropyl-5-methyl-2-hexenal could be considered the characteristic aroma compounds of crystal malt. The results of this present study would help to establish a standard to assess the quality traits of crystal malt sample.