Premature flocculation of yeast can occur during fermentation whilst fermentable sugars remain high (although this is not always the case) and results in a dramatic reduction in the number of yeast cells in suspension, leading to an incomplete fermentation and product quality issues for the brewer. It is widely acknowledged that certain polysaccha-ride/protein complexes arising from malt husk can promote premature yeast flocculation (PYF) in susceptible yeasts. What are less clear are the malting conditions that promote the formation of these factors. This paper discusses an investigation into the factors in malt processing that contribute to the formation of PYF-positive malts. Micro, pilot scale, and commercial malting samples were subject to a laboratory scale PYF test to look for positive samples. It was observed that the PYF factor could be removed from PYF-positive malt during steeping, but this factor returned during the early stages of germination. At the end of germination, the factor haddisappeared, suggesting that the factor had been degraded, possibly by the action of microbial enzymes. At the pilot scale, high CO_2 levels were encouraged by not aerating during steeping and re-circulating the air. In contrast to expectations, these processing environments did not encourage the PYF factors to form. In fact, the control malts were more PYF-positive than the test malt. However, samples taken from commercial scale malting where high CO_2 levels were present did turn out PYF-positive. Samples from the same batch of barley processed separately under "normal" conditions were PYF-negative. It is clear from our pilot malting experiments that adverse processing conditions such as elevated CO_2 did not result in the formation of PYF-positive malt, whereas on the commercial scale, elevated CO_2 did produce PYF-positive malt. Variations in microbial populations on the barley and different processing conditions on the two sites may account for this variation. It is well known that many differentmethods exist for PYF testing and individual establishments have tailored these to the re-quirements of the brewery in which the malt is destined to be used. While these methods work well for predicting the likelihood of malts promoting the PYF condition in their laboratories or breweries, the subtle differences in methods make it difficult for an individual to compare results from different establishments. In order to address this issue, a PYF network has been initiated. This network has been set up to try to establish a common testing method and to encourage knowledge transfer between researchers and industry.
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