Malt induced premature yeast flocculation: its origins, detection and impacts upon fermentation (PhD)

Abstract

Premature yeast flocculation (PYF) is a sporadic problem encountered during industrial brewing fermentations. Current hypothesis states that factors thought to arise from fungal infection of the barley in the field and/or the malt in the maltings cause yeast to flocculate prematurely and/or heavily before the depletion of the sugars in the wort. This results in poorly attenuated worts, with higher residual extract and lower alcohol by volume, flavour abnormalities (i.e. diacetyl, SO₂), lower carbonation levels, disruption of process cycle times and potential issues with the re-use of the yeast in subsequent fermentations. Consequently, premature yeast flocculation generates significant financial and logistical problems both to the brewer and the maltster.

In the current study, a small-scale fermentation assay was developed and optimised to predict the PYF potential of malts, as well as to investigate the importance of the yeast strain in the incidence and severity of the phenomenon. Furthermore, the impacts of the PYF factor(s) (i.e. arabinoxylans, antimicrobial peptides) on yeast fermentation performance and metabolite uptake were also studied, whilst the Biolog detection system was investigated as a potential rapid tool to detect PYF.

The results obtained suggested that our in-house assay can be successfully used to predict the PYF potential of malts 69 or 40 h post-pitching depending upon the yeast strain used. Whilst ale yeasts were not found to be susceptible to PYF, lager yeasts exhibited different degrees of susceptibility, even to the same PYF factor(s). More specifically, the more flocculent lager yeast SMA was found to be more susceptible than the medium flocculent lager yeast W34/70. However, interestingly, the fermentation performance of a PYF+ wort could be significantly improved by using a non-flocculent and a relatively PYF-insensitive lager yeast. It was also shown that worts with lower amount of glucose and maltose could be responsible for poor fermentation profiles and/or heavy PYF as well as elevated residual sugars and lower fermentability. The observation that linoleic acid (6 mg.l^-1) exacerbated PYF (P = 0.047) and made its detection more rapid was found to be contrary to the “titration hypothesis”  which hypothesised that the addition of fatty acids might “titrate” out antimicrobial peptides so that they can no longer bind to the yeast cells. High gravity fermentations with worts inducing PYF did not have a significant effect (P > 0.05) on yeast physiological characteristics or fermentation performance, suggesting that the PYF+ sample used in this study was inducing PYF though the ‘bridging’ polysaccharide mechanism rather than through the antimicrobial peptides. The Biolog system can be used for the metabolic characterisation of different flocculence lager yeasts incubated in different fermentation media, whilst wort composition had a significant effect in redox reduction reactions.