Sofie Malfliet

Bacterial community dynamics during industrial malting, with an emphasis on lactic acid bacteria.

Characterization of the microflora during malting is an essential step towards process management and optimization. Up till now, however, microbial characterization in the malting process has mostly been done using culture-dependent methods, probably leading to biased estimates of microbial diversity. The aim of this study was to characterize the bacterial communities using two culture-independent methods, including Terminal Restriction Fragment Length Polymorphism (T-RFLP) and 454 pyrosequencing, targeting the 16S rRNA gene. Studied samples originated from two harvest years and two malting houses malting the same batch of barley. Besides targeting the entire bacterial community (T-RFLP), emphasis was put on lactic acid bacteria (LAB) (T-RFLP and 454 pyrosequencing). The overall bacterial community richness was limited, but the community structure changed during the process. Zooming in on the LAB community using 454 pyrosequencing revealed a total of 47 species-level operational taxonomic units (OTUs). LAB diversity appeared relatively limited since 88% of the sequences were covered by the same five OTUs (representing members of Weissella, Lactobacillus and Leuconostoc) present in all samples investigated. Fluctuations in the relative abundances of the dominant LAB were observed with the process conditions. In addition, both the year of harvest and malting house influenced the LAB community structure.

Assessing the xylanolytic bacterial diversity during the malting process

The presence of microorganisms producing cell wall hydrolyzing enzymes such as xylanases during malting can improve mash filtration behavior and consequently have potential for more efficient wort production. In this study, the xylanolytic bacterial community during malting was assessed by isolation and cultivation on growth media containing arabinoxylan, and identification by 16S rRNA gene sequencing. A total of 33 species-level operational taxonomic units (OTUs) were found, taking into account a 3% sequence dissimilarity cut-off, belonging to four phyla (Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria) and 25 genera. Predominant OTUs represented xylanolytic bacteria identified as Sphingobacterium multivorum, Stenotrophomonas maltophilia, Aeromonas hydrophila and Pseudomonas fulva. DNA fingerprinting of all xylanolytic isolates belonging to S. multivorum obtained in this study revealed shifts in S. multivorum populations during the process. Xylanase activity was determined for a selection of isolates, with Cellulomonas flavigena showing the highest activity. The xylanase of this species was isolated and purified 23.2-fold by ultrafiltration, 40% ammonium sulfate precipitation and DEAE-FF ion-exchange chromatography and appeared relatively thermostable. This study will enhance our understanding of the role of microorganisms in the barley germination process. In addition, this study may provide a basis for microflora management during malting.

Impact of Mashing-off Temperature and Alternative Kettle-Hopping Regimes on Hop α-Acids Utilization upon Wort Boiling

In this study, the influence of the mashing-off temperature (78 versus 95°C) on α-acids behavior and isomerization during wort boiling at the 10-L scale was investigated. In addition, various kettle-hopping regimes were applied to determine their possible impact on the isomerization of α-acids into iso-α-acids. As determined by quantitative HPLC as a function of wort-boiling time, mashing-off at 95°C always resulted in significantly higher hop α-acids utilization at the end of wort boiling (approx. 58%) compared with conventional mashing-off at 78°C (approx. 42%). Moreover, with mashing-off at 95°C, iso-α-acid profiles obtained at the end of wort boiling were potentially favorable in view of a finer, less lingering beer bitterness (relatively less isocohumulone) and prolonged bitterness stability upon aging (proportionally less trans-isomers). In regard to alternative hopping regimes, dosed addition of hop material as a function of wort boiling time resulted in similar α-acids utilizations compared with full hopping at the onset of the boil. Our results point to the potential of alternative ways of kettle hopping, in particular when mashing-off at 95°C, for more economic hopping combined with enhanced bitterness quality and stability.