Yeasts in an industrial malting ecosystem

Abstract

The malting ecosystem consists of two components: the germinating cereal grains and the complex microbial community. Yeasts and yeast-like fungi are an important part of this ecosystem, but the composition and the effects of this microbial group have been largely unknown. In this study we surveyed the development of yeasts and yeast-like fungi in four industrial scale malting processes. A total of 136 malting process samples were collected and examined for the presence of yeasts growing at 15, 25 and 37 degrees C. More than 700 colonies were isolated and characterized. The isolates were discriminated by PCR-fingerprinting with microsatellite primer (M13). Yeasts representing different fingerprint types were identified by sequence analysis of the D1/D2 domain of the 26S rRNA gene. Furthermore, identified yeasts were screened for the production of alpha-amylase, beta-glucanase, cellulase and xylanase. A numerous and diverse yeast community consisting of both ascomycetous (25) and basidiomycetous (18) species was detected in the various stages of the malting process. The most frequently isolated ascomycetous yeasts belonged to the genera Candida, Clavispora, Galactomyces, Hanseniaspora, Issatchenkia, Pichia, Saccharomyces and Williopsis and the basidiomycetous yeasts to Bulleromyces, Filobasidium, Cryptococcus, Rhodotorula, Sporobolomyces and Trichosporon. In addition, two ascomycetous yeast-like fungi (black yeasts) belonging to the genera Aureobasidium and Exophiala were commonly detected. Yeasts and yeast-like fungi produced extracellular hydrolytic enzymes with a potentially positive contribution to the malt enzyme spectrum. Knowledge of the microbial diversity provides a basis for microflora management and understanding of the role of microbes in the cereal germination process.

Indigenous Microbial Community of Barley Greatly Influences Grain Germination and Malt Quality

“…Van Campenhout 58 estimated that approximately 75% of malt xylanase activity was derived from the microbial community and only 25% from the grain. Our recent study 32 showed that the indigenous yeast community may also have a significant impact on the production of cell wall-hydrolysing enzymes. Fungal activity was also involved in the increased proteolysis.…”

“…Finally, the removal of water during kilning halts both grain and microbial activities. However, kilning has little effect on the viable counts of microbes, which are generally higher in the finished malt than that in the native barley 31,32,46 . The kilning regime has been identified as a significant factor in controlling the microbial community 52 .…”

“…In addition to germinating barley, the malting process involves another metabolically active component: the diverse microbial community that naturally colonizes the grains. The indigenous community harbours a wide range of microbes including numerous species of Gram-negative and -positive bacteria, yeasts and filamentous fungi 17,20,32,41,46 . In addition, each process step can be a source of additional microbes and their metabolites 32,45 .…”

“…Conditions that enable grain germination also favour microbial growth. Imbibed water rapidly activates the dormant microbes present in barley and therefore steeping can be regarded as a crucial step in malting with respect to microbial activity 32,41,45 . Microbial activity remains high during germination.…”

Yeasts isolated from industrial maltings can suppress Fusarium growth and formation of gushing factors

“…Our previous study revealed that a numerous and diverse yeast community consisting of both ascomycetous and basidiomycetous species was a signiWcant part of the malting ecosystem [35]. Several yeasts produced plant cell walldegrading enzymes with potentially positive contribution to malt processability.…”