Grain

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Microbial Populations on Barley

Malting Process

Microbial communities found on barley are very diverse and change significantly throughout the malting process, with more diversity occurring further along in the malting process. In the barley fields, Gram-negative bacteria (especially Erwinia herbicola) are abundant. Climate is said to have the most impact on which species grow in barley fields. For example Fusarium species are encouraged by high humidity, and are associated with mycotoxins and gushing problems in beer. During dry storage of barley, spore forming bacteria tend to survive. Xerophilic fungi (fungi that doesn't need much water) also survive on dry barley during storage .

Microbial communities change quite a bit during the malting process. It is thought that the factors affecting this are the initial microbe populations, interactions between species, the variances in the malted barley characteristics/processes, and additives. Different malting houses are likely to form "in-house" microflora populations. The first step in malting, which is steeping, sees the first large change in microbial populations. The steeping process favors lactic acid bacteria (LAB), which are seen in extremely small numbers before steeping. Particularly these are Leuconostoc species. In the case of yeasts, Basidiomycota fungi grow, as wellas Fusarium .

The second step of malting, which is germination, sees starches converted to sugars, which causes microbial populations to increase once again. Where Leuconostoc species of bacteria were dominate during steeping, the germination process is where Lactobacillus species begin to dominate the microbial community. A greater diversity of species also occurs during this step in malting. As far as fungi, Ascomycetous yeasts begin to dominate, where as Alternaria and Cladosporium decline .

During kilning, microbial populations are reduced by a factor of 10-100, but are still higher than that of field barley. The high degree of variability is dependent on the kilning temperature and procedure. Heat resistant microbes, perhaps by forming biofilms, can survive the hot temperatures. These include Rhizopus and Mucor species .

Malted Barley

Most historical studies on microbe populations on malt have been done using lab media such as agar plates, which are thought to not be as effective at analyzing the overall population as recent DNA sequencing approaches, such as "454 amplicon pyrosequencing". A recent study took this approach to measuring bacterial diversity on barley and malt (fungi was not analysed). This study looked at two malting houses during two different years (2010 and 2011). It found that on malt, the largest communities of bacteria were found to include Enterobacter, Sphingobacterium, Weissella, Lactobacillus, Lactococcus, Streptococcus, Acinetobacter, and Strenotrophomonas, Leuconostoc, Pseudomonas, Wautersiella, Cryseobacterium, Curtobacterium, and Propionobacterium .

Interestingly, the study also found that the majority genera of microbes was different between the two years sampled. In 2010, there were more Firmicutes species (which includes LAB) and more Actinobacteria, and less Bacteroidetes. In 2011, there were fewer of both Firmicutes and Actinobacteria, but more Bacteroidetes (~15 mins in).

The study examined LAB populations specifically due to their potential to help with malt stability and quality . Streptococcus was the most abundant genus of bacteria in all samples. In 2010, Lactobacillus was more abundant than it was in 2011. Lactococcus and Weisella were more abundant in 2011 than in 2010. Maltster 1 had a great abundance of both Lactococcus and Weisella in 2011 over Maltster 2. In 2010, Lactobacillus was more abundant for Maltster 2 than it was for Maltster 1. This showed that microbial populations differ not only between harvest years, but also malt houses (~17 mins in).

Another study focused on microbial populations in an American coolship brewery detected only Pediococcus on grain samples, but the lack of detection of other microbes as attributed to weak amplification .

• See this study for a list of microbes that were found on malt using DNA methods.
• See Table 2 of this study for a lit of microbes that were found using traditional plating methods.
• See this video presentation by Bart Lievens at the 2015 Belgian Brewers Conference:

Mash and Wort

During mashing, the population of microbes diminishes greatly due to near pasteurization temperatures. However, thermotolerant microbes do survive. These are usually homofermentative LAB . These microbes can have both positive and negative impact on wort production. Mash acidification by thermotolerant L. amylovorus has shown to improve enzymatic conversion of starches to sugars, increased extract and fermentability, increased TSN and FAN, and even improved head retention and increased shelf stability . Bacterial growth during mashing can also have a negative impact. For example, the thermotolerant Gram-positive and facultative anaerobe Bacillus coagulans can cause the mash to go sour and form nitrosamines. B. coagulans forms nitrosamines without oxygen between the temperatures of 86°F/30°C and 154°F/68°C, and can withstand a pH of 4.0 or higher . Clostridium, which may not necessarily originate from the malt itself, can create butyric acid off flavors during the mash or during kettle souring . High bacterial growth can cause lautering issues, probably due to the production of dextrans by the bacteria .

See Also

Additional Articles on MTF Wiki

• Culturing Lacto From Grains
• Lactobacillus
• Pediococcus

External Resources

• Homebrew Talk Wiki page on Grain.