SUMMARY Brewing beer involves microbial activity at every stage, from raw material production and malting to stability in the package. Most of these activities are desirable, as beer is the result of a traditional food fermentation, but others represent threats to the quality of the final product and must be controlled actively through careful management, the daily task of maltsters and brewers globally. This review collates current knowledge relevant to the biology of brewing yeast, fermentation management, and the microbial ecology of beer and brewing.

The Microbiology of Malting and Brewing

Cereal fungal infection, mycotoxins, and lactic acid bacteria mediated bioprotection: From crop farming to cereal products

Barley grain carries a numerous, variable, and complex microbial population that mainly consists of bacteria, yeasts, and filamentous fungi and that can partly be detected and quantified using plating methods and microscopic and molecular techniques. The extent and the activity of this microflora are determined by the altering state of the grain and the environmental conditions in the malt production chain. Three ecological systems can be distinguished: the growing cereal in the field, the dry barley grain under storage, and the germinating barley kernel during actual malting. Microorganisms interact with the malting process both by their presence and by their metabolic activity. In this respect, interference with the oxygen uptake by the barley grain and secretion of enzymes, hormones, toxins, and acids that may affect the plant physiological processes have been studied. As a result of the interaction, microorganisms can cause important losses and influence malt quality as measured by brewhouse performance and beer quality. Of particular concern is the occurrence of mycotoxins that may affect the safety of malt. The development of the microflora during malt production can to a certain extent be controlled by the selection of appropriate process conditions. Physical and chemical treatments to inactivate the microbial population on the barley grain are suggested. Recent developments, however, aim to control the microbial activity during malt production by promoting the growth of desirable microbial cultures, selected either as biocontrol agents inhibiting mycotoxin-producing molds or as starter cultures actively contributing to malt modification. Such techniques may offer natural opportunities to improve the quality and safety of malt.

From field barley to malt: detection and specification of microbial activity for quality aspects.

Recent advances in the malting and brewing industry

The biochemical and physiological reactions of germination have long been utilised to produce barley malt for brewing and other purposes. Also some oat malt has been produced as raw-material of ale and stout production. The main goals of malting have been the degradation of grain storage components to soften the kernel structure, synthesis of amylolytic enzymes and production of nutrients for brewing yeast. Also flavour and colour attributes have been important. During the recent years interest has arisen also in the secondary metabolites produced during germination, which can have valuable health promoting properties and act as bioactive or functional compounds in foods. By using a tailored germination/malting process a desired combination of valuable properties may be obtained in germinating grains or seeds. All this requires knowledge and know-how of the germination process and the biochemistry behind it. This paper reviews the scientific knowledge about germination/malting of oat with special emphasis on changes in grain characteristics.

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Germination: a means to improve the functionality of oat

The germination of grains that had been steeped in solutions/suspensions of antibiotics was improved relative to water-steeped controls. The effect of the antibiotics was most striking for dormant grain that had been steeped for 90 min rather than 60 min. By incubating decorticated grains under different amounts of water it was shown that restriction of air to decorticated dormant grains reduced germination much more than was the case with decorticated, mature grains. Incubating the dishes of grain in oxygen, rather than air, overcame the inhibitory effect of excess water, confirming that it was the restriction of the oxygen supply by the water that checked germination. It was shown that sprays of dilute sulphuric acid favoured grain germination by acting on the outer layers of the grain. When acid-sprayed and untreated samples were decorticated and incubated their germination rates under different test conditions were essentially identical. In contrast gibberellic acid was shown to exert its germination-stimulating effects and sodium hydroxide its inhibitory effects on the internal tissues. By comparing the respiration rates of ground samples of grain (entire and decorticated; untreated and surface-sterilised) it was shown that microbes in the surface layers have substantial oxygen uptake rates. It seems that the competition of these microbes with the grain tissues for oxygen is a major cause of dormancy.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1993.tb01160.x

Microbes and grain germination

Treatments were applied to dormant grains pre-drying, during dry storage and in the immediate pre-steep period. The effects on rates of recovery from dormancy were assessed. Only applications of sulphuric acid and gibberellic acid in the pro-drying period subsequently enhanced germination. No treatments applied during dry storage were beneficial. It appeared that oxygen was not needed during storage for grain maturation. Atmospheres of carbon dioxide applied during warm storage reduced subsequent germination but this gas was without effect on grain stored at ambient temperatures. Steeping grains in dilute solutions of mineral acids and some sulphur containing compounds improved their germinability.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1993.tb01152.x

The use of chemical agents to overcome dormancy in malting barley

To obtain reproducible results germination tests should be carried out at closely controlled temperatures, and in a water-saturated atmosphere. Tests carried out on a substratum of agar gel are more reproducible than those carried out on paper. It is proposed that the 1 ml and 3 ml (agar) tests, carried out at 18°C and in a water-saturated atmosphere, be evaluated in collaborative trials alongside the recommended 4 ml and 8 ml tests with a view to replacing the latter with the former, if their superiority is confirmed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.1992.tb01104.x

Peter J. Doran

Papers

Microbes and grain germination

The use of chemical agents to overcome dormancy in malting barley

Studies on germination tests