MICROBES ON BARLEY GRAINS

Experiments on the incorporation of radioactivity from [C-14]-glucose, the reduction of triphenyltetrazolium chloride (TTZ) and oxygen uptake have demonstrated that in undamaged grains neither glucose nor TTZ pass inward through the testa and are, respectively, utilized and reduced by microbes associated with the husk and pericarp. The microbes on the husk have a strong but variable respiration. Using a TTZ reagent to quantify the surface microbes, by their ability to reduce TTZ to a coloured formazan, it was shown that acidity checks and warm steeping and aeration favour the development of microbes on steeped grain. Using measurements of the respiration rates of strips of husk, incubation water and steep water it has been shown that most, if not all, of the oxygen uptake associated with husk and steep liquor is due to microbes. The build up of this population when grain or husk is wetted can be checked or prevented by acidification or by a mixture of antibiotics. However, neither of these agents kill all the microbes. Under many conditions microbial respiration is limited by the supply of respiratory substrates derived from the husk. To explain our results, and those of others, it is necessary to suppose that the microbial populations of individual grains vary widely; that initially the population consists of vegetative cells, which will grow immediately the grain is wetted, and spores which will only grow after a lag period; that initially nutrients for the microbes on the husk and in the steep liquor are plentiful, but that after longer wet periods both microbial multiplication and respiration are nutrient-limited. The results have implications for the choice of steeping regimes in malting.