Barley is used, in many countries, for brewing and also, in Scotland, to make whisky. For both purposes, barley is first turned into malt, through partial germination, allowing breakdown of the internal grain structure (modification), followed by oven-drying (kilning) to temporarily arrest enzyme activity. The malt is then milled fairly coarsely prior to extraction in hot water, in the first stage of brewing or distilling.
The suitability of a barley sample for these processes is partly due to the environment, that is the field and climatic conditions, in which it was grown, but also to genetic factors, so the malting, brewing and distilling industries will only accept grain from certain varieties. By understanding the genetic factors determining malting quality potential and how they are inherited, Institute scientists can provide knowledge and tools for barley breeders.
Alcohol results from fermentation of sugars derived from breakdown of starch. The starch in barley is in the form of granules, embedded in a matrix of protein and contained in a cellular structure, in the inner part (endosperm) of the grain. The cell walls and some of the protein must be broken down during malting, so that the starch can be gelatinised (that is, the granules swell, burst and become soluble) and broken down by enzymes during hot water extraction.
The total amount of material that can be brought into solution is called the Hot Water Extract (HWE) and is the key determinant of good malting quality. This is assessed in the laboratory, using ‘scaled-down’ versions of malting and initial brewing processes. High HWE can be due to rapid enzyme production during malting but, in NW Europe, generally results from a friable grain structure, easy to modify, and moderate enzyme levels. Viewed under Scanning Electron Microscopy (SEM) such a structure appears less compacted with the starch granules more clearly seen.
Left: SEM of malted barley grain showing compacted endosperm structure with starch granules heavily embedded in protein matrix. Right: SEM of barley grain after micro-malting showing a friable structure with starch granules readily accessible for enzymic degradation.
Good malting varieties also have thin cell walls with low levels of the polysaccharide, b-glucan, the content of which is determined by several genetic factors, or Quantitative Trait Loci (QTLs). Some have been mapped to barley chromosomes, for example in a population studied at SCRI, there were effects on chromosomes 1H, 5H and 7H. Genes coding for enzymes implicated in b-glucan synthesis, have been mapped to similar areas of 1H and 7H, but further work is required to confirm cause and effect. Grain protein is largely affected by N fertiliser application, but there are also genetic effects. A QTL on chromosome 5H, co-located with the one for b-glucan. This may indicate pleiotropic effects of a single factor, as synthesis of major grain constituents occurs simultaneously. The importance of this area of chromosome 5H, for malting quality, was also shown in another population at the Institute, by detection of a QTL for HWE.
