Understanding components of specific weight in barley grains – opportunities for improving grain quality and processing efficiency (PhD)

Quality requirements for malting barley include germination rate, per cent admixture (other grains and contaminant particles), nitrogen levels, cultivar, moisture content, uniformity, skinning level, disease/weathering damage and specific weight (SW). Whilst the majority of these malting barley quality requirements are well understood in terms of end use, SW is not.

Specific weight is one of the longest-standing measures of grain quality for cereals and oilseeds; it is a measure of the weight of grain per unit volume and is reported in kilograms per hectolitre (kg hl^-1). An increased SW is thought to be beneficial for malt output, however, the relationships between SW and measures of malt output or efficiency are not well understood.

The aim of this research is to enhance the understanding of SW as a measure of grain quality and establish which aspects of barley grain determine this measure, then relate this to the malting process and outputs, to understand how SW influences malting.

Firstly, SW has been broken down into its two components: grain density and packing efficiency. This is a key part of the research because both components can change independently. Different grain parameters influence each of the components, therefore, both need to be considered together when investigating SW differences or similarities between samples.

The packing efficiency and grain density of nine spring barley cultivars were investigated, this demonstrated that grain density contributed 48.5% and packing efficiency 36.5% to the variation in SW. It was hypothesised that the packing efficiency of grains was primarily influenced by grain morphometrics, and grain density influenced by composition. The ways in which composition changed with grain density was investigated by stratifying grains according to their density, resulting in several fractions with different densities. Compositional analyses were carried out on these groups which showed that grain nitrogen level and the proportional volume of starch B-type granules contributed 47% to the observed variation in grain density.

Specific weight is also known to be affected by growing conditions, with year-to-year variation observed. Therefore, the effect of a moderate, but prolonged water stress on SW was investigated under glasshouse conditions. Plant development was altered by the stress, but SW was maintained through compensatory mechanisms.

To investigate how changes in SW affect malt quality parameters, SW was manipulated through selection for different grain size and weights. Specific weight was shown to be strongly correlated with the predicted spirit yield and hot water extract of the malt. These are two fundamental measures of malt quality. Grain density also correlated with these two malt quality measures, but grain packing efficiency did not. This indicates that it is grain density rather than the packing efficiency of the grain that is beneficial component of SW for malting. Therefore, if breeding is continued to enhance malt quality through increasing SW, this should be targeted through increasing the grain density component rather than packing efficiency.