Barley specific weight pulled apart by PhD

Specific weight (SW) is one of the longest-standing ways of judging cereal grain quality. To better understand it in barley, a recently completely PhD project has pulled apart its components. Dhan Bhandari, who manages cereal quality research at AHDB, outlines the main findings.

PhD student Aaron Hoyle started to investigate the components of SW in malting barley back in 2016. Based at SRUC, his work has revealed the complexity of this top-line malting quality indicator.

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 SW. However, relatively little is known about this latter characteristic.

SW is the weight of grain in a defined volume. Reported in kilograms per hectolitre (kg/hl), a high SW was believed, by some, to indicate a high starch content. However, it has recently been shown that a high SW may not be due to an increased starch content in barley grains. Despite this, a high SW helps increase the weight of grains in malting vessels – and this can increase the output per processed batch during the malting process.

The link between SW and malt quality parameters had yet to be made. Certainly, the headline SW can mask tremendous variation in the traits responsible for malting efficiency. So, Aaron worked with the team at SRUC to study two key SW components: packing efficiency and grain density. The former measures how much of the bulk volume is occupied by the grain, and the latter concerns what the grain is made of.

Nine spring malting barley varieties, known to give a wide range of SW and screenings, were grown in field trials. The harvested grain was analysed in meticulous detail. For example, for each variety, 100 representatively sampled grains were measured (length, width and depth) and weighed, with significant differences observed.

The result demonstrated that grain density contributed 48.5% and packing efficiency 36.5% to the variation in SW.

Further analysis revealed that grain nitrogen level and the proportional volume of the smaller starch B-type granules contributed 47% to the observed variation in grain density.

In addition to variety, growing conditions are known to affect SW. In this study, plants were exposed to moderate but prolonged water stress under glasshouse conditions. Although this altered plant development, SW was maintained through compensatory mechanisms.

Experiments that used different grain sizes and weights to manipulate SW showed a strong correlation between SW and the predicted spirit yield and hot water extract of the malt – two fundamental measures of malt quality.

Although grain density correlated with these two malt quality measures, packing efficiency did not. This indicates that grain density is the most influential SW component. A finding of particular interest to plant breeders and maltsters.

This studentship project has improved our understanding of the relative importance of each component of SW. It has also trained a new scientist to help deliver innovations to agriculture for decades.

Going with the grain: high-powered microscopes can reveal the building blocks of barley. These grain cross-sections show ‘A-type’ (large discs) and ‘B-type’ (small balls) starch granules, as well as various other components (e.g. cell walls and parts of the protein matrix). From the same spring barley variety (Laureate), the images show that no two grains are the same.