Grain skinning in malting barley: the influence of husk adhesion on grain and malt quality

Abstract

Barley (Hordeum vulgare L.) is a crop of prime importance to the malting, brewing and distilling sectors. Modern varieties are susceptible to a defect called “skinning”, a condition in which the adhesion of the husk to the caryopsis has been weakened, and partial or complete loss of the husk occurs. Malting of skinned grains is inefficient and results in production of poor quality malt and financial losses to the farmers and maltsters. Although the malting industry recognises that grain skinning causes inefficiencies and loss of production, to date there has been no quantitative assessment of these effects. Furthermore, there is little understanding of how the condition of harvested grain (e.g. moisture content) influences the occurrence of skinning, and thus its potential value in the malting process. The overall aim of this research was to investigate how grain condition at harvest influences the quality of husk adhesion and how skinning severity and husk-loss type impacts the malting process and quality of the malt produced. This thesis details an investigation into the effects of moisture content (MC) to determine whether grain or tissue moisture content affects grain skinning. In the three varieties examined in the field, MCs were between 22% and 45% and skinning was positively correlated with MC; in the four varieties examined in the glasshouse, MCs were between 11% and 25% and skinning was negatively correlated with MC. Light microscopy examination of the grains has confirmed that two distinctly different mechanisms of skinning: in the glasshouse, parenchyma cells were damaged whereas in the field tissue has separated along the cementing layer, without causing cell damage. Micromalting studies were conducted on two varieties (Concerto and Chronicle), with four skinning severities: intact (0% skinned), mild (16% skinned), skinned (50% skinned) and severe (90% skinned), to examine the effect of skinning severity on the quality of the malt produced. Quality was determined by measuring friability and homogeneity, hot water extract (HWE) and α-amylase content. Under- and over-modification of the samples was also investigated at a single grain level, using the Carlsberg Malt Modification Method, thin layer chromatography and scanning electron microscopy. Hot water extract increased with the increase in severity of skinning, however when it was adjusted to account for lost husk biomass HWE then decreased with skinning severity. Friability, homogeneity and the α amylase content also decreased with higher skinning severity. Detailed investigation into modification indicated that malt quality declines with increase in skinning due to a large proportion of undermodified grains in skinned malt. No signs of overmodification were detected. Lastly, two most common husk-loss types in the bulk (missing lemma and huskless) were investigated and compared to grain with attached husk to determine if the type of husk damage is of importance during malting. The difference in germination rate and vigour, production of α-amylase and the rate of water uptake were investigated. Huskless grains had significantly lower levels of germination rate and α-amylase production, compared to the grains with lemma missing and unbroken husks. The main findings from this project are that grain skinning reduces malting efficiency and the quality of the malt produced due to undermodification of the grains and lack of homogeneity in the malted bulk. For the farmer, reduction in the skinning risk could be achieved through careful observation of the moisture during harvest. Maltsters would be able to achieve improved homogeneity and efficiency through careful examination of the bulks for the types of skinning and alterations in the malting regimes employed. On the methods currently used in micro-malting tests and variety evaluation, it is evident that malting of samples with high levels of skinned grain will give a HWE compared to samples with intact grain. Therefore, a corrected measure of HWE is required to take into account the proportion of skinned grain, or mass of husk-loss, in order to determine the true malting efficiency of the sample, or variety.