Grain skinning in malting barley: the influence of husk adhesion on grain and malt quality
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Date
04/07/2020Author
Okoro, Paulina Marta
Metadata
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.