The Survival of Limit Dextrinase during Fermentation in the Production of Scotch Whisky

Experimental Whisky Fermentations: Influence of Wort Pretreatments

In addition to ethanol yield, the production of flavour congeners during fermentation is a major consideration for Scotch whisky producers. Experimental whisky fermentations can provide useful information to the industry, and this is the focus of this paper. This study investigated the impact of wort pretreatments (boiled, autoclaved, filtered) on fermentation performance and flavour development in Scotch whisky distillates as an alternative to freezing wort for storage. Our study showed that no significant sensorial differences were detected in low wines (first distillates), while the chemical compositions showed clear changes in increased levels of esters and higher alcohols in boiled and autoclaved wort. In contrast, filtered wort comprised overall lower levels of congeners. Regarding alcohol yield, all three pretreatments resulted in decreased yields. In practice, the pretreatment of wort prior to fermentation requires additional process operations, while freezing requires large storage units. The pretreatments adopted in this study significantly influence the composition of the malt wort used for experimental whisky fermentations, and this results in a poorer fermentation performance compared with untreated wort. We recommend the use of fresh or frozen wort as the best options for small-scale fermentation trials.

Exploring the factors that affect the themostability of barley limit dextrinase - Inhibitor complex

Barley Limit dextrinase (Hordeum vulgare HvLD) is the unique endogenous starch-debranching enzyme, determining the production of a high degree of fermentation. The activity of HvLD is regulated by an endogenous LD inhibitor protein (LDI). In beer production, free LD is easy to inactivate in mashing process under the condition of high temperature. The binding of LD with LDI protects it against heat inactivation. Exploring the factors affecting the themostability of HvLD-LDI complex is important for beer production. In this work, the themostability of HvLD-LDI complex at different NaCl concentrations and temperatures were explored by molecular dynamics simulation and binding free energy calculation. In NaCl solution, the complex exhibits higher conformational stability at 343 K and 363 K than those in pure water. Root mean square fluctuation (RMSF) analysis identified the thermal sensitive regions of HvLD and LDI. The binding free energy results suggest that the LD-LDI complex is more stable in NaCl solution than those in pure water at high temperature. The residues with high contribution to the complex were identified. The structural and dynamic details will help us to understand the driving forces that lead to the themostability of HvLD-LDI complex at different temperatures and different salt concentrations, which will facilitate the optimization conditions of beer production for maintaining the thermal stability and activity of HvLD.

Association mapping of malting quality traits in UK spring and winter barley cultivar collections

Historical malting quality data was collated from UK national and recommended list trial data and used in a GWAS. 25 QTL were identified, with the majority from spring barley cultivar sets. In Europe, the most economically significant use of barley is the production of malt for use in the brewing and distilling industries. As such, selection for traits related to malting quality is of great commercial interest. In order to study the genetic basis of variation for malting quality traits in UK cultivars, a historical set of trial data was collated from national and recommended list trials from the period 1988 to 2016. This data was used to estimate variety means for 20 quality related traits in 451 spring barley cultivars, and 407 winter cultivars. Genotypes for these cultivars were generated using iSelect 9k and 50k genotyping platforms, and a genome wide association scan performed to identify malting quality quantitative trait loci (QTL). 24 QTL were identified in spring barley cultivars, and 2 from the winter set. A number of these correspond to known malting quality related genes but the remainder represents novel genetic variation that is accessible to breeders for the genetic improvement of new cultivars.

A Chromogenic Assay Suitable for High-Throughput Determination of Limit Dextrinase Activity in Barley Malt Extracts

Twenty-four malt samples were assayed for limit dextrinase activity using a chromogenic assay developed recently in our group. The assay utilizes a small soluble chromogenic substrate which is hydrolyzed selectively by limit dextrinase in a coupled assay to release the chromophore 2-chloro-4-nitrophenol. The release of the chromophore, corresponding to the activity of limit dextrinase, can be followed by measuring the UV absorption at 405 nm. The 24 malt samples represented a wide variation of limit dextrinase activities, and these activities could be clearly differentiated by the assay. The results obtained were comparable with the results obtained from a commercially available assay, Limit-Dextrizyme from Megazyme International Ireland. Furthermore, the improved assay uses a soluble substrate. That makes it well suited for high-throughput screening as it can be handled in a 96-well plate format.

Genetic diversity and QTL mapping of thermostability of limit dextrinase in barley

Limit dextrinase (LD) is an essential amylolytic enzyme for the complete degradation of starch, and it is closely associated with malt quality. A survey of 51 cultivated barley and 40 Tibetan wild barley genotypes showed a wide genetic diversity of LD activity and LD thermostability. Compared with cultivated barley, Tibetan wild barley showed lower LD activity and higher LD thermostability. A doubled haploid population composed of 496 DArT and 28 microsatellite markers was used for mapping Quantitative Trait Loci (QTLs). Parental line Yerong showed low LD activity and high LD thermostability, but Franklin exhibited high LD activity and low LD thermostability. Three QTLs associated with thermostable LD were identified. The major QTL is close to the LD gene on chromosome 7H. The two minor QTLs colocalized with previously reported QTLs determining malt-extract and diastatic power on chromosomes 1H and 2H, respectively. These QTLs may be useful for a better understanding of the genetic control of LD activity and LD thermostability in barley.

Genotypic and environmental variation in barley limit dextrinase activity and its relation to malt quality

Variation in the limit dextrinase activity of barley malt, and the relationships between limit dextrinase activity and malt quality parameters were investigated using eight cultivars grown at seven diverse locations in China for two successive years. Limit dextrinase activity varied with genotype and location, with the levels ranging from 0.245 U/g to 0.980 U/g. The results showed that the variation in limit dextrinase activity was more attributable to the environment (location and year) than to the genotype. The response of limit dextrinase activity to the environment differed markedly among cultivars, and was reflected by large difference in coefficient of variation of cultivars across diverse locations. Regression analysis showed that limit dextrinase activity was negatively correlated with malt viscosity (r=-0.52, P<0.01), positively correlated with Kolbach index (r=0.38, P<0.01) and malt extract (r=0.30, P<0.05), but had no significant correlation with malt protein content and diastatic power.

The proteinaceous inhibitor of limit dextrinase in barley and malt

Barley limit dextrinase catalyses hydrolysis of alpha-1,6-D-glucosidic bonds in branched poly- or oligosaccharides from starch. A specific inhibitor of this enzyme is found in mature barley kernels, but disappears after several days of germination. Two forms of this proteinaceous inhibitor, identical in amino acid sequence, have been isolated and characterized. They differ in attachment of cysteine or glutathione to a sulfhydryl group, possibly that of cysteine residue 59 of the inhibitor. They can form a 1:1 complex with limit dextrinase and are believed to interact specifically with the enzyme active site. The inhibitor present in mature barley can effectively reduce enzyme activity in barley germinated for a short time and in commercial malt.