Relationships of barley proteins soluble in sodium dodecyl sulphate to malting quality and varietal identification

Extraction, Composition, Functionality, and Utilization of Brewer’s Spent Grain Protein in Food Formulations

In recent years, brewer’s spent grain (BSG) has gained attention as a plant-based protein source because it occurs in large quantities as a by-product of beer brewing. BSG can contribute to future food requirements and support the development of a circular economy. In light of the dynamic developments in this area, this review aims to understand the proteins present in BSG, and the effect of extraction techniques and conditions on the composition, physicochemical, and techno-functional properties of the obtained protein extracts. The water-insoluble hordeins and glutelins form the major protein fractions in BSG. Depending on the beer brewing process, the extraction technique, and conditions, the BSG protein isolates predominantly contain B, C, and ϒ hordeins, and exhibit a broad molecular weight distribution ranging between <5 kDa and >250 kDa. While the BSG isolates obtained through chemical extraction methods seem promising to obtain gelled food products, physical and enzymatic modifications of BSG proteins through ultrasound and proteolytic hydrolysis offer an effective way to produce soluble and functional protein isolates with good emulsifying and foaming capabilities. Specifically tailored protein extracts to suit different applications can thus be obtained from BSG, highlighting that it is a highly valuable protein source.

Constitutive differences between steely and mealy barley samples associated with endosperm modification

BACKGROUND

Structurally different areas may occur in the endosperm of the barley grain, and they can be visually classified as either mealy or steely. Barleys with a high proportion of grains that are mostly steely often show uneven physical-chemical modification of the endosperm during malting. To study the relationship between steeliness and endosperm modification, two samples of barley cv. Scarlett with contrasting malting quality were analysed.

RESULTS

The proportions of steely grains were 77% and 46% in the two samples, which were then defined as steely sample and mealy sample, respectively. The steely sample showed slower modification during malting (in terms of beta-glucan degradation, friability increase, and Calcofluor staining), lower hot water extract (HWE) and acrospire growth, and higher extract viscosity. Endosperm permeation to large molecules (tested with the fluorescein isothiocyanate-dextran conjugate, FITC-D) closely followed cell wall modification in the steely sample, but this was not so in the mealy sample.

CONCLUSIONS

Higher steeliness was associated with higher levels of C hordeins in the grain of barley cv. Scarlett. It is proposed that such hordeins can increase the permeability to large molecules (FITC-D) but slow modification. Like steeliness and the level of C hordeins, permeability to FITC-D appears to be more linked to environmental rather than genetic effects. Although a more general association of C hordeins with steeliness of malting barley still has to be ascertained, the negative role of C hordeins in malting quality has been confirmed.

Expression of the gamma-zein protein of maize in seeds of transgenic barley: effects on grain composition and properties

A cDNA clone encoding the gamma-zein protein of maize was expressed in developing grain of barley using the starchy endosperm cell-specific promoter from the wheat Glu-1D-1 (HMW subunit 1Dx5) gene. Seven transgenic lines were recovered from 226 bombarded immature embryos, of which two were sterile and four tetraploid, while five were shown to express the gamma-zein protein based on western blotting. Southern blot analysis showed the presence of between about three and twelve transgene insertions. Detailed comparative studies of five null and five homozygous transformed sub-lines from transgenic line A showed that gamma-zein accounted for over 4% of the total prolamin fraction, corresponding to about 1.9% of the total grain N. Comparison of the proteins present in the gel protein fraction demonstrated that the gamma-zein was incorporated into polymers, as in maize. However, there was no effect on grain hardness measured using the Perten Single Kernel Characterisation System or on the vitreousness measured by visual inspection. This contrasts with the situation in maize where a clear association with vitreousness has been reported.

Qualitative and quantitative changes in barley seed protein patterns during the malting process analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with respect to malting quality

Seeds of two barley cultivars, similar in total protein content and malt extract yield but different in their final attenuation values, were malted. Samples taken at daily intervals during the malting process were extracted sequentially with Tris-HCl buffer, aqueous 2-propanol, aqueous 2-propanol containing 0.5% dithiothreitol, and 4 M urea, containing 0.5% dithiothreitol and 1% Nonidet P-40. The protein composition of these extracts was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and computer densitometry to determine whether differences observed in the rate or extent of protein modification are related to the malting quality character final attenuation. It was found that, common to both cultivars, the albumin and globulin proteins were relatively resistant to proteolysis, whereas the hordeins suffered a dramatic breakdown during malting, with the D hordein being degraded most rapidly, followed by the B and C hordeins. Besides these similarities, differences between both cultivars were observed in the relative rates of D hordein degradation, as this rate was considerably higher in the cultivar with high malting quality. Similar, but much less distinct kinetics were seen with certain B hordeins. Since a possible relationship might exist between the rate of proteolysis of the D hordeins and the character final attenuation, we analyzed a larger number of barley cultivars with different final attenuation values with a simplified technique. For the ten cultivars examined, differences during germination were again seen in the rates of modification of the D hordeins. However, significant correlations between the D hordein breakdown and final attenuation values were not obtained, so that we propose that there exists at best a loose correlation between the relative rate of proteolysis of these proteins and the malting quality character final attenuation.

Detection of polypeptides and amylase isoenzyme modifications related to malting quality during malting process of barley by two-dimensional electrophoresis and isoelectric focusing with immobilized pH gradients

Two cultivars ("Alexis" and "Lenka") of contrasting final attenuation values were malted, and the protein and amylase isoenzyme composition, as well as the change in protein and amylase isoenzyme composition during malting, was investigated by two-dimensional polyacrylamide gel electrophoresis of total proteins, and isoelectric focusing of amylase isoenzymes, respectively. Isoelectric focusing demonstrated that significant differences exist between the amylase isoenzyme patterns of the two cultivars, suggesting a correlation between the presence of certain amylase isoenzyme bands and final attenuation. This finding was confirmed by analysis of 36 barley cultivars with a wide range of quality. It was shown that all cultivars which are of low or, at best, moderate final attenuation values exhibit the amylase band "B" (isoelectric point approximately 6.8), whereas those cultivars which are predominantly of high malting grade do not possess this "B" isoenzyme band, but exhibit the pronounced "A" isoenzyme band (isoelectric point approximately 6.5) instead, suggesting that these isoenzymes (which we suppose to be beta-amylases) can be utilized to predict the final attenuation values of unknown barley samples or new lines. However, "final attenuation" is a complex function. Preliminary results of two-dimensional gel electrophoresis indicate that other factors, such as total amount of amylases, or a 19 kDa A hordein-like polypeptide, which was degraded faster in the low malting grade cultivar "Lenka", may also have a role in determining quality.

Two-dimensional polyacrylamide gel electrophoresis, with immobilized pH gradients in the first dimension, of barley seed proteins: discrimination of cultivars with different malting grades

The suitability of high-resolution two-dimensional gel electrophoresis for barley cultivar discrimination and for classification with respect to their malting properties was studied. Seed proteins of 14 barley cultivars with different malting qualities were extracted with urea/dithiothreitol/Nonidet P-40 buffer and subjected to two-dimensional gel electrophoresis with immobilized pH gradients in the first dimension (IPG-DALT). The results of IPG-DALT were compared to the protein patterns obtained by a standard technique, sodium dodecyl sulfate polyacrylamide gel electrophoresis of hordeins. Sodium dodecyl sulfate-gel electrophoresis yielded seven different "B" and four different "C" hordein patterns; "A" and "D" hordein patterns were uniform in all cultivars tested. Four cultivars could be distinguished unequivocally, the others were classified into three groups containing between two and five cultivars. In contrast to these findings. IPG-DALT yielded three different "A", eight different "B", four different "C" and two different "D" hordein patterns. When the "A", "B", "C" and "D" hordein patterns were combined, ten cultivars exhibited unique hordein patterns whereas the remaining ones were classified into two groups containing two cultivars each. Moreover, when albumin and globulin proteins were used for evaluation in addition to the hordeins, all cultivars could be discriminated by IPG-DALT. IPG-DALT, performed on small-scale and/or ready-made gels, proved to be an ideal complementary system to one-dimensional electrophoretic methods for routine seed testing purposes because of its speed, reliability, and simplicity. IPG-DALT was also applied to study the relationship between the different polypeptide patterns and the malting quality. Although cultivars with identical one-dimensional protein patterns but different malting quality could be successfully differentiated by IPG-DALT, a direct correlation between specific protein spots or protein patterns to the malting quality was not found within the cultivars tested.

Barley cultivar discrimination: II. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectric focusing with immobilized pH gradients

Isoelectric focusing performed with immobilized pH gradients was found superior to other commonly used electrophoretic methods for discrimination of 55 European winter and spring barley cultivars. Hordeins, the alcohol-soluble proteins, yielded 32 different patterns, allowing identification of 22 cultivars and classification of the remaining ones into ten groups of two to eight cultivars each. Only 21 different hordein patterns were observed using horizontal sodium dodecyl sulfate-polyacrylamide gel electrophoresis, followed by silver staining. Twelve cultivars exhibited unique hordein patterns, the remaining nine groups contained 2-11 cultivars. Resolution of isoelectric focusing with immobilized pH gradients was further enhanced in some cases when the patterns of urea/dithiothreitol-soluble proteins were used instead of the hordein patterns. However, evaluation was more complicated because of the larger number of protein bands detected.