Disulphide-bonded ‘gel protein’ aggregates in barley: Quality-related differences in composition and reductive dissociation

Mashing performance as a function of malt particle size in beer production

Significant innovations have occurred over the past 50 years in the malting and brewing industries, focused on optimization of the beer mashing, boiling and fermentation processes. One of the challenges faced in beer brewing has been in the malting process to obtain the desired malt and wort quality to produce high-quality beer products. The hydrolytic enzymes produced during grain germination are mostly entrapped inside the cellular matrices of the grain. The intra-grain diffusion of enzymes for in-situ hydrolysis, as well as diffusion of enzymes to wort, depends upon the malt size and malt size fractions obtained after milling. This review investigates the relationship between varying barley grain particle size distribution and the efficiency of the malting and mashing processes. Recommended ideal particle size of barley grain before and after milling are proposed based on the review of existing literature. Each brewing batch of grains with a proportion of >80% plump grains (>2.5 mm in size) is suggested to be the optimal size before milling, whereas the optimum grain particle size after milling ranged between 0.25 and 0.5 mm. The current review will summarize the theoretical aspects for malt milling and the particle size characteristics for optimizing the brewing process.

Barley Protein Properties, Extraction and Applications, with a Focus on Brewers' Spent Grain Protein

Barley is the most commonly used grain in the brewing industry for the production of beer-type beverages. This review will explore the extraction and application of proteins from barley, particularly those from brewers’ spent grain, as well as describing the variety of proteins present. As brewers’ spent grain is the most voluminous by-product of the brewing industry, the valorisation and utilisation of spent grain protein is of great interest in terms of sustainability, although at present, BSG is mainly sold cheaply for use in animal feed formulations. There is an ongoing global effort to minimise processing waste and increase up-cycling of processing side-streams. However, sustainability in the brewing industry is complex, with an innate need for a large volume of resources such as water and energy. In addition to this, large volumes of a by-product are produced at nearly every step of the process. The extraction and characterisation of proteins from BSG is of great interest due to the high protein quality and the potential for a wide variety of applications, including foods for human consumption such as bread, biscuits and snack-type products.

Milling, water uptake, and modification properties of different barley (Hordeum vulgare L.) lots in relation to grain composition and structure

Milling properties, water uptake, and modification in malting were studied in 14 barley (Hordeum vulgare L.) lots from two consecutive crop years. In all barley lots studied, grains with lower β-glucan and protein content and higher starch content produced finer flours upon milling. Grains with lower β-glucan content also hydrated more rapidly during steeping. A detailed study of two cultivars from two crop years indicated that similar environmental conditions could induce a higher β-glucan content and concentration of aggregated B hordein in the peripheral endosperm and a lower proportion of C hordein entrapped among aggregated hordeins deeper within the endosperm. These characteristics were associated with production of coarser flours during milling as well as with slower water uptake and lower modification. However, the data do not distinguish between the effect of β-glucan content and that of hordein localization. Distribution of β-glucan or total protein within the kernel was not linked to hydration or modification.

Quantification of Hordeins by ELISA: the correct standard makes a magnitude of difference

BACKGROUND

Coeliacs require a life-long gluten-free diet supported by accurate measurement of gluten (hordein) in gluten-free food. The gluten-free food industry, with a value in excess of $6 billion in 2011, currently depends on two ELISA protocols calibrated against standards that may not be representative of the sample being assayed.

AIM

The factors affecting the accuracy of ELISA analysis of hordeins in beer were examined.

RESULTS

A simple alcohol-dithiothreitol extraction protocol successfully extracts the majority of hordeins from barley flour and malt. Primary hordein standards were purified by FPLC. ELISA detected different classes of purified hordeins with vastly different sensitivity. The dissociation constant (Kd) for a given ELISA reaction with different hordeins varied by three orders of magnitude. The Kd of the same hordein determined by ELISA using different antibodies varied by up to two orders of magnitude. The choice of either ELISA kit or hordein standard may bias the results and confound interpretation.

CONCLUSIONS

Accurate determination of hordein requires that the hordein standard used to calibrate the ELISA reaction be identical in composition to the hordeins present in the test substance. In practice it is not feasible to isolate a representative hordein standard from each test food. We suggest that mass spectrometry is more reliable than ELISA, as ELISA enumerates only the concentration of particular amino-acid epitopes which may vary between different hordeins and may not be related to the absolute hordein concentration. MS quantification is undertaken using peptides that are specific and unique enabling the quantification of individual hordein isoforms.

Shotgun proteome analysis of beer and the immunogenic potential of beer polypeptides

The majority of beer proteins originate from barley (Hordeum vulgare) which is used for brewing. Barley is known to contain celiacogenic gliadin-like prolamins (hordeins) along with other immunogenic proteins which endure malt proteases and the harsh conditions of brewing. In addition, a multitude of peptides that may retain or even amplify the immune-stimulating potential is released in beer because of proteolysis. The comprehensive annotation of the beer proteome is challenged both by the high concentration range of the protein entities and by a severe degree of processing-induced modifications. Overcoming the pitfalls of the classical two-dimensional electrophoresis approach coupled to mass spectrometry (MS), the gel-free shotgun proteomic analysis expanded the current inventory of a popular Italian beer to 33 gene products, including traces of intact B- and D-hordeins and 10 proteins from Saccharomyces spp. The high performance liquid chromatography-electrospray MS/MS peptidomic analysis of the low-molecular weight beer components disclosed a panel of hordein-derived peptides that encrypt gluten-like sequence motifs, potentially harmful to celiacs. The presence of antigliadin IgA-immunoresponsive prolamins was assayed by Western and dot blot using sera of N=4 celiac patients. Gliadin-reactive T-cell lines isolated from the intestine of N=5 celiacs activated an IFN-γ response when challenged with deamidated beer polypeptides.

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.