The Impact of Malt Derived Proteins on Beer Foam Quality. Part I. The Effect of Germination and Kilning on the Level of Protein Z4, Protein Z7 and LTP1

Proteomics and Metabolomics Reveal that an Abundant α-Glucosidase Drives Sorghum Fermentability for Beer Brewing

Sorghum (Sorghum bicolor), a grass native to Africa, is a popular alternative to barley for brewing beer. The importance of sorghum to beer brewing is increasing because it is a naturally gluten-free cereal, and climate change is expected to cause a reduction in the production of barley over the coming decades. However, there are challenges associated with the use of sorghum instead of barley in beer brewing. Here, we used proteomics and metabolomics to gain insights into the sorghum brewing process to advise processes for efficient beer production from sorghum. We found that during malting, sorghum synthesizes the amylases and proteases necessary for brewing. Proteomics revealed that mashing with sorghum malt required higher temperatures than barley malt for efficient protein solubilization. Both α- and β-amylase were considerably less abundant in sorghum wort than in barley wort, correlating with lower maltose concentrations in sorghum wort. However, metabolomics revealed higher glucose concentrations in sorghum wort than in barley wort, consistent with the presence of an abundant α-glucosidase detected by proteomics in sorghum malt. Our results indicate that sorghum can be a viable grain for industrial fermented beverage production, but that its use requires careful process optimization for efficient production of fermentable wort and high-quality beer.

Variation in quality of grains used in malting and brewing

Cereal grains have been domesticated largely from food grains to feed and malting grains. Barley (Hordeum vulgare L.) remains unparalleled in its success as a primary brewing grain. However, there is renewed interest in "alternative" grains for brewing (and distilling) due to attention being placed on flavor, quality, and health (i.e., gluten issues) aspects that they may offer. This review covers basic and general information on "alternative grains" for malting and brewing, as well as an in-depth look at several major biochemical aspects of these grains including starch, protein, polyphenols, and lipids. These traits are described in terms of their effects on processing and flavor, as well as the prospects for improvement through breeding. These aspects have been studied extensively in barley, but little is known about the functional properties in other crops for malting and brewing. In addition, the complex nature of malting and brewing produces a large number of brewing targets but requires extensive processing, laboratory analysis, and accompanying sensory analysis. However, if a better understanding of the potential of alternative crops that can be used in malting and brewing is needed, then significantly more research is required.

Quality attributes for barley malt: "The backbone of beer"

Malting is the process of preparing barley for brewing through partial germination followed by drying. This process softens the grain cell wall and stimulates the production of diastatic enzymes, which convert starch into malt extract. The suitability of a barley grain for malt production depends upon a large number of quality parameters that are crucial for the identification and release of high-quality malt varieties. Maintaining tight control of these quality attributes is essential to ensure high processing efficiency and final product quality in brewery and malt house. Therefore, we have summarized the basic malting process and various physiological and biochemical quality parameters that are desirable for better malt quality. This study may provide an understanding of the process, problems faced, and opportunities to maltsters and researchers to improve the malt efficiency by altering the malting process or malt varieties.

A Comprehensive Phylogenetic Analysis of the Serpin Superfamily

Serine protease inhibitors (serpins) are found in all kingdoms of life and play essential roles in multiple physiological processes. Owing to the diversity of the superfamily, phylogenetic analysis is challenging and prokaryotic serpins have been speculated to have been acquired from Metazoa through horizontal gene transfer due to their unexpectedly high homology. Here, we have leveraged a structural alignment of diverse serpins to generate a comprehensive 6,000-sequence phylogeny that encompasses serpins from all kingdoms of life. We show that in addition to a central “hub” of highly conserved serpins, there has been extensive diversification of the superfamily into many novel functional clades. Our analysis indicates that the hub proteins are ancient and are similar because of convergent evolution, rather than the alternative hypothesis of horizontal gene transfer. This work clarifies longstanding questions in the evolution of serpins and provides new directions for research in the field of serpin biology.

Posttranslational Modifications Drive Protein Stability to Control the Dynamic Beer Brewing Proteome

Mashing is a key step in beer brewing in which starch and proteins are solubilized from malted barley in a hot water extraction and digested to oligomaltose and free amino nitrogen. We used SWATH-MS to measure the abundance and site-specific modifications of proteins throughout a small-scale pale ale mash. Proteins extracted from the malt at low temperatures early in the mash decreased precipitously in abundance at higher temperatures late in the mash due to temperature/time-induced unfolding and aggregation. We validated these observations using experimental manipulation of time and temperature parameters in a microscale pale ale mash. Correlation analysis of temperature/time-dependent abundance showed that sequence and structure were the main features that controlled protein abundance profiles. Partial proteolysis by barley proteases was common early in the mash. The resulting proteolytically clipped proteins were particularly sensitive and were preferentially lost at high temperatures late in the mash, while intact proteins remained soluble. The beer brewing proteome is therefore driven by the interplay between protein solubilization and proteolysis, which are in turn determined by barley variety, growth conditions, and brewing process parameters.

Construction of a comprehensive beer proteome map using sequential filter-aided sample preparation coupled with liquid chromatography tandem mass spectrometry

The quality traits of beer, which include flavor, texture, foam stability, gushing, and haze formation, rely on contributions from beer proteins and peptides. Large-scale proteomic analysis of beer is gaining importance, not only with respect to authenticity of raw material in beer but also to improve quality control during beer production. In this work, foam proteins were first isolated from beer by virtue of their high hydrophobicity. Then sequential filter-aided sample preparation coupled with liquid chromatography and tandem mass spectrometry was used to analyze both beer protein and foam protein. Finally, 4692 proteins were identified as beer proteins, and 3906 proteins were identified as foam proteins. In total, 7113 proteins were identified in the beer sample. Several proteins contributing to beer quality traits, including lipid transfer protein, serpin, hordein, gliadin, and glutenin, were detected in our proteins list. This work constructed a comprehensive beer proteome map that may help to evaluate potential health risks related to beer consumption in celiac patients.

Plant serpin protease inhibitors: specificity and duality of function

The serpins are a family of structurally conserved protease inhibitors found in all animal and plant kingdoms. After interaction with their cognate substrate(s), their native energetically stressed state is relaxed by hydrolysis, resulting in a semi-stable covalent bond that disables the protease. The inherent flexible serpin structure supports additional non-inhibitory functions. This review will focus on several biological functions attributed to plant serpins, ranging from specific cell death protease inhibitors to a stabilizing role for β-amylase in seeds. Functional conservation of a particular serpin type, the LR serpins, is suggested by its compelling ubiquity throughout the plant kingdom. The multiple target specificity of plant serpins including the LR serpins enables them to perform dual functions that are not mutually exclusive both as a regulator of cell death and as a protective anti-pathogenic protein.

Noncanonical interactions between serpin and β-amylase in barley grain improve β-amylase activity in vitro

Serpin protease inhibitors and β-amylase starch hydrolases are very abundant seed proteins in the endosperm of grasses. β-amylase is a crucial enzyme in the beer industry providing maltose for fermenting yeast. In animals and plants, inhibitory serpins form covalent linkages that inactivate their cognate proteases. Additionally, in animals, noninhibitory functions for serpins are observed such as metabolite carriers and chaperones. The function of serpins in seeds has yet to be unveiled. In developing endosperm, serpin Z4 and β-amylase showed similar in vivo spatio-temporal accumulation properties and colocalize in the cytosol of transformed tobacco leaves. A molecular interaction between recombinant proteins of serpin Z4 and β-amylase was revealed by surface plasmon resonance and microscale thermophoresis yielding a dissociation constant of 10-7 M. Importantly, the addition of serpin Z4 significantly changes β-amylase enzymatic properties by increasing its maximal catalytic velocity. The presence of serpin Z4 stabilizes β-amylase activity during heat treatment without affecting its critical denaturing temperature. Oxidative stress, simulated by the addition of CuCl2, leads to the formation of high molecular weight polymers of β-amylase similar to those detected in vivo. The polymers were cross-linked through disulfide bonds, the formation of which was repressed when serpin Z4 was present. The results suggest an unprecedented function for a plant seed serpin as a β-amylase-specific chaperone-like partner that could optimize β-amylase activity upon germination. This report is the first to describe a noninhibitory function for a serpin in plants.

Process Proteomics of Beer Reveals a Dynamic Proteome with Extensive Modifications

Modern beer production is a complex industrial process. However, some of its biochemical details remain unclear. Using mass spectrometry proteomics, we have performed a global untargeted analysis of the proteins present across time during nanoscale beer production. Samples included sweet wort produced by a high temperature infusion mash, hopped wort, and bright beer. This analysis identified over 200 unique proteins from barley and yeast, emphasizing the complexity of the process and product. We then used data independent SWATH-MS to quantitatively compare the relative abundance of these proteins throughout the process. This identified large and significant changes in the proteome at each process step. These changes described enrichment of proteins by their biophysical properties, and identified the appearance of dominant yeast proteins during fermentation. Altered levels of malt modification also quantitatively changed the proteomes throughout the process. Detailed inspection of the proteomic data revealed that many proteins were modified by protease digestion, glycation, or oxidation during the processing steps. This work demonstrates the opportunities offered by modern mass spectrometry proteomics in understanding the ancient process of beer production.

Study of quantitative changes of cereal allergenic proteins after food processing

BACKGROUND

Within last few years, the occurrence of food allergens and corresponding food allergies has been increasing, therefore research into the individual allergens is required. In the present work, the effect of cereal processing on the amounts of allergenic proteins is studied by modern proteomic-based approaches. The most important wheat and barley allergens are low-molecular-weight (LMW) proteins. Therefore we investigated the relative quantitative changes of these proteins after food technological processing, namely wheat couscous production and barley malting.

RESULTS

A comparative study using mass spectrometry in connection with the technique of isobaric tag for relative and absolute quantification (iTRAQ) revealed that the amount of wheat allergenic LMW proteins decreased significantly during couscous production (approximately to 5-26% of their initial content in wheat flour). After barley malting, the amounts of the majority of LMW proteins decreased as well, although to a lesser extent than in the case of wheat/couscous. The level of two allergens even slightly increased.

CONCLUSION

Suggested proteomic strategy proved as universal and sensitive method for fast and reliable identification of various cereal allergens and monitoring of their quantitative changes during food processing. Such information is important for consumers who suffer from allergies.

Characterization of barley serpin Z7 that plays multiple roles in malt and beer

Barley protein Z7 (BSZ7) is a well-known serine protease inhibitor that was regarded as a major effector of beer foam stability. Moreover, it has also been suggested to participate in haze formation and affect wort filterability. The present study purified BSZ7 from barley malt and characterized its secondary structure and modification, as well as its relationship with peroxidase, to elucidate the molecular base of BSZ7 that supports its multiple roles in malt and beer. It was found that after 30 min of heating, the secondary structure was not affected. BSZ7 has no inhibiting effect on nonspecific protease originated from malt, suggesting its negative role in wort filterability was accomplished by other means. Furthermore, the glycation of BSZ7 by the Maillard reaction may make some contribution to its survival during wort boiling. The interaction of BSZ7 with polysaccharides and polyphenols found by adding experiment may explain how it acts as a negative factor on wort filterability. Greater understanding of BSZ7 and other proteins of malts will lead to better improvements in brewing quality.

Beer and wort proteomics

Proteome analysis provides a way to identify proteins related to the quality traits of beer. A number of protein species in beer and wort have been identified by two-dimensional gel electrophoresis combined with enzyme digestion such as trypsin, followed by mass spectrometry analyses and/or liquid chromatography mass/mass spectrometry. In addition, low molecular weight polypeptides in beer have been identified by the combination of non-enzyme digestion and mass analyses. These data sets of various molecular weight polypeptides (i.e., proteomes) provide a platform for analyzing protein functions in beer. Several novel proteins related to beer quality traits such as foam stability and haze formation have been identified by analyzing these proteomes. Some of the proteins have been applied to the development of efficient protein or DNA markers for trait selection in malting barley breeding. In this chapter, recent proteome studies of beer and wort are reviewed, and the methods and protocols of beer and wort proteome analysis are described.

The impact of different ale brewer's yeast strains on the proteome of immature beer

Background

It is well known that brewer’s yeast affects the taste and aroma of beer. However, the influence of brewer’s yeast on the protein composition of beer is currently unknown. In this study, changes of the proteome of immature beer, i.e. beer that has not been matured after fermentation, by ale brewer’s yeast strains with different abilities to degrade fermentable sugars were investigated.

Results

Beers were fermented from standard hopped wort (13° Plato) using two ale brewer’s yeast (Saccharomyces cerevisiae) strains with different attenuation degrees. Both immature beers had the same alcohol and protein concentrations. Immature beer and unfermented wort proteins were analysed by 2-DE and compared in order to determine protein changes arising from fermentation. Distinct protein spots in the beer and wort proteomes were identified using Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF-MS) and MS/MS and revealed common beer proteins, such as lipid transfer proteins (LTP1 and LTP2), protein Z and amylase-protease inhibitors. During fermentation, two protein spots, corresponding to LTP2, disappeared, while three protein spots were exclusively found in beer. These three proteins, all derived from yeast, were identified as cell wall associated proteins, that is Exg1 (an exo-β-1,3-glucanase), Bgl2 (an endo-β-1,2-glucanase), and Uth1 (a cell wall biogenesis protein).

Conclusion

Yeast strain dependent changes in the immature beer proteome were identified, i.e. Bgl2 was present in beer brewed with KVL011, while lacking in WLP001 beer.

Proteomic analysis of differences in barley (Hordeum vulgare) malts with distinct filterability by DIGE

Filterability is an essential quality parameter of barley malt and significantly impacts productive efficiency and quality of beer. In the study, differences of metabolic capability, rather than of initial contents of macromolecules in barleys, were found to be the main reason for malt filterability gap between the widely used cultivars Dan'er and Metcalfe in China. Comparative proteomics based on fluorescent difference gel electrophoresis (DIGE) was employed to quantitatively analyze proteins of four commercial malts belonging to the two cultivars, and 51 cultivar-differential spots were identified to 40 metabolic proteins by MALDI-TOF/TOF mass spectrometry, mainly including hydrolases and pathogen-related proteins. According to their function analysis and abundance comparison between cultivars, filterability-beneficial and -adverse proteins were putatively proposed. Two most remarkable differential proteins, β-amylase and serpin Z7, were further investigated to verify their effects on Dan'er malt filterability. These results provide biological markers for barley breeders and maltsters to improve malt filterability.

BIOLOGICAL SIGNIFICANCE

To the best of our knowledge, this is the first report of comprehensive investigation of metabolic proteins related to wort filterability of barley malts, and sheds light on clues for filterability improvement. Visible differences in the expression level of metabolic proteins between Dan'er and Metcalfe malts using 2D-DIGE signify a valuable tool for cultivar comparison, illustration of key proteins responsible for filterability and even other qualities of barley malts. And with these explorations on biomarkers of malt filterability and other aspects, there will be higher efficiency and quality of beer brewing, less application of exogenous hydrolases and more expending market for Chinese malting barleys. This article is part of a Special Issue entitled: Translational Plant Proteomics.