What is in a Beer? Proteomic Characterization and Relative Quantification of Hordein (Gluten) in Beer

Barley based gluten free beer - A blessing or an uncontrollable risk?

Recent reports have highlighted that beer labelled "gluten-free", crafted with enzymatic treatments to remove gluten, may contain polypeptides that could be immunotoxic to individuals with coeliac disease. As strict adherence to a gluten-free diet is the only way to manage this condition, accurate labelling is crucial to those with coeliac disease. This paper aims to discuss the presence, levels and immunogenicity of gluten peptides found in gluten-reduced barley beers. While advances have been made in the detection and quantification of gluten peptides in beer, there are still challenges to the interpretation of gluten measurements as well as to assess whether peptides are immunotoxic in vivo. To make progress, future efforts should involve a combination of in vivo toxicity assessment of the degraded proteins, development of standardised gluten-free production strategies to minimise variability in gluten fragment presence, guidance on how to control the outcome as well as to develop appropriate reference materials and calibrators.

From grain to malt: Tracking changes of ultra-low-gluten barley storage proteins after malting

Barley (Hordeum vulgare L.) is a major cereal crop produced globally. Hordeins, the major storage proteins in barley, can trigger immune responses leading to celiac disease or symptoms associated with food allergy. Here, proteomics approaches were employed to investigate the proteome level changes of grain and malt from the malting barley cultivar, Sloop, and single-, double- and triple hordein-reduced lines. The triple hordein-reduced line is an ultra-low gluten barley cultivar, Kebari®. Using discovery proteomics, 2,688 and 3,034 proteins in the barley and malt samples were detected respectively. Through the application of targeted proteomics, a significant reduction in the quantity of B-, D-, and γ-hordeins, as well as avenin-like proteins, was observed in the ultra-low gluten malt sample. A compensation mechanism was observed evidenced by increased biosynthesis of seed storage globulins, specifically vicilin-like globulins. Overall, this study has provided insights into protein compositional changes after malting in celiac-friendly barley varieties.

Low Gluten Beers Contain Variable Gluten and Immunogenic Epitope Content

Gluten content labels inform food choice and people practicing a gluten-free diet rely upon them to avoid illness. The regulations differ between jurisdictions, especially concerning fermented foodstuffs such as beer. Gluten abundance is typically measured using ELISAs, which have come into question when testing fermented or hydrolysed foodstuffs such as beer. Mass spectrometry can be used to directly identify gluten peptides and reveal false negatives recorded by ELISA. In this survey of gluten in control and gluten-free beers, gluten protein fragments that contain known immunogenic epitopes were detected using liquid chromatography-mass spectrometry in multiple beers that claim to be gluten-free and have sufficiently low gluten content, as measured by ELISA, to qualify as being gluten-free in some jurisdictions. In fact, several purportedly gluten-free beers showed equivalent or higher hordein content than some of the untreated, control beers. The shortcomings of ELISAs for beer gluten testing are summarised, the mismatch between ELISA and mass spectrometry results are explored, and the suitability of existing regulations as they pertain to the gluten content in fermented foods in different jurisdictions are discussed.

Quantitative TMT-based proteomics revealing host, dietary and microbial proteins in bovine faeces including barley serpin Z4, a prominent component in the head of beer

There has been little information about the proteome of bovine faeces or about the contribution to the faecal proteome of proteins from the host, the feed or the intestinal microbiome. Here, the bovine faecal proteome and the origin of its component proteins was assessed, while also determining the effect of treating barley, the major carbohydrate in the feed, with either ammonia (ATB) or sodium propionate (PTB) preservative. Healthy continental crossbreed steers were allocated to two groups and fed on either of the barley-based diets. Five faecal samples from each group were collected on Day 81 of the trial and analysed by quantitative proteomics using nLC-ESI-MS/MS after tandem mass tag labelling. In total, 281 bovine proteins, 199 barley proteins, 176 bacterial proteins and 190 archaeal proteins were identified in the faeces. Mucosal pentraxin, albumin and digestive enzymes were among bovine proteins identified. Serpin Z4 a protease inhibitor was the most abundant barley protein identified which is also found in barley-based beer, while numerous microbial proteins were identified, many originating bacteria from Clostridium, while Methanobrevibacter was the dominant archaeal genus. Thirty-nine proteins were differentially abundant between groups, the majority being more abundant in the PTB group compared to the ATB group. SIGNIFICANCE: Proteomic examination of faeces is becoming a valuable means to assess the health of the gastro-intestinal tract in several species, but knowledge on the proteins present in bovine faeces is limited. This investigation aimed to characterise the proteome of bovine faecal extracts in order to evaluate the potential for investigations of the proteome as a means to assess the health, disease and welfare of cattle in the future. The investigation was able to identify proteins in bovine faeces that had been (i) produced by the individual cattle, (ii) present in the barley-based feed eaten by the cattle or (iii) produced by bacteria and other microbes in the rumen or intestines. Bovine proteins identified included mucosal pentraxin, serum albumin and a variety of digestive enzymes. Barley proteins found in the faeces included serpin Z4, a protease inhibitor that is also found in beer having survived the brewing process. Bacterial and archaeal proteins in the faecal extracts were related to several pathways related to the metabolism of carbohydrates. The recognition of the range of proteins that can be identified in bovine faeces raises the possibility that non-invasive sample collection of this material could provide a novel diagnostic approach to cattle health and welfare.

Proteomic identification of beer brewing products in the ground layer of Danish Golden Age paintings

The application of mass spectrometry-based proteomics to artworks provides accurate and detailed characterization of protein-based materials used in their production. This is highly valuable to plan conservation strategies and reconstruct the artwork's history. In this work, the proteomic analysis of canvas paintings from the Danish Golden Age led to the confident identification of cereal and yeast proteins in the ground layer. This proteomic profile points to a (by-)product of beer brewing, in agreement with local artists' manuals. The use of this unconventional binder can be connected to the workshops within the Royal Danish Academy of Fine Arts. The mass spectrometric dataset generated from proteomics was also processed with a metabolomics workflow. The spectral matches observed supported the proteomic conclusions, and, in at least one sample, suggested the use of drying oils. These results highlight the value of untargeted proteomics in heritage science, correlating unconventional artistic materials with local culture and practices.

High-level production of Aspergillus niger prolyl endopeptidase from agricultural residue and its application in beer brewing

BACKGROUND

Prolyl endopeptidase from Aspergillus niger (AN-PEP) is a prominent serine proteinase with various potential applications in the food and pharmaceutical industries. However, the availability of efficient and low-cost AN-PEP remains a challenge owing to its low yield and high fermentation cost.

RESULTS

Here, AN-PEP was recombinantly expressed in Trichoderma reesei (rAN-PEP) under the control of the cbh1 promoter and its secretion signal. After 4 days of shaking flask cultivation with the model cellulose Avicel PH101 as the sole carbon source, the extracellular prolyl endopeptidase activity reached up to 16.148 U/mL, which is the highest titer reported to date and the secretion of the enzyme is faster in T. reesei than in other eukaryotic expression systems including A. niger and Komagataella phaffii. Most importantly, when cultivated on the low-cost agricultural residue corn cob, the recombinant strain was found to secret a remarkable amount of rAN-PEP (37.125 U/mL) that is twice the activity under the pure cellulose condition. Furthermore, treatment with rAN-PEP during beer brewing lowered the content of gluten below the ELISA kit detection limit (< 10 mg/kg) and thereby, reduced turbidity, which would be beneficial for improving the non-biological stability of beer.

CONCLUSION

Our research provides a promising approach for industrial production of AN-PEP and other enzymes (proteins) from renewable lignocellulosic biomass, which provides a new idea with relevant researchers for the utilization of agricultural residues.

Addition of Hop (Humulus Lupulus L.) Bitter Acids Yields Modification of Malt Protein Aggregate Profiles during Wort Boiling

Hop bitter acids are used in the brewing industry to give beer bitterness. However, much of this bitterness is lost during processing, specifically during the wort boiling step. One of the major causes might be the interaction with protein-protein complexes. Therefore, the aim of this study was to clarify the role of hop bitter acids in protein aggregate formation using a proteomic approach. The effect of hop addition on protein composition was analyzed by liquid chromatography-mass spectrometry/MS (LC-MS/MS), and further analyses were performed to characterize the wort before and after boiling. Addition of hop bitter acids yielded a change in wort protein profiles, and hop bitter acids were found to bind primarily to less abundant proteins which are not related to beer quality traits, such as foam or haze. Wort protein aggregate profiles were revealed, and findings from this study suggested the precipitation of particular proteins in the aggregates during boiling when hops were added.

A Comprehensive Comparison of Gluten-Free Brewing Techniques: Differences in Gluten Reduction Ability, Analytical Attributes, and Hedonic Perception

This study provides a comprehensive comparison among the most common gluten-free (GF) brewing practices, with a focus on the impact of each treatment on physicochemical parameters and consumer acceptability of the final beer. In addition, the influence of a longer cold maturation on the natural reduction of the gluten content was investigated. Prolyl endopeptidase addition was found to be the most effective treatment in reducing gluten levels (−75.93%), followed by silica gel (−53.09%), longer cold maturation (−4.32%), and tannins (−1.85%). Nonetheless, none of the treated beer samples was gluten-free (gluten content > 20 ppm) due to the high nitrogen content of the original wort. The silica gel application treatment affected the physicochemical and sensory characteristics of the final beer the least. According to the difference from control test results, no significant difference in terms of overall liking, appearance, odor/aroma, or taste was observed between the silica gel-treated sample and control beer (p > 0.05). On the other hand, the application of enzymes and tannins significantly affected the appearance and the beer odor/aroma. Nevertheless, all beer samples received positive sensory acceptance scores.

Application of Mass Spectrometry-Based Proteomics to Barley Research

Barley (Hordeum vulgare) is the fourth most cultivated crop in the world in terms of production volume, and it is also the most important raw material of the malting and brewing industries. Barley belongs to the grass (Poaceae) family and plays an important role in food security and food safety for both humans and livestock. With the global population set to reach 9.7 billion by 2050, but with less available and/or suitable land for agriculture, the use of biotechnology tools in breeding programs are of considerable importance in the quest to meet the growing food gap. Proteomics as a member of the "omics" technologies has become popular for the investigation of proteins in cereal crops and particularly barley and its related products such as malt and beer. This technology has been applied to study how proteins in barley respond to adverse environmental conditions including abiotic and/or biotic stresses, how they are impacted during food processing including malting and brewing, and the presence of proteins implicated in celiac disease. Moreover, proteomics can be used in the future to inform breeding programs that aim to enhance the nutritional value and broaden the application of this crop in new food and beverage products. Mass spectrometry analysis is a valuable tool that, along with genomics and transcriptomics, can inform plant breeding strategies that aim to produce superior barley varieties. In this review, recent studies employing both qualitative and quantitative mass spectrometry approaches are explored with a focus on their application in cultivation, manufacturing, processing, quality, and the safety of barley and its related products.

The post-translational modification landscape of commercial beers

Beer is one of the most popular beverages worldwide. As a product of variable agricultural ingredients and processes, beer has high molecular complexity. We used DIA/SWATH-MS to investigate the proteomic complexity and diversity of 23 commercial Australian beers. While the overall complexity of the beer proteome was modest, with contributions from barley and yeast proteins, we uncovered a very high diversity of post-translational modifications (PTMs), especially proteolysis, glycation, and glycosylation. Proteolysis was widespread throughout barley proteins, but showed clear site-specificity. Oligohexose modifications were common on lysines in barley proteins, consistent with glycation by maltooligosaccharides released from starch during malting or mashing. O-glycosylation consistent with oligomannose was abundant on secreted yeast glycoproteins. We developed and used data analysis pipelines to efficiently extract and quantify site-specific PTMs from SWATH-MS data, and showed incorporating these features into proteomic analyses extended analytical precision. We found that the key differentiator of the beer glyco/proteome was the brewery, with beer from independent breweries having a distinct profile to beer from multinational breweries. Within a given brewery, beer styles also had distinct glyco/proteomes. Targeting our analyses to beers from a single brewery, Newstead Brewing Co., allowed us to identify beer style-specific features of the glyco/proteome. Specifically, we found that proteins in darker beers tended to have low glycation and high proteolysis. Finally, we objectively quantified features of foam formation and stability, and showed that these quality properties correlated with the concentration of abundant surface-active proteins from barley and yeast.

Tracking Gluten throughout Brewing Using N-Terminal Labeling Mass Spectrometry

Gluten-containing grains cause adverse health effects in individuals with celiac disease. Fermentation of these grains results in gluten-derived polypeptides with largely uncharacterized sizes and sequences, which may still trigger an immune response. This research used N-terminal labeling mass spectrometry to characterize protein hydrolysates during each stage of bench-scale brewing, including malting, mashing, boiling, fermentation, and aging. Gluten hydrolysates from each brewing step were tracked, and the immunotoxic potential was evaluated by sequence comparison with peptides known to stimulate celiac immune responses. The results indicate that proteolysis and precipitation of gliadins occurring during brewing differ by protein region and brewing stage. The termini of gliadins were hydrolyzed throughout the entire brewing process, but the central regions remained relatively stable. Most hydrolysis occurred during malting, and most precipitation occurred during boiling. The addition of yeast yielded new cleavage sites but did not result in complete hydrolysis. Consistent detection of peptides within the clinically important regions of gliadin corroborated the hydrolytic resistance of this region. N-terminal labeling mass spectrometry served as a novel approach to track the fate of gliadin/gluten throughout bench-scale brewing. Consistently identified fragments could serve as improved targets for the detection of hydrolyzed gluten in fermented products.

Peptidomics of an industrial gluten-free barley malt beer and its non-gluten-free counterpart: Characterisation and immunogenicity

Recent research suggests that gluten-free beers by prolyl-endopeptidase treatment may not be safe for coeliac disease (CD) patients. Therefore, the gluten peptidome of an industrial gluten-free prolyl-endopeptidase treated malt beer (<10 ppm gluten) was compared to its untreated counterpart (58 ppm gluten) as a reference. NanoLC-HRMS analysis revealed the presence of 155 and 158 gluten peptides in the treated and reference beer, respectively. Characterisation of the peptides in treated beer showed that prolyl-endopeptidase activity was not complete with many peptides containing (multiple) internal proline-residues. Yet, prolyl-endopeptidase treatment did eliminate complete CD-immunogenic motifs, however, 18 peptides still contained partial, and potentially unsafe, motifs. In the reference beer respectively 7 and 37 gluten peptides carried (multiple) complete and/or partial CD-immunogenic motifs. Worrying is that many of these partial immunogenic gluten peptides do not contain a recognition epitope for the R5-antibody and would be overlooked in the current ELISA analysis for gluten quantification.

Benchtop micro-mashing: high-throughput, robust, experimental beer brewing

Brewing science is undergoing a renaissance with the use of modern analytical chemistry and microbiology techniques. However, these modern analytical tools and techniques are not necessarily aligned with the scale and scope of brewing science. In particular, brewing processes can be time consuming, ingredient intensive, and require specialised technical equipment. These drawbacks compound with the need for appropriate numbers of replicates for adequately powered experimental design. Here, we describe a micro-scale mash method that can be performed using a common laboratory benchtop shaker/incubator, allowing for high throughput mashing and easy sample replication for statistical analysis. Proteomic profiles at both the protein and peptide levels were consistent between the 1 mL micro-mash and a 23 L Braumeister mash, and both mash scales produced wort with equivalent fermentable sugar and free amino acid profiles. The experimental flexibility offered by our micro-mash method allowed us to investigate the effects of altered mash parameters on the beer brewing proteome.

Compositional and immunogenic evaluation of fractionated wheat beers using mass spectrometry

The safety and regulatory status of fermented products derived from gluten-containing grains for patients with celiac disease remains controversial. Bottom-up mass spectrometry (MS) has complemented immunoassays for the compositional and immunogenic analyses of wheat beers. However, uncharacterized proteolysis during brewing followed by the secondary digestion for MS has made the analysis and data interpretation complicated. In this study, the composition and immunogenic potential of seven commercially available wheat beers were evaluated using bottom-up MS with the aid of fractionation and a multi-step peptide search strategy to identify peptides generated by various types of proteolysis. Gluten-derived peptides accounted for approximately 50% and 20% of the total number of wheat-derived and barley-derived peptides, respectively, in the investigated beers. Although relatively large polypeptides cannot be thoroughly characterized using traditional bottom-up proteomics, up to 50% of peptides identified contained celiac-immunogenic motifs, and consumption of wheat beers would pose risks for celiac patients.

Immunogenic Potential of Beer Types Brewed With Hordeum and Triticum spp. Malt Disclosed by Proteomics

The protein/peptide composition of five beer kinds, including two experimental beer-like products brewed with einkorn (Triticum monococcum), a beer labeled as “gluten-free,” a traditional all-barley malt and a wheat (T. aestivum) containing beer, was characterized with HPLC-ESI MS/MS-based proteomics. To enlarge the characterization of the components, the polypeptides were fractionated according to their molecular size (cut-off 6 kDa). All the beer types contained a variety of polypeptides arising from all the gliadin subfamilies (α-/β-, γ-, and ω-gliadins) able to induce an immune response in celiac disease (CD) patients in addition to a panel of IgE-reactive food allergens. Wheat storage proteins were heavily hydrolyzed in the beer samples brewed with einkorn. The presence of gluten-like fragments, also including the 25-mer and 33-mer-like of α-gliadin, was confirmed in beer brewed with barley and wheat malt as well as in the gluten-free beer. Both CD-toxic and allergenic peptides of all beer samples were drastically degraded when subjected to a simulated gastroduodenal (GD) digestion. After in vitro digestion, the level of gluten-like peptides assayed with the G12 competitive ELISA, was below the threshold (20 ppm) for a food to be considered as “gluten-free.” A few gliadin-derived epitopes occurred in the digests of beers crafted with wheat or Norberto-ID331 line of einkorn. In contrast, digests of all barley malt and gluten-free beers did not contain detectable gluten-like epitopes, but only minor fragments of hordeins and IgE-reactive food allergens. All beer samples evoked a weak immune response on gliadin-reactive celiac T cells isolated from intestinal biopsies of celiac patients. Compared to undigested polypeptides, the response was markedly reduced by GD digestion. Although the consumption of a moderate amount of beer brewed with barley or einkorn could deliver a relatively low amount of CD-toxic epitopes, the findings of this study emphasize the urgent need of a reliable and accurate quantification of gluten epitopes in all types of beer, also including the gluten-free one, to compute realistically the contribution of beer to the overall gluten intake, which can be responsible of intestinal tissue damages in celiacs.

Mapping Coeliac Toxic Motifs in the Prolamin Seed Storage Proteins of Barley, Rye, and Oats Using a Curated Sequence Database

Wheat gluten, and related prolamin proteins in rye, barley and oats cause the immune-mediated gluten intolerance syndrome, coeliac disease. Foods labelled as gluten-free which can be safely consumed by coeliac patients, must not contain gluten above a level of 20 mg/Kg. Current immunoassay methods for detection of gluten can give conflicting results and may underestimate levels of gluten in foods. Mass spectrometry methods have great potential as an orthogonal method, but require curated protein sequence databases to support method development. The GluPro database has been updated to include avenin-like sequences from bread wheat (n = 685; GluPro v1.1) and genes from the sequenced wheat genome (n = 699; GluPro v 1.2) and Triticum turgidum ssp durum (n = 210; GluPro v 2.1). Companion databases have been developed for prolamin sequences from barley (n = 64; GluPro v 3.0), rye (n = 41; GluPro v 4.0), and oats (n = 27; GluPro v 5.0) and combined to provide a complete cereal prolamin database, GluPro v 6.1 comprising 1,041 sequences. Analysis of the coeliac toxic motifs in the curated sequences showed that they were absent from the minor avenin-like proteins in bread and durum wheat and barley, unlike the related avenin proteins from oats. A comparison of prolamin proteins from the different cereal species also showed α- and γ-gliadins in bread and durum wheat, and the sulphur poor prolamins in all cereals had the highest density of coeliac toxic motifs. Analysis of ion-mobility mass spectrometry data for bread wheat (cvs Chinese Spring and Hereward) showed an increased number of identifications when using the GluPro v1.0, 1.1 and 1.2 databases compared to the limited number of verified sequences bread wheat sequences in reviewed UniProt. This family of databases will provide a basis for proteomic profiling of gluten proteins from all the gluten containing cereals and support identification of specific peptide markers for use in development of new methods for gluten quantitation based on coeliac toxic motifs found in all relevant cereal species.

In Celiac Disease Patients the In Vivo Challenge with the Diploid Triticum monococcum Elicits a Reduced Immune Response Compared to Hexaploid Wheat

SCOPE

Gluten from the diploid wheat Triticum monococcum (TM) has low content of immunostimulatory sequences and a high gastro-intestinal digestibility. Gluten-reactive T cells elicited by diploid and hexaploid (Triticum aestivum-TA) wheat in celiac disease (CD) patients upon a brief oral challenge are analyzed.

METHODS AND RESULTS

Seventeen patients with CD (median age 13 years) consumed for 3 days sandwiches made with TM (cultivar Norberto-ID331, N=11), or TA (cultivar Sagittario, N=11) flours, corresponding to 12 gr of gluten/die. Immunostimulatory properties are assessed in blood by measuring the IFN-γ-secreting T cells by EliSpot and the expression of inflammatory cytokines/receptors (IL-12A, IL-15, IL-18RAP, IFN-γ) by qPCR. TA mobilizes a remarkable number of gliadin-specific, IFN-γ-secreting T cells (p<0.05), while no significant cell mobilization is induced by TM (p=ns). Similar results are obtained in response to five immunogenic peptides from α-, ω-, and γ-gliadins, although with a large individual variability. An increased mRNA expression for IL-12A and IFN-γ is detected in the group eating TA compared to those consuming TM (p<0.05).

CONCLUSIONS

Although T. monococcum is a cereal not suitable for the diet of celiacs, this diploid wheat elicits a reduced in vivo T-cell response compared to T. aestivum in celiac patients.

Characterization and Relative Quantitation of Wheat, Rye, and Barley Gluten Protein Types by Liquid Chromatography-Tandem Mass Spectrometry

The consumption of wheat, rye, and barley may cause adverse reactions to wheat such as celiac disease, non-celiac gluten/wheat sensitivity, or wheat allergy. The storage proteins (gluten) are known as major triggers, but also other functional protein groups such as α-amylase/trypsin-inhibitors or enzymes are possibly harmful for people suffering of adverse reactions to wheat. Gluten is widely used as a collective term for the complex protein mixture of wheat, rye or barley and can be subdivided into the following gluten protein types (GPTs): α-gliadins, γ-gliadins, ω5-gliadins, ω1,2-gliadins, high- and low-molecular-weight glutenin subunits of wheat, ω-secalins, high-molecular-weight secalins, γ-75k-secalins and γ-40k-secalins of rye, and C-hordeins, γ-hordeins, B-hordeins, and D-hordeins of barley. GPTs isolated from the flours are useful as reference materials for clinical studies, diagnostics or in food analyses and to elucidate disease mechanisms. A combined strategy of protein separation according to solubility followed by preparative reversed-phase high-performance liquid chromatography was employed to purify the GPTs according to hydrophobicity. Due to the heterogeneity of gluten proteins and their partly polymeric nature, it is a challenge to obtain highly purified GPTs with only one protein group. Therefore, it is essential to characterize and identify the proteins and their proportions in each GPT. In this study, the complexity of gluten from wheat, rye, and barley was demonstrated by identification of the individual proteins employing an undirected proteomics strategy involving liquid chromatography-tandem mass spectrometry of tryptic and chymotryptic hydrolysates of the GPTs. Different protein groups were obtained and the relative composition of the GPTs was revealed. Multiple reaction monitoring liquid chromatography-tandem mass spectrometry was used for the relative quantitation of the most abundant gluten proteins. These analyses also allowed the identification of known wheat allergens and celiac disease-active peptides. Combined with functional assays, these findings may shed light on the mechanisms of gluten/wheat-related disorders and may be useful to characterize reference materials for analytical or diagnostic assays more precisely.

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.

Detection and Quantitation of Gluten in Fermented-Hydrolyzed Foods by Antibody-Based Methods: Challenges, Progress, and a Potential Path Forward

Celiac disease (CD) affects ~1 in 141 individuals in the United States, requiring adherence to a strict gluten-free diet. The Codex Standard and the European Commission states that gluten level of gluten-free foods must not exceed 20 ppm. The FDA requires food bearing the labeling claim “gluten-free” to contain <20 ppm gluten. Accurate quantitation of gluten in fermented-hydrolyzed foods by antibody-based methods is a challenge due to the lack of appropriate reference materials and variable proteolysis. The recent uses of proteases (e.g., proline endopeptidases or PEP) to hydrolyze immunopathogenic sequences of gluten proteins further complicates the quantitation of immunopathogenic gluten. The commercially available antibody-based methods routinely used to detect and quantitate gluten are not able to distinguish between different hydrolytic patterns arising from differences in fermentation processes. This is a severe limitation that makes accurate quantitation and, ultimately, a detailed evaluation of any potential health risk associated with consuming the food difficult. Utilizing gluten-specific antibodies, a recently developed multiplex-competitive ELISA along with western blot analysis provides a potential path forward in this direction. These complimentary antibody-based technologies provide insight into the extent of proteolysis resulting from various fermentation processes and have the potential to aid in the selection of appropriate hydrolytic calibration standards, leading to accurate gluten quantitation in fermented-hydrolyzed foods.

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.

Hordein Accumulation in Developing Barley Grains

The temporal pattern of accumulation of hordein storage proteins in developing barley grains was studied by enzyme-linked immunosorbent assay (ELISA), western blot and liquid chromatography tandem mass spectrometry (LC-MS/MS). Hordein accumulation was compared to the pattern seen for two abundant control proteins, serpin Z4 (an early accumulator) and lipid transferase protein (LTP1, a late accumulator). Hordeins were detected from 6 days post-anthesis (DPA) and peaked at 30 DPA. Changes in fresh weight indicate that desiccation begins at 20 DPA and by 37 DPA fresh weight had decreased by 35%. ELISA analysis of hordein content, expressed on a protein basis, increased to a maximum at 30 DPA followed by a 17% decrease by 37 DPA. The accumulation of 39 tryptic and 29 chymotryptic hordein peptides representing all classes of hordein was studied by LC-MS/MS. Most peptides increased to a maximum at 30 DPA, and either remained at the maximum or did not decrease significantly. Only five tryptic peptides, members of the related B1- and γ1-hordeins decreased significantly by 21-51% at 37 DPA. Thus, the concentration of some specific peptides was reduced while remaining members of the same family were not affected. The N-terminal signal region was removed by proteolysis during co-translation. In addition to a suite of previously characterized hordeins, two novel barley B-hordein isoforms mapping to wheat low molecular weight glutenins (LMW-GS-like B-hordeins), and two avenin-like proteins (ALPs) sharing homology with wheat ALPs, were identified. These identified isoforms have not previously been mapped in the barley genome. Cereal storage proteins provide significant nutritional content for human consumption and seed germination. In barley, the bulk of the storage proteins comprise the hordein family and the final hordein concentration affects the quality of baked and brewed products. It is therefore important to study the accumulation of hordeins as this knowledge may assist plant breeding for improved health outcomes (by minimizing triggering of detrimental immune responses), nutrition and food processing properties.

Proteomic profiling of barley spent grains guides enzymatic solubilization of the remaining proteins

Within the brewing industry, malted barley is being increasingly replaced by raw barley supplemented with exogenous enzymes to lessen reliance on the time-consuming, high water and energy cost of malting. Regardless of the initial grain of choice, malted or raw, the resultant bulk spent grains are rich in proteins (up to 25% dry weight). Efficient enzymatic solubilization of these proteins requires knowledge of the protein composition within the spent grains. Therefore, a comprehensive proteomic profiling was performed on spent grains derived from (i) malted barley (spent grain A, SGA) and (ii) enzymatically treated raw barley (spent grain B, SGB); data are available via ProteomeXchange with identifier PXD008090. Results from complementary shotgun proteomics and 2D gel electrophoresis showed that the most abundant proteins in both spent grains were storage proteins (hordeins and embryo globulins); these were present at an average of two fold higher in spent grain B. Quantities of other major proteins were generally consistent in both spent grains A and B. Subsequent in silico protein sequence analysis of the predominant proteins facilitated knowledge-based protease selection to enhance spent grain solubilization. Among tested proteases, Alcalase 2.4 L digestion resulted in the highest remaining protein solubilization with 9.2 and 11.7% net dry weight loss in SGA and SGB respectively within 2 h. Thus, Alcalase alone can significantly reduce spent grain side stream, which makes it a possible solution to increase the value of this low-value side stream from the brewing and malt extract beverage manufacturing industry.

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.

Liquid Chromatography-Mass Spectrometry Analysis Reveals Hydrolyzed Gluten in Beers Crafted To Remove Gluten

During brewing, gluten proteins may be solubilized, modified, complexed, hydrolyzed, and/or precipitate. Gluten fragments that persist in conventional beers render them unsuitable for people with celiac disease (CD) or gluten intolerance. Barley-based beers crafted to remove gluten using proprietary precipitation and/or application of enzymes, e.g. prolyl endopeptidases (PEP) that degrade the proline-rich gluten molecules, are available commercially. Gluten measurement in fermented products remains controversial. The industry standard, a competitive ELISA, may indicate gluten values <20 mg/kg, which is deemed safe for people with CD. However, in this study, liquid chromatography-mass spectrometry analyses revealed gluten peptides derived from hydrolyzed fragments, many >30 kDa in size. Barley gluten (hordeins) were detected in all beers analyzed with peptides representing all hordein classes detected in conventional beers but also, alarmingly, in many gluten-reduced beers. It is evident that PEP digestion was incomplete in several commercial beers, and peptides comprising missed cleavages were identified, warranting further optimization of PEP application in an industrial setting.

A multiplex competitive ELISA for the detection and characterization of gluten in fermented-hydrolyzed foods

A novel competitive ELISA was developed utilizing the G12, R5, 2D4, MIoBS, and Skerritt antibody-HRP conjugates employed in nine commercial ELISA test kits that are routinely used for gluten detection. This novel multiplex competitive ELISA simultaneously measures gliadin-, deamidated gliadin-, and glutenin-specific epitopes. The assay was used to evaluate 20 wheat beers, 20 barley beers, 6 barley beers processed to reduce gluten, 15 soy sauces, 6 teriyaki sauces, 6 Worcestershire sauces, 6 vinegars, and 8 sourdough breads. For wheat beers, the apparent gluten concentration values obtained by the G12 and Skerritt antibodies were typically higher than those obtained using the R5 antibodies. The sourdough bread samples resulted in higher apparent gluten concentration values with the Skerritt antibody, while the values generated by the G12 and R5 antibodies were comparable. Although the soy-based sauces showed non-specific inhibition with the multiple R5 and G12 antibodies, their overall profile was distinguishable from the other categories of fermented foods. Cluster analysis of the apparent gluten concentration values obtained by the multiplex competitive ELISA, as well as the relative response of the nine gluten-specific antibodies used in the assay to different gluten proteins/peptides, distinguishes among the different categories of fermented-hydrolyzed foods by recognizing the differences in the protein/peptide profiles characteristic of each product. This novel gluten-based multiplex competitive ELISA provides insight into the extent of proteolysis resulting from various fermentation processes, which is essential for accurate gluten quantification in fermented-hydrolyzed foods. Graphical abstract A novel multiplex competitive ELISA for the detection and characterization of gluten in fermented-hydrolyzed foods.

Food for thought: Selecting the right enzyme for the digestion of gluten

Gluten describes a complex mixture of proteins found in wheat, rye, barley and oats that pose a health risk to people affected by conditions such as coeliac disease and non-coeliac gluten sensitivity. Complete digestion of gluten proteins is of critical importance during quantitative analysis. To this end, chymotrypsin was investigated for its ability to efficiently and reproducibly digest specific classes of gluten in barley. Using proteomics a chymotryptic peptide marker panel was elucidated and subjected to relative quantification using LC-MRM-MS. Thorough investigation of peptide markers revealed robust and reproducible quantification with CVs <15% was possible, however a greater proportion of non-specific cleavage variants were observed relative to trypsin. The selected peptide markers were assessed to ensure their efficient liberation from their parent proteins. While trypsin remains the preferred enzyme for quantification of the avenin-like A proteins, the B-, D- and γ-hordeins, chymotrypsin was the enzyme of choice for the C-hordeins.

Comparison of Gluten Extraction Protocols Assessed by LC-MS/MS Analysis

The efficiency of gluten extraction is of critical importance to the results derived from any analytical method for gluten detection and quantitation, whether it employs reagent-based technology (antibodies) or analytical instrumentation (mass spectrometry). If the target proteins are not efficiently extracted, the end result will be an under-estimation in the gluten content posing a health risk to people affected by conditions such as celiac disease (CD) and nonceliac gluten sensitivity (NCGS). Five different extraction protocols were investigated using LC-MRM-MS for their ability to efficiently and reproducibly extract gluten. The rapid and simple "IPA/DTT" protocol and related "two-step" protocol were enriched for gluten proteins, 55/86% (trypsin/chymotrypsin) and 41/68% of all protein identifications, respectively, with both methods showing high reproducibility (CV < 15%). When using multistep protocols, it was critical to examine all fractions, as coextraction of proteins occurred across fractions, with significant levels of proteins existing in unexpected fractions and not all proteins within a particular gluten class behaving the same.

Proposal for C-Hordein as Reference Material in Gluten Quantification

The concentration of residual barley prolamin (hordein) in gluten-free products is overestimated by the R5 ELISA method when calibrated against the wheat gliadin standard. The reason for this may be that the composition of the gliadin standard is different from the composition of hordeins. This study showed that the recognition of whole hordein by R5 antibody mainly came from C-hordein, which is more reactive than the other hordeins. The proportion of C-hordein in total hordein ranged from 16 to 33% of common Finnish barley cultivars used in this study and was always higher than that of ω-gliadin, the homologous protein class in the gliadin standard, which may account for the overestimation. Thus, a hordein standard is needed for barley prolamin quantification instead of the gliadin standard. When gluten-free oat flour was spiked with barley flour, the prolamin concentration was overestimated 1.8-2.5 times with the gliadin standard, whereas estimates in the correct range were obtained when the standard was 40% C-hordein mixed with an inert protein. A preparative-scale method was developed to isolate and purify C-hordein, and C-hordein is proposed as a reference material to calibrate barley prolamin quantification in R5-based assays.

Isolation and characterization of gluten protein types from wheat, rye, barley and oats for use as reference materials

Gluten proteins from wheat, rye, barley and, in rare cases, oats, are responsible for triggering hypersensitivity reactions such as celiac disease, non-celiac gluten sensitivity and wheat allergy. Well-defined reference materials (RM) are essential for clinical studies, diagnostics, elucidation of disease mechanisms and food analyses to ensure the safety of gluten-free foods. Various RM are currently used, but a thorough characterization of the gluten source, content and composition is often missing. However, this characterization is essential due to the complexity and heterogeneity of gluten to avoid ambiguous results caused by differences in the RM used. A comprehensive strategy to isolate gluten protein fractions and gluten protein types (GPT) from wheat, rye, barley and oat flours was developed to obtain well-defined RM for clinical assays and gluten-free compliance testing. All isolated GPT (ω5-gliadins, ω1,2-gliadins, α-gliadins, γ-gliadins and high- and low-molecular-weight glutenin subunits from wheat, ω-secalins, γ-75k-secalins, γ-40k-secalins and high-molecular-weight secalins from rye, C-hordeins, γ-hordeins, B-hordeins and D-hordeins from barley and avenins from oats) were fully characterized using analytical reversed-phase high-performance liquid chromatography (RP-HPLC), sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), N-terminal sequencing, electrospray-ionization quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS) and untargeted LC-MS/MS of chymotryptic hydrolyzates of the single GPT. Taken together, the analytical methods confirmed that all GPT were reproducibly isolated in high purity from the flours and were suitable to be used as RM, e.g., for calibration of LC-MS/MS methods or enzyme-linked immunosorbent assays (ELISAs).

Comparing Multiple Reaction Monitoring and Sequential Window Acquisition of All Theoretical Mass Spectra for the Relative Quantification of Barley Gluten in Selectively Bred Barley Lines

Celiac disease (CD) is a disease of the small intestine that occurs in genetically susceptible subjects triggered by the ingestion of cereal gluten proteins for which the only treatment is strict adherence to a life-long gluten-free diet. Barley contains four gluten protein families, and the existence of barley genotypes that do not accumulate the B-, C-, and D-hordeins paved the way for the development of an ultralow gluten phenotype. Using conventional breeding strategies, three null mutations behaving as recessive alleles were combined to create a hordein triple-null barley variety. Proteomics has become an invaluable tool for characterization and quantification of the protein complement of cereal grains. In this study multiple reaction monitoring (MRM) mass spectrometry, viewed as the gold standard for peptide quantification, was compared to the data-independent acquisition strategy known as SWATH-MS (sequential window acquisition of all theoretical mass spectra). SWATH-MS was comparable (p < 0.001) to MRM-MS for 32/33 peptides assessed across the four families of hordeins (gluten) in eight barley lines. The results of SWATH-MS analysis further confirmed the absence of the B-, C-, and D-hordeins in the triple-null barley line and showed significantly reduced levels ranging from <1% to 16% relative to wild-type (WT) cv Sloop for the minor γ-hordein class. SWATH-MS represents a valuable tool for quantitative proteomics based on its ability to generate reproducible data comparable with MRM-MS, but has the added benefits of allowing reinterrogation of data to improve analytical performance, ask new questions, and in this case perform quantification of trypsin-resistant proteins (C-hordeins) through analysis of their semi- or nontryptic fragments.