INVESTIGATION OF HORDEINS DURING BREWING AND THEIR INFLUENCE ON BEER HAZE BY PROTEOME ANALYSIS

Haze in Beer: Its Formation and Alleviating Strategies, from a Protein-Polyphenol Complex Angle

Beer is one of the oldest and most widely consumed alcoholic beverages. Haze formation in beer is a serious quality problem, as it largely shortens the shelf life and flavor of beer. This paper reviews the factors affecting haze formation and strategies for reducing haze. Haze formation is mainly associated with specific chemical components in malt barley grains, such as proteins. The main factor causing haze formation is a cross-linking of haze active (HA) proteins and HA polyphenols. Many HA proteins and their editing genes or loci have been identified by proteomics and quantitative trait locus (QTL) analysis, respectively. Although some technical approaches have been available for reducing haze formation in beer, including silica and polyvinylpolypyrrolidone (PVPP) adsorbent treatments, the cost of beer production will increase and some flavor will be lost due to reduced relevant polyphenols and proteins. Therefore, breeding the malt barley cultivar with lower HA protein and/or HA polyphenols is the most efficient approach for controlling haze formation. Owing to the completion of barley whole genome sequencing and the rapid development of modern molecular breeding technology, several candidate genes controlling haze formation have been identified, providing a new solution for reducing beer haze.

Proteome-Based Analysis of Colloidal Instability Enables the Detection of Haze-Active Proteins in Beer

Colloidal haze is a serious quality defect of bright beers that considerably reduces their shelf life and is thought to be triggered by hordeins, a class of proline-rich barley proteins. In this work, the proteomes of fresh and old beers were investigated in bottled pilsners and compared to the protein inventory of haze to identify specific haze-active proteins. Haze isolates dissolved in rehydration buffer contained high concentrations of proteins and sugars but provided protein gels with weak spot signals. Consequently, a treatment for the chemical deglycation with trifluoromethanesulfonic acid was applied, which resulted in the identification of protein Z4, LTP1, CMb, CMe, pUP13, 3a, and Bwiph as constituents of the haze proteome. Because only one hordein was detectable and the proline content in haze hydrolysates was lower than those of barley prolamins, our results suggest that this class of proteins is of minor importance for haze development.

Haze activity of different barley trypsin inhibitors of the chloroform/methanol type (BTI-CMe)

Our previous study found that the critical protein in SE (silica eluted) proteins is BTI-CMe, and assumed that SE-ve malt for brewing may improve the haze stability in beer. In this study, we investigated the difference in gene sequence and corresponding amino acid sequence of BTI-CMe between SE+ve and SE-ve types. The results showed that there were 7 amino acid differences between Yerong (SE-ve) and Franklin (SE+ve). Two types BTI-CMe were expressed in vitro and purified successfully. By adding the purified BTI-CMe into commercial beer, we found that both original turbidity and alcohol chill haze degree of beer were increased. BTI-CMe of SE-ve haplotype showed a lower level of haze formation in beer than SE+ve haplotype. Response surface methodology (RSM) was conducted to determine the relationship between BTI-CMe and tannic acid, and their effects on haze formation. It was found that (1) higher content of BTI-CMe and/or tannic acid in beer would give rise to higher turbidity; (2) there was a significant interaction between BTI-CMe and tannic acid; (3) haze activity disparity of BTI-CMe between two types was significantly and positively correlated with the tannic acid concentration.

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.

The B-hordein prolamin family of barley

The spectrum of B-hordein prolamins and genes in the single barley cultivar Barke is described from an in silico analysis of 1452 B-hordein ESTs and available genomic DNA. Eleven unique B-hordein proteins are derived from EST contigs. Ten contigs encode apparent full-length B-hordeins and the eleventh contains a premature stop codon that will lead to a truncated B-hordein. The 11 sequences are placed within the two previously described classes, i.e., the B1- and B3-type B-hordeins. The number of ESTs assigned to each sequence is used as an estimate of relative gene transcription and expression. Three of the sequences account for 79% of the total ESTs, with one sequence comprises 32% of the total ESTs and has a variant C-terminus caused by an undefined sequence change history near the 3' coding terminus. The 70× difference in EST distribution among sequences points to the importance of understanding differential rates of expression within closely related gene families. Analysis of available genomic sequences confirms the EST assembly and reveals one full-length and two partial sequences of pseudogenes as evidenced by no matching ESTs for the sequences and premature stop codons and frame shifts.