The art of brewing alcoholic beverages has existed for thousands of years. The process of beer brewing begins with barley grains, which are malted to allow partial germination, triggering expression of key enzymes. The germinated grains are then dried and milled. Next, starch, proteins, and other molecules are solubilized during mashing. During mashing, solubilized enzymes degrade starch to fermentable sugars, and digest proteins to produce peptides and free amino acids. Fermentable sugars and free amino acids are required for efficient yeast growth during fermentation.
After the mash, the wort is removed, and hops are added for bitterness and aroma, and the wort is boiled. After boiling, the wort is inoculated with yeast, and fermentation proceeds to produce bright beer. Typically this bright beer is then filtered, carbonated, packaged, and sold. Many proteins originating from the barley grain and the yeast are present in beer, and these have been reported to affect the quality of the final product. However, some of the biochemical details of this process remain unclear. To better understand what happens during the various steps of the brewing process, Schultz et al. used mass spectrometry proteomics to perform a global untargeted analysis of the proteins present across time during beer production and described this work in a recent paper (1). Samples analyzed included sweet wort produced by a high temperature infusion mash, hopped wort, and bright beer.
Beer and wort are complex mixtures of soluble and suspended proteins, carbohydrates, and small organic molecules, and proteomic analysis requires that the proteins and peptides are efficiently separated from these other components before analysis. For this work, the researchers separated the proteins in these samples from other components by TCA precipitation, and then further pretreated the samples for MS analysis by denaturation, reduction/alkylation and digestion to peptides by trypsin, and desalting. This process identified 210 unique quantifiable proteins from barley and yeast. Proteins identified were predominantly from barley including lipid transfer proteins, hordeins, and α-amylase inhibitors. Many yeast proteins were identified in bright beer samples, including secreted cell wall enzymes and abundant intracellular metabolic enzymes.
Using this method, the Schultz et al. 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.
1. Schultz, B. et al. (2018) Process Proteomics of Beer Reveals a Dynamic Proteome with Extensive Modifications. J. Proteome Res. 17, 1647–53.
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