Analysis of the Gluten Map in Beers by LC-MS/MS Combined with Optimized Extraction Protocol

Gluten, which accounts for 70%-80% of the total protein content of wheat, is the main family of storage proteins found in the starchy endosperm of Pooideae subfamily, such as wheat, barley, rye and oats. As described in previous research, it could be divided into two fractions based on their different solubility in aqueous alcohol. Alcohol-soluble proteins of cereal grains that are rich in proline (Pro) and glutamine (Gln) are termed prolamins. The word of prolamins is a collective name of alcohol-soluble, water-insoluble storage proteins from wheat (gliadin), barley (hordein), rye (secalin), and oat (avenin). The alcohol-insoluble fraction of wheat gluten is called glutenin and the corresponding fractions from other species glutenins. Celiac disease (CD) is defined as a chronic Tcell mediated enteropathy caused by exposure to dietary gluten in genetically predisposed individuals, which account for approximately 1% of the global population. Clinical symptoms of CD can vary of patients and may include intestinal symptoms of diarrhea or steatorrhea, as well as extraintestinal symptoms, such as bone pain or osteoporosis. The only effective treatment for individuals with CD is lifelong adherence to strict gluten-free diet. Beer is a popular alcohol beverage fermented with cereals such as barley and wheat which contains antigenic proteins of CD. Gluten-free beer is also specially provided by manufacturers for CD sufferers. During the whole process of beer manufacturing, hordeins and other storage proteins are degraded by proteases which are activated during malting. The high temperature during mashing may also cause the modification of gluten proteins. So the gluten proteins can be degraded to produce soluble polypeptides and free amino acids, and the epitopes used for the ELISA protocol are also damaged, which hinders the accurate detection of gluten proteins by ELISA protocols. In this work, 0.1 mol/L ammonium acetate in methanol and acetone was selected as the optimized solution to individually obtain alcohol-insoluble and alcohol-soluble fractions of gluten in beer samples to maximally acquire the gluten information. The reproducibility of the optimized gluten extraction protocol was proved to be good. Deamidated gluten peptides and gluten fragments were proved to be present in beer. Then, the gluten map of eight commercial beers was explored using optimized gluten extraction protocol followed by liquid chromatography-mass spectrometry (LC-MS/MS) analysis. It was confirmed that combining both the protein fraction and the peptide fraction could significantly enhance the gluten protein identifications. Gluten proteins were identified in three fractions for all types of beer even in gluten-free beer. Mass spectrometry may be the supplemented method to ELISA methods to confirm the safety of gluten-free beers.