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A High-Maltose Broth Method for Studying the Effects of Amino Acids on Fermentability1

Brewery fermentations require sufficient yeast growth during fermentation to achieve the timelines and beer quality expected. It is only possible to obtain the necessary growth when yeast are provided with an adequate supply of all nutrients, including amino acids, minerals, and fermentable sugars. However, a broth made from 100% high-maltose syrup (HMS), and thus containing no micronutrients, showed excellent fermentability under standard laboratory conditions (Congress wort specific gravity, high pitching rates, and continuous stirring), indicating no requirement for micronutrients under these conditions. The aim of this study was to develop a lab-scale fermentation test, based on reduced pitching rates and use of adjunct sugars, that would indicate the need for micronutrients during fermentation. Malts from three barley varieties and of varying quality were used to investigate the effectiveness of the test. Amino acid levels in worts and fermented worts were measured using ultra-performance liquid chromatography. Effects of amino acids on fermentability were most apparent when a pitching rate of 0.45 g of compressed yeast per 100 mL of broth and a broth with a 40:60 ratio of HMS to Congress wort were used. At least 1,000 mg of amino acids per L was necessary to completely ferment an 8.5°P broth. Individual amino acids were absorbed in the order expected, with the exception of glutamine, a type A amino acid, which was absorbed at a slower rate than the type B amino acids histidine and methionine. The broth method also found that proline was absorbed even under standard fermentation conditions once other amino acids had been depleted.