Malting Sorghum with Aspergillus Oryzae Enhances Gluten-Free Wort Yield and Extract

Abstract

Production of gluten-free beers is growing in the international market as an alternative for celiacs. In this investigation, white sorghum was malted conventionally and after inoculation of soaked kernels with 1 or 2% (w/w) koji (Aspergillus oryzae) for four days. The resulting sorghum diastatic malts were compared with barley malt and characterized in terms of α-amylase, β-amylase, and amyloglucosidase activities and employed to produce sweet worts following the double mashing procedure. Yields or wort extracts, fermentable carbohydrates, and free amino nitrogen (FAN) characterizations were assessed. The addition of 1% (w/w) A. oryzae to sorghum kernels before malting did not affect germination capacity (97.3%), whereas inoculation with 2% (w/w) resulted in about 5% less germination. At the end of the five-day malting procedure, the malts had similar dry matter losses. The α-amylase activity of malts was positively affected by the addition of A. oryzae, whereas the β-amylase was unaffected. Amyloglucosidase activity of malts was positively or negatively affected, respectively, by the addition of 1 or 2% koji. Worts produced from the sorghum malt inoculated with 1% koji generated 27% more fermentable carbohydrates and 24% more FAN after the double mashing procedure. More importantly, 1% koji inoculation malt only generated 8.8% less fermentable sugars compared with the barley malt. The major difference between the barley wort and the different sorghum worts was in maltose concentration, which clearly favored the barley wort. The sorghum malt inoculated with 2% koji did not further increase yield of fermentable sugars. This research clearly indicates that the inoculation of 1% koji during the malting of sorghum is advantageous in terms of diastatic activity and generation of fermentable carbohydrate in worts produced following the double mashing procedure. The regular sorghum and the sorghum malt + 1% koji produced 12°P worts with 40 and 21% less fermentable sugars compared with the barley control wort. Therefore, the supplementation of the sorghum malt with koji has potential to increase extracts and fermentable sugars in gluten-free worts.