Characterisation and improvement of the quality of Rwandese traditional beer « ikigage » made from sorghum

Abstract

Ikigage is a Rwandese traditional beer made from sorghum malt and local plants, mainly Vernonia amygdalina “umubirizi”. However, this beer remains less attractive than Western beers of pils type because of poor hygienic quality, variations of organoleptic quality and limited shelf life. The aim of this work is to characterize ikigage beer in order to improve its hygienic quality and to reduce the organoleptic variations using the local raw materials. The first part of our study shows that ikigage beer marketed in Rwanda is characterized by the presence of Stapylococcus aureus and many micro-organisms of fecal origin (Escherichia coli and fecal streptococci), by the low ethanol content, high total acidity and an important amounts of proteins. The yeasts (Saccharomyces cerevisiae and Issatckenkia orientalis), followed by the lactic acid bacteria (Lactobacillus fermentum and Lactobacillus buchneri), are the predominant microorganisms involved in fermentation of ikigage beer fermentation. Besides, one of the major problems in sorghum beer brewing is the efficient conversion of the starch extracts into fermentable sugars due to the weak activity of β-amylase in sorghum malt. The use of Eleusine coracana “uburo” (Musama variety) malt (30%), associated with mashing decantation procedure, increases the content of sugar fermentable, maltose particularly, in sorghum wort, and consequently ethanol content in the beer. The second part of our study shows that V. amygdalina, known for its antibacterial properties and its bitterness similar to hops, contributes to the production of methyl salycilate, beta-damascenone and many terpenes compounds (δ-3-carene, β-farnesene, farnesol, β-citronellol and linalool), in the sorghum beer “ikigage”. This work reveals also the presence of 14 polyfunctional thiols in beers brewed with ‘non-Western’ raw materials. Among them, the well-known hop constituent 3-methyl-2-buten-1-thiol emerged as a key flavour in the unhopped beers containing V. amygdalina. V. amygdalina also contributes to the production of 1-butanethiol and 4-sulfanyl-4-methyl-2-pentanone in sorghum beer. However, contrary to hops, V. amygdalina addition during boiling also appears to strongly inhibit the production of 2-sulfanylethyl acetate. The third part of our study shows that the use of S. cerevisiae in combination with I. orientalis and L. fermentum as starter allows producing ikigage beer having the GRAS (generally recognized as safe) statute while preserving the organoleptic characteristics similar to those of local traditional beer “ikigage” produced by peasants