Effect of Reduction of Carbonyl Compounds by Yeast on Flavor Stability of Happoshu

In Japan, beverages with higher than ∼67% malt are considered beer. Those with lower are called Happoshu, although the malt ratio in most Happoshu brands is below 25%. Organoleptic tests on aged Happoshu were performed, and the amounts of carbonyl compounds were determined using gas chromatography, HPLC, and liquid chromatography-mass spectrometry. These results led to the selection of 3-methylbutanal as one of the most decisive indices for flavor staleness, which appears as a caramel-like or papery flavor in Happoshu. The brewing trials suggested that the suspended yeast cell concentration at the end of primary fermentation is a decisive factor for the reduction of 3-methylbutanal, as well as the reduction of yeast cell activity. Yeast strains that were preferable for beer conventional brewing were not always suitable for Happoshu fermentation. The fermentation progress was found to be particularly important in stabilizing the flavor of Happoshu. One of the mechanisms proposed is that the yeast plays an important role in decreasing the amount of staling compounds and their precursors.