As flavor compounds or precursors of flavor compounds, free amino acids can result in the formation of further flavor. For example, glutamic acid produced by the metabolism of glutamine will accumulate during the late fermentation period and give delicious flavor. Additionally, branched-chain amino acids are also increased. Under the effect of transaminase they can produce aketone acid, which is the precursor for malt flavor aldehydes, fruit flavor alcohols, and other fermented flavor compounds (Wu et al., 2006).