Breeding of branched short-chain fatty acids non-producing natto bacteria and its application to production of natto with light smells

Abstract

Natto contains branched short-chain fatty acids (BCFAS), such as isobutyric acid, isovaleric acid, and 2-methylbutyric acid. These BCFAS have unpleasant smell. To produce light-smelling natto, we tried to develop BCFAS non-producing natto bacteria. BCFAS are presumed to be synthesized from branched amino acids, such as valine, leucine, and isoleucine. Therefore we constructed mutant strains, yqiT1 and ywaA1, which lack leucine dehyrogenase (LDH) and branched-chain amino acid aminotransferase by insertion mutation, respectively. YqiT1 produced much smaller amount of BCFAS than parent strain. While, ywaA1 produced BCFAS as much as parent strain. From this result, we concluded that LDH catalyzes the first step of the BCFAS biosynthesis pathway. Next we deleted the LDH gene of O-2 strain by means of homologous recombination and transduction. Natto made by the use of the mutant strain, B2, which had a deletion in the LDH gene, contained only 0.7 mg BCFAS/100g, while natto made by the use of the parent strain contained 70.7 mg BCFAS/100g. The natto fermented by B2 strain was valued highly as a light-smelling natto by sensary evaluation. Finally, we obtained the LDH-defective mutants by chemical mutagenesis to utilize in commercial production of natto. The mutants produced little BCFAS like B2, and the natto fermented by the mutants had lighter smells.