Abstract
Most high γ-aminobutyric acid (GABA) producers are Lactobacillus brevis of plant origin, which may be not able to ferment milk well due to its poor proteolytic nature as evidenced by the absence of genes encoding extracellular proteinases in its genome. In the present study, two glutamic acid decarboxylase (GAD) genes, gadA and gadB, were found in high GABA-producing L. brevis NPS-QW-145. Co-culturing of this organism with conventional dairy starters was carried out to manufacture GABA-rich fermented milk. It was observed that all the selected strains of Streptococcus thermophilus, but not Lactobacillus delbrueckii subsp. bulgaricus, improved the viability of L. brevis NPS-QW-145 in milk. Only certain strains of S. thermophilus improved the gadA mRNA level in L. brevis NPS-QW-145, thus enhanced GABA biosynthesis by the latter. These results suggest that certain S. thermophilus strains are highly recommended to co-culture with high GABA producer for manufacturing GABA-rich fermented milk.
Highlights
Most high γ-aminobutyric acid (GABA) producers are Lactobacillus brevis of plant origin, which may be not able to ferment milk well due to its poor proteolytic nature as evidenced by the absence of genes encoding extracellular proteinases in its genome
This may suggest that L. brevis of plant origin may not able to survive in milk environments because of its poor proteolytic nature
We report a new strategy of manufacturing GABA-rich fermented milk by co-culturing high GABA producer with dairy starter including S. thermophilus and L. bulgaricus in skimmed milk supplemented with monosodium glutamate (MSG), and provide new insights into the effects of dairy starters on the cell viability of L. brevis NPS-QW-145 and its GABA biosynthesis ability in milk
Summary
Most high γ-aminobutyric acid (GABA) producers are Lactobacillus brevis of plant origin, which may be not able to ferment milk well due to its poor proteolytic nature as evidenced by the absence of genes encoding extracellular proteinases in its genome. Certain strains of S. thermophilus improved the gadA mRNA level in L. brevis NPS-QW-145, enhanced GABA biosynthesis by the latter These results suggest that certain S. thermophilus strains are highly recommended to co-culture with high GABA producer for manufacturing GABA-rich fermented milk. Genomic analysis indicated the absence of genes encoding extracellular or cell wall-anchored proteinases in the sequenced L. brevis ATCC 367 (a starter culture for beer, sourdough and silage) and L. brevis KB290 (an isolate from traditional Japanese fermented vegetable) This may suggest that L. brevis of plant origin may not able to survive in milk environments because of its poor proteolytic nature. Monosodium glutamate (MSG) is normally added to milk as the substrate for manufacturing GABA-rich fermented milk because of low content of free glutamate in milk[8]
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