Studies On Enrichment Of γ-Aminobutyric Acid In Bread By Sourdough Fermentation

Bread is one of the staple foods in China, which is wildly popular in the world. γ-Aminobutyric acid(GABA) has a variety of physiological functions. Therefore, the addition of GABA in bread can effectively reduce the incidence of cardiovascular disease and blood pressure. This study was aim to isolate and screen the high GABA producing lactic acid bacteria(LAB) from the collected zymotic fluid of traditional fermented rice noodles by modified paper chromatography(MPC) and high performance liquid chromatography(HPLC). In order to make a functional bread enriched of GABA, the selected high GABA-producing LAB would be used for sourdough fermentation to enrich GABA. In addition, the sourdough characteristics and the effect of sourdough fermentation on bread were investigated.In this study, a total of 621 LAB strains were isolated from zymotic fluid of traditional fermented rice noodles. The results of MPC indicated that 3 strains had GABA-producing ability, including H-1, H-5 and H-18. The highest GABA content was found in the fermented broth of H-18, reaching 3.66 g/L, followed by H-5(1.37 g/L). However, H-1 showed the lowest GABA content(0.47 g/L). Based on 16 S r DNA gene sequencing, H-18 was identificated as Lactobacillus buchneri(L.bu), while the others were identificated as Lactobacillus brevis. H-18(L.bu) was used for the following application of sourdough fermentation since it had a greater GABA-producing ability.The free amino acid(FAA) content of different fermentation substrates was determined by HPLC. The highest content of total FAA, glutamic acid and GABA were all found in mung flour, which was selected as sourdough fermentation substrates for L.bu to enrich GABA. Optimization of sourdough fermentation was carried out by response surface experiment based on single factor experiments. Considering convenience of the actual operation, the mung dough yield of 480, fermentation for 40 h at 28°C were found to be the optimal mung sourdough fermentation conditions. In this condition, the fitted values of GABA content for mung sourdough fermented by L.bu was 5.697 mg/g mung powder. The result of experiment in practice was 5.593 mg/g mung powder, being close to the fitted values.The content of total FAA of mung dough without fermentation(MD) was 234.53 mg/100 g, while it ran up to 634.43 mg/100 g in the dough fermented by L.bu(MSD). What’s more, the essential amino acid accounted for 34.13%(MSD), higher than MD(18.77%). Compared with MD(6.82 mg/100g), the GABA content in MSD was imporved to 116.50 mg/100 g, equivalent to 5.593 mg/g mung powder, realizing the GABA enrichment in the mung sourdough fermentation.Compared to wheat bread(WB), the specific volume and texture properties of mung bread(MB) were siginificantly deceased. While the quality of mung bean bread can be effectively promoted by the mung sourdough fermentation. The hardness, gumminess and chewiness of mung sourdough bread(MSB) decreased significantly, while elasticity increased after sourdough fermentation, improving the texture quality of mung bread.In bread, MSB in comparison with WB and MB, the FAA was in the order as WB(73.14)<MB(88.72) <MSB(257.58 mg/100g). What’s the most important was that the GABA content in MSB increased to 25.02 mg/100 g, siginificantly higher than WB(4.39 mg/100 g) and MB(4.91 mg/100g).Through GC-MS analysis, a total of 57,50 and 57 varities of volatile flavor compounds were detected in WB, MB and MSB, reapectively. MSB had higher varities and contents of volatile flavor compounds than MB. Compared with MB, there are 14 kinds of new volatile flavor compounds were found in WB, forming the unique flavor of mung bread. Sourdough fermentation showed significant effect on improving mung bread flavor. There were 19 kinds of volatile flavor compounds only appeared in MSB, which were not determined in MB, such as 2, 3-pentanedione, trans-2-2-nonenal, 1-octene-3-ketone and peach aldehyde. In addition, the results of sensory evaluation showed that sourdough fermentation can effectively improve the specific volume, texture and flavor of mung bread so as to improve the sensory score and overall acceptability. This study ultimately made a functional mung sourdough bread enriched with GABA, meeting the needs of consumers in different aspacts.