The Isolation And Identification Of Lactic Acid Bacteria From Naturally Fermented Tofu Whey And The Application Of The Strains In The Acidic Whey Tofu

Tofu is a kind of traditional Chinese food consumed as a daily basis,which is usually prepared with magnesium chloride（MgCl₂）or calcium sulfate（CaSO₄）.However,acidic whey tofu is coagulated with naturally fermented tofu whey and often applied as raw material to make sufu in Yunnan and Hunan province.The sufu made by acidic whey is greatly popular among consumers because of the good texture and flavor.However,the natural fermentation process of tofu whey is vulnerable to the environment during the production process,leading to a high variance in tofu quality between batches,therefore further influencing the stability of sufu.The present study aims to screen and isolate high-acid-producing lactic acid bacteria from the naturally fermented tofu whey and produce the acidic whey tofu that has similar properties in texture with naturally fermented acidic whey tofu,therefore reducing the difference of the acidic whey tofu and promoting the development of the industry of sufu.Firstly,lactic acid bacteria were screened and identified from 10 naturally fermented tofu whey samples.Lactobacillus fermentrum,Lactobacillus paracasei,Lactobacillus plantarum,Lactobacillus delbrueckii,Lactobacillus mucosae and Lactobacillus parabuchneri were identified in the level of species.The pH changes of tofu whey were monitored during the fermentation with all these strains.The results demonstrated that L.plantarum possessed the highigest acid-producing ability owing to the lowest pH after 24 h fermentation,followed by L.fermentrum,along with L.paracasei,L.delbrueckii,L.mucosae and L.parabuchneri.Therefore,L.plantarum was choosen to produce of acidic whey tofu subsequently,while the L.paracasei with weaker acid-producing ability was used as a control.Secondly,the changes of organic acids,isoflavones and sugars were monitored during the fermentation of tofu whey with L.paracasei and L.plantarum respectively.Results showed that the total organic acids increased largely after 24 h fermentation at 37°C,especially for the lactic acid and acetic acid.Compared with L.paracasei,L.plantarum had stronger acid-producing ability.All the sugars including fructose,sucrose,stachyose,raffinose,glucose in the tofu whey were utilized during the fermentation process,especially for the stachyose and sucrose.Besides,isoflavones could be transformed into the isoflavone aglycones during the fermentation with lactic acid bacteria and L.plantarum had stronger capability of transformation isoflavones into isoflavone aglycones.Thirdly,in order to get the best soybean curd coagulated with fermented tofu whey by L.paracasei and L.plantarum respectively,the pH,water holding capacity,hardness and microstructure of the soybean curd were chosen to determine the best shaping temperature and the addition of coagulant.The results showed that both bacteria could produce the best soybean curd when the holding temperature was 37°C and the ratio of the coagulant is 10%（v/v）of the soymilk.Rheological experiments showed that soybean curd coagulated with fermented tofu whey by L.plantarum needed shorter time to coagulate and had great hardness.At last,the tofu coagulated with fermented tofu whey by L.paracasei and L.plantarum under the optical conditions were compared with naturally fermented acidic whey tofu.The results showed that the tofu coagulated with fermented tofu whey by L.plantarum were similar with tofu coagulated with naturally fermented tofu whey in p H,water holding capacity,hardness,microstructure and rheology qualities.In conclusion,the L.plantarum was possible to ferment tofu whey and produce stable acidic tofu that was similar in texture properties to naturally fermented tofu,therefore,the research could be used as a reference for industrial production.