Studies On The Isolation Of Lactic Acid Bacteria Specific For Vegetable And Application In Carrot Fermentation

Lactic acid bacteria(LAB)are generally regarded as safe(GRAS)and serve many functions such as improving product flavor and nutrition,enhancing the intestinal function of humans and animals,increasing immunity,and preventing cancer.In this study,LAB with high acid yield,fast growth rate,and strong antibacterial activity were isolated from traditional fermented foods.The species of the selected strains were identified by physiological and biochemical as well as molecular biological methods Single-factor and response surface optimization experimental designs were used to obtain the optimal parameters in LAB fermentation of carrot pulp,and the aroma components were analyzed.LAB fermented carrot powder was prepared by spray drying,and its aroma components and microstructure were studied.The main conclusions are as follows:(1)Thwenty-three LAB strains were isolated from traditional fermented foods.Selection of LAB were based on acid-producing ability,growth rate,and antibacterial ability.LAB LC1 and LT1 with the strongest acid-producing abilities were selected,and both strains showed strong inhibitory abilities against E.coli ATCC 25922 and S.aureus ATCC 6538.No mutural inhibition was observed in the antagonism test with LC1 and LT1,indicating no antagonism between the two strains.Through physiological and biochemical as well as 16 S rDNA identification,it was determined that strain LC1 was Lactobacillus casei(L.casei)and LT1 was Lactobacillus reuteri(L.reuteri).(2)The two strains grew well in carrot pulp fermentation.The final pH value of the LC1 fermented carrot pulp was 3.43 and the total acid content reached 12.68 g/L;the final pH of the LT1 fermented carrot pulp was 3.90 and the total acid content was 6.03 g/L.Antioxidation assay on carrot pulp showed that the potassium ferricyanide reducing power and DPPH free radical scavenging rate of carrot pulp increased significantly after autoclaving.After carrot pulp fermentation by LC1 and LT1,the hydroxyl radical and DPPH radical scavenging rate of the carrot pulp did not change significantly,but the reducing ability of potassium ferricyanide increased markedly.(3)Fermentation of carrot juice by L.casei LC1 and L.reuteri LT1.After single-factor and response surface optimization experiments,the optimal carrot juice fermentation parameters were obtained: strain ratio LC1: LT1 at 1:1,fermentation time of 8h,inoculum size of 3.1%,and fermentation temperature at 26.7°C.The carrot pulp obtained under the optimum conditions showed bright orange color,uniform tissue state,soft texture,and sweet and sour taste.The original herbal flavors of carrot pulp disappeared,and some unique aroma of LAB fermentation made the carrot pulp more acceptable to cunsumers.Compared with current products on the market,this product is unconcentrated reduced carrot juice,without food additives such as sucrose,citric acid and essence,and is more nutritional and healthier.(4)Physical and chemical properties of spray-dried carrot powder,such as flour extraction rate,water content,and color,were measured and used as the primary indicators,while bulk density,hygroscopicity,and dissolution time were meaused and used as the secondary indicators.The optimal maltodextrin dosage was 40%.The microstructure of carrot powder showed that the powder was spherical before fermentation,with uniform shape and smooth surface.After fermentation,the shape of carrot powder particles became irregular,the spherical surface was invaded,and many small holes appeared on the surface.These changes may be beneficial to the absorption of nutrients by human body from carrot powder.(5)The study of the aroma components of carrot pulp and carrot powder showed that LAB fermentation had significant effects on the aroma components in carrots.The volatile components of carrot powder and carrot pulp also differed significantly.The floral and fruit aroma in the original carrot pulp disappeared after high-temperature spray drying,while the components related to sweet smell increased greatly in the carrot powder.In this paper,two strains of lactic acid bacteria were selected and applied to the fermentation of carrot juice.Good-tasted,nutritional and healthy fermented carrot juice drink was obtained.In addition,the fermentation of these two strains can significantly improve the yield of carrot powder prepared by spray drying and have good economic benefit.