| Zizyphus jujube is a Chinese specialty that is famous for its dual use of medicine and food.And It has the highest cultivated area and output in the world.By 2017,the planting area of Zizyphus jujube in China had reached 3 million hectares,with an annual output of 8.5 million tons.At present,the demand for dried Zizyphus jujube in the market is already saturated.About 20%of the Zizyphus jujube are rotted and deteriorated due to the inability to sell,resulting in serious economic losses.Zizyphus jujube is rich in nutrients and contains a variety of functional ingredients.The sugar content of dried Zizyphus jujube is as high as 60%or more,which is no less than the total sugar content of some starchy raw materials.Citric acid is the most commonly used acidity additive in food industry,and its demand is huge.The industrial production of citric acid is mostly obtained by liquid submerged fermentation of Aspergillus niger using starch raw materials as a carbon source.Finding cheap and suitable fermented raw materials is the primary purpose for many producers to reduce the cost of citric acid production and obtain maximum economic benefits.The paper used the low grade dried Zizyphus jujube as raw materials to study the liquid fermentation production process of citric acid.The main researches were carried out on the extraction of jujube juice,the screening of fermenting strains,the optimization of nutrient components of jujube juice fermentation matrix,the determination of fermentation process parameters and the pretreatment of jujube residue to produce citric acid by fermentation.After research and analysis,the following results were obtained:1.The jujube juice was extracted by adding water at 1:6(mass ratio),beating,adjusting its pH to 3.5,and adding 0.2%pectinase(enzyme activity>20u/mg).The suitable enzymatic hydrolysis conditions of jujube juice were determined by two-factor and three-level orthogonal test,which was enzymatically hydrolyzed at 40℃ for 4 h.After enzymatic hydrolysis,the content of reducing sugar,total nitrogen and phosphorus in jujube juice were 90.14 g·L-1,1.09 g·L-1 and 0.32 g·L-1 respectively.The contents of Fe,Mn,Cu and Zn were relatively low,which were 0.021 g·L-1,0.003 g·L-1,0.0006 g·L-1 and 0.0025 g·L-1 respectively.Compared with the industrial components required for the industrial production of citric acid,the nutrient composition of jujube juice was initially consistent with the basic nutrient conditions for Aspergillus niger growth and accumulation of citric acid.2.Through the preliminary screening and rescreening of 9 strains of Aspergillus niger that could ferment citric acid,and taking jujube juice as the matrix,Aspergillus niger strain H-2 produce acid amount up to 41.12 g·L-1,the optimal temperature for 30℃,fermentation period of 84 h,which was the best fermentation strain.Aspergillus niger H-2 had the best effect in producing citric acid,which was identified as the fermentation strain.In the process of producing citric acid from jujube juice,Aspergillus niger strain H-2 began to produce citric acid after 24 hours of growth.After entering the acid-producing stage,the mycelium spheres were small in shape and tightly packed,which was consistent with the basic morphological characteristics of high-yield strains.Qualitative analysis of the product by HPLC showed that the most citric acid produced by strain H-2 was citric acid.3.The optimization experiment of citric acid fermentation medium was carried out with jujube juice as the substrate.It was determined that the inorganic nitrogen source was added as NH4CI and the organic nitrogen source was soybean meal.The addition of CuSO4·5H2O,MgSO4·7H2O metal ions had a certain promoting effect.CH3OH as a growth-promoting factor,added to jujube juice could significantly improve the acid production capacity of citric acid.Further,through the Box-Behnken design and response surface test,the content of each component of citric acid fermented by jujube juice was determined as follows:the reducing sugar of jujube juice was adjusted to 128 g·L-1,KH2PO4 3.2 g·L-1,NH4Cl 1.6 g·L-1,soybean meal 8.7 g·L-1,CuSO4·5H2O 1 mg·L-1,MgSO4·7H2O 0.5 g·L-1,CH3OH 3%.4.The process parameters for the production of citric acid by jujube juice were determined as the shaking speed of 200 r min-1,the filling volume of 80 mL,the inoculum volume of 2%,and the initial pH of the adjustment was 5.5.The citric acid yield was 58.86 g·L-1 after fermentation for 84 h under the optimal fermentation conditions of jujube juice,which was significantly higher than that of the unoptimized group.Observing the Aspergillus niger strain under the optimal fermentation conditions,the growth state was good,the morphology of the hyphae was regular,and the hyphae was tightly packed,indicating that the condition was suitable for the accumulation of citric acid by the Aspergillus niger strain.The citric acid production of Aspergillus niger fermented jujube juice was analyzed.It was found that most of the fermentation broth was citric acid,and the content of misacid was less than 2%,mainly oxalic acid and fumaric acid.5.Study on fermentation of citric acid by using jujube residue as raw material,the paper carried out a preliminary study on the fermentation of citric acid by using the jujube residue remaining after extracting jujube juice.The effect of treating jujube residue with alkaline hydrogen peroxide was significant,the optimal processing condition was:temperature of 56℃ and 4%hydrogen peroxide dosage,time of 200 min.Through SEM image,FTIR and XRD pattern analysis,the effect of hydrogen peroxide treatment was significant,and the final reducing sugar content could reach 21.16%.Aspergillus niger could grow in the jujube residue hydrolysate matrix to form a normal hyphae which could accumulate citric acid.At the end of fermentation,the biomass of the bacteria reached 25.01 g·L-1,and the accumulation of citric acid was 27.45 g·L-1.It indicated that the jujube residue can be used for fermentation of Aspergillus niger to produce citric acid after adjusting the composition. |
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