| Originated in China,Chinese jujube is a very popular fruit,which is rich in soluble sugars,proteins,vitamins,minerals and phenolic acids,and has high edible value and medicinal functions.As the world's largest producer and consumer of Chinese jujube.In 2020,the cultivated area of the country has reached more than 14000 square kilometers,and the yield has increased to 7.464 million tons(dried Chinese.jujube).The processing of Chinese jujube is mainly dry,accounting for 75%of the total processing of Chinese jujube,12%of others,and only 13%of the deep processing of Chinese jujube.Therefore,the deep processing of Chinese jujube is the key affecting the development of the Chinese jujube industry.It is estimated that every year about 30%of the Chinese jujube can not be directly sold or processed for reasons such as small fruit,bruises,cracked fruit or insects,etc.,resulting in a large amount of waste of raw materials.Chinese jujube is rich in sugar and other nutrients,with a sugar content of more than.45%(dry jujube),which is a high-quality raw material for microbial fermentation.The purpose of this paper is to produce citric acid,which is widely used in food and chemical industry,with dried Chinese jujube as raw material and microbial fermentation technology.The purposes are as follow:(1)To impove utilization rate of Chinese jujube and solve the problem that it is difficult to sell fruit in the production of a large number of inferior Chinese jujube;(2)Convert the high content of sugar into high quality citric acid in jujube and turn waste into treasure;(3)To further complete the theory and production technology of deep processing and citric acid microbial fermentation of jujube.After a lot of experiments,the research results are as follows:(1)Using dried Shaanxi jujube as raw material,the jujube juice which apply to ferment citric acid was obtained by adding water and enzymatic hydrolysis.The main components were reducing sugar content of 151.92 g/L and pH 4.07;The contents of K,Ca,Mg,Fe,Zn,Cu and Mn were 104.57,62.34,30.64,4.81,1.71,0.20 and 2.94 mg/L,respectively.The organic acids were lactic acid,malic acid,tartaric acid,fumaric acid,citric acid and succinic acid,the contents of which were 0.76,0.21,0.19,0.15,0.17 and 0.06 g/L,respectively.(2)Aspergillus niger M2160(A.niger CICC 2160),which was suitable for static fermentation of citric acid on the surface of jujube juice,was screened out from four strains of Aspergillus niger,which were commonly used in industry.After inoculating 2%(v/v)Aspergillus niger M2160 spore suspension in jujube juice for 192 h,the highest citric acid content was 13.51 g/L,and the citric acid yield was 11.16%.It was found that oxalic acid,malic acid,succinic acid and fumaric acid were more in citric acid fermentation broth by measuring the changes of 8 kinds of organic acids.(3)Through an in Further took the initial pH(A)and the addition of NH4Cl(B),MgCl2(C),and CH3OH(D)as the response factors.Taking the citric acid content in the fermentation broth of 192 h fermentation as the response value to conduct a four-factor three-level response surface optimization experiment,the optimal nutrient conditions for citric acid fermentation by Chinese jujube juice were determined as pH 3.51,NH4Cl addition amount 6.01 g/L,MgCl2 addition amount 0.39 g/L,and CH3OH addition amount 2.89%,and the citric acid content in the fermentation broth reached 31.98±0.33 g/L.The experiment studied the influence of the thickness of the fermentation broth on the citric acid production by A.niger,and found that the suitable thickness of the liquid was 6.00 cm,the content of citric acid in the fermentation broth after 192 h fermentation is 39.83±0.35 g/L.(4)After the Chinese jujube juice fermentation broth had removed the bacteria and other solid impurities,the pH of the crude extract was 1.45,the content of reducing sugar was 5.87 g/L.The contents of oxalic acid,lactic acid,malic acid,citric acid,tartaric acid,succinic acid and fumaric acid were 2.56,0.79,0.98,39.52,0.21,0.35 and 0.50 g/L,respectively.It was chromaticity parameters L0*,a0*and b0*were 180.02,38.17 and-29.73.The contents of K,Ca,Mg and Cu in the detectable minerals were 1649.72,43.53,109.26 and 9.69 mg/L.(5)After the crude citric acid extract was fermented by Saccharomyces cerevisiae,neutralized by Ca(OH)2,activated carbon adsorption and anion-cation exchange resin adsorption,the reducing sugars,minerals,miscellaneous acids and pigment impurities had been basically removed,the pure citric acid solution was obtained.The content of citric acid in the pure liquid was 32.83 g/L.500 mL of pure liquid could produce 13.69 g of citric acid monohydrate solid product with a purity of 98.13%. |
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