| Inulin is a renewable and abundant raw material,which can be converted into fructose and glucose by complete hydrolysis of inulinase.Inulin is generally found in the roots and tubers of some non-grain crop plants.In China,many regions have recently initiated the large-scale cultivation of Jerusalem artichoke and chicory,because these plants grow well in poor soil,show resistances to wind,alkali and plant disease.In this paper,inulin is used as a raw material to produce D-lactic acid by enzymatic hydrolysis and fermentation.Compared with the prosperous development of L-lactic acid production,the demand of D-lactic acid still exceeds supply.Some work has been done focused on the blank of inulin biorefinery,including screening of inulinase producing strain from soil sample,optimization of inulinase production and exploration of inulinase propertities.In addition,this paper also focuses on the study of enzymatic hydrolysis and D-lactic acid production process,which establishes a method for highly efficient utilization of inulin and rapid production of D-lactic acid.The production of D-lactic acid from non-food biomass material is not just beneficial to improve the level of deep processing of Jerusalem artichoke on the fields of agricultural and forestry industrial structure,more conductive to broaden the bio-based chemicals and bio-based raw materials,which contributes to not only achieving the organic combination of forestry ecological protection and industrial development,but also easing the fossil resources and food resources to achieve sustainable economic and social development.The main contents and results of this paper are summarized as follows:(1)A strain with high activity of inulinase was isolated from the soil samples of Jerusalem artichoke,which was identified and named as Penicillium oxalicum XL01 by 18 S rRNA.The expression of extracellular inulinase can be induced by the strain XL01 with inulin as the sole carbon source.The highest levels of enzyme(inulinase:46.2 IU/m L,invertase: 33.6 IU/mL)were obtained in 7 d of fermentation at 30°C,natural pH,2.0%(v/v)inoculation,220 rpm with 20 g/L inulin and 20 g/L beef extract as a fermentation medium.After optimization,activities increased by 246% and 297%,respectively.(2)The enzymatic hydrolysis of inulin between commercial inulinase and self-produced inulinase were compared.The results showed that the monosaccharides yield of self-produced inulinase was higher than that of commercial inulinase under the same inulinase dosage(30 IU/g inulin).The high monosaccharides yield benefits to complete hydrolysis of inulin.After 72 h of enzymatic hydrolysis of 120 g/L inulin,16.4 g/L glucose,23.9 g/L fructose,and a large number of oligosaccharides were obtained with commercial inulinase at 50°C and pH 4.8.However,20.7 g/L glucose and 84.9 g/L fructose were obtained with self-produced inulinase at 50°C and pH 5.0.Apparently,the enzymatic hydrolysis of inulin is more complete with self-produced inulinase.(3)The effects of AHF,SHF and SSF on the D-lactic acid production were studied.The SSF process showed the best performance.D-lactic acid yields of the three strategies were 21.5%,46.2% and 99.5% using 80 g/L inulin as substrate.After optimization of SSF process(30°C,pH 5.2 and 30 IU/g inulin inulinase),120 g/L inulin was completely consumed by L.bulgaricus CGMCC 1.6970 giving 123.6 g/L D-lactic acid with a productivity of 1.72 g/(L·h).The values of D-lactic acid concentration,cell growth were similar by using these two kinds of inulinases.(4)The Logistic equation can be used to simulate the growth of L.bulgaricus.The kinetic model of D-lactic acid production by SSF was established on the basis of the model of inulin hydrolysis by using Logistic equation,Luedeking-Piret equation and Luedeking-Piret-like equation.The model of inulin hydrolysis contained reversible inhibition of fructose and enzyme inactivation.The fitting curve and test point were consistent with the suggested simulation,which indicated that the study has achieved satisfied results. |
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