| Kitchen wastes have been the most abundant and problematic organic solid waste in the world since they are the main source of decay, odor and leachate in collection and transportation due to its high volatile solids and moisture content. However, as kitchen wastes contain rich nutrition, including carbohydrate, lipid, protein and other compounds, it could be a potential raw material for recycling utilization. Compared to the landfill, incineration and compost, lactic acid fermentation is a sustainable technology for kitchen wastes. It can effectively achieve kitchen wastes minimization and generate lactic acid for energy recovery. Lactic acid has both hydroxyl and carboxyl groups with one chiral carbon atom, and it is widely used in the food, pharmaceutical and general chemical industries. In addition, lactic acid can be polymerized to form the biodegradable and recyclable polyester polylactic acid, which is considered a potential substitute for plastics manufactured from petroleum. Thus, it is necessary to develop an efficient lactic acid fermentation process to stabilize the kitchen wastes.The present study dealt with lactic acid production using kitchen wastes as substrate. A homofermentative Lactobacillus plantarum strain, isolated from kitchen wastes, was employed as the starter culture in the quick storage of kitchen wastes to suppress both pathogenic and spoilage microorganisms. Response surface methodology was adopted to optimize lactic acid production during simultaneous saccharification and fermentation of kitchen wastes by examining five independent variables (α-amylase/solid ratio, protease/solid ratio, yeast extract concentration, temperature and CaCO3 concentration). The combination of Plackett-Burman design and response surface methodology was employed to screen and optimize various variables for lactic acid bacteria partitioning in aqueous two-phase system under different operating conditions. Moreover, PEG/DEX systems were also used to extract lactic acid from kitchen wastes fermentation.⑴A strain of lactic acid bacteria was initially isolated from 10 day fermented kitchen wastes on MRS agar. After identified by normally physiological and biochemical tests and API 50 CH galleries, the strain was a homofermentative Lactobacillus plantarum species and was named Lactobacillus plantarum BP04. Experiments from growth curve indicated that this strain could grow quickly and reach stationary phase within 12 h. In addition, its lactate production capacity could be up to 138.09 g/L.⑵Lactobacillus plantarum BP04 was employed as starter culture in kitchen wastes storage with different inoculant levels at 0, 2 and 10% (v/w) to suppress the outgrowth of pathogenic and spoilage bacteria. Inoculation was effective in accelerating pH drop and reducing the growth period of enterobacteria to 9.7 and 2 days, corresponding to inoculant levels at 0, 2 and 10% (v/w). Increasing inoculum levels were found to inhibit the growth of Lactobacillus brevis and Leuconostoc lactis. HPLC analysis revealed that lactic acid was the predominant organic acid during the treatment of kitchen wastes. Its concentration varied among the fermented processes reflecting variations of microbial activity in the fermented media.⑶Central composite design using response surface methodology was employed to optimize parameters ofα-amylase, protease, temperature, CaCO3 and yeast extract in simultaneous saccharification and fermentation process. A satisfactory fit of the quadratic model was realized. Lactic acid biosynthesis was significantly affected by interaction of protease×temperature. Protease, temperature and CaCO3 had significantly linear effects on lactic acid production whileα-amylase and yeast extract had insignificant effects. Yeast extract was proved to be unnecessary in eatery food waste fermentation. The optimum condition was found to beα-amylase at 13.86 U/g dried food waste, protease at 2.12 U/g dried food waste, temperature at 29.31℃and CaCO3 at 62.67 g/l with the maximum lactic acid concentration and yield at 98.51 g/l and 88.75%, respectively. Increase of inoculum size would be suitable by accelerating depletion of initial soluble carbohydrate to enhanceα-amylase efficiency in kitchen wastes fermentation. Increasing magnitudes of CaCO3 supplement could improved the reducing sugars depletion rate and led to the balance between the rates of hydrolysis and fermentation favorable to hydrolysis.⑷The combination of Plackett-Burman design and response surface methodology was used to examine partitioning parameters for Lactobacillus plantarum BP04 in aqueous two-phase system with polyethylene glycol (PEG)/dextran (DEX) as the biphasic system. One-sided partition of cells to DEX phase was observed in all trials. Results from Plackett-Burman design presented that the molecular weight of PEG and DEX had highly significant and negative effects on volume ratio while cell density, electrolytes, settling time and pH were insignificant. PEG10000 and DEX20000 were further optimized by response surface methodological approach. Volume ratio was strongly affected by the variation of polymer concentration whereas the interaction between PEG10000 and DEX20000 was insignificant. The optimum condition was found to be PEG10000 at 6% (w/w) and DEX20000 at 13.85% (w/w) at the experimental range with the minimal volume ratio at 0.815.⑸PEG/DEX systems were also used to extract lactic acid from kitchen wastes fermentation. Results showed that PEG/DEX was a good biocompatible system which could effectively extract lactic acid from fermented media. PEG10000/DEX20000 biphasic system had little effect on the growth of Lactobacillus plantarum BP04, whereas lactic acid production rate was about 0.20 g /(L·h) between 12 and 48 h, much lower than 0.631 g/(L·h) in conventional fermentation system. The variation of PEG and DEX concentrations influenced cells growth and lactic acid biosynthesis indistinctively. Volume ratio kept stable throughout the processes, despite the changes of raw material and product concentrations. Different from little influence of PEG molecular weight on fermentation, the increase of DEX molecular weight from 20000 to 40000 led to declines of lactate conversion rate from 0.631 g/(L·h) to 0.518 g/(L·h) and lactic acid concentration from 33 g/L To 22 g/L. Increasing magnitudes of inoculum size could shorten the period of lag phase and enhance the production rate. When inoculum sizes were at 1%,4% and 8% (w/w), the corresponding lactic acid conversion rate were 0.558 g/(L·h) and 0.602 g/(L·h) and 0.649 g/(L·h), respectively. A repeated extractive fermentation was carried out in PEG10000/ DEX20000 biphasic system with four top-phase replacements. Results presented that when cell density reached the stationary phase in the first extractive fermentation, the lactate production in the aqueous two-phase system was maintained.
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