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Anaerobic co-fermentation of swine manure (SM) and apple waste (AW) restricts by the slow hydrolysis of substrates with complex structures, which subsequently leads to low lactic acid (LA) production. Therefore, a novel strategy based on enzymatic pretreatment for improving LA production from anaerobic co-fermentation of SM and AW was proposed in this study. The results indicated that the maximal LA concentration increased from 35.89 ± 1.84 to 42.70 ± 2.18 g/L with the increase of enzyme loading from 0 to 300 U/g VS substrate . Mechanism exploration indicated that enzymatic pretreatment significantly promoted the release and hydrolysis of insoluble organic matter from fermentation substrate, thus providing an abundance of reaction intermediates that were directly available for LA production. Additionally, bacteria analysis revealed that the high concentration of LA was associated with the prevalence of Lactobacillus. This study offered an environmental-friendly strategy for promoting SM and AW hydrolysis and provided a viable approach for recovering valuable products., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Lactic acid (LA) is an important chemical with broad market applications. To optimize LA production, food waste has been explored as feedstock. Due to the wide variety of food waste types, most current research studies have obtained different conclusions. This study focuses on carbohydrate-rich fruit and vegetable waste (FVW) and lipid-rich kitchen waste (KW), and the effect of inoculum, temperature, micro-oxygen, and initial pH were compared. FVW has a greater potential for LA production than KW. As an inoculum, lactic acid bacteria (LAB) significantly increased the maximum LA concentration (27.6 g/L) by 50.8 % compared with anaerobic sludge (AS). FVW exhibited optimal LA production at 37 °C with micro-oxygen. Adjustment of initial pH from 4 to 8 alleviated the inhibitory effect of accumulated LA, resulting in a 46.2 % increase in maximum LA production in FVW. The expression of functional genes associated with metabolism, genetic information processing, and environmental information processing was higher at 37 °C compared to 50 °C., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Micro-aeration was proven to be an environmentally friendly strategy for efficiently enhancing volatile fatty acids (VFAs) and lactic acid (LA) production. The roles of micro-aeration on mono-digestion of swine manure (SM) for VFAs production and co-digestion of SM with corn silage (CS) for LA production were investigated, respectively. In this study, micro-aeration increased the maximum VFAs concentration by 20.3% to 35.71 g COD/L, and shortened the time to reach the maximum from 18 days to 10 days. Micro-aeration limited the conversion of LA into VFAs, leading to LA accumulation effectively to be 26.08 g COD/L. Microbial community analysis suggested that Clostridium and Terrisporobacter were always the dominant bacteria with or without micro-aeration for VFAs production, but the relative abundance increased notably during the same period. However, Bifidobacterium, which could use the higher productivity metabolism pathway, i.e., Bifidum pathway to produce LA, increased from lower than 1% to 22.9% by micro-aeration., (Copyright © 2021 Elsevier Ltd. All rights reserved.)

Anaerobic co-fermentation is a favorable way to convert agricultural waste, such as swine manure (SM) and apple waste (AW), into lactic acid (LA) through microbial action. However, the limited hydrolysis of organic matter remains a main challenge in the anaerobic co-fermentation process. Therefore, this work aims to deeply understand the impact of cellulase (C) and protease (P) ratios on LA production during the anaerobic co-fermentation of SM with AW. Results showed that the combined use of cellulase and protease significantly improved the hydrolysis during the enzymatic pretreatment, thus enhancing the LA production in anaerobic acidification. The highest LA reached 41.02 ± 2.09 g/L within 12 days at the ratio of C/P = 1:3, which was approximately 1.26-fold of that in the control. After a C/P = 1:3 pretreatment, a significant SCOD release of 45.34 ± 2.87 g/L was achieved, which was 1.13 times the amount in the control. Moreover, improved LA production was also attributed to the release of large amounts of soluble carbohydrates and proteins with enzymatic pretreated SM and AW. The bacterial community analysis revealed that the hydrolytic bacteria Romboutsia and Clostridium_sensu_stricto_1 were enriched after enzyme pretreatment, and Lactobacillus was the dominant bacteria for LA production. This study provides an eco-friendly technology to enhance hydrolysis by enzymatic pretreatment and improve LA production during anaerobic fermentation. [Display omitted] • Cellulase and protease pretreatment promoted LA production from SM and AW. • The highest LA yield at the ratio of C/P = 1:3 was 1.26 times of that control. • The SCOD concentration at the ratio of C/P = 1:3 was 1.13 times that of the control. • Enzymatic pretreatment was beneficial to the enrichment of hydrolytic bacteria. • Lactobacillus was dominant genus for LA production during the co-fermentation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The potential of starch and inexpensive nitrogen for LA production was investigated. • LA production could be enhanced by gelatinization and enzyme pretreatment. • CSL as nitrogen source showed higher LA concentration than PKC or SM. • Lactobacillus was dominant for LA production in the anaerobic co-fermentation. • It was higher economic benefits from the co-fermentation of CS with CSL. Anaerobic co-fermentation is an interesting process that can efficiently valorize complex organic substrates by converting them into lactic acid. One of the main issues, however, is the high substrate cost, and its reduction is of great significance to the practical application. Therefore, this study investigated the feasibility of lactic acid production from carbohydrate-enrich corn starch, tapioca starch, potato starch, and rice starch with inexpensive nitrogen-rich agro-industrial waste of corn steep liquor, palm kernel cake, and soybean meal. Fermentation results indicated that the co-fermentation of starch with corn steep liquor significantly enhanced the lactic acid concentration compared with the co-fermentation of starch with palm kernel cake or soybean meal (p < 0.05). The highest lactic acid concentrations of 68.89 ± 6.30, 71.78 ± 13.74, 67.90 ± 2.94, and 68.83 ± 2.68 g/L were achieved under the co-fermentation of corn starch, tapioca starch, potato starch, and rice starch with corn steep liquor, respectively. The microbial community structure showed the dominance of Lactobacillus (60.78%-95.97%) in the co-fermentation of different substrates, which is believed to be beneficial to the growth of such fermentative bacteria with the main products of lactic acid. Furthermore, the economic analysis demonstrated that the co-fermentation of corn starch and corn steep liquor had better economic benefits than other treatments. This study provides a promising way to reduce lactic acid production costs, which will change the economic feasibility of lactic acid fermentation. [ABSTRACT FROM AUTHOR]

• 1. Co-AF of SM with AW for LA production was optimized using BBD-RSM. • 2. The optimum conditions were 60.4% AW in feedstock, 34.7 ℃, and 5.0% VS. • 3. LA concentration of 31.18 g/L was obtained in the optimum condition. • 4. Experimental result was in good agreement with the predicted value of the model. • 5. Both Lactobacillus and Clostridium were dominant in the co-AF. Co-anaerobic fermentation (co-AF) of swine manure (SM) and apple waste (AW) has been proved to be beneficial for lactic acid (LA) production. In order to further improve the LA production, three important parameters, namely AW in feedstock, temperature, volatile solids (VS) of feedstock, were evaluated using Box-Behnken design and response surface methodology. The quadratic regression model was developed and interactive effects was found between the three parameters. Results showed that the maximum concentration, 31.18 g LA/L (with LA yield of 0.62 g/g VS), was obtained under optimum conditions of 60.4% AW in feedstock, 34.7 ℃, and 5.0% VS. At the optimum conditions, the solubilization of organic matter was enhanced compared with mono-fermentation of SM. Microbial community structure of the reactor diverged greatly with fermentation time. Clostridium and Lactobacillus were dominant bacteria in the fermentation process, resulting in a remarkably LA accumulation. [ABSTRACT FROM AUTHOR]