Continuous mixed-culture lactic acid fermentation of dairy effluent: Effects of magnetite supplementation and recycle

Lactic acid (LA) is an important industrial compound used in pharmaceuticals and food. It is considered as an alternative to petrochemical polymers. The global LA market reached USD 3.2 billion in 2021 and is expected to grow to USD 6.9 billion by 2030. LA can be produced through chemical synthesis or microbial fermentation, with the latter being the primary method. However, current commercial LA fermentation processes are costly, energy-intensive, and time-consuming. To address these issues, studies have explored mixed-culture fermentation using organic waste as a substrate, eliminating the need for sterilization. However, mixed-culture fermentation faces challenges in maintaining high LA selectivity and substrate utilization efficiency. To promote the dominance of lactic acid bacteria (LAB) and achieve high LA selectivity, low pH conditions and reduced retention times have been used. However, these strategies may hinder LA yield and productivity due to factors like feedback inhibition, low substrate conversion efficiency, and nutrient deficiencies. Previous studies have shown that iron supplementation, particularly iron oxides like magnetite, can enhance enzyme activity in fermentative bacteria. However, the application of magnetite supplementation and recycling in continuous mixed-culture fermentation has not been reported. This study aims to investigate how magnetite supplementation and recycling affect the performance of mixed-culture LA fermentation using whey as a substrate. The study will analyze various fermentation parameters and microbial community structures to gain insights into the fermentation process.

乳酸（Lactic acid，LA）是一类重要的工业用化学品，广泛应用于制药与食品行业，同时被视作石油基聚合物的替代材料。2021年全球乳酸市场规模达32亿美元，预计到2030年将增长至69亿美元。乳酸可通过化学合成或微生物发酵两种途径制备，其中微生物发酵为当前主流生产方式。但当前商业化的乳酸发酵工艺存在成本高昂、能耗高且耗时较长的痛点。为解决上述问题，已有研究探索采用有机废弃物作为底物开展混合培养发酵，从而省去灭菌步骤。然而，混合培养发酵面临着难以维持高乳酸选择性与底物利用效率的挑战。为强化乳酸菌（Lactic Acid Bacteria，LAB）的群落主导地位并实现高乳酸选择性，现有研究多采用低pH环境与缩短停留时间的策略。但此类策略可能因反馈抑制、底物转化效率低下以及营养缺陷等因素，降低乳酸的产率与生产强度。已有研究表明，添加铁源（尤其是磁铁矿这类铁氧化物）可提升发酵细菌的酶活性。但目前尚未见将磁铁矿添加与回收工艺应用于连续式混合培养发酵的相关报道。本研究旨在探究磁铁矿添加与回收工艺对以乳清为底物的混合培养乳酸发酵性能的影响，并通过分析各类发酵参数与微生物群落结构，深入解析该发酵过程的作用机制。