Introduction
Sophorolipids (SLs) as renewable glycolipid biosurfactants are mainly produced by microorganisms of the Saccharomyces ssp. (Van Bogaert et al., 2007). Glucose and fatty acids are the main precursors for the synthesis of SLs. First, fatty acids are catalyzed by P450 monooxygenase into hydroxy fatty acids, and then UDP-glucoses are sequentially connected to form non-acetylated acidic SLs via the action of glucose transferase I and II (Lodens et al., 2020). Finally, the functional activities of acetyltransferase and extracellular lactonase lead to the acetylation and lactonization of SLs. Up to 20 different structural forms of SLs are known to exist, and this variation is the result of differences in acetylation, hydroxyl position, the length of the fatty acid chain, and the unsaturation degree of fatty acids (Hu & Ju, 2001). Thus, the production of SLs is a complex and multiphase fermentation process, involving a gas phase (air), a solid phase (cells, SLs crystals), a hydrophilic liquid phase (acidic SLs, glucose) and a hydrophobic liquid phase (lactonic SLs, oil), which pose a significant challenge to the efficient and stable production of SLs (Tian, Li, Chen, Mohsin & Chu, 2021).
At present, the optimization of microbial fermentation can be divided into three different aspects: (1) obtain high-performance producers via mutagenic breeding or genetic engineering, (2) develop and utilize cheap substrates to reduce the costs of fermentation , and (3) optimize the fermentation process to achieve the efficient synthesis of product (Dolman, Kaisermann, Martin & Winterburn, 2017; Li, Chen, Tian & Chu, 2020; Tian et al., 2021; Wang et al., 2020a). Of these processes, the rational and precise regulation of fermentation remains the most significant factor in achieving high-efficiency production. The identification of key process parameters form the basis of process control and optimization. By regulating key process parameters, it is possible to achieve the regulation of cell metabolism in a flexible manner, this allowing high titer, productivity, and yield (Wang et al., 2020c). The continuous development of sensing detection and information processing technologies, along with the real-time detection of conventional environmental parameters by on-line sensors, cellular macro-physiological, and metabolic parameters, has led to the availability of key parameters on-line, including living cell amount, oxygen uptake rate (OUR), carbon dioxide evolution rate (CER), and respiratory quotient (RQ). This information creates a database for the fermentation process (Chen, Lin, Tian, Li & Chu, 2019; Feng et al., 2021).
However, the mining of sensitive process parameters still relies upon correlation analysis and manual experience and has yet to be studied from the perspective of big data analysis (Davila, Marchal & Vandecasteele, 1997). On the other hand, it is gratifying that various data processing and analytical methods are gradually being introduced into the fermentation process. For example, linear and non-linear algorithms, neural networks, support vector machines, and other mathematical models, can quickly process many on-line and off-line parameters, and can therefore be correlated with regulatory processes during the fermentation process (Safarian, Saryazdi, Unnthorsson & Richter, 2021; Zhang et al., 2020). Overall, the current options for regulating the fermentation process arise predominantly from the process control perspective and lack rationality (Kim, Yun & Kim, 2009). Other options include guidance provided by cellular metabolic characteristics, but this approach lacks universal applicability.
In terms of SLs fermentation, the supplementation of glucose and oil substrates is essential for the synthesis of SLs. During the late fermentation stage, and with the gradual accumulation of SLs, the rheological properties of the fermentation broth undergoes significant changes, thus increasing viscosity. These changes exert a key impact on mass transfer and mixing, thus resulting in a limited oxygen supply, consequently, the productivity of SLs synthesis decreases notably. The development of a semi-continuous fermentation process could significantly alleviate the influence of oxygen limitation on SLs synthesis (Zhang et al., 2018). It has been found that controlling the content of oil, and the ratio of oil to SLs, can exert influence on the morphology of SLs (crystalline or non-crystalline types), thereby significantly changing the sedimentation characteristics of SLs (Chen et al., 2021b). In turn, this affects the efficiency of semi-continuous fermentation. Therefore, it is vital that we are able to precisely control the process of oil supplementation so that we can achieve the efficient production of SLs.
In this study, we established a multi-scale parameter detection system for the SLs fermentation process by applying a range of on-line sensors, mainly including a near-infrared spectrometer and a process mass spectrometer. First, we studied the differences in macro-physiological and metabolic parameters under different rates of oil supplementation. Subsequently, we used a range of process parameters to construct a data-mechanism fusion model, which was accomplished by integrating data modeling with cellular metabolic mechanisms, for feedback feeding of oil and glucose. Finally, this model was applied to semi-continuous fermentation to achieve a highly efficient production system for SLs.