Food Waste and Waste Cooking Oil Valorization for Sustainable Production of Sophorolipids


Student thesis: Doctoral Thesis

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Award date4 Jul 2023


The high human population has led to higher demand for chemical products and therefore, derived problems with more waste formation, especially food waste (FW) and waste cooking oil (WCO). On the other hand, the wide use of chemical products generated from fossil-based resources has also derived environmental problems. Bio-based resources may help to overcome these problems because they are renewable, associated with reduced greenhouse gas emissions and, if not overexploited, sustainable. Sophorolipids (SL) is a green biosurfactant with low eco-toxicity and high biodegradability which shows its potential to replace its fossil-based counterpart. However, the high production cost has hampered its competitiveness. In this thesis, we focused on the green production of lactonic SL via WCO and FW valorization and streamlining of the production bioprocess for more economic production.

In order to reduce production costs and protect the environment, bakery waste oil (BWO) was used as the hydrophobic secondary/waste feedstocks to produce sophorolipids (SLs). The first project aims at optimizing the fermentation medium using BWO as the carbon source for SL production by Starmerella bombicola. To obtain the optimal conditions, multiple linear regression was used to analyse the influence of different parameters. It was found that the inoculum concentration of 2% v v− 1 and BWO and glucose concentrations of 60 g L− 1 and 100 g L− 1, respectively, resulted in an increase of 19.6% in the lactonic SL (67.8 ± 11.5 g L− 1) production. Further optimization indicated the profound influence of pH regulation at 3.5 by KOH, i.e., compared with NaOH. KOH led to higher concentrations of biomass (p < 0.05), more BWO consumption, and thus, an increase of 42.2% in SL titre (96.4 ± 9.1 g L− 1) and corresponding volumetric and specific productivities of 0.446 g L− 1 h− 1 and 0.027 g g CDW−1 h−1, respectively. Multiple regression analysis demonstrated that pH and the concentration of BWO as the feeding medium were the most influential parameters in fermentative SLs production. This study demonstrated that KOH offered benefits to improve SLs titre by maintaining high biomass during the bioprocess, displayed the importance of intracellular potassium in cell viability and improved the valorisation of the BWO.

To further reduce the production cost and improve sustainability, commercialized (citywide, central collected) food waste obtained from the supermarket was used as the hydrophilic secondary/waste feedstocks for SLs production. However, in order to adopt FW, efforts have to be put to overcome the inhibitory effect caused by the indigenously existing inhibitor(s). In the second project, the optimization of the process flow on FW from preparation to SL production was demonstrated which included, 1) identification of the inhibitor in FW; 2) introduction of the washing step and 3) further optimize of the process flow to produce SLs.

Upon the FW hydrolysate has been produced, the components of the hydrolysate were assessed. It was identified that food waste contains lactic acid, acetic acid and ethanol. This indicated the nature of waste streams may impede their efficient utilization as fermentation feedstock. In a control experiment, lactic acid exerted maximally inhibited lactonic SL production by 85.6% and inhibited cell growth by 62.6%. To overcome the inhibitory effect, the introduction of the washing step with deionized water during the preparation of FW was demonstrated. HPLC analysis has shown that when the washing step was introduced, the concentration of those inhibitors was found to drop in residual FW. This indicated the feasibility of the washing step to remove the potential inhibitor(s) in the FW.

Further optimization of the process flow has demonstrated that the lactonic SL production was observed only if the washing step have been carried out which further identified the dose-dependent inhibitory effect of lactic acid on lactonic SL production. After further optimization, the best condition for FW treatment was identified which include a washing step to remove the lactic acid, centrifugation of the washout solution to retrieve the residue and adjusting pH of the sterilized hydrolysate to 5.6 by KOH prior to the SL bioprocess. Based on the modification of the process flow, the highest concentration of 137.5 ± 9.7 g L-1 lactonic SL was achieved.

Although biorefinery development is the best option to minimize waste disposal by using waste as feedstock, not all biorefinery concepts are economically viable. This thesis provides insights into better utilization of hydrophilic and hydrophobic waste streams as the feedstock for sustainable production of SLs. Through the optimization of SLs bioprocess in BWO and FW, the productivity of the SL from the waste stream has been improved, therefore making the SLs production more sustainable and facilitating
the transformation from a linear to a circular economy for global industries and society.