Solvent recovery and reutilization in bioprocess have great potential to reduce production cost and environmental harm. However, solvent recovery process is often energy-intensive and costly. In this study, we developed an integrated bioprocess for fermentative production of protopanaxadiol (PPD) from ethanol waste recycled in down-stream extraction process, achieving solvent recovery and reutilization in a cost-effective manner. In this integrated process, PPD isolation and purification were achieved by foam separation and resin chromatography, respectively; ethanol solution used as PPD extractant and chromatography eluent was recycled to be reused directly as yeast carbon source for PPD biosynthesis. Notably, in 3 batches of 5 L-fermentation productions, the recycled ethanol could compensate 81.3% of the ethanol used in fermentation. Since the lost PPD during chromatography elution was returned into the next-batch fermentation together with the recycled ethanol, the overall yield of 3- batch production (85.78%) was higher than the yield of single batch production (~75%). This study demonstrates a promising integrated bioprocess for triterpene compounds production from ethanol waste.

Introduction

Protopanaxadiol (PPD), a major bioactive component of Panax ginseng, is regarded as promising high-value pharmaceutical compound for tumor inhibition and depression treatment (Li et al., 2006; Popovich and Kitts, 2004; Hui et al., 2012b). Traditional PPD production relies on phytoextraction that is followed by hydrolysis or enzymolysis, and is a labor-intensive and highly polluting process. Synthetic biology technology provides an alternative method for producing PPD in engineered Saccharomyces cerevisiae (S. cerevisiae) (Xu et al., 2013). In recent years, considerable progress has been achieved for engineering yeast for high production of PPD (Dai et al., 2013; Zhao et al., 2016, 2017). However, the down-stream bioprocess of PPD isolation and purification has not yet been studied. The down-stream bioprocess is known to be relatively solvent intensive. It has been reported that for the production of 1 kg of commercially available active pharmaceutical ingredients a median value of 45 kg of material needs to be used, of which about 50% has been organic solvents (Henderson et al., 2007). Recovery of waste solvent through crystallization or membrane separation can offer significant benefits with regards to reduced storage and waste costs as well as increased compliance with environmental legislation. However, these recovery processes are highly energy-intensive and costly (Constable et al., 2007). Therefore, it is of great interest to find a new way to reutilize the wasted organic solvent to be recycled during down-stream bioprocess.