Abstract
BackgroundDelactosed whey permeate (DWP) is a side stream of whey processing, which often is discarded as waste, despite of its high residual content of lactose, typically 10–20%. Microbial fermentation is one of the most promising approaches for valorizing nutrient rich industrial waste streams, including those generated by the dairies. Here we present a novel microbial platform specifically designed to generate useful compounds from dairy waste. As a starting point we use Corynebacterium glutamicum, an important workhorse used for production of amino acids and other important compounds, which we have rewired and complemented with genes needed for lactose utilization. To demonstrate the potential of this novel platform we produce ethanol from lactose in DWP.ResultsFirst, we introduced the lacSZ operon from Streptococcus thermophilus, encoding a lactose transporter and a β-galactosidase, and achieved slow growth on lactose. The strain could metabolize the glucose moiety of lactose, and galactose accumulated in the medium. After complementing with the Leloir pathway (galMKTE) from Lactococcus lactis, co-metabolization of galactose and glucose was accomplished. To further improve the growth and increase the sugar utilization rate, the strain underwent adaptive evolution in lactose minimal medium for 100 generations. The outcome was strain JS95 that grew fast in lactose mineral medium. Nevertheless, JS95 still grew poorly in DWP. The growth and final biomass accumulation were greatly stimulated after supplementation with NH4+, Mn2+, Fe2+ and trace minerals. In only 24 h of cultivation, a high cell density (OD600 of 56.8 ± 1.3) was attained. To demonstrate the usefulness of the platform, we introduced a plasmid expressing pyruvate decarboxylase and alcohol dehydrogenase, and managed to channel the metabolic flux towards ethanol. Under oxygen-deprived conditions, non-growing suspended cells could convert 100 g/L lactose into 46.1 ± 1.4 g/L ethanol in DWP, a yield of 88% of the theoretical. The resting cells could be re-used at least three times, and the ethanol productivities obtained were 0.96 g/L/h, 2.2 g/L/h, and 1.6 g/L/h, respectively.ConclusionsAn efficient process for producing ethanol from DWP, based on C. glutamicum, was demonstrated. The results obtained clearly show a great potential for this newly developed platform for producing value-added chemicals from dairy waste.
Highlights
Delactosed whey permeate (DWP) is a side stream of whey processing, which often is discarded as waste, despite of its high residual content of lactose, typically 10–20%
Barrett et al heterologously expressed both the lactose- and galactose-utilizing pathways from lactic acid bacteria in C. glutamicum and successfully employed the engineered C. glutamicum to produce l-lysine on a whey-based medium. This strain exhibited slow growth on lactose, and the plasmid-based expression vector was unstable [32]. These results indicate that further work is needed before a robust C. glutamicum strain capable of efficiently transforming dairy waste into valuable products is ready
Construction of a lactose metabolizing C. glutamicum strain and assessment of its genetic stability To enable C. glutamicum to grow on lactose, we decided to introduce the genes encoding lactose permease and β-galactosidase (lacSZ) operon from S. thermophilus, which encodes a lactose permease and a β-galactosidase
Summary
Delactosed whey permeate (DWP) is a side stream of whey processing, which often is discarded as waste, despite of its high residual content of lactose, typically 10–20%. Microbial fermentation is one of the most promising approaches for valorizing nutrient rich industrial waste streams, including those generated by the dairies. We present a novel microbial platform designed to generate useful compounds from dairy waste. DWP, a concentrated residual of the process, contains 10–20% lactose and minerals, is of little economic value but of high polluting load, and is commonly used as animal feed [11]. As DWP still contains large amounts of lactose, efforts have been put to develop valorization processes, for instance, processes where the sugar is converted into valuable chemicals through microbial fermentation [12,13,14,15]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
