Yeast for the Production of Biochemicals and Biofuels

Demands for bioenergy resources, nutraceuticals, biofertilizers, and other industrially important chemicals have escalated globally due to rapid urbanization, industrialization, and a growing awareness about bio-based, environment-friendly products [1,2,3]. Microbial cell factories, especially yeast strains, have attracted researchers and industrialists to synthesize natural products, e.g., pigments, chemicals, enzymes, biofuels, etc., because of their ability to utilize inexpensive waste feedstock as nitrogen and energy sources, thereby mitigating the concerns of waste management with simultaneous revenue generation [1]. Wild yeast strains have the innate quality of naturally decomposing complex organic matter into easily assimilable molecules, making them cultivable on agro-industrial waste hydrolysates via optimized bioprocessing [1,4]. Prior knowledge of the organisms’ nutritional requirements, fermentation conditions, and the underlying cellular biosynthetic pathways has led to the enhanced production of valuable commodities with desired properties [5,6]. This Special Issue is focused on the use of various yeast and fungal strains for the synthesis of various biochemicals and biofuels that are lucrative to industry (in energy, healthcare, cosmetics, the food sector, etc.) and the health-conscious population, who prefer bio-based products above chemically synthesized alternatives (Figure 1). Seven articles were published in this issue that highlight the sustainable and environment-friendly production of various yeast-based commodities.

The first and third research articles (chronological order) focus on the use of Saccharomyces cerevisiae for 2,3-butanediol (BD) production, and the fourth on γ-bisabolene production from S. cerevisiae using various strategies. Liu et al. (contribution 1) show that the addition of exogenous short-chain fatty acids, acetic acid, and dissolved oxygen impacted the BD production from S. cerevisiae W141. Their optimized results for the 3.25 ± 0.03 g/L BD yield involved the use of 4.52 mg/L dissolved oxygen and 1.0 g/L acetic acid, ascertaining that the bioprocess was industrially feasible and sustainable. Ao et al. (contribution 2) demonstrate a 3.12-fold enhancement in BD production by cultivating S. cerevisiae L7 by employing the Plackett–Burman multifactorial design combined with a central composite design. Fedbatch fermentation using glucose and acetic acid feed was proven to elevate BD yield, and the results provide strong support for large-scale BD production by Saccharomyces cerevisiae.

The fourth article describes γ-bisabolene production, which is an approved food additive and may be considered as a substitute for D2 diesel in the hydrogenated state. A new γ-bisabolene synthase was identified in this strain and expressed in peroxisomes along with the mevalonate pathway to achieve a γ-bisabolene titer of 125.0 mg/L (contribution 3). Furthermore, the peroxisome autophagy gene was deleted, the squalene synthase expression was decreased, and acetyl-CoA supply was elevated to obtain maximal γ-bisabolene production at 584.14 mg/L (shake-flask) and 2.69 g/L (fed-batch), being the highest record known to date. The second article in the series also demonstrated the significance of genetic engineering strategy for using yeast or fungal strains for production of important healthcare products that can provide a strong foundation to industry for sustainable large-scale production (contribution 4).

The fifth article (contribution 5) discusses the use of different carbon sources to produce torularhodin from oleaginous red yeast Rhodosporidium mucilaginosa using an optimized extraction strategy involving treatment with acid, bases, enzymes, along with heat and ultrasound. The heat and acid combination were considered most suitable for extracting the torularhodin pigment, which was purified using a silica cartridge and eluted with a mixture of methanol–hexane–acetone in a ratio of 2:1:2, and its absorption coefficient (E^1%_1cm) was determined as 3342 in chloroform. The authors demonstrate that various carbon sources influenced the carotenoids content and constituents in this yeast, although torularhodin was still the dominant pigment.

The research topic also includes two interesting review articles on yeast-mediated bioenergy production along with waste valorization (contributions 6–7). The review article by Zhang et al. (contribution 7) focuses on the challenges and opportunities of bioethanol production, whereas Ahuja et al. (contribution 6) highlights the use of yeast for the production of biodiesel, bioethanol, and biogas. The reviews emphasize the need for advanced or integrated technologies to address the current global energy demands by employing waste feedstock for a net zero emission target and to achieve a circular economy, as also solicited by experts [3,7,8].

All the articles in this research article have unanimously vouched for using wild-type or engineered yeast strains for environmentally friendly, cost-effective, and sustainable production of biochemicals and biofuels via optimized bioprocessing strategies.