Bioethanol is one of the most commonly used biofuels in transportation sector to reduce greenhouse gases. S. cerevisiae is the most employed yeast for ethanol production at industrial level though ethanol is produced by an array of other yeasts, bacteria, and fungi. This paper reviews the current and nonmolecular trends in ethanol production using S. cerevisiae. Ethanol has been produced from wide range of substrates such as molasses, starch based substrate, sweet sorghum cane extract, lignocellulose, and other wastes. The inhibitors in lignocellulosic hydrolysates can be reduced by repeated sequential fermentation, treatment with reducing agents and activated charcoal, overliming, anion exchanger, evaporation, enzymatic treatment with peroxidase and laccase, in situ detoxification by fermenting microbes, and different extraction methods. Coculturing S. cerevisiae with other yeasts or microbes is targeted to optimize ethanol production, shorten fermentation time, and reduce process cost. Immobilization of yeast cells has been considered as potential alternative for enhancing ethanol productivity, because immobilizing yeasts reduce risk of contamination, make the separation of cell mass from the bulk liquid easy, retain stability of cell activities, minimize production costs, enable biocatalyst recycling, reduce fermentation time, and protect the cells from inhibitors. The effects of growth variables of the yeast and supplementation of external nitrogen sources on ethanol optimization are also reviewed.

The term single cell protein (SCP) refers to the dried microbial cells or total protein extracted from pure microbial culture (algae, bacteria, filamentous fungi and yeasts) which serves as food or/and feed supplements. Different substrate and fermentation optimizations are being carried out to maximize SCP production. However, little attention was given to coculturing. SCPs are produced better using coculture than monoculture. This paper reviews the positive roles of coculture in SCP production. First, it results in better saccharification of substrates and efficient carbon source utilization. Second, filling substrate utilization gap is another contribution of coculture. Third, it upgrades biomass and enrich SCP with nutrients than monoculture. Fourth, it reduces fermentation time and production cost by reducing cost of substrate treatment. Key words: Single cell protein, biomass, coculture, monoculture, mixed culture.

Yeasts are important microorganisms used for ethanol production; however, they are not equally efficient in the amount of ethanol production under different environmental conditions. It is, therefore, necessary to screen for elite strains to utilize them for commercial production of these commodities. In this study, yeasts were isolated from different Ethiopian traditional fermented alcoholic beverages (teji, tella, shamiata and areqe tinisis), milk and ergo, teff and maize dough, soil and compost, flowers, and fruits to evaluate their potential use for ethanol fermentation process. Isolates were screened for efficient ethanol production and the selected ones were identified using phenotypic and genetic characters using D1/D2 region of LSU rDNA sequence analysis. The yeast isolates were evaluated based on their growth and fermentation of different carbon sources. Response surface methodology (RSM) was applied to optimize temperature, pH and incubation time using central composite design (CCD) in Design-Expert 7.0.0. A total of 211 yeasts colonies were isolated of which 60% were ethanologenic yeasts (ethanol producers) and 40% were non-ethanol producers. The yeast population detected from various sources was in the range of 105CFU from traditional foods and beverages to that of 103CFU from fruits and soil samples. The data also showed that the number of colony types (diversity) did not correlate with population density. The highly fermentative isolates were taxonomically characterized into four genera, of which 65% of the isolates (ETP37, ETP50; ETP53, ETP89, ETP94) were categorized under Saccharomyces cerevisiae, and the remaining were Pichia fermentans ETP22, Kluyveromyces marxianus ETP87, and Candida humilis ETP122. The S. cerevisiae isolates produced ethanol (7.6-9.0 g/L) similar with K. marxianus ETP87 producing 7.97 g/L; comparable to the ethanol produced from commercial baker's yeast (8.43 g/L) from 20 g/L dextrose; whereas C. humilis ETP122 and P. fermentans ETP22 produced 5.37 g/L and 6.43 g/L ethanol, respectively. S. cerevisiae ETP53, K. marxianus ETP87, P. fermentans ETP22 and C. humilis ETP122 tolerated 10% extraneous ethanol but the percentage of ethanol tolerance considerably decreased upon 15%. S. cerevisiae ETP53 produced ethanol optimally at pH 5.0, 60 h, and 34oC. pH 4.8, temperature 36oC, and 65 h of time were optimal growth conditions of ethanol fermentation by K. marxianus ETP87. The ethanol fermentation conditions of P. fermentans ETP22 was similar to S. cerevisiae ETP53 though the ethanol titer of S. cerevisiae ETP53 was higher than P. fermentans ETP22. Therefore, S. cerevisiae ETP53, K. marxianus and P. fermentans ETP22 are good candidates for ethanol production.

Ethanol production from non-food substrate is strongly recommended to avoid competition with food production. Whey, which is rich in nutrients, is one of the non-food substrate for ethanol production by Kluyveromyces spp. The purpose of this study was to optimize ethanol from different crude (non-deproteinized, non-pH adjusted, and non-diluted) whey using K. marxianus ETP87 which was isolated from traditional yoghurt. The sterilized and non-sterilized whey were employed for K. marxianus ETP87 substrate to evaluate the yeast competition potential with lactic acid and other microflora in whey. The effect of pH and temperature on ethanol productivity from whey was also investigated. Peptone, yeast extract, ammonium sulfate ((NH₄)₂SO₄), and urea were supplemented to whey in order to investigate the requirement of additional nutrient for ethanol optimization. The ethanol obtained from non-sterilized whey was slightly and statistically lower than sterilized whey. The whey storage at 4 °C didn’t guarantee the constant lactose presence at longer preservation time. Significantly high amount of ethanol was attained from whey without pH adjustment (3.9) even if it was lower than pH controlled (5.0) whey. The thermophilic yeast, K. marxianus ETP87, yielded high ethanol between 30 and 35 °C, and the yeast was able to produce high ethanol until 45 °C, and significantly lower ethanol was recorded at 50 °C. The ammonium sulfate and peptone enhanced ethanol productivity, whereas yeast extract and urea depressed the yeast ethanol fermentation capability. The K. marxianus ETP87, the yeast isolated from traditional yoghurt, is capable of producing ethanol from non-sterilized and non-deproteinized substrates.

Besides organic compounds such as lactose and proteins, cheese whey is rich in other nutrients. Damping of these valuable compounds to the environment, first, harms the environment, and second, it wastes valuable resources. Therefore, this review aims to find out the current progress on the valorization of cheese whey for ethanol production. Efficient ethanol-producing yeasts like Saccharomyces cerevisiae have no pathway to utilize lactose and, therefore, they can be co-cultured with microbes that can produce β-galactosidase. In addition, chemical, biological, and physical hydrolysis of lactose can be used to produce ethanol from cheese whey. Ethanol production from unsterilized or unpasteurized whey is very promising and this reduces the production cost significantly. This suggests that the ethanol-producing yeasts are competent against the lactic acid bacteria that are commonly found in cheese whey. Despite the presence of central metabolic genes associated with ethanol production from different sugars in some yeasts, these yeasts can’t ferment the different sugars and this is basically due to a lack of the different sugar transport systems in the yeasts. Therefore, additions of different sugars to whey to increase the sugar content for economical ethanol production are impaired by catabolite repressions. However, catabolite repression can be significantly reduced by metabolic engineering by targeting sugar transporter proteins like the major facilitator superfamily (MFS), particularly LAC, CEL2, HGT, RAG, and KHT. Therefore, this enhances ethanol production from cheese whey supplemented with a variety of sugars. Currently, nanoparticles and metal-organic frameworks coated immobilization of S. cerevisiae produced higher ethanol from lignocellulosic substrates than the classical carries such as alginates; however, studies of such immobilizing materials on Kluveromyces spp for ethanol production are very limited, and open for research. Electro-fermentation, an emerging bioprocess to control microbial fermentative metabolism, boosts ethanol production, enables the production of 14% (v/v) ethanol, and shortens the fermentation time of high sugar-containing whey. Generally, utilizing efficient yeast (possibly by adaptive evolution and genetic engineering) at optimal fermenting conditions enabled to production of economical ethanol from cheese whey that contains higher sugars (greater than 15%) at the large-scale cheese processing industries.