Yeasts, particularly strains of Saccharomyces cerevisiae, stand as the primary organisms in the realm of bioethanol production. This section delves into various aspects of yeast physiology, with a focus on nutrition, growth, and metabolism, all integral components of alcohol fermentations. For an in-depth exploration of yeasts, Walker’s work in 1998, 2009, and 2010 serves as a valuable resource.
Yeast cells, for their growth and fermentation, demand an array of essential nutrients, broadly categorized into macronutrients and micronutrients. Macronutrients, required at the millimolar level, encompass carbon, nitrogen, oxygen, sulfur, phosphorus, potassium, and magnesium. On the other hand, micronutrients, needed at the micromolar level, include trace elements like Ca, Cu, Fe, Mn, and Zn.
In simple nutritional media, yeast finds an optimal environment for growth, thriving on carbon and nitrogen-backbone compounds, inorganic ions, and a few growth factors. Growth factors, such as vitamins, nucleosides, nucleotides, amino acids, fatty acids, sterols, and various compounds, play pivotal roles in catalytic and structural functions.
Yeasts, including S. cerevisiae, flourish in warm, dilute, sugary, acidic, and aerobic settings. Industrial strains of S. cerevisiae exhibit optimal growth between 20-30ºC and at pH levels ranging from 4.5 to 5.5. Despite being not strictly facultative anaerobes, S. cerevisiae requires oxygen for membrane biosynthesis, particularly in fatty acid and sterol biosynthesis.
S. cerevisiae, with its ellipsoid shape and eukaryotic features, reproduces asexually through budding and sexually through cell conjugation. The growth process involves nutrient transportation, assimilation, and the integration of various functions to enable cell division. The budding cycle, while optimized for laboratory conditions at approximately 90 minutes, extends considerably in industrial fermenters due to the challenging physico-chemical environment.
In the context of alcoholic fermentations by S. cerevisiae, the primary fermentable sugars derived from first-generation feedstocks include sucrose, glucose, and fructose. In contrast, second-generation feedstocks offer glucose, xylose, and arabinose. Notably, S. cerevisiae faces challenges in fermenting pentose sugars like xylose and arabinose, necessitating microbiological and molecular genetic interventions.
The metabolic pathway guiding glucose to pyruvate, known as glycolysis or the Embden Meyerhof Parnas pathway, represents a crucial aspect of alcohol fermentation. The theoretical conversion to ethanol from glucose involves intricate biochemical steps, resulting in ethanol production at around 92% of the theoretical maximum. However, industrial fermentation practices typically achieve yields around 90%, redirecting fermentable carbon to new yeast biomass and minor metabolites.
The overall glucose-to-ethanol pathway encompasses various fermentative enzymes, ensuring the reoxidation of reduced co-enzyme NADH to NAD+. Terminal fermentative reactions lead to the conversion of pyruvate to acetaldehyde, further reduced by alcohol dehydrogenase to ethanol. This process maintains redox balance and supports glycolysis.
Distinct from Saccharomyces cerevisiae, Zymomonas bacteria employ the Entner-Doudoroff pathway for ethanol production. While both organisms convert sugars to ethanol via homoethanol pathways, their routes differ significantly.
In alcohol fermentations, yeast produces a spectrum of fermentation metabolites alongside ethanol and carbon dioxide. While these metabolites contribute to flavor in beverage production, they are undesirable in bioethanol production due to ethanol yield loss. Undesirable metabolites include fusel alcohols, polyols, esters, organic acids, vicinyl diketones, and aldehydes.
Efforts to mitigate undesirable metabolite production, particularly glycerol, involve strategies like simultaneous saccharification and fermentation (SSF) processes. These methods prevent osmostress and limit glycerol production as an undesired by-product. Construction of yeast strains with reduced glycerol production further contributes to optimizing ethanol yield in bioethanol plants.
The intricate interplay of yeast physiology, metabolic pathways, and fermentation strategies underscores the ongoing efforts to enhance bioethanol production efficiency. As research advances, innovations in genetic engineering and fermentation practices promise a more sustainable and optimized future for bioethanol production.