In the realm of industrial bioethanol fermentation, the choice of fermentation system plays a pivotal role in shaping the efficiency and yield of the bioethanol production process. Several systems are at the disposal of bioethanol producers, each with its own set of advantages and disadvantages. These encompass batch, continuous, semi-continuous, and immobilized systems, each tailored to specific production needs.
The table below delves into the pros and cons of these fermentation systems, shedding light on their applicability and limitations. In addition to the mentioned systems, variations and adaptations, such as the modified Melle-Boinot system prevalent in Brazilian fuel ethanol plants, bring further diversity to the fermentation landscape.
- Simultaneous Saccharification and Fermentation (SSF): A method combining the saccharification of biomass into sugars with fermentation in a single process.
- Direct Microbial Conversion Technologies (DMC): Technologies streamlining microbial conversion processes for enhanced efficiency.
- Very High Gravity Fermentations (VHG): Fermentation processes operating at high sugar concentrations, contributing to increased ethanol concentrations.
Regardless of the chosen fermentation system, the primary goal for bioethanol producers remains consistent: achieving rapid and efficient conversion of sugars into ethanol. Monitoring various parameters during fermentation, including yeast cell density, sugar consumption, pH, temperature, foaming, and alcohol content, becomes imperative to ensure a controlled and optimal process.
To maintain this control, distilleries closely monitor and regulate key parameters, notably temperature and pH. Parameters such as spirit yield, conversion efficiencies, and the correlation between initial sugar concentration and final ethanol yield are crucial for assessing fermentation performance.
The predicted spirit yield (PSY), a key metric, estimates the expected alcohol production from a given amount of cereal. Traditionally measured through lab-scale processes, near-infrared analyzers now offer rapid predictions. Values for wheat, for instance, typically range from 385-400 liters per tonne.
From an agronomic perspective, bioethanol feedstocks are also evaluated based on potential ethanol yields per hectare of cultivable land. Sweet sorghum, wheat, Jerusalem artichoke, sugar beet, chicory, and potato are examples, each offering unique yields per hectare.
Moving into the microbiological realm of fermentation, optimizing yeast viability and vitality while minimizing contaminant bacteria is paramount. The presence of wild yeasts and lactic acid bacteria, particularly Lactobacillus spp., can significantly impact ethanol yield. Lactic acid bacteria, in competition with yeast for sugars, result in ethanol loss.
Sterilization of fermenters, yeast mixing vessels, and associated pipelines is crucial for bacteria control. Acid-washing yeast slurries is a common practice in distilleries to reduce bacterial contamination. Bioethanol plant operators implement various measures, including the use of preventative antibiotics (where permitted), chemical cleaners, sanitizers, sterilants, and heat sterilization to ensure optimal hygiene.
In the context of yeast selection for bioethanol fermentations, using the correct strain is vital. Commercially available strains of Saccharomyces cerevisiae, designed for high ethanol concentrations, effective fermentation in high solids, and robust stability, are now commonplace. Some plants engage in yeast recycling, eliminating the need for frequent yeast purchases. Others conduct in-house yeast propagation through rigorous aeration to stimulate yeast growth.
Ongoing research and development focus on enhancing stress resistance in industrial yeast strains, particularly in terms of temperature and ethanol tolerance, and the ability to withstand chemical inhibitors present in lignocellulose hydrolysates. Genetic engineering plays a pivotal role in these developments, aiming to equip yeast cells with the resilience needed to thrive in challenging environments and overcome substrate toxicity. The works of Bettiga et al. (2008) exemplify the intensity of research endeavors in this critical domain. As the bioethanol landscape evolves, these innovations pave the way for more robust and efficient fermentation processes.