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The effects of toxicity on ethanol production

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The production of ethanol is often impacted by the toxicity of feedstocks used. It is essential to understand feedstock-related toxicities’ adverse effects on ethanol production and to take measures to minimize their effects. 

Feedstock toxicity, a challenge for productivity and yield

Organic acids, the principal inhibitors of fermentation

Organic acids – such as lactic acid, acetic acid, propionic acid, and butyric acid – are the primary inhibitors coming from the substrates. They are highly present in cane molasses and beet substrates and present, to a lesser degree, in c-starch substrates. Organic acids in the medium negatively impact yeast growth and fermentations’ overall performance leading to lower ethanol productivity and yield 

Osmotic pressure inhibitors

In addition to organic acids, feedstocks may contain sulfites and sodium present at high concentrations. The presence of such compounds in fermentation increases osmotic pressure, which also inhibits yeast growth 

The importance of yeast robustness and fermentation pH

To minimize the impact of inhibitors, it is critical to use a robust yeast able to tolerate unfavorable conditions. This will lessen the impact of organic acids and other toxic compounds present in the medium on yeast growth and conversion of sugars to ethanol.  

pH is also a key factor that influences yeast growth and fermentation efficiency. The acid form for an organic acid (couple acid/base) is the toxic form, and the lower the pH the higher the proportion of acid form. Indeed, yeasts require a large amount of energy to eliminate protons released by the acids in the cell. So, the lower the pH the higher the inhibition of an organic acid. However, increasing pH too much causes a risk for bacteria growth – another hazard factor for fermentation. pH has to be optimized to a reach balance favoring yeast growth, limiting the effects of organic acids on the yeast, and inhibiting bacteria growth. 

Strategies to minimize toxicity

Different methods can be implemented to reduce the impact of toxicity on ethanol production:  

  • Adapting the pHideal between 4.0 and 5.5 for the yeast – with careful attention to contaminants, to achieve favorable conditions for yeast growth and fermentation, 
  • Adapting the feeding rate, to which the substrate is injected into the fermenter, 
  • Regularly testing the feedstocks, as organic acids concentrations change in molasses and heavy substrates depending on their storage conditions and times, 
  • Using nutrition to supply the yeast with the correct number of vitamins and minerals for efficient fermentation. 

In conclusion, feedstock toxicity poses significant challenges to ethanol production. The toxicity of feedstocks inhibits yeast growth and reduces fermentation productivity, leading to a decline in yield and profitability. Therefore, it is crucial to recognize the toxic compounds present in the feedstock to adopt preventive measures and minimize their effects. The use of robust yeast, nutritional support, and pH and process optimization are the primary approaches for withstanding the effects of feedstock toxicity on ethanol production while boosting profitability. 

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