Fermentation by-products or non-metabolized feed components can inhibit the ethanol production and yeast growth. These secondary components become more concentrated when used and this limits the recycling process of distillery residue.

Acetate and lactate are the most important inhibitory fermentation by-products

[125] . Certain inhibitors are high in a few substances, e. g., sulfite waste liquor may be high in sulphurous acid and furfural. Blackstrap molasses may contain high concentrations of calcium salts. High temperature, sugar concentration, and ster­ilization in the presence of salts (especially phosphates) and proteins can produce components toxic to yeast [22].

When important individual inhibitors are not present, a combination of inhib­itors or generalized osmotic pressure effects shall be the limiting factors. High salt concentrations also encourage the production of undesirable by-products such as

glycerol [193]. A 16-20% non-fermentable dissolved solids content sets the practical upper limit for most yeasts in the absence of toxic inhibitors [52].