Unfavorable oxidation-reduction potential (ORP) has also been cited as a cause of poor fer­mentability (Leonard and Hajny 1945). Collingsworth and Reid (1935) found that the addition of reducing agents to media improved their fermentability. Three methods have been pro­posed for overcoming unfavorable ORP in fermentation media: phytochemical reduction by large amounts of yeast; use of reducing agents; and production of reducing substances from sugars by either caramelization or alkali degradation.

When Na2SO3, NaHSO2, Na3SO3.5H2O, Na2S2O3, Na2S2O5, KHSO3, Na2S, sulfite waste liquor, alkali-decomposed sugar, ascorbic acid, cysteine, or reduced iron filings were added to wort hydrolysates, an improved fermentation was observed, which underscored the effect of ORP (Leonard and Hajny 1945; Leonard and Peterson 1947). Diethanolamine, triethanol­amine, pyridine, aniline, dimethylaniline, and similar substances also showed favorable action toward fermentation under the same conditions. The amount of reducing agent required is dependent upon the length and temperature of heat treatment period (Leonard and Hajny 1945; Leonard and Peterson 1947). The mechanism of detoxification by reducing agents is not clear. However, researchers have found that toxic and oxidizing compounds such as furfural and HMF would be reduced to their less inhibitory alcohol forms inside yeast cells associated with oxidation of NAD(P)H, and redirect yeast energy to fixing the damage caused by furans and by intracellular reduced NAD(P)H and ATP levels (Nilsson et al. 2005; Almeida et al. 2007).