80 



OXIDATIOX-REDUCTION POTENTIALS 



dyes the potential during rapid fermentation fell to + 0-03 v. or + 0-04 v. at pH 6-4^ 

 various phosphorus compounds accelerating the fall in potential. Phospho-glyceric 

 acid was found to be most effective. When the phosphorus compound was used up 

 the potential rose, but in the case of arsenates, which also accelerated the potential 

 fall, there was no rise in potential as long as any glucose remained unfermented. 



Some interesting observations on the electrode potentials in cultures of different 

 yeasts have been made by Kluyver and Hoogerheide (1934). They inserted an 

 electrode in a AVarburg manometer and investigated the potentials reached under 

 aerobic and anaerobic conditions. In the case of nearly all yeasts the potentials 

 established under anaerobic conditions were from + 0-08 v. to + 0-10 v. at pH 5-4. 

 irrespective of conditions in the culture. In aerobic cultures the potential did not 

 fall to as low a level and differences in conditions of fermentation and in different 

 species of yeast were reflected in the potentials reached, as shown in table 17. 



Fromageot and Desnuelle (1935) made some observations by means of indicators 

 on the potential reached in top yeast fermentations carried out anaerobically, and 

 found at pH 6-4 potentials ranging from — 0-145 v. to — 0-185 v. Certain reduction 

 reactions which are thermodynamically possible at this Ej, level were found to occur. 



The keeping qualities of beer have been stated to be dependent upon the 

 electrode potential (Clerck, 1934 ; Hartong, 1934 ; Mendlik, 1934 ; Pozen, 1935). 



TABLE 18 

 Lowest Potentials reached in Aerobic Cultures of Different Yeasts, etc. 



The oxidation-reduction potential reached in yeast suspensions was found by 

 Fromageot and Bost (1937) to be dependent upon the dye added, but Kakukawa 

 (1938) found the potential reached in living yeast suspensions to be independent of 

 the presence of indicator dyes. Dehydrogenations produced by wood-destroying 

 moulds {Merulius and Fomes) have been studied by Vitucci, Pallares and Nord (1946). 

 Resazurin proved a convenient oxidation-reduction potential indicator and electrode 

 potentials were measured in growing cultures. 



WARBURG MANOMETRIC TECHNIQUE 



There is a considerable number of methods of measuring the respiratory activities 

 of tissues, cells, enzyme systems, etc., dependent upon alterations in pressure when 

 oxygen gas is absorbed and carbon dioxide is liberated. The variations in experimen- 

 tal technique to suit individual needs are very numerous, but the general principles are 

 similar. Many workers have made contributions to the technique and papers by 

 Barcroft, Dickens. Elliott, Fenn, Keilin. Krebs, Roughton, Summerson. Thunberg 



