128 OXIDATION-REDUC TION POTENTIALS 



conditions, shaking to suspend the cream, etc., has led to reliable official methods 

 of grading. Recently, however, an official test utilising Resazurin has been adopted 

 by the Ministry of Agriculture. Resazurin is blue (in the oxidised form) at pH 6-5 

 and red at pH 5-3. Its first reduction product, Resorufi.n, is pink at pH C-5 and 

 yellow at pH 4-8, whilst Dihydroresorufin is colourless. Fundamentally Resazurin 

 is similar to methylene blue both in constitution and behaviour but v/hereas 

 Methylene blue is reduced at an E^ range of + 0-12 to + 0-03 v., Resazurin is reduced 

 before the potential has fallen so far, i.e., at a range of + 0-18 to + 0-09 v. 



Fresh milk containing dicp^lved oxygen has an E^ of +0-2 v. so that any 

 reducing system in the milk, either natural or from an infecting organism will reduce 

 Resazurin more quickly than methylene blue. So that, whilst methylene blue 

 reduction can be ascribed to bacterial contamination Resazurin reduction may occur 

 due to pus and tissue cells present in mastitis. Although the methylene blue test 

 may be a more accurate indicator of the extent of bacterial contamination the 

 Resazurin test will be a more delicate criterion of udder disease, a common cause of 

 faults in cheese-making. (Davis, 1939.) 



Twigg (1945) has studied the oxidation-reduction potential behaviour of 

 Resazurin, and Vitucci and others (1946) have reported the effect of the dye on moulds. 



CHEDDAR CHEESE 



Some interesting observations on the significance of oxidation-reduction 

 potentials in cheese making have been described by Davies, Davis, Dearden and 

 Mattick (1931, 1932, 1934). The bacteria most active in the metabiotic stimulation 

 of the pigment of " rusty spot " are those producing a low electrode potential, 

 whilst the fault known as " oihness " in dairy products may be prevented by the 

 growth of suitable bacteria which so lower the potential that fat oxidation is 

 prevented. These oxidative taints do not appear when the E^ is less than + 0-3 v. 

 at pH 6-5, whilst " anaerobic " faults, such as rusty spot, do not appear when the 

 Eh is above — 0-1 v. at pH 5-0. Gassy fermentations are prevented by adding 

 oxidants such as nitrates. The electrode potentials in the interior of a ripening 

 cheddar cheese follow a well-defined course and it is concluded that there is first 

 the establishment of an almost pure culture of Streptococcus lactis, followed by a 

 lactobacillus in the later stages of ripening. 



HEMOLYSIN 



Neil and Mallory (1926) have found that the hsemolysin produced in certain 

 cultures of hsemolytic streptococci is inactivated by oxygenation but is re-activated 

 by treatment with reducing agents such as sodium hydrosulphite. Todd (1932, 

 1933, 1934) has extended this study and finds that there are several kinds of strepto- 

 lysin. Those of greatest interest from the point of view of human disease are the 

 streptolysins of group A hsemolytic streptococci (Lancefield's classification). These 

 group A strains produce two kinds of streptolysin. One of these is not inactivated 

 by oxygenation and does not appear to function as an antigen. The oxygen-labile 

 hsemolysin is readily inactivated and re-activated by changes in the oxidation- 

 reduction conditions of the system ; and it gives rise to an antibody wdien injected 

 into animals. This antibody neutralises the hsemolytic properties of the strepto- 

 lysin and its presence in the serum of patients is of great value in the diagnosis of 

 infections with hsemolytic streptococci. 



