MILK TESTING 127 



available for the reduction of sulphates and for the fixation of carbon dioxide, of 

 which the mechanism is not yet explained. Elementary sulphur is reduced to sul- 

 phides and it is necessary to explain how the sulphur is brought into contact with the 

 cellular enzymes ; solution of the sulphur in unsaturated fats has been suggested. 



Reference should be made to Kluyver (1931), Bunker (1936), Zobell (1937), 

 Butlin, Adams and Thomas (1949). 



MILK TESTING AND GRADING 



The importance of being able to assess rapidly the cleanliness and keeping 

 qualities of milk needs no emphasis. For a comprehensive review Wilson's (1935) 

 monograph should be studied. One method of grading milk, the plate count, by 

 attempting to count the number of bacteria present, is time-consuming, and difficult, 

 leading to fallacious results. The nature of the infecting organisms is of equal import- 

 ance to their numbers when considering the keeping quality of the milk. It follows 

 that an oxidation-reduction indicator method by giving a measure of the metabolic 

 activities of organisms present is likely to give a fair indication of their effect in 

 milk-spoilage. Hobbs (1939) has studied the part played by bacteria in the reduction 

 of methylene blue by milk samples. In good samples of milk coliform organisms and 

 Staphylococcus aureus were the most common contaminants with other staphylococci, 

 micrococci and streptococci present in smaller numbers. The most rapidly reducing 

 organisms were the coliform group followed by Streptococcus lactis, faecal streptococci, 

 Staphylococcus aureus, Staphylococcus albus, Staphylococcus citreus, micrococci, group 

 C haemolytic streptococci, Streptococcus agalactiae and aerobic spore bearers. 



The oxidation-reduction potentials of raw milk incubated anaerobically fell 

 fairly slowly but the potential fell much more rapidly when the milk contained coliform 

 organisms or Streptococcus lactis even with aerobic incubation. The potential fell to a 

 much lower level with coliform organisms than in sterile milk incubated anaerobically. 



The question arises whether the reductions effected in milk are due to natural 

 systems in the milk or to the contaminating bacteria. There are at least four natural 

 reducing systems in milk. The observation by Schardinger that unboiled milk will 

 reduce methylene blue in the presence of formaldehyde may be due to xanthine 

 oxidase ; fat may augment its action both by absorbing the enzyme and substrate 

 and by dissolving the colourless leuco-methylene blue, hence enabling the reduction 

 of the methylene blue to go nearer to completion by the usual mass-action law. 

 The enzyme responsible was found to be associated with the cream but was removed 

 by churning. Lactose, glutathione and ascorbic acid may also contribute to the 

 reducing powers of raw milks. When autoclaved milk develops reducing powers 

 possibly due in part to aldehydes formed from lactose and protein hydrolysis 

 products. In addition there has been discovered photochemical reduction by both 

 raw and heated milk activated by exposure to sunlight. 



Raw and pasteurised milks do not themselves, however, reduce methylene blue 

 at any rapid rate under aerobic conditions unless they are contaminated with bacteria. 

 The reducing activity of contaminated milk is due to the metabolic processes of the 

 contaminating bacteria and may be followed potentiometrically or, a matter of 

 greater practical importance, by the reduction of dyes. Methylene blue has been 

 commonly used and with suitable precautions as to standardisation of reagents and 



