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of peroxides are gradually formed in tinctures of guaiacum and that 
these substances react with the unchanged guaiacum in the presence 
of a peroxidase or suitable oxygen carrier, giving rise to the formation 
of guaiacum blue. On the other hand, there is abundant evidence 
at hand to show that milk contains substances capable of inducing 
the oxidation of guaiacum and other readily oxidizable substances 
by means of hydrogen peroxide or ozonized oil of turpentine. These 
substances are destroyed by boiling and are known as the peroxidases. 
A great many reagents have been proposed for the detection- and 
approximate estimation of the peroxidases in milk, with the view, 
primarily, of distinguishing between fresh or raw and heated (pas- 
teurized) or boiled milk. Among these may be mentioned guaia- 
cum (33), potassium iodide, and starch (34), paraphenylene-dia- 
mine (35), ortol (36), paradiethyl-paraphenylene-diamine (37), ursol 
(38), guaiacol (39), amidol (Leffmann) (35), phenolphthalin (40), 
benzidine (41), etc. These reagents are used in connection with 
small quantities of hydrogen peroxide or some peroxide compound 
such as the persulphates, perborates, or ozonized oil of turpentine, 
and with fresh unheated milk they all give characteristic changes of 
color which are not shown by milks which have been heated to 80° C. 
or higher. 
Whether the peroxidases of milk give rise to any changes in the 
composition of the milk can at present only be conjectured. It may 
be of course that they gradually effect the oxidation of reducing 
substances in the milk. According to some authors they gradually 
disappear when the milk turns sour. It has been our experience, 
however, that they pass practically unchanged into the whey when 
milk curdles as the result of the lactic acid fermentation. In the 
present state of our knowledge the various tests which have been pro- 
posed for the peroxidases of milk are chiefly useful in enabling us to 
form an idea of the condition of the milk, whether it has been heated 
beyond certain temperatures or not, although according to Gillet 
(15) even normal fresh milks vary in the amounts of peroxidases 
which they contain, and this has also been our own experience with 
this reaction. 
Reductases . — According to Seligmann (42) raw milk possesses re- 
ducing properties; for example, it reduces Schardinger’s (43) reagent, 
which consists of a solution of methylene blue containing small 
amounts of formaldehyde. By some authors these reducing substances 
have been regarded as ferments, reductases, by others as due to 
bacteria, and by still others they have been looked upon as identical 
with catalase, the ferment in milk which decomposes hydrogen per- 
oxide. By the use of a weak alcoholic solution of methlyene blue, 
Smidt (44) claims to have been able to distinguish between the re- 
duction brought about by bacteria and that caused by ferments. This 
