Tke Fermentations of Milk. 
807 
Workers, and their properties studied ; amongst them there appears 
to be at least one true yeast {Saccharomyces) having the power to 
ferment milk sugar into alcohol. It seems probable that the yeasts 
which produce this change do so by first producing a soluble 
ferment (enzyme) by their growth, which has power to invert or 
change the milk sugar into a sugar fermentable into alcohol. It is 
well known that common yeast acts in this way upon common cane 
sugar, inverting it by action of a soluble ferment (invertase) before 
fermenting it. 
Amongst the milk fermemts which the researches of recent years 
have made us acquainted with are many which have the power of 
curdling milk without rendering it acid, some of tliem, on the other 
hand, actually rendering it slightly alkaline. Numbers of species 
which do this liave been studied by Duclaux, who has given 
to a large class of them the generic name Tyrothrix. They are 
considered to curdle milk by producing a soluble ferment or enzyme 
resembling the active principle of rennet, and as a matter of fact 
the clear liquid obtained by filtering a culture of these bacteria 
through biscuit ware will curdle milk, though no bacteria are present 
in it, as they have been removed by the filtration. Not only so, 
but in many cases such a solution contains a second enzyme, which 
dissolves or digests the curd more or less slowly after it has been 
formed. Warington has made the generalisation that all the 
bacteria which possess the double power of first curdling milk, and 
then dissolving the curd, ai-e of the class which liquefy gelatine 
when grown upon it, and this conclusion has been confirmed by 
Duclaux. It seems likely that all the gelatine-liquefying bacteria 
have the property of digesting casein, and it is further probable 
that they do this in order to prepare it for their own nourishment. 
These digestive bacteria, if so they may be called, are of great 
importance in the ripexino op cheese. Speaking very generally, it 
may be said that cheeses are ripened by simultaneous processes of 
two distinct kinds. On every crack and cranny exposed to access 
of air grows one or another, perhaps several, species of mould, 
torula, bacillus, or bacterium. By directly feeding upon and alter- 
ing the casein these organisms produce a small quantity of strongly 
tasting and strongly smelling decomposition products of casein, 
which, like a pinch of spice, communicate a pungent flavour to the 
whole body of cheese. At the same time some of these microbes 
secrete or produce enzymes which gradually difl'use through the 
substance of the cheese from the outside to the centre, softening, 
peptonising, or partially digesting the hard and tasteless curd as 
they penetrate through the mass. Into a detailed account of the 
chemistry of cheese ripening we cannot now enter ; it would require 
another article at least as long as this to give an account of what 
has already been made clear about the maturing of different 
varieties of cheese. 
VOL. lU. T. S. — 12 3 I 
