“I was much surprised, I admit,” Emile rejoined, “to find how two things, orange juice and milk, each excellent by itself, could make such a nasty drink when mixed.”
“In future, my friends, you will know that anything sour makes milk turn. What I brought about with lemon juice you effected with orange, which contains, though in small quantity and masked by the sweet flavor of the fruit, exactly the same ingredient that gives the lemon its sour taste.
“The juice of sorrel leaves, that of green grapes, and of unripe fruits in general, vinegar, and in fact everything with a similar taste, make milk turn at once. These sour-tasting substances are called acids. Vinegar is an acid; that which gives its sourness to the lemon is another; green grapes contain a third; sorrel leaves furnish a fourth. The number of acids is very considerable. All those that we need to know anything about have this same sharp flavor, sometimes stronger, sometimes weaker; all, in short, make milk curdle just as I showed you with the acid of the lemon.
“From theory let us turn to practice. Cleanliness in everything is of the first importance, but in the care of milk especially must one be scrupulous in this particular. The vessels for holding it and keeping it any length of time must be carefully and thoroughly cleaned as often as they are used, if one [[304]]would avoid the risk of its turning. Suppose a few drops of old milk or some remnants of any kind of food are left in a pot, tucked away where they are hard to get at: these impurities soon turn sour, especially in warm weather, and the milk, finding an acid substance in the vessel, quickly spoils and curdles. How often the milk itself is blamed for this accident when want of cleanliness is the sole cause!
“Milk contains three principal substances, namely: cream, or fatty matter from which butter is made; casein, or curds, used for making cheese; and, finally, a substance with a slightly sweet taste called sugar of milk. These three ingredients taken away, hardly anything is left but water. To separate these three, one proceeds as follows:
“Left standing in a cool place and exposed to the air, milk becomes covered, sooner or later, according to the season, with a thick oily layer that takes the name of cream. This is the material from which butter is made. It rises to the surface unaided and separates when simply exposed to the air. It is removed with a skimmer.
“What is left is skimmed milk, of the same whiteness, the same appearance, as the original milk, but deprived of its fatty matter. Into this skimmed milk let us pour a few drops of some acid, lemon juice for example. The milk turns and thick white flakes are formed. Those flakes are the curd, the casein, in short the material of which cheese is composed. [[305]]
“After the casein has been removed there remains nothing but a transparent liquid that might be taken for water slightly tinted with yellow. This liquid is called whey. It contains little besides water with a small quantity of sugar of milk which gives it a slightly sweet taste. It is especially in Switzerland that sugar of milk is obtained on a large scale by the evaporation of the liquid that remains after removing the cream and curds from the milk. In spite of its name this substance has nothing in common with ordinary sugar, the white loaf-sugar we use; it is a dull-white substance, rather hard, crunching under the teeth, and of a slightly sugary taste. It is used only in pharmacy.
“Cream and casein constitute the nutritive ingredients of milk, and determine its food value. The milk that is richest in these constituents is sheep’s milk, next comes goats’ milk, and last of all cows’ milk. Although of little value to us, sugar of milk claims our attention for a moment on account of the change it undergoes to the great detriment of the milk itself. Little by little, especially when exposed to the heat of summer, this sugary matter sours and becomes an acid. That is what makes milk sour if kept too long. Of course when this sourness shows itself the milk soon curdles. Coagulation takes place as if an acid had been added to the milk. Hence, to keep milk for some time and prevent its turning sour of its own accord, this acidulation of the sugar of milk must be delayed. This is done by taking care to boil the milk a little every day.” [[306]]