566 EVIL EFFECT OF THE AIR UPON BUTTER. 



This is the first stage in the decomposition, which, when once begun, 

 generally spreads or extends with increasing rapidity.* 



Again, I have also stated that this fluid (oleic) acid of butter absorbs 

 oxygen with great rapidity from the air (p. 560), and changes rapidly into 

 other compounds. This is the second stage, and is succeeded by others, 

 which it is unnecessary to enumerate. 



To this action of the air is partly to be ascribed that peculiar kind of 

 rancidity, which, without penetrating into the interior of well packed 

 butter, is yet perceptible on its external surface, wherever the air has 

 come in contact with it. A knowledge of this action of the atmosphere, 

 therefore, urges strongly the necessity of closely incorporating and knead- 

 ing together the butter in the cask or firkin — that no air holes or openings 

 for air be left — that the cask itself be not only water-tight but air-tight — 

 and that it should never be finally closed till the butter has shrunk in as 

 far as it is likely to do, and until the vacancies, which may have 

 arisen between the butter and the cask, have been carefully filled up 

 again. 



§ 17. Of the natural and artificial curdling of milk. 



When milk is left to itself for a certain length of time it becomes sour 

 and curdles. The curd and whey, however, do not readily separate un- 

 less a gentle heat be applied, when the curd contracts in bulk, and either 

 squeezes out and floats upon the whey, or, when cut into pieces or placed 

 in a perforated cheese-vat, allows the whey freely to flow from it. If 

 the mixed curd and whey from the entire milk be allowed to simmer for 

 a length of time at a slow fire, the buttery part will separate from the 

 cheese, and will float on the top in the form of a fluid oil. 



1°. Natural curdling, — The natural curdling of milk is produced by 

 the lactic acid, which, as we have seen (j). 544), is always formed from 

 the milk-sugar when milk is allowed to stand for any length of time iu 

 the air. It does not curdle immediately upon becoming sour, for a reason 

 which I shall presently explain. « 



2°. Artificial curdling. — But it is not usual in the manufacture of 

 cheese to allow the milk to sour and curdle of its own accord. The pro- 

 cess is generally hastened by the artificial addition of acid, or of some 

 substance, such as rennet, by which the natural production of acid is ac- 

 celerated. Almost any acid substance will have the effect of curdling 

 milk. Muriatic acid (spirit of salt), diluted with water, is said to be ex- 

 tensively, though not universally, employed in Holland for this pur- 

 pose. In other countries vinegar, f tartaric acid, lemon juice, cream of 



* I do not know whether a converse change is ever observed in butter by long keeping in 

 contact with brine— whether it ever becomes very sensibly harder. Tallow, as is well 

 known to candle-makers, and especially to the manufacturers of stearin candles, becomes 

 harder by keeping, indeed sometimes is unfit for use until it is a year old— candles in a damp 



Klace become harder by keeping — and in tallow that has lain long in a wet mine the oily part 

 as been found entirely changed into the solid fat of tallow (Beetz) A similar change, 

 therefore, is not impossible nor inexplicable in butter also— only if it ever do take place, we 

 should expect the changed butter to be less solid and dense than before. 



t " To coagulate a cotyla of milk we add a cyathus of sweet vinegar" (Dioscorides). Milk 

 is also curdled by ardent spirits, by the juice of the fig, and by a decoction of the flowers of 

 the artichoke, of the white and yellow bed-straw (gcdium), and of the crowfoot (ranunculus 

 Jlammula and lingula). The Tuscan ewe-cheese is curdled with the juice of the fresh, or 

 with a decoction of the dried flowers of the wild thistle, or with the flowers of the artichoke, 

 which gives a cheese of finer colour and less pungent taste. 



