CONTROL OF CHEESE-MAKING. 385 



drawn ; when the curd pulls out in strings it is ready for 

 vatting. 



The Fermentation Test for Milk. — In order to ascertain the 

 fitness of milk for making good cheese, the appearance of the 

 curd on souring should be noted ; the test is carried out by 

 filling test tubes, which must be scrupulously clean, and prefer- 

 ably sterilised, with milk, and after covering them, keeping them 

 for twelve hours at a temperature of 40° C. (104° F.), and exam- 

 ining the curd produced. A good milk will either not have 

 curdled in the time, or will have produced a homogeneous curd, 

 with the development of a clean acid smell. If there is much 

 separation of whey, if the curd is granular, or especially if the 

 curd contains many bubbles or is spongy, the milk will not 

 make good cheese ; a strong unpleasant smell is also very undesir- 

 able. All these conditions indicate the presence of undesirable 

 organisms ; to some extent the effects of these may be counter- 

 acted by the addition of a starter (a pure cultivation of lactic 

 acid bacilli) before renneting, but a milk giving a very gassy 

 curd in the fermentation test will not produce good cheese. 



Other Products Derived from Milk. 



Commercial Milk-sugar — Preparation. — Where whey is a 

 bye-product, in cheese making countries, it is treated for the 

 manufacture of milk-sugar. This is done by allowing it to stand 

 so that the cream present may rise to the surface, heating it, 

 and removing the cream and a portion of the proteins. The 

 whey is neutralised with lime, and a little alum added, which 

 precipitates a further amount of proteins. The whey is then 

 boQed down in vacuum pans, and the sugar allowed to crystallise 

 out. Milk-sugar is purified by re-crystallisation from water or, 

 where alcohol is cheap, by dissolving it in water and precipi- 

 tating with alcohol. 



The milk-sugar of commerce is usually in the form of fine 

 powder, but it is also sold in crystals ; it consists of essentially 

 pure sugar, but may contain sensible amounts of lactic acid, ash, 

 and, sometimes, proteins. Its chief uses are for the preparation 

 of infants' food and the manufacture of penta-nitro-lactose, which 

 forms a part of some high explosives. 



Examination of Commercial Milk-sugar. — Add 6 or 7 

 grammes of the finely-powdered sugar to about 50 c.c. of dis- 

 tilled water ; stir vigorously with a thermometer for ten seconds 

 and allow the solution to settle for twenty seconds ; read the 

 fall in temperature on dissolution and rapidly filter the solution. 

 When sufficient clear filtrate is obtained, fill a 200 mm. polari- 

 scope tube, and polarise as soon as possible. Take polarimetric 



