3 i4 THE CHEMISTRY OF DAIRYING 



kept in a warm place overnight and then bottled. The corks 

 must be tied down, as a large amount of carbon dioxide is 

 formed. 



Effects of Heat. The more important effects of heat on 

 milk have already been described, but they may be summarised 

 as follows : 



(1) Alteration of density, due to expansion of the liquid. 



(2) Caramelisation of sugar. This change begins at about 

 70 C. ; it continues as long as this temperature is maintained, 

 and it is accelerated at higher temperatures. On prolonged 

 boiling the milk becomes brown in colour, and distinctly acid. 

 Even traces of lacto-caramel impart a disagreeable flavour to 

 the milk. This is often noticeable in milk that has been over- 

 pasteurised. 



(3) Thin cream. The fat globules in milk tend to aggregate 

 together on standing. Heat causes these aggregations to 

 break up, with the result that the cream rises slowly and is 

 thin. 



(4) Formation of haptogen membrane. This skin or scum 

 which forms on the surface of the milk consists of coagulated 

 albumin, together with some fat and partially dried casein. 

 The proteins probably also suffer some partial decomposition, 

 as traces of sulphuretted hydrogen are evolved. 



(5) Alteration of curd. Milk which has been boiled or 

 even heated to lower temperatures is not so readily coagulated 

 by rennet, and the curd which is produced is soft and crumbly. 

 This effect may be an indirect one due to precipitation of 

 calcium phosphate. 



(6) Sterilisation, partial or complete according to the tem- 

 perature, and destruction of all the enzymes naturally present 

 in the milk. 



Condensed Milk. This product would be more correctly 

 described as concentrated milk. It is prepared by evaporating 

 off a portion of the water. The process is carried out in 

 vacuum pans under diminished pressure in order that the 

 temperature may not rise above 65 C. This ensures practi- 

 cally complete sterilisation, and does not caramelise the milk 

 sugar ; but it does not prevent the formation of clots due to 



