THE ANALYSIS OF CHEESE 87 



at about 22 acidity, and the curd vatted when the 

 whey draining off has an acidity of about 100. 



The acidity of the curd may also be judged by the 

 hot-iron test ; an iron is heated and allowed to cool till 

 it can just be touched with the finger ; the acidity of 

 the curd is judged by the length of the string which 

 is formed when the iron is pressed on the curd and 

 withdrawn. 



The strength of rennet is determined by weighing out 

 0.5 gramme of a solid extract, or measuring 5 c.c. of a 

 liquid extract, and diluting to 100 c.c. i c.c. of the 

 solution is added to 100 c.c. of separated milk of 

 acidity 20 at a temperature of 35 C. ; the tempera- 

 ture is kept at 35, and the milk slowly stirred with a 

 thermometer till it curdles, which is indicated by the 

 path of the thermometer becoming visible ; the time 

 which has elapsed since the addition of the rennet is 

 noted, and the strength of the rennet calculated as 

 parts of rennet which will cause curdling in forty 

 minutes by the following formulae : 



800,000 ,. -, 

 Strength ^ tor solid extracts, 



80,000, ,. -T 

 or 7^ lor liquid extracts, 



where T time in minutes. 



The curdling should not take less than five minutes 

 nor more than ten minutes, and should this be the case 

 less or more of the rennet solution should be used, and 

 the results increased or decreased proportionately. 



