RELATION OF BACTERIA TO DAIRY PRODUCTS 565 



Several attempts have been made to determine how much 

 buttermilk may be added to skim-milk without causing it to 

 curdle when heated. We have found that not over 5 per 

 cent of buttermilk could be added to sweet skim-milk and the 

 mixture heated without curdling. 



No satisfactory way of pasteurizing buttermilk without sepa- 

 rating the curd from the whey is known, but when sweet cream 

 is pasteurized before ripening and churning there is no need of 

 pasteurizing the buttermilk. 



Tests for pasteurized milk (Hastings) 



Starch's test. The test which is usually employed by in- 

 spectors and chemists is the Storch test, named for its dis- 

 coverer. Two solutions are necessary 1st, a solution of 

 hydrogen peroxide. This substance is sold on the market in 

 the form of a 3 per cent solution of the pure hydrogen peroxide. 

 For use this is diluted with 14 parts of water. This solution 

 kept in the light gradually loses its strength, hence should be 

 renewed at intervals of six weeks. If 0.1 per cent of sulfuric 

 acid is added, its keeping properties will be greatly increased. 

 The second reagent needed is an organic compound, paraphe- 

 nylendiamine, a crystalline substance, colorless when perfectly 

 fresh, but gradually decomposing and taking on a brownish 

 color. One part of this substance is dissolved in 50 parts of 

 hot water and the solution filtered through paper. 



The test can be carried out as follows : 20 c.c. of the milk 

 are placed in a tea cup ; three to five drops of the hydrogen 

 peroxide added, and mixed with the milk. One or two drops 

 of the paraphenylendiamine solution is then added. In raw 

 milk a grayish blue color develops at once, which in one-half 

 minute changes to an indigo blue. If the milk has been heated 

 to 176 F. or above, no color will develop immediately after the 

 addition of the paraphenylendiamine. A color will appear in 



