33 



SIXTH LECTURE. 

 THK UELATION OF BACTERIA AND ENZYMS TO CHEESE MAKING. 



1. Bacteria needed for cheese maJcing. — For the successful makiniz; of 

 Cheddar cheese hictic-acid bacteria are necessar\' (reference No. 9, 

 J). 248). There are man}' varieties of these bacteria, but they are 

 aUke in being able to convert milk sugar into lactic acid. The acid of 

 milk is noticeable to the taste when it reaches 0.3 per cent. The milk 

 begins to thicken when the acid reaches 0.4 per cent, and above 

 0.5 per cent the curd becomes solid. The chemical action of the lactic 

 acid is given in Lecture 1, paragraph 3. Less than one-half of 1 per 

 cent of milk sugar is required to furnish the lactic acid needed in mak- 

 ing cheese. The most favorable temperature for growth of the bac- 

 teria is 90 to 95° F. (references No. 3, p. 26, and No. 9, p. 64). The 

 lactic-acid bacteria are of value by preventing the growth of injurious 

 forms of fermentation. 



[Suggestions to lecturer: Have each student determine the amount 

 of acid (see Lecture 5, paragraph 2) in fresh milk, and then at hourly 

 intervals at room temperature for six or eight hours.] 



2. Bacteria undesirahle for cheese mailing. — Among bacterial fer- 

 ments that act injuriously in cheese making are the following: (1) 

 Those producing gas abundantly, and thereby causing ' ' floating " curds 

 and "hufhng" cheese (references No. 9, p. 69, and No. 11, p. 169); (2) 

 those digesting the curd; (3) those producing offensive flavors (ref- 

 erence No. 11, p. 177); and (4) those producing poisonous products 

 (reference No. 11, p. 179). Many injurious fermentations can be 

 detected. (See Lecture 5, paragraph 4.) 



3. Two classes of ferments. — Two classes of ferments are recognized : 

 (1) Organized and (2) unorganized. Organized ferments are living 

 organisms capable of producing fermentations, such as bacteria (ref- 

 erence No. 3, p. 22). An unorganized ferment or enzym is a chemi- 

 cal substance or ferment without life, capable of causing many 

 changes, the enzym itself undergoing little or no change. They are 

 produced by bacteria and in higher forms of plants and animals. 

 There are two enzyms of special interest in cheese making: (1) Rennet 

 enzyms and (2) galactase. 



4. Rennet enzym is an unorganized ferment capable of coagulating 

 the calcium casein of milk. On account of this property rennet is a 

 fundamental substance in Cheddar cheese making. Rennet is used in 

 the form of an extract. 



(a) Source of rennet extract. The usual source is the fourth stom- 

 ach of a suckling calf. The enzym may be extracted by treatment 

 with dilute salt water. Reliable commercial rennet extracts are pref- 

 erable to home-made extracts, being of more uniform strength. Ren- 

 net should be kept in a cool, dark place (references No. 8, p. 32, and 



