400 BACTERIA IN DAIRY PRODUCTS 



CHEESE 



The characteristic flavor and aroma of each of the great number of cheeses which 

 have been developed by empirical methods are almost entirely due to the growth of 

 bacteria and other micro-organisms. In each type of cheese there is usually a def- 

 inite sequence of organisms which develop at successive stages of ripening. Some of 

 these organisms have little to do with the flavor production but are of importance in 

 controlling the course of the fermentation and in preparing the ground for the bac- 

 teria or molds which give the cheese its characteristic flavor. The nature of this 

 biological sequence and the resulting type of cheese are determined in the making 

 process, especially by the temperatures used, the water retained, the amount of salt, 

 and the method by which it is incorporated, and by the temperature of curing. 



The simplest cheeses are those of the cottage or Neufchatel type which undergo 

 no ripening and depend on a lactic fermentation of the milk for their flavor. The acidity 

 of the curd has a protective action, but deterioration may be caused by surface growth 

 of Oiditim lactis or by anaerobic spore-formers which may give sour and bitter flavors. 



Hard cheeses such as Cheddar, Edam, Emmental, or Swiss and Parmesan, 

 which are made from milk curdled with rennet, undergo a complicated curing proc- 

 ess in which the hard, rubbery texture is materially altered and the insoluble para- 

 casein is converted into soluble products among which are albumoses, peptone, amino 

 acids, ammonia, and carbon dioxide. In this proteolysis the natural enzymes of the 

 milk, the pepsin of the rennet, and possibly the lactic bacteria play an important 

 part. 



In Cheddar, which is the cheese most generally made in this country, the manu- 

 facturing process encourages the lactic fermentation. The high buffering action of 

 the curd permits a continuation of the fermentation until the sugar is entirely ex- 

 hausted. In the first few days after the cheese is made the lactic streptococci reach 

 enormous numbers.' As the ripening progresses this type declines followed by a 

 large increase of the lactobacilli and sometimes of liquefying cocci. These bacteria 

 are probably responsible for the active ester production which goes on simultaneously 

 with the proteolysis and is closely associated with the development of flavor.^ 



It has not been definitely proved that any one variety of bacteria is responsible 

 for the characteristic Cheddar flavor, but it is known that this flavor is lacking in 

 cheese made from pasteurized milk. In the Emmental or Swiss cheese the mak- 

 ing process includes heating for some time at 5o°-55° C. The growth of all bacteria 

 is checked and the greater part destroyed. The large mass of curd cools slowly in 

 the press and the growth of the lactobacilli naturally present in the milk or introduced 

 as starters, or with the rennet is encouraged. These bacteria rapidly ferment the lac- 

 tose to lactates and produce conditions which tend to inhibit colon-aerogenes and 

 other gas-forming bacteria. In the warm room (70°-^ 2° F.) propionic bacteria de- 

 velop, converting the lactates to propionic acid and carbon dioxide. 



In Cheddar cheese any gas formed escapes through the porous curd, but in Swiss 



■ Hastings, E. G., Evans, A. C, and Hart, E. B.: Wisconsin Agric. Expcr. Sla. Research Bull. 

 25. 1912. 



' Suzuki, S. K., Hastinj^s, E. G., and Hart, E. B.: ibid., Bull. 11. 1910. 



