Production of Cheese

Cheeses are produced by microbial fermentations. sion or omission of secondary microbial species late in

Cheese consists of milk curds that have been separated the fermentation process.

from the liquid portion of milk (whey). The curdling of Ripening of cheeses involves additional enzymatic

milk is accomplished by using the enzyme rennin and transformations after formation of the cheese curd, us-

lactic acid bacterial starter cultures. Rennin is obtained ing enzymes produced by lactic acid bacteria or en-

from calf stomachs or by microbial production. zymes from other sources. Unripened cheeses do not re-

Cheeses are classified as: (1) soft, if they have a high quire additional enzymatic transformations. Cottage

water content (50% to 80%); (2) semihard, if the water cheese and cream cheese are produced by using a starter

content is about 45%; and (3) hard, if they have a low culture similar to the one used for the production of cul-

water content (less than 40%). Cheeses are also classi- tured buttermilk and are soft cheeses that do not require

fied as unripened, if they are produced by a single-step ripening. Sometimes a cheese is soaked in brine to en-

fermentation, or as ripened, if additional microbial courage development of selected bacterial and fungal

growth is required during maturation of the cheese to populations during ripening. Limburger is a soft cheese

achieve the desired taste, texture, and aroma. Processed produced in this manner. During ripening, the curds are

cheeses are made by blending various cheeses to softened by proteolytic and lipolytic enzymes and the

achieve a desired product. If the water content is ele- cheese acquires its characteristic aroma. The production

vated during processing, thereby diluting the nutritive of Parmesan cheese also involves brine curing,

content of the product, the product is called a Various fungi are also used in the ripening of differ-

"processed cheese food" rather than a cheese. ent cheeses. The unripened cheese is normally inocu-

The natural production of cheeses involves a lactic lated with fungal spores and incubated in a warm moist

acid fermentation with various mixtures of Streptococcus room to favor the growth of filamentous fungi. For ex-

and Lactobacillus species used as starter cultures to initi- ample, blue cheeses are produced by using Penicillium

ate fermentation. The flavors of different cheeses result species, Roquefort cheese is produced by using P. roque-

from the use of different microbial starter cultures, forti, and camembert and brie are produced by using P.

varying incubation times and conditions, and the inclu- camemberti and P. candidum.

FIG. 6-21 The propionic acid fermentation produces propionic acid and carbon dioxide. The carbon dioxide produced in this fermentation forms the holes in Swiss cheese.

drates (FIG. 6-21). These bacteria have the ability to carry out this fermentation pathway beginning with lactic acid as the substrate. The ability to utilize the end product of another fermentation pathway is quite unusual, but it allows species of Propionibac- terium to carry out late fermentation during the pro­duction of cheese.

Lactic acid bacteria convert the initial substrates in milk to lactic acid. Propionic acid bacteria then con­vert the lactic acid to propionic acid and carbon diox­ide. Propionic acid bacteria begin their fermentation only after the cheese curd has formed through the ac­tion of lactic acid bacteria. The release of carbon diox­ide during this late fermentation forms gas bubbles in the semisolid cheese curd, forming the holes in Swiss cheese. The propionic acid formed during this

fermentation also gives Swiss cheese its characteristic flavor.

Mixed-acid Fermentation

The mixed-acid fermentation pathway is carried out by Escherichia coli, as well as hundreds of other bacte­rial species. In the mixed-acid fermentation pathway the pyruvate formed during glycolysis is converted to various products—ethanol, acetate (acetic acid), formate (formic acid), lactate (lactic acid), molecular hydrogen (H₂), and carbon dioxide (C0₂). It is called the mixed-acid fermentation because of these many [ products. The proportions of the products vary, de­pending on the bacterial species.

Mixed-acid fermentation can be detected by the Methyl Red (MR) test, which is based on the color