Causative Agent Of Lactococcus Lactis

Dry and semidry sausages

Employs a starter culture containing species of Lactobacillus and Pediococcus; meat is stuffed into casings, incubated, heated, and then dried.

the production of lactic acid, inhibits the growth of many other microorganisms in the milk. Aesthetic features of the milk change as well because lactic acid lowers the pH, which in turn causes the milk proteins to coagulate or curdle, and sours the flavor. Historically, people most likely acquired a taste for sour curdled milk, an early stage of milk spoilage, out of necessity. Later, they began experimenting to intentionally produce more desirable fermented milk products. ■ milk spoilage, p. 768

Today, with high quality control standards and the use of pasteurized milk, which has been heated to kill most bacteria, the commercial production of fermented milk products does not rely on naturally introduced lactic acid bacteria. Instead, starter cultures containing one or more strains of lactic acid bacteria are added to the milk. These strains are carefully selected to produce the most desirable flavors and textures, enhancing the quality of the fermented milk product. Precious starter cultures must be carefully maintained and protected against contamination, particularly by bacteriophages, which can damage or destroy them. Currently efforts are focused on selecting or developing bacteriophage-resistant strains of lactic acid bacteria. ■ bacteriophage, p. 323

Cheese Cheese-making probably originated in Asia more than 8,000 years ago, yet even with today's modern production methods, it has remained an art requiring experience, timing, and patience. Cheese can be made from the milk of a wide variety of animals, but most common cheeses are made with cow's milk. Sheep or goat's milk may be used to give characteristic flavors. Cheeses are classified as very hard, hard, semisoft, and soft, according to their percentage of water.

Cottage cheese is one of the simplest cheeses to make. Pasteurized milk is inoculated with a starter culture, usually containing Lactococcus (.Streptococcus) cremoris and L. lactis, and then incubated until fermentation of the lactose produces enough lactic acid to cause the proteins in milk to coagulate. The coagulated proteins, or curd, are heated and cut into small pieces to facilitate drainage and removal of the liquid waste portion, or whey. Unlike most cheeses that undergo further microbial processes called ripening or curing, cottage cheese is unripened.

The initial steps in the production of ripened cheese are the same as those of cottage cheese, except the enzyme rennin is added to the fermenting milk to hasten protein coagulation (figure 32.3). Rennin is an enzyme naturally found in the stomach of young calves, where it aids in digestion of the mother's milk. Today, rennin is commercially produced using genetically engineered microorganisms. After the whey is separated from the curds, the curds are salted and pressed, and then they are shaped into the traditional forms of cheese, usually bricks or wheels. The molded cheese is then ripened to encourage characteristic changes in texture and flavor. ■ protein production using genetically engineered microorganisms, p. 222

32.3 Microorganisms in Food and Beverage Production 805

Depending on the type of cheese, ripening can take from several weeks to several years. Changes imparted during this time are generally due to the metabolic activities of naturally occurring or starter lactic acid bacteria. Longer ripening gives rise to more acidic, sharper cheeses. Some cheeses are inoculated with other bacteria or fungi that impart characteristics particular to the kind of cheese. For example, the bacterium Propionibacterium shermanii ripens Swiss cheese and gives it the characteristic holes, or eyes, and a nutty flavor. This bacterium ferments organic compounds to produce propionic acid and CO2. The CO2 gas causes the holes in the cheese, while the propionic acid imparts the typical flavor. Propionic acid also inhibits spoilage organisms. Roquefort, Gorgonzola, and Stilton cheeses are ripened by the fungus Penicillium roque-fortii. Growth of the fungus along cracks in the cheese gives these cheeses the distinctive bluish-green veins. Brie and Camembert are ripened by a white fungus such as P. candidum or P. camemberti inoculated on the surface of the cheese. The mycelia of the fungus produce enzymes that alter texture and

Lactic acid production and rennin activity cause the milk proteins to coagulate. The coagulated mixture is cut to facilitate the separation of the solid curds and liquid whey.

The curds are heated and cut into small pieces. The liquid whey is removed by draining.

Figure 32.3 Commercial Production of Cheese Why does a longer ripening process give rise to a sharper cheese?

Curds are salted and pressed into blocks or wheels for aging.

Figure 32.3 Commercial Production of Cheese Why does a longer ripening process give rise to a sharper cheese?

806 Chapter 32 Food Microbiology flavor as they gradually work their way into the cheese. Limburger cheese is made in a similar manner, but with the bacterium Brevibacterium linens.

The fermentation and the ripening process must both be carefully controlled to guard against growth of contaminating microorganisms that can produce undesired flavors, textures, and appearances. Fortunately, salting, low oxygen levels, acidity, and microbial metabolites such as propionic acid inhibit the growth of most pathogens in cheese.

From a microbiological standpoint, an interesting challenge in cheese-making is how to dispose of the copious volumes of whey produced during cheese production. Roughly 9 liters of protein and carbohydrate-rich whey are generated for every liter of curd. If large volumes of untreated whey are dumped into a body of water, aerobic microorganisms metabolize the organic matter, depleting the water's dissolved oxygen during the process, which results in the death of fish and other aquatic animals that require oxygen. Therefore, before disposal, whey must be treated to decrease its organic content. Alternatively, whey is sometimes spread on agricultural land. This disposal method poses environmental problems when the whey has a high salt or acid content. Commercial uses for whey, such as animal feed and protein food additives, are being developed in response to environmental and economic concerns associated with its disposal. ■ biochemical oxygen demand, p. 786

Yogurt To produce yogurt, pasteurized milk is concentrated slightly and then inoculated with a starter culture containing Streptococcus thermophilus and Lactobacillus bulgaricus. The mixture is incubated at 40°C to 45°C for several hours, during which time these thermophilic bacteria grow rapidly and produce lactic acid and other end products, such as acetaldehyde, that contribute to the flavor. Carefully controlled incubation conditions favoring the balanced growth of S. thermophilus and L. bulgar-icus ensure the proper levels of acid and flavor compounds.

Acidophilus Milk Traditional acidophilus milk is the product of fermentation by Lactobacillus acidophilus. The more readily available sweet acidophilus milk retains the flavor of fresh milk because it is not fermented. Instead, a culture of L. acidophilus is added immediately before packaging. The bacteria are simply included for their purported health benefits. Some evidence suggests they may aid in the digestion of lactose as well as prevent and reduce the severity of some diarrheal illnesses, but the role they play in the complex interactions of the human intestinal tract is not clear. Unlike most lactic acid bacteria used as starter cultures, L. acidophilus can potentially colonize the intestinal tract.

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