Although mesophilic lactobacilli are undoubtedly inhabitants of raw milk and the dairy environment, upon acidification of raw milk they are frequently overgrown by strong acidifiers of the genus Lactococcus. However, they do gain access to the cheesemaking process, because they are often found as secondary flora during the ripening of different cheese varieties. This is especially true for raw-milk cheese, but mesophilic lactobacilli are also common in cheese manufactured with modern technologies, using pasteurization of the milk, defined-strain starters, and hygienic processing. The starter is responsible for the acidification during the first stages of cheese manufacture and may reach up to 109 colony-forming units (cfu) per gram of cheese. During ripening, however, the number of starter cfu generally decreases rather quickly to lower than 107 g-1, whereas subsequently the nonstarter adventitious lactobacilli grow out and may reach numbers higher than those of the starter culture (21).
The secondary flora in Cheddar cheese has been examined most extensively; it consists mostly of mesophilic lactobacilli and sometimes also pediococci. These bacteria are collectively referred to as nonstarter lactic acid bacteria (NSLAB). Isolates from this group belong to the species Lactobacillus paracasei, Lb. plantarum, Lb. rhamnosus, and Lb. cur-vatus. The composition of the NSLAB in the cheese varies with the day of manufacture and with the age of the cheese (16,22). Adventitious NSLAB have also been reported for Emmental, Comte, and other types of cheeses (23-25).
The NSLAB have the unique ability to grow under the highly selective conditions prevailing in a ripening cheese. Lactose is largely depleted in the first hours of cheese manufacture by the fermentation of the starter bacteria. The pH is between 4.9 and 5.3, the temperature below 13oC, the moisture content less than 50%, the salt concentration in moisture is 4-6%, and oxygen is barely available. All in all, ripening cheese seems a hostile environment for microorganisms. Yet the adventitious lactobacilli manage to grow, obviously with a low rate, but the generally long ripening period allows them enough time to reach considerably high levels of cfu per gram of cheese. They apparently consume compounds other than lactose, such as lactate, citrate, glycerol, amino sugars, amino acids, and perhaps even on material released from lysed starter bacteria. Although the NSLAB, like other lactobacilli, exhibit fastidious nutritional requirements, they clearly find ample opportunities for growth in a ripening cheese. They possess a wide range of hydrolytic enzymes and are able to effect proteolysis and lipolysis (22,26).
Because NSLAB dominate the microflora of many long-ripened cheeses, they are believed to contribute to the maturation of cheese. The numbers of NSLAB are reported to be higher in Cheddar cheeses made from raw milk than in those from pasteurized milk (27). Differences in flavor between these cheeses, with a more intense flavor in raw milk cheeses, suggest that the indigenous NSLAB play an important role in flavor development. Indeed, they have been shown to contribute to the formation of small peptides and amino acids, which are the precursors of the flavor components (28).
The observation that the presence of NSLAB in cheese on the one hand leads to a desirable flavor and on the other hand may induce possible defects or spoilage (29-31) makes it a delicate choice for the cheese maker to use a certain lactobacillus as adjunct starter. This strain should be selected with care because only a limited number of the NSLAB present in cheese combine all the required properties with the concomitant lack of imperfections. McSweeney et al. (32) were successful in improving the flavor of Cheddar cheese by using strains isolated from raw milk cheese. This improvement was believed to be due to increased formation of amino acids. Cheese made from milk inoculated with strains of Lb. plantarum or Lb. casei subsp. pseudoplantarum received the best gradings (33).
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