Despite their importance in several cheese types, much less is known about the metabolism of Penicilium spp. and brevibacteria used to make mold-ripened and surface-ripened cheeses. These organisms are not really starter cultures, since they do not contribute to acid development, but they are just as integral to the cheese making process as are the lactic starter cultures. Accordingly, their main role in cheese manufacture is to produce flavors and cause desirable changes in texture and appearance of the finished cheese (also see Chaps. 6 and 11).
The mold responsible for the well-known blue-veined appearance of Roquefort, Gorgonzola, and other blue cheese types is P. roqueforti. Although spores of P. roqueforti are added to milk or curds before the lactic fermentation, mold growth does not occur until after the lactic culture has fermented all or most of the available lactose to lactic acid. Lactic acid serves as an energy source for the mold. Importantly, consumption of lactic acid causes the pH to rise from about 4.6 to as high as 6.2 (Marth and Yousef, 1991). As P. roqueforti grows in cheese, substantial proteolysis occurs through elaboration of several extracellular protein-ases, endopeptidases, and exopeptidases. Amino acids can be subsequently metabolized releasing amines, ammonia, and other possible flavor compounds (that also may raise the pH). However, the most characteristic blue cheese flavors are generated from lipid metabolism (Gripon, 1987). As much as 20% of triglycerides in milk are hydrolyzed by lipases produced by P. roqueforti. Although free volatile fatty acids may themselves contribute to cheese flavor, their metabolism, via P-oxidation pathways, results in formation of a variety of methylketones. It is this class of compounds that is responsible for the flavor of blue cheese.
Just as in blue-veined cheeses, growth of P. camemberti in the manufacture of Camembert and Brie cheeses occurs as a secondary fermentation, and again, lactic acid is used as an energy source. The subsequent rise in pH (from 4.6 to as high as pH 7.0 at the surface) because of lactate consumption and ammonia production provides opportunities for other organisms to grow, and the surface microflora of Camembert cheese can be quite diverse. The proteinases and peptidases produced by P. camemberti are similar to those produced by P. roqueforti (Gripon, 1987). Although P. roqueforti grows throughout the cheese mass (because of deliberate aeration during cheese making), growth of P. camemberti is confined to the surface; therefore, protein breakdown in the interior of cheese is dependent on diffusion of excreted enzymes. Production of ammonia, methanethiol, and other sulfur compounds, presumably derived from amino acids, are also characteristic of Camembert cheese. Lipolysis of triglycerides and fatty acid metabolism by P. camemberti are just as important in surface-ripened cheese as in blue-veined cheese, and methylketones are abundant (Gripon, 1987).
Although B. linens is primarily used in the manufacture of Muenster, brick, and other surface-ripened cheeses, its potential use as a flavor adjunct has led to re newed interest in the metabolism of this organism (Rattray and Fox, 1999). Most attention has focused on proteinases and peptidases produced by B. linens and subsequent formation of volatile flavor compounds from amino acid metabolism. Unlike lactic acid bacteria that produce a single proteinase (PrtP), B. linens produces several extracellular and intracellular proteinases and peptidases. Metabolism of released amino acids results in formation of many sulfur-containing compounds, including hydrogen sulfide, methanethiol, and other volatile flavors that are characteristic not only of surface-ripened cheese but which are also important in Cheddar cheese. The ability of B. linens to produce these flavor compounds, along with a high level of proteolytic activity, have led to the use of this organism as a flavor adjunct in Cheddar-type cheeses (Weimer et al., 1999).
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