Bluemold Ripened Cheese

Although blue mold-ripened cheeses are made throughout the world, three specific types have achieved a significant measure of fame to warrant their own name (and have DPO status). Roquefort, perhaps the most well-known of all blue cheeses, must be made according to a strict set of manufacturing requirements. For example, the milk must come from specially-bred sheep that have grazed in the Causses region of France. It is neither pasteurized, standardized, nor homogenized, and the cheese must ultimately be aged in caves within that same region. In Italy, the manufacture of Gorgonzola cheese is similarly restricted to the Po Valley region of Northern Italy and its manufacture is also subject to specified procedures. This cheese is made from cow's milk, but is otherwise very similar to Roquefort. Gorgonzola, however, is usually not quite as strongly flavored as Roquefort. Finally, the English representative to the blue-mold family is called Stilton. It also is made from cow's milk and only in three counties of central England.

Other blue mold-ripened cheeses are ordinarily referred to simply as blue cheese, although regional varieties, often times of excellent singular quality, exist worldwide.

In general, the manufacture of blue mold-ripened cheese requires several specialized steps. In large part, the goal is to ensure that the aerobic mold can grow well within the interior of the cheese, where the Eh is ordinarily low.Although the blue mold organism, P.roque-forti, is capable of growing at high CO2 and low O2 levels, air incorporation is still an important feature of the process. The coagulated milk is cut, when very firm, into larger than normal-sized curds (as much as 2.5 cm). As noted earlier, the larger the curds at cutting, the higher the moisture will be in the cheese. However, an additional effect is to create a more open or porous texture such that diffusion of air is increased.

Frequently, the mesophilic lactic starter culture, in addition to containing L. lactis, will also include heterofermentative Leuconostoc or citrate-fermenting lactic acid bacteria. These bacteria produce CO2 that contributes to the open texture that enhances oxygen diffusion and gas exchange. Spores of P. roquefortii can be added to the curds or to the milk, along with the lactic starter culture, prior to setting. Later, once the cheese is hooped and the lactose fermentation is complete, the cheese wheels are brine- or dry-salted. At this point, a set of large bore needles (diameter about 0.24 cm) may be used to pierce the cheese and provide a means for air to penetrate the inner portions. Later, when the cheese is cut vertically, one can see that mold growth followed the spike lines where oxygen was available.

Next, the cheese is aged for several weeks in a warm (10°C to 12°C), humid (90% to 95% relative humidity) environment that promotes growth of the P.roqueforti throughout the interior of the cheese. Lower ripening temperatures (4°C to 8°C) and longer times may also be used. Since the only way to arrest further mold growth is to cut off its supply of oxygen, the ripened cheeses are then wrapped in oxygen-impermeable foil. Although some consumers like blue cheese flavor, they may not be fond of the moldy appearance; thus, some manufacturers may limit growth of P. roqueforti, giving a somewhat whiter cheese with only a few streaks of blue. Even consumers who enjoy blue veined cheese may object to extensive growth at the surface, so the wheels are often cleaned prior to final packaging. Pimaricin may also be applied to control surface growth.

Although other organisms, including yeast and bacteria, are often present in these cheeses, especially at the surface, the flavor compounds characteristic of blue cheese clearly are generated primarily via mold growth and metabolism. Fungi, in general, are prolific producers of proteases, peptidases, and lipases, and P. roqueforti is no exception. Thus, release and subsequent metabolism of protein hydrolysis and lipolysis products are important in blue cheese flavor. Of particular importance are ammonia and amines, derived from amino acid metabolism, and methyl ketones, derived from free fatty acids.

Free fatty acids, themselves, may also contribute to cheese flavor, but their metabolism, via p-oxidation pathways to 2-heptanone, 2-nonanone, and other ketones, are primarily responsible for the flavor of blue cheese.As much as 20% of the triglycerides in the milk may be hydrolyzed.When blue cheese is made from raw milk, natural milk lipases may also contribute to formation of free fatty acids. These lipases are loosely associated with the surface of the casein micelles and are dislodged by agitation. In addition, lipases' substrates (triglycerides) are exposed when the fat globule is disrupted.There-fore, some manufacturers add a portion of homogenized raw cream to the cheese milk to accelerate flavor development.

Finally, it is important to note that mold growth during blue cheese ripening is accompanied not just by flavor development, but also by a marked increase in pH. This occurs because P. roqueforti, not having lactose on which to grow (all of the lactose is fermented by the starter culture), instead uses lactic acid as an energy source.Therefore, consumption of lactic acid causes the pH to rise from about 4.6 to above 6.0. Production of ammonia and other amines also contributes to the increase in pH. This increase in pH, in turn, promotes flavor production because many of the fungal decar-boxylases and other enzymes involved in methyl ketone formation have neutral pH optima. However, the rise of pH to near neutrality may also have food safety and preservation consequences in that acid-sensitive organisms may be able to grow once the pH reaches non-inhibitory levels.

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