Cheese with Eyes

Swiss cheese, the most famous of the eye-containing cheeses, is considered by experienced cheese makers to be the easiest cheese to make but the hardest cheese to make well. This is because the manufacture of high quality Swiss cheese, and proper eye development, in particular, depends on two rather independent processes. First, Swiss cheese requires excellent curd handling technique, such that conditions are correct for eyes to form. Second, there must be precise control over the microorganisms that are involved in the fermentation and produce the gas that ultimately results in eye formation. It is possible, easy in fact, to produce a cheese that tastes like Swiss cheese, but that has either small eyes, large eyes, irregularly-shaped eyes, too many eyes, too few eyes, or no eyes.The goal is to produce a cheese with the correct flavor and body char acteristics, and that has the right amount of uniformly distributed, evenly-shaped round eyes with the desired size.The trick is to manage the early cheese-making steps such that the curd has just the right texture or elasticity necessary to accommodate the carbon dioxide that is produced much later in the process.

The manufacture of Swiss cheese seems easy because the steps involved in curd making appear rather simple and straightforward. However, care must be taken from the outset. First, the milk is usually standardized to 3% fat. If too much or not enough fat is present in the milk, the body will be too soft or too hard, leading to poor eye development. Second, the milk is given a modest heat treatment, about 56°C for fifteen seconds, which is below that used for pasteurization but which still is effective at inactivating many of the milk-borne bacteria that could otherwise cause problems during the ripening period. Finally, the quality of the cheese also depends heavily on the performance of the bacteria used in the mixed species starter culture, as well as the ratio of those organisms.

To achieve the texture and body characteristics necessary for proper eye development, Swiss cheese must have the correct pH and moisture content and retain (or lose) the right amount of calcium.The curds are cut to about rice-sized particles, and then are cooked to a much higher temperature than for Cheddar-type cheeses.Typically,the temperature will be raised over a forty-minute period from 32°C to 35°C at setting to as high as 55°C, and then is held at that temperature during stir-out for another forty-five minutes or until the curd is sufficiently dry.Thermophilic cultures that are tolerant of high temperature must be used (see below), although there are some manufacturers who cook to a lower temperature and add mesophilic L. lactis subsp. lactis.

It is important to note that reducing the moisture in the curd will also result in less lactose in the curd. As noted above, controlling the amount of lactose in the curd is an effective way to limit the amount of acid that will ultimately be produced during fermentation, a consideration that will be further discussed later. High cooking temperatures also slow acid development, remove water, and prevent solu-bilization of calcium phosphate into the whey. The whey is drained at the end of cooking, while the pH is still high (about 6.3). This keeps phosphate in the curd, which promotes buffering and keeps the pH from becoming too low later during the fermentation.

In the traditional process, when the curds are sufficiently firm, they are collected or dipped into cheese cloth, which is then placed into a large round form. In a modified version used by many U.S. manufacturers, the curds are allowed to settle under the whey, sometimes with weights applied, then the whey is drained, the matted curd is cut into block-sized sections, and placed into forms. In the larger operations, curd-whey mixtures are pumped into draining vats and then the curd is filled into large forms. For all of the processes, the cheese blocks are then pressed and held for up sixteen hours at near ambient temperature.

Unlike Cheddar cheese, in which the fermentation occurs in the vat or on draining tables, for Swiss cheese there has been no opportunity, up this point, for fermentation. Rather, the fermentation occurs after the cheese is out of the vat and filled into forms. In addition, the fermentation takes a much longer time, as much as twenty-four hours. The cheese, however, will still be warm following the cooking step, so the internal temperature may be as high as 35°C for several hours.The actual fermentation is quite different from that which occurs during Cheddar cheese manufacture. In Cheddar cheese, the culture contains mesophilic lactococci, and although more than one species may be present (e.g., L. lactis subsp. lactis and L. lactis subsp. cre-moris), they are obviously closely related and serve an almost identical function. In contrast, the Swiss cheese starter culture contains three different organisms, from three different genera: Streptococcus thermophilus, Lactobacillus helveticus, and Propionibacterium freudenre-ichii subsp. shermanii. All three are essential, and all are responsible for the fermentation pattern that is unique to Swiss cheese.

The culture that is initially added to the milk contains different proportions of each organ-ism.The S. thermophilus generally outnumbers the L. helveticus by as much as ten to one. Therefore, at the outset, growth of S. ther-mophilus occurs first, in part because it is present at a higher concentration, but also because its simple physiological requirements are more easily met, especially compared to the more fastidious L. helveticus. As noted in Chapter 4, S. thermophilus and L. helveticus (or Lactobacillus delbrueckii subsp. bulgari-cus) have a synergistic relationship, such that growth of one organism promotes growth of the other. In Swiss cheese, S. thermophilus growth is stimulated by amino acids and pep-tides released from casein via L. helveticus pro-teinases. Growth of the latter does not commence until the pH and Eh within the cheese are sufficiently reduced by S. thermophilus, a period that may take as long as twelve hours.

Another factor that influences the outcome of the Swiss cheese fermentation relates directly to the metabolic properties of these two organisms. In fact, one of the most interesting peculiarities of all lactic acid bacteria occurs when S. thermophilus ferments lactose in Swiss cheese (actually, the same phenomena also occurs in Mozzarella cheese, yogurt, or anytime S. thermophilus grows in milk). This organism actively transports lactose from the extracellular environment across the cell membrane and into the intracellular cytoplasm.The enzyme, p-galactosidase, then immediately hy-drolyzes the accumulated lactose to glucose and galactose. Glucose is phosphorylated to glucose-6-phosphate, which then feeds into the glycolytic pathway and is rapidly metabolized to lactic acid. However, most strains of S. thermophilus do not express the enzymes necessary for galactose phosphorylation and metabolism (enzymes that comprise the Leloir pathway), and instead secrete or efflux the galactose back into the extracellular medium. In the case of Swiss cheese, galactose will appear in the curd almost as fast as lactose is consumed. Researchers have learned that the excretion of galactose not only coincides with lactose consumption, but that the efflux reaction actually provides a driving force for lactose uptake (described in Chapter 2).

Thus, in the first several hours of the fermentations, S. thermophilus grows and ferments lactose and excretes galactose (Figure 5-7). L. hel-veticus begins active growth after about eight to twelve hours and competes with S. ther-mophilus for the remaining lactose, which is subsequently consumed. However, one of the main roles of L. helveticus is to then ferment the galactose left behind by S. thermophilus, such that after about eighteen to twenty-four hours all of the carbohydrate in the curd has been fermented. If the curd contained just the right amount of lactose at the start of the fermentation (i.e., after cooking), and all of the lactose and its constituent monosaccharides were fermented to completion, then the final pH should be very near 5.2 ± 0.1. If there is too much lac-

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