Technological Production Phases of Dairy Products

Milk Pre-treatment and Standardization

Although most industrial dairy products are produced from pasteurized milk, a large number of raw milk cheeses are increasingly described as celebrated traditional on-farm-made cheeses and commercially proposed as gastronomic specialities, emphasizing their distinctive flavor and suggesting the best way to consume them. In the microbiological literature these types of cheese are attracting increasing coverage, showing once more the importance of the biodiversity of raw milk native microflora to achieve the roundest, most pleasant and palatable traits. Studies are needed on molecular methods for monitoring both useful and dangerous microorganisms during practices performed in raw milk cheese making, such as milk storage/ ripening (usually at a low temperature) or milk skimming for reducing the fat content. Indeed, cold storage is known to cause physical and chemical changes to milk, and to be selective for the development of psychrotrophic microflora. Great interest is merited by the case of partial milk-skimming by natural creaming typically performed within the manufacture of famous Italian semi-fat hard and long-ripened cheeses (Parmigiano-Reggiano and Grana Padano), promoting fat floating to the surface of the raw milk contained in wide trays for as long as six to 12 hours. Within this singular and uncommon practice, fat droplets appearing on the surface lead to microbial enrichment of the cream and a reduction in the microbial content of the skimmed milk lying below (Mucchetti and Neviani 2006).

Starter Addition

For starter cultures, a particularly valid nomenclature is proposed by Limosowtin, et al. (1996), who define "Mixed Strain Starters" (MSS) as the cultures that include many species and strains in unknown proportions, and "Defined Strain Starters" (DSS) as those containing known quantities of known strains. Within the first category, the authors list "Artisanal or Natural Mixed Starter Cultures" (NMSS), "Thermophilic Mixed Starter Cultures" (TMSS) and "Mesophilic Mixed Starter Cultures" (MMSS); within the second (DSS) "Thermophilic Defined Starter Cultures" (TDSS) and "Mesophilic Defined Starter Cultures" (MDSS). It can be assumed that most DSS, as well as many single strain starters, are commercial starters produced by specialized laboratories; MSS - mainly NMSS - are prepared and managed by the dairies, often following traditional procedures that, for the same type of cheese, can differ somewhat among the various manufacturers. Many cheeses are traditionally produced without starter addition nor by using back-slopping practices. Natural whey cultures are microbial cultures naturally occurring in the whey extracted in previous cheese making, stored at room temperature or variously handled, and then used in the manufacture of the following day with a back-slopping practice. Instead, natural artisanal milk cultures are generally obtained by incubating, at appropriate temperatures, a large amount of raw milk after mild heating to destroy undesirable microflora. Indeed, they commonly contain strains of thermoduric microorganisms, such as thermobacteria, thermophilic streptococci and enterococci. Moreover, the dairy industry can benefit from the use of other microbial cultures called "adjuncts;" they are not essential for the technological process in itself, but are selected and used with specific additional purposes. They can be "reinforcing cultures" to be used to accelerate or standardize acidification; "flavoring cultures" to enhance aroma production; "protective cultures" to inhibit pathogens or spoilage agents, or "health cultures" to enrich the product with probiotic strains.

The addition of starters and/or adjuncts to cheese milk causes an immediate change in the microbiota of the technological ecosystem concerned, as loads of at least 106 cells per ml of milk of each important microbial type are applied. Thus, the structure of the population under investigation is strongly - and often permanently - influenced after the inoculum.

Milk Clotting

Rennet-coagulated cheeses represent, by far, the greater part of solid dairy products. Liquid, powder or tablet rennets are usually special preparations with little to no significant microbial content. Among traditional cheeses a number of more or less long ripened products are manufactured by the use of a rough type of rennet prepared in the form of a paste from the fourth stomach (abomasum) of suckling goat kid or lambkin. These rennets, in addition to chymosin and other proteases, contain some lipases that, during cheese ripening, are responsible for reactions that produce a distinct piquant taste. Several studies have shown that these rennet pastes might have a microbial content, representing a special additional source of microorganisms for the cheese (P. Deiana, personal communication). Modern research on the microbial content of raw rennet is, to our knowledge, nonexistent and no information is available about the possible presence of stressed or unculturable microorganisms.

In any case, depending on the type of cheese, coagulation time is variable, generally predefined through the quantity and quality (clotting strength) of the rennet. Milk fat droplets, whey with water-soluble components and microbial cells are entrapped within the casein network, e.g. inside the pores among aggregates of micelles. Microbial growth may also occur, of course, in a good nutritional environment and with favorable temperatures, no longer as planktonic cells but growing as colonies in a solid matrix. Sampling at this stage of cheese making may be of interest for chemical investigation. However, microbial populations at the end of the clotting process cannot yet be referred to cheese, due to the fact that part of the microorganisms entrapped in the coagulum will be eliminated from the system by subsequent technological phases.

Curd Cutting, Cooking and Draining

Once the coagulum firmness required for the specific cheese variety has been reached, the curd is cut with knives or wire-tools into small pieces. Cut size also strongly depends on the cheese type, as the firmer and larger the curd pieces, the higher the moisture content of the cheese. According to the Italian dairy tradition, curd piece size is typically named after fruits or seeds of similar size: walnut, little walnut, almond, hazelnut, pea, small pea and grain. Long ripening cheeses require curd cutting at the smallest size (grain); fresh or brief-ripening cheeses at the largest one (walnut). With cutting, caseins continue to interact and squeeze out the whey entrapped (with all its water-soluble components, lactose included) and some microbial cells as well (more cocci than rods). Curd pieces shrink, become firmer and, depending again on the cheese variety, they can be differently processed. Syneresis may be enhanced by lowering the pH (hence, counting on starter effectiveness), increasing the temperature and stirring the curd (performing, in this case, the process of curd cooking). Alternatively, the curd pieces can be promptly separated from the whey, drained and subjected to the subsequent technological phase of molding. However, curd treatments are always ecologically important, involving selective pressures with major effects upon microorganisms and their activities.

Molding

Molding can be ecologically important due to the possibility of contamination occurring during curd handling. It is generally recognized that chemical cleaning and sanitation of tanks, vats and other tools used within cheese making - made of proper modern materials - can reduce contamination considerably, whereas it is more difficult to achieve satisfactory results when the cheese is exposed to the work environment. This is considered the main source of adventitious cheese microflora, commonly including non-starter LAB, and is responsible for important activities during cheese ripening. Molding is, therefore, another stage of cheese making during which the cheese microflora can change.

Salt Addition

Salt is added in cheese making to improve its taste and to lower the water activity. In some cases it is added as a solid to milled curd; in others, cheeses removed from the forms are brined, and in further cases the cheese molds are dry salted by rubbing or sprinkling salt on their surfaces.

Salting causes selective pressure upon microorganisms. Brines are often a source of cheese contamination by salt-tolerant microorganisms: yeasts such as Debaryomyces hansenii and its imperfect form Candida famata; bacteria such as staphylococci, micrococci, enterococci, corynebacteria and some LAB.

Ripening

It is well-known that cheese ripening occurs in a variety of environmental conditions, depending on cheese type, and often in natural or cellar conditions, where it is uncontrolled and difficult to reproduce. Moreover, this final process of cheese making is difficult to describe because it consists of a complex succession of events conditioned by the previous technological stages, with the contribution of secondary adventitious microflora, under the influence of the cheese storage environment and, in some cases, caused by curing practices. Then, further complication may be encountered in some cases, where ripening proceeds without heterogeneity within the same cheese mold: blue-veined cheeses include portions strongly affected by both growth and activity of Penicillium roqueforti; surface-ripened cheeses are characterized by centripetal ripening due to diffusion of enzymes produced by the surface microflora. Such complexity requires polyphasic analytical approaches including physical, chemiometric, molecular and cultural procedures to be performed on several samples from various parts of the same cheese mold.

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