MICROBIOLOGY Study Guide - 2012

CHAPTER 15. FOOD MICROBIOLOGY

15.1. MICROBIOLOGY OF MILK AND DAIRY PRODUCTS

15.1.6. Cheeses

Cheese is produced by coagulating milk using rennet (or a milk-clotting enzyme of microbial origin), followed by separating the whey from the curd, Processing, and ripening. Based on technological features and the COMPOSITION OF THE microflora involved in ripening, cheeses are classified into the following groups.

1. Hard cheeses with a low second-cooking Temperature (35–41 °C) include Dutch, Kostroma, Uglich, Edam, Gouda, etc. Starter cultures for this group comprise mesophilic lactococci and leuconostocs: Lac. lactis, Lac. cremoris, Lac. diacetylactis, Leu. cremoris (Composition 1). In addition, accompanying lactobacilli of the species L. plantarum, L. casei, L. brevis participate in the ripening process. Rennet and starter microorganisms drive the primary biochemical processes during cheese ripening: The breakdown of lactose into lactic acid, the proteolysis of curd Proteins, and The formation of a fine-eyed texture (eyes) through CO₂ production by aroma-forming cultures—Lac. diacetylactis, Leu. cremoris.

2. Hard cheeses with a high second-cooking temperature (50–56 °C) include Swiss, Altai, Maasdam, Gruyère, etc. Starter cultures for these cheeses contain thermophilic lactic acid Bacteria: thermophilic streptococci (Streptococcus thermophilus) and Swiss bacilli (Lactobacillus helveticus), as well as propionic acid bacteria of the species Propionibacterium freudenreichii.

A distinctive feature of Swiss-type cheeses is the involvement of thermophilic lactic acid rods in ripening, which exhibit high proteolytic activity and carry out profound proteolysis of the curd proteins. Consequently, cheeses with a high second-cooking temperature contain more free Amino Acids and non-protein nitrogen compounds than hard cheeses produced with a low second-cooking temperature. The characteristic sweet-and-spicy flavor of Swiss-type cheeses is attributed to significant levels of Proline, hydroxyproline, Alanine, and glutamic acid, which impart a sweet or savory taste.

Propionic acid bacteria play a special role in shaping the specific flavor, aroma, texture, and consistency of the cheese. During the Second Stage of ripening, they ferment lactates, leading to the accumulation of volatile Fatty acids—propionic and acetic acids—in the cheese. The gases generated during Propionic Acid Fermentation—hydrogen and carbon dioxide—contribute to The Development of a well-defined texture featuring large eyes up to 1–4 cm in diameter.

3. Cheeses with a high level of Lactic acid fermentation. This group is subdivided into cheeses without curd cheddarization, such as Russian, Russky, and Cantal, and cheeses with curd cheddarization, such as Cheddar, Cheshire, Gloucester, and Dunlop.

The technological characteristics of Russian-type cheeses include: The Use of Composition 1 starter; prolonged stirring of the cheese grains (40–60 min) to intensify the lactic acid process; bulk molding of the grains, which promotes the Formation of the characteristic slit-like voids typical of this cheese variety; and partial salting in the grain.

The technological features of Cheddar-type cheeses comprise: the use of a starter culture consisting of mesophilic lactic acid bacteria without gas-producing strains (Lac. lactis, Lac. cremoris, L. plantarum); prolonged stirring of the cheese grains; cheddarization of the curd (holding the cheese blocks at an optimum temperature for LAB to accumulate lactic acid); milling of the curd; and its subsequent salting and pressing.

During the manufacturing of hard cheeses, LAB introduced via the starter multiply most intensively during pressing and within the first 5–10 days of ripening. Their population peaks at 1.0–1.5 × 10⁹ CFU/g. Afterward, a gradual die-off of LAB begins and continues until the end of ripening. In Swiss-type cheeses, the population of propionic acid bacteria reaches a maximum of 200 million/g during the second half of ripening, as these bacteria utilize lactates as a nutrient source.

4. Soft cheeses. Soft cheeses include mold-ripened, smear-ripened, brined, and acid-coagulated varieties, among others.

Mold-ripened cheeses. This group encompasses A large number of cheeses that ripen with the participation of mold Fungi: Russian Camembert, White Dessert (Russia), Blue, Gorgonzola, Taleggio (Italy), Danablu and Micella (Denmark), Stilton (UK), Roquefort, Brie, Camembert (France), etc.

The production of Roquefort cheese involves a Composition 1 starter and mycelial fungi of the species Penicillium roqueforti. This green mold proliferates throughout the cheese mass. To create favorable growth conditions for P. roqueforti, the cheese wheels are pierced throughout their thickness after pressing. Microbiological processes are most active in the cheese During the first day of ripening: the total count of lactobacilli reaches 5–6 × 10⁹ per gram of curd. After 30 days of ripening, their population drops to 2.5 × 10⁷ per gram. Spore formation of Penicillium roqueforti inside the cheese becomes visible 2 to 3 weeks after production. This microflora drives enzymatic processes that form the organoleptic characteristics specific to this cheese variety. In addition to the lactococci and leuconostocs introduced into the mixture, Yeasts, micrococci, enterococci, and lactobacilli of the species L. casei and L. plantarum—which play a positive role in cheese ripening—have also been detected in Roquefort.

Camembert is a soft, cylindrical cheese that ripens with a white surface mold. The production of Camembert cheese utilizes a starter based on mesophilic lactococci and leuconostocs, along with a liquid culture of Penicillium camamberti. Sometimes, a suspension of Penicillium camamberti spores is applied to The surface of the cheeses using a cotton swab or a spray bottle.

During the initial stage of Camembert ripening, LAB multiply intensively. Their maximum population (2.5 × 10⁹ per gram) is observed approximately 3 hours after pressing, with the cream-producing lactococcus dominating among all cultures. As a result of lactobacilli METABOLISM, the pH of the cheese mass drops to 4.8. After 6–7 days of ripening, a visible turf of Penicillium camamberti appears On the surface. Mold growth continues for 15–20 days of ripening.

Smear-ripened cheeses are soft and semi-hard cheeses that ripen with a surface smear. These include Dorogobuzh, Smolensk, Dorozhny (Russia), Nemunas, Rambynas (Lithuania), Maroilles, Munster (France), Limburger (Belgium), Romadour (Germany), etc.

Cheese smear represents a community of salt-tolerant microorganisms that multiply under aerobic conditions on the surface of cheeses. This community consists of three main groups of microorganisms: yeasts, brevibacteria, and micrococci. The Qualitative and quantitative composition of the smear microflora changes as the cheese ripens.

The production of smear-ripened cheeses employs a starter based on mesophilic lactococci and leuconostocs. The peak of the lactic acid process occurs during the molding and primary ripening stages. During this period, the count of lactococcal Cells reaches 10⁹ per gram of cheese mass.

During the Cytology/cytology/16.html">Early stages of ripening, yeasts belonging to the genera Kluyveromyces, Debaryomyces, Saccharomyces, Trichosporon, and others proliferate on the cheese surface. The Yeast population peaks (10⁷–10⁹ per gram) on the 4th to 5th day of ripening. Subsequently, the yeast count begins to decline, although they are still detected in small quantities until the end of ripening. Yeasts utilize lactates for Nutrition, thereby increasing the pH of the cheese matrix. This creates favorable conditions for the Development of the primary smear microorganism—Brevibacterium linens. Brevibacteria begin to multiply on the cheese surface by the 6th day of ripening, reaching a population of 10⁹ per gram or more, and accounting for up to 90–95% of the total smear microflora.

The smear microflora also includes micrococci: M. freudenreichii, M. caseolyticus, M. varians. Most micrococcal strains exhibit high proteolytic, aminopeptidase, and lipolytic activity.

Brined (pickled) cheeses. A distinctive feature of brined cheeses is that they mature and are stored in a concentrated brine, which imparts a sharp, salty flavor, a crumbly texture, and a white color. This cheese group includes Bryndza, Georgian, Imeretian, Ossetian, Stolovy, Chanakh, Suluguni, Feta, etc. Brined cheeses are characterized by an elevated salt content (4–7%) and moisture content in the matured product (47–53%). Halotolerant strains of lactococci capable of carrying out lactic acid fermentation in the presence of elevated sodium chloride levels are selected for brined cheese starter cultures.

During the ripening of cheeses, various defects in flavor, aroma, and consistency may occur. The most common among them are the following.

Bitter taste in cheese is caused by the excessive accumulation of specific bitter Polypeptides resulting from casein breakdown under the action of rennet and LAB peptide Hydrolases, which exhibit weak proteolytic activity. Enterococci (Enterococcus faecalis) present in raw milk that survive pasteurization are capable of accumulating large amounts of bitter Peptides. To prevent this defect, starter cultures containing lactic acid bacteria with high proteolytic activity should be used, and high-quality raw milk must be selected for cheese production.

Sour or excessively sour taste is the most common defect in cheeses produced with a low temperature of the second scalding. It mainly arises from processing overripe milk, adding excessive amounts of starter culture, or using starter cultures with elevated acid-producing activity. A sour taste in Swiss and Soviet cheeses may also occur due to the absence or poor development of propionic acid fermentation.

To prevent this defect, it is necessary to systematically monitor the quality of milk and starter cultures, and to regulate the lactic acid fermentation process by diluting the whey with Water. Cheese manufacturing should be managed to achieve optimal active acidity (pH) and moisture content in the cheese after pressing.

Brittle texture (splitting). This defect is characterized by the appearance of cracks longer than 1 cm in the cheese. When subjected to minor external loads, the size of these cracks increases. Quite often, a brittle texture is combined with a lack of eyes in the cheese. One of the conditions contributing to a brittle texture is insufficient cohesion of the cheese mass, which is due to a low Calcium and phosphorus content in the cheese. As noted above, a high rate of the lactic acid process during cheese production leads to a decrease in bound Ca and P and a low minimum pH value in the cheese. Splitting has been observed in cheeses with a pH below 5.17 after pressing.

High salt concentrations and delayed gas production caused by coliforms or heterofermentative lactobacilli also contribute to the occurrence of splitting.

Measures to prevent this defect include: maintaining pH at an optimal level by using milk with appropriate acidity; selecting the proper dose of starter culture and duration of curd processing; establishing optimal cheese moisture; and deacidifying the whey with water when necessary.

Fistulae (cavities). Voids up to 0.5 cm in size form inside the cheese and may expand, creating external openings. These cavities become contaminated with foreign microorganisms, primarily mold spores, yeasts, and putrefactive bacteria. The causes of fistula formation include: processing milk with elevated acidity, overdrying the cheese curd, poor curd adhesiveness, and excessive gas production.

Early gas formation is a defect caused by the intensive development of gas-producing microflora during the first 5–10 days after production. This defect manifests as the formation of a large number of small eyes inside the cheese, causing the cheese to swell and the rind to frequently crack. The causative agents are yeasts and coliform bacteria, which ferment milk sugar during the first three days after pressing. Since yeasts and coliforms are destroyed during milk pasteurization, the primary conditions for early gas formation are post-pasteurization milk contamination by microorganisms and low starter activity. Among coliforms, the genera Klebsiella and Enterobacter pose the greatest danger because they produce three times more gas than E. coli. One of the most important preventative measures against early gas formation is effective washing and disinfection of equipment and pipelines that come into contact with milk and the cheese curd. Another effective measure is increasing The rate of the lactic acid process, especially in the early stages of production. In this case, not only The activity of the starter used plays an important role, but also the preliminary ripening of the milk.

Late gas formation. This defect occurs in cheeses with high and medium temperatures of the second scalding due to the intensive multiplication of spore-forming clostridia within the cheese mass, primarily butyric acid bacteria—Clostridium butyricum and C. tyrobutyricum. Both species produce Butyric acid and large amounts of gases—H₂ and CO₂. The cheese eye pattern becomes spongy or fissured, the texture softens and becomes smeary, the cheese develops a harsh, unpleasant odor of butyric acid and a rancid taste, the cheese wheel swells, and the rind cracks. Clostridia multiply in the cheese during the later stages of ripening, when milk sugar has been completely fermented and the pH begins to rise due to the accumulation of alkaline protein breakdown products. Late gas formation can be prevented by using high-quality feed, bacteria removal from raw milk via bactofugation (which removes up to 90% of bacterial spores), adding substances that inhibit the growth of butyric acid bacteria (peroxidase-catalase Treatment), and selecting lactobacilli antagonists against butyric acid bacteria for starter formulations (antagonistic starters containing L. plantarum).

Lack of eyes (blind cheese). Blind cheese often results from the omission or suppression of gas-producing cultures—Lac. diacetylactis and Leuconostoc cremoris in cheeses with a low second-scalding temperature, or Propionibacterium schermann in cheeses with a high second-scalding temperature—from the starter composition.

The suppression of these microorganisms can be caused by bacteriophage infection or an elevated sodium chloride content (propionic acid bacteria are particularly sensitive to it). At too low a pH value of the cheese mass, the proper eye pattern may fail to form.

Molding. With improper cheese care and violation of ripening regimes, colonies of various mold fungi appear on the cheese surface. Mold spores enter the cheese surface from the air and from shelves. Effective disinfection of shelves and filtration of air entering the ripening room are necessary. To eliminate the growth of mold fungi, the cheese surface is treated with potassium sorbate. Potassium sorbate (0.5–1.0%) is also added to the brining vats.

Rind pitting (spotty mold). This defect is caused by the mold fungus Geotrichum candidum (synonyms Oospora lactis or Endomyces lactis). Initially, small round white spots appear on the cheese surface. Subsequently, the mold mycelium penetrates deep into the cheese rind, resulting in the formation of holes 5–10 mm in diameter. In cases of severe contamination with spotty mold, the rind can be completely destroyed.