**1. Introduction**

Cheese belongs to dairy products representing an important part of human balance diet for hundreds of years. It contains the essential macronutrients and micronutrients. Different factors as environmental conditions, geographical region, processing technologies and the local customs determine the development and release of the huge variety of cheese. One prevention method of milk spoilage over the centuries has been the production of cheese, meanwhile the nutritional benefits are conserving [1].

Cheese is the result of a fermentation process driven by lactic acid bacteria. Different bacteria, yeast and molds constitute the complex microbial ecosystem of this product. Throughout the ripening, the microbes release proteinases and lipases that are involved in the protein and fat hydrolysis, resulting amino acids and fatty acids. These compounds represent important flavour precursors for cheeses [2].

The cheese microbiota is involved in the development of the appearance texture, flavour and aroma. Both starter culture organisms and allochthonous microorganisms are part of the specific microbial ecosystem.

The quality and the processing of the milk, the environmental conditions (temperature, humidity, salt), the applied technology influence the quantity and the diversity of the specific species of microorganisms. Also, this fact is affected by the microbial conditions in the manufactures [3].

Cheese can be considered a privileged product with beneficial or negative impact on health, influenced by the nutritional composition, salt and bioactive compound content. The negative effect is related to the presence of pathogenic bacteria such as *Listeria* and *Escherichia coli* [1].

## **2. Factors controlling microbial growth in cheeses**

In various types of cheese, the growth and development of microbes are influenced by physical and chemical parameters (water activity, pH, ripening temperature, redox potential), chemical composition (salt content, presence of nitrates) and by the metabolites as organic acids and the protein like compounds released by starter cultures named bacteriocins [2, 4].

Water activity (aw) of cheese range between 0.917–0.988. Vast majority of non-starter bacteria, spoilage and pathogens are favoured by this condition. The optimum aw of starter bacteria is under these values. Osmophilic yeasts support aw less than 0.60 [4]. The aw is related to sodium chloride (NaCl) and the produced metabolites (lactate, acetate, and propionate) content. These compounds decrease the vapour pressure of water as the result of their dissolution in the moisture of the cheese. Higher the concentration of these metabolites, the reduced the aw is. Without vacuum or sealed packaging, the moisture content is reduced, due to evaporation, causing a gradient in the aw of the cheese. This gradient commonly is proportional with the cheese size and is lower in the external part. At low aw the lag phase of bacterial growth is prolonged [2].

The salt content decreases the aw value of the cheese and consequently is an antimicrobial agent. For example, for the brined-cured cheeses at the beginning of the ripening process, the salt concentration is much higher on the cheese surface than in the inside the block. For this reason, salt tolerant microorganisms can grow on the surface of this type of cheeses. Surface-ripened cheeses microbiota include *Brevibacterium linens* and *Debaryomyces hansenii* growing at 15% NaCl with aw ~0.916 [4]. Law salt content was associated with the development of coryneforms, micrococci, and staphylococci. The growth of these microorganisms is supported by 10–15% NaCl. 10% NaCl doesn't affect the development of *P. camemberti*. 20% NaCl is tolerated by several strains of *P. roqueforti,* whereas *Geotrichum candidum* is susceptible to salt [2]. Concentration of 15 g/L NaCl is used in manufacturing of Protected Designation of Origin (PDO) Serpa cheese [5]. NaCl can have a negative impact on cheese, affecting the enzymatic and microbial activities. This lead to the change of biochemistry processes as lactose metabolism, hydrolysis of fats and casein, the flavour compound development and the cheese pH formation. Paracasein aggregation or hydration is influenced by the salt -, calcium content and pH. These determinate the water binding properties of the casein matrix, the textural, rheological and cooking aspects and the ability to synerese [6].

Most bacteria grow best at neutral pHs. In acidic conditions (pH < 4) only several bacteria are able to grow, for example *Lactobacillus* spp. Molds and yeasts prefer pHs lower than 4.5. Viability of bacteria is affected by metabolites. Various weak organic acids (as sorbic and propionic acid) are causing bacterial lysis. These

#### *Microbial Diversity of Traditionally Processed Cheese from Northeastern Region of Transylvania… DOI: http://dx.doi.org/10.5772/intechopen.97591*

compounds cross the bacterial cell membrane releasing protons inside the cell and leading to acidification. In cheese environment, the main occurring organic acids are acetic, lactic and propionic acids. Propionic acid exerts antimicrobial effect against molds [4]. The slight acidic range in curd (pH = 4.5–5.3) contributes significantly to the growth and development of bacteria in cheese [7]. Microbial growth is determined also, by the ripening temperature. The optimum temperatures for the frequently used lactic acid bacteria, belonging to mesophilic and thermophilic bacterial groups, is 35°C and 55°C respectively.

Maturation temperature should play a role in preventing the growth of undesirable microbes as secondary flora, spoilage and pathogenic microorganisms, thus avoiding the losses [4].

The food hygiene is taken very seriously in the dairy industry. The quality of dairy products depends on raw milk quality and the processing conditions. Before ripening the number of microorganisms showed higher values in artisanal soft cheese from Germany named kochkäse, made from raw milk in contrast with cheese from pasteurised milk. After maturation, the amount of microorganisms was identical in the two samples. There is a close correlation between the temperature and ripening time. The ripening time is decreased at 35°C with the rising microorganism population. The maturation temperature at 15°C favour the lactic acid bacteria growth [8].

Redox potential (Eredox) values in cheese is approximately equal with Eredox = −250 mV, during the maturation process is reduced, due to unknown mechanisms. The redox potential is different inside and outside of the cheese that specifies the microbiota.

It was shown that the cheese surface is characterized by aerobic bacteria namely *Brevibacterium* spp., *Bacillus* spp. and *Micrococcus* spp. The predominant microbes inside the cheese are anaerobic or facultative aerobic microorganisms [4].

For the prevention of bacterial gas formation by coliforms and *Clostridium tyrobutyricum*, different chemicals are added to milk. In case of cheeses as Gouda and Edam cheeses potassium or sodium nitrate is added in amount of 20 g/100 L raw milk). In the milk or curd, the xanthine oxidoreductase catalyses the formation of nitrites from nitrates, that inhibit the aforementioned progress. Also carcinogenic compound as nitrosamine may resulted from the reaction of nitrites with aromatic amino acids [2].

Environmental factors influence the growth and development of lactic acid bacteria in cheese. The environmental factors and the various amount of NaCl, glucose and lactose affect the growth and biofilm formation of bacterial strains originated from Serbian traditionally processed cheese (*Enterococcus hirae* KGPMF9, *Lactococcus lactis* subsp*. lactis* KGPMF23, *Lactobacillus fermentum* KGPMF29, *Streptococcus uberis* KGPMF2). The limiting values for the factors were 6.5% of NaCl and 4°C. Except for *E. hirae* KGPMF9. *S. uberis* KGPMF2 different concentration of the carbohydrates and temperature of 37°C induced the planktonic growth and biofilm formation of the analysed bacteria. In case of biofilm formation basic media in contrast to acidic media was limiting factor [9].
