The ability of microorganisms to grow plays an important role in dairy production [1, 2]. The microorganisms, owing to enzymes they produce, impact the texture, smell, and flavor of a dairy product. The probiotic characteristics of such a product play an important role, too. To ensure successful reproduction of microorganisms, appropriate growth conditions must be provided.
Reproductive capacity is best assessed by using the probability theory. In this case, the probability of division of one cell living in specific conditions, characterized by the presence and concentration of a substrate, water activity, active acidity, the salt weight fraction, and a number of other parameters that influence the cell’s life, is calculated [3, 5, 7].
This can be done on the basis of either special or previously conducted experiments provided that the conditions of such experiments were recorded. Both methods require compiling a rather large database that helps predict the behavior of bacteria in any given conditions. As complicated as it may seem at first glance, this task requires a strictly formalized approach to the description of the properties of both microorganisms and their environment. The present-day methods of mathematical modeling make it possible to predict the behavior of objects and their interaction with the environment [6, 10].
As regards the growth of microorganisms, a distinction should be made between a closed (uncontrolled) and controlled environment. Partially controlled systems can also exist. An uncontrolled system is such that is not exposed to external influences or when such exposure is negligible. The ideal uncontrolled system is a thermally insulated and hermetically sealed tub containing a substrate with the original number of microorganisms. Nominally, cheese mass at the ripening stage can be considered such a system [11, 13]. The main physical and chemical processes in cheese are influenced by ferments, i.e., chemical components that make up the cheese mass. Microorganisms are actively involved in this process as they take up nutrients, release metabolic products, and change the environment. Their activity during cheese ripening can only be affected by changing the temperature. A decrease in the temperature results in the reduced reproductive rate; an increase in the temperature accelerates the rate of cell division.
The majority of cheeses ripen within a temperature range of 8–20оС. During cheese ripening, its moisture content changes owing to water evaporation off the surface. This content is not large as opposed to the total cheese mass, but it can be of paramount importance as it influences the life of microorganisms.
Therefore, cheese can be referred to a group of systems with partially controlled parameters. In practical terms, it means that the living conditions of microorganisms inside cheese mass can only be controlled by changing its ripening and storage temperature.
The manufacture of fermented milk products is controlled more easily. Fermented milk products are normally manufactured in tanks equipped with a temperature control system (cooling and heating) and agitators. This setup makes it possible to stir the mass during production and influence the temperature. Moreover, various ingredients that influence the living conditions of microorganisms can be added to the mixture. Such ingredients may be salt, sugar, flavoring agents, preservatives, emulsifiers, stabilizers, etc. This system, although isolated from external influences, can be controlled in a wider context. However, the volume of this system and, consequently, its resources are limited, which means that only a certain number of microorganisms can be grown in this volume. Their maximum concentration is limited not only by the nutrients in the substrate but also by a variety of other factors.
