Cheeses are analyzed as vacuum drying objects. An experimental vacuum drier and its elements are schematized. The operating principle of the experimental setup is described. Moisture is demonstrated to be among the most important components of cheese. The physicochemical composition of cheeses is considered. The forms and energy of moisture binding in cheese are discussed. The hygroscopic and thermophysical properties of cheeses are reported. The kinetics of the vacuum drying of cheeses has been investigated. The vacuum drying of cheeses includes two stages: the drying rate is constant at the first stage and decreases at the second stage. The temperature curves of cheeses have been plotted in the temperature–moisture weight fraction coordinates. Drying curves in the heat load–time, temperature–time, and moisture weight fraction–time coordinates have been obtained and analyzed for various cheeses. Cheese drying rate curves have been constructed by graphical differentiation. The maximum cheese drying rates have been determined. Equilibrium moisture content values for cheese drying have been found. The cheese shrinkage ratio has been correlated with the thickness of the cheese bed being dried and with the shape and size of cheese pieces. Cheese shrinkage at both stages of vacuum drying proceeds uniformly. Raising the drying temperature above the prescribed temperature reduces the shrinkage ratio.
kinetics, vacuum drying, cheeses, temperature, shrinkage, moisture, dryers, heat, drying curves
INTRODUCTION
When considering cheese as a vacuum drying object, note that the variation of the properties of cheese during drying depends on the physicochemical properties, structure, and binding forms of moisture in the material and on the thermophysical characteristics of the cheese, including specific features of mass and energy transfer. The basic structural elements of cheese are macrograins, interlayers between macrograins, microvoids, and micrograins. The main element of each macrograin is a protein network, whose cells contain numerous included micrograins as fat drops, lipoid drops, and crystals.
The passage of fat from milk into cheese depends on many factors. Other conditions being equal, medium-size fat globules primarily pass into cheese, followed by small and large ones [1, 2]. Milk fat is viewed as the most valuable constituent of milk, even though milk proteins are still more valuable from the standpoint of nutrition physiology. The following four factors contribute to the particular significance of milk fat in milk and dairy products: economic attractiveness, nutritional value, taste, and the physical properties of dairy products that are due to the presence of fat [3].
During cheese ripening, all cheese components undergo profound changes and, as a result, the given cheese brand acquires its characteristic texture and pattern [4].
The moisture content of cheese depends on processing conditions, namely, renneting temperature and time, second-heating temperature, partial salting of cheese curds, water addition in the second heating, and cheese curds processing time. As the curdling temperature and second-heating temperature are decreased, the moisture capacity of the cheese curds and the water content of the finished product increase. As the temperature is raised, the water content of the cheese decreases. Moisture is lost at the salting stage due to osmotic water transfer and at the ripening stage via evaporation. The intensity of the microbiological and biochemical processes occurring in a cheese depends on the initial moisture content of the cheese after pressing [4, 5].
For most hard and semihard cheeses, the weight fraction of fat in dry matter is 45–50% and the weight fraction of moisture is 40–44%.
Fat in cheese is mainly in the form of micrograins 10–15 μm in diameter. There are also larger fat inclusions, which are referred to as fat drops and are uniformly distributed throughout the cheese bulk. The fat drops and lipoid micrograins in cheese are milk fat destabilized during cheese making and ripening. This is true because, above 20°C, cheese fat can melt out of cheese curd, and this is the main obstacle in thermal cheese dehydration.
OBJECTS AND METHODS OF THE STUDY
The objects of our study were the Sovetskii, Shveitsarskii, Altaiskii, Gornyi, Moskovskii, Gollandskii, Kostromskoi, Poshekhonskii, Yaros-lavskii, and Ozernyi brands of cheese. The study was carried out using standard, commonly accepted, and modified physicochemical, rheological, microbiolo-gical, and biochemical methods.
The drying techniques and drier designs employed in cheese making are very diverse. First of all, any drier design should ensure uniform heating and drying of the product and reliable control of its temperature and moisture content. The driers should have a sufficiently high output capacity, but at the same time they should be economical in terms of heat and electricity requirements and should be metal-intensive to the least possible extent. Present-day driers should be multipurpose, capable of drying various materials [6, 7].
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