INTRODUCTION
It is common knowledge that the process of milk coagulation represents the transition of a micellar colloidal system of caseins to a gel-like state. This process underlies many dairy technologies, particularly, cheese making, largely predetermining cheese quality [1]. The latter circumstance makes the coagulation of milk proteins the subject of thorough research for scientists specializing in the properties of milk and dairy products.
The estimation of the beginning of gelation in milk is topical both for scientific research and for process monitoring in industry. Suffice it to note that the exact estimation of the beginning of coagulation directly in the cheese vat makes it possible, in principle, to adjust automatically the process schedule as the physic-chemical indicators of milk change (for example, the protein content) in order to economize on both milk-clotting preparations and cheese production times. Very important is also the correct and unambiguous determination of the duration of the main coagulation stages for research purposes, in particular, for building adequate models of the coagulation process.
Despite the very long history of dairy technology, the methods of objective monitoring of the milk coagulation process appeared relatively recently. We may say that the main method of determining the time of coagulation was visual observation using a cheesemaking knife or a cheesemaker's finger practically until the mid-20th century. Note that the first experience of objective observation over the rennet clotting of milk with a viscometer dates back to 1932 [2].
At present, there are a large number of various methods for objective monitoring of milk coagulation. They all may conventionally be divided into several groups characterized by the choice of milk parameters that undergo changes during coagulation [3–5]. Basically, these are certainly rheological and optical methods, based on observing changes in milk structure during its coagulation. In addition, the rheological methods record structural changes by the system's response to applied mechanical stresses or deforma-tions, and the optical methods, by the change in the absorption or dissipation of visible light, as well as in the emission of the infrared band. The wide use of these two methods depends, primarily, on the possibility of their use directly online. Moreover, these methods ensure obtaining well-correlated data [6–8].
Another quite widespread method to monitor milk coagulation due to the possibility to use it online is the hot-wire method, developed in 1985 [9]. Its essence is that the temperature of an electric current–conducting wire placed in milk increases during coagulation because of a decrease in a convective heat sink resulting from the formation of a structure in milk. This method is indirect rheological in essence; therefore, the results obtained with it correlate well with data obtained both rheologically and optically [10]. Thanks to its simplicity and possibility to be used online, this method continues to be topical nowadays [11].
