Yogurt production is traditional branch in our food industry. Yogurt is a fermented milk product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus. One of them is invented and used for the first time in our country and that's why it is named after Bulgaria. Yogurt starter culture is a symbiotic combination of strains of bacteria mentioned above without genetic modification. One of the main parameters that characterize starter culture quality is pH, which is measured on-line in industry. A control system of starter culture pH value is proposed. To guarantee the quality of pre-fermentation product, the pH value is maintained in admissible range. The proposed system is a part of contemporary high-technology in the manufacture of fermented dairy products.
A new highly effective symbiotic starter culture is investigated. For production of the original yogurt, natural strains S.thermophilus 13а and Lb. bulgaricus 2-11 are used. They are isolated from home-made yogurts manufactured in Rhodopes - mountain region in Bulgaria. The new control system is developed on the basis of experimental data - laboratory measurements of lactose, lactic acid and biomass concentrations, as well as on-line measurements of starter culture pH. A mass-balance model is derived using experimental data. The dynamics of each variable is presented by differential equation. The procedure applied for model identification is realized using program package on the basis of Evolutionary algorithm that minimizes mean square errors between model and experimental data. From expert's point of view, a mathematical relationship between lactose concentration and pH value in the reactor is determined. This relationship gives the possibility the dynamical equation of pH to be derived. In this way, the relationship between the control output (pH) and control input - the dilution rate (D) is received.