Darja Pečar

Testing the corrosion resistance of stainless steels during the fermentation of probiotic drink.

BACKGROUND Over recent years, food producers have devoted much attention to the production of safe foods. Simultaneously, using advanced process technologies, it has been necessary to carefully select materials for use in process equipment. Milk and its products are exposed to metal surfaces from the time they are processed, through the various stages of handling and manufacture, to the packaging of the finished products for market. The selection of suitable materials for daily use in the dairy industry cannot be governed solely by their price and mechanical properties but must also take into consideration their influence on the quality of milk products. RESULTS This paper presents the results of testing the corrosion resistance of three stainless steels during the fermentation of a specific probiotic drink, kefir. Experiments were conducted using preliminarily activated kefir grains as a starter culture. Corrosion resistance was studied using a gravimetric and two electrochemical methods. The two steels showing the best corrosion performance differed mainly in their Mo and Nb contents. CONCLUSION The results indicated that Nb played the most protective role against corrosion during kefir fermentation, since the steel containing Nb but no Mo showed the lowest corrosion rate.

Kinetic modeling of benzoic acid esterification using functionalized silica gel

ABSTRACT In our study, we synthesized two types of solid acid catalysts. Silica gel with a particle size of 0.063–0.2 mm was silanized first at reflux (S1) and second at room temperature (S3) using (3-mecaptopropyl)trimethoxysilane in toluene, and further oxidized with H2O2, and methanesulfonic acid. Characterization of the synthesized catalysts was performed using adsorption/desorption of nitrogen (BET), Fourier transform infrared spectroscopy, scanning electron microscopy, and thermogravimetric analysis. Both catalysts were used in esterification reactions of benzoic acid with methanol. All reactions were performed in a batch reactor at temperatures, ϑ = 55, 60, and 64.5°C, stirrer speed, fs = 400 min−1, and catalyst loading, m = 5, 10, and 15 g. The concentration of produced methylbenzoate was determined by HPLC. The study was supplemented with a kinetic study of the reaction. First-order kinetics was confirmed for this esterification reaction. Using catalyst S1, higher conversion was reached, while catalyst S3 is much more suitable for reuse.

OXIDATION OF MENTHOL: REACTION-RATE DETERMINATION BASED ON THERMODYNAMIC PROFILES

This study investigated the reaction kinetics of menthol oxidation. For this purpose the thermal profiles obtained from the measurements within the reaction calorimeter were used for the first time. In our case the reaction was performed under isothermal conditions using potassium dichromate as a catalyst. Due to the low solubility of menthol in water, acetone was added to the diluted sulfuric acid as a co-solvent. This caused the reaction medium to become more homogeneous and thus more appropriate for kinetic analyses without considering a mass transfer term.

Mathematical model of CO2 release during milk fermentation using natural kefir grains: Modeling CO2 release during milk fermentation

BACKGROUND Milk fermentation takes place in the presence of various micro-organisms, producing a variety of dairy products. The oldest of them is kefir, which is usually produced by the fermentation of milk with kefir grains. Carbon dioxide (CO2 ), as one of the process products, also contributes to the characteristic flavor of kefir. The amount of CO2 generated during fermentation depends on bioprocessing conditions and may change, which is not desirable at the industrial level. RESULTS In this study we developed a simplified mathematical model of CO2 release in the milk-fermentation process. An intuitive approach based on superposition and experimental analysis was used for the modeling. The chemical system studied was considered as a two-input (temperature, rotational frequency of the stirrer) one-output (CO2 concentration) dynamic system. CONCLUSION Based on an analysis of CO2 release transients in the case of non-simultaneous stepwise changed input quantities, two differential equations were defined that describe the influence of the two input quantities on the output quantity. The simulation results were verified by experiments. The proposed model can be used for a comprehensive analysis of the process that is being studied and for the design and synthesis of advanced control systems, which will ensure a controlled CO2 release at the industrial level.