A new method for controlled release of carbon dioxide during the fermentation process of a probiotic beverage

V magistrskem delu smo obravnavali tematiko sproščanja ogljikovega dioksida (CO2) med fermentacijo mleka s kefirnimi zrni. CO2 ima velik vpliv na organoleptične lastnosti mlečnega napitka kefirja, zaradi česar je smiselno spremljanje njegove koncentracije v fermentacijskem mediju. Vsebnost CO2 lahko zaradi različnih razlogov tudi niha, kar na industrijskem nivoju ni zaželeno, zato smo pri naših raziskavah razvili model, ki je primeren za nadaljnjo analizo tega procesa ter za načrtovanje in sintezo regulacijskega sistema za vodeno dinamiko sproščanja CO2. Vse eksperimente smo izvajali v avtomatiziranem šaržnem reaktorju, ki je deloval kot odprt sistem. Fermentacije so potekale pri različnih temperaturah in vrtilnih frekvencah mešala, sledili smo koncentracijo nastalega CO2. Dobljene koncentracijske profile smo najprej analizirali iz mikrobiološkega vidika. Ugotovili smo, da lahko značilnosti krivulje uspešno opišemo z Gompertz-ovim kinetičnim modelom in njegovimi parametri (maksimalno doseženo koncentracijo, maksimalno proizvodnost CO2 in čas trajanja lag faze). Potrdili smo tudi odvisnost sproščanja CO2 od obeh procesnih parametrov. V nadaljevanju smo na osnovi analize eksperimentalnih podatkov razvili poenostavljen matematični model sproščanja CO2. Temperaturo in vrtilno frekvenco mešala smo obravnavali kot vhodni spremenljivki, koncentracijo plina kot izhodno. Serija eksperimentov pri konstantnih vhodnih spremenljivkah je pokazala, da lahko odziv izhodne spremenljivke v vseh primerih opišemo z eksponentnim izrazom s pripadajočimi parametri. Sledila je določitev vpliva stopničnega spreminjanja obeh vhodnih spremenljivk na izhodno. Primerjali smo koncentracijske profile CO2 v primeru konstantnih in spremenljivih vhodnih spremenljivk. Na tej osnovi smo določili prenosni funkciji, ki povezujeta spremembo vhodnih z odzivom izhodne spremenljivke. Dosegli smo zelo dobro ujemanje eksperimentalno in simulacijsko dobljenih profilov.In our master\u27s thesis we dealt with the topic of carbon dioxide (CO2) release during milk fermentation with kefir grains. CO2 has a great impact on organoleptic properties of the milk based beverage kefir, whitch makes it meaningful to trace the concetration in the fermentation medium. The CO2 content can vary due to different causes, whitch, on an industrial level, is unwanted. In our research we developed a model, that is suitable for further analysis of the process and for design and synthesis of a regulation system allowing guided dynamics of CO2 release. All experiments were carried out in an automated batch reactor, that functioned as an aerobic system. Fermentations were conducted at different temperatures and stirrer rotation speeds, and the CO2 concentration was measured. The resulting concentration profiles were first analysed from a microbiological point of view. We determined that the curve characteristics are successfully described by the Gompertz model and its parameters (maximum concentration, maximum CO2 productivity and the duration of lag phase). We confirmed the dependence of the CO2 release on both of the process parameters. Based on the analysis of experimental data we developed a simplified mathematical model for CO2 release. Temperature and stirrer rotation speed were determined as input variables, gas concentration as the output variable. A series of experiments with the input variable held at an constant value showed, that the response of the output variable can be described with an exponential equation and its parameters. This was followed by introducing a step change in both input variables and the derermination of the impact on the output response. We compared the CO2 concentration profiles of constant and changing input variables. On this basis, we determined the transfer function connecting the change in the input variable with the response of the output variable. We have reached a very good agreement between the experimental and simulated values