# Use of a Heating System to Control the Probiotic Beverage Production in Batch bioreactor
## 1 Introduction  
Bioreactors of three basic forms have been used widely in the fields of environmental protection, healthcare, industrial biotechnology, and space exploration: batch, fed-batch and continuous ones [1, 2, 3]. For batch bioreactors, it is typical that their content has no contact with external substances or organisms during the execution of the biological process, i.e., the bioreactor is closed during the operation, and no addition or removal in/from the bioreactor is possible. This mode of operation allows easy construction of batch bioreactors, which is reflected in low production costs and easy maintenance. Therefore, batch bioreactors are the most widespread and most used among all types of bioreactors [4, 5, 6, 7, 8, 9]. In fed-batch bioreactors, it is possible to add substances or organisms during the biological process's execution to cultivate one or more nutrients [3, 10, 11, 12, 13]. All products remain in the bioreactor till the end of the fermentation process. Continuous bioreactors (also named flow bioreactors) enable the inflow and outflow of substances or organisms into/from the reactor as a flowing stream. The microorganisms or cells in bioreactors are fed continuously [14, 15, 16]. In our article, we focused on batch bioreactors for the production of probiotic dairy beverages. Depending on the type of microorganism, the carbon dioxide, acetic acid, diacetyl, acetaldehyde, ethyl alcohol, and several other substances are formed during the fermentation process, and these give the products their characteristic fresh taste and aroma. To obtain high-quality milk products, some main parameters of the fermentation process must follow the prescribed reference values. One of the quantities which correlates well with the vital fermentation process parameters is CO\({}_{2}\) release. Thus, the control of the proper dynamic profile of the CO\({}_{2}\) release can also assure proper values of the fermentation process parameters. The quality and quantity of the produced probiotic beverages and the time required to obtain them depend on the time course of the fermentation process. Based on experiences, technologists working in this field know what the time course of the fermentation process should be to make production as optimal as possible. The main problem in using the present batch bioreactors is that they do not enable control of the fermentation process after the beginning of the operation. The whole fermentation process subordinates only from the compounds selected before the start of the fermentation process. However, the transient state of the fermentation process can also be influenced by batch bioreactors. Despite the fact that it is not possible to supply microorganisms or substrates during operation, we can change some physical and chemical quantities in the bioreactor during operation: temperature (by heating or cooling), homogenization (by mixing), ventilation (by supplying oxygen). Many laboratory tests were performed, from which we tried to determine which physical/chemical quantity can most influence the transient state of the fermentation process. It has been shown that changing the temperature has a great influence on fermentation. Changing the bioreactor temperature is also easy to realize, as most bioreactors are equipped with a heating system. This was why we decided to develop a control system that will influence the fermentation process by changing the bioreactor's temperature. In the initial phase of the study, we also performed a thorough review of relevant journal publications in the period 2000-2020. The purpose of this review was to determine the directions of research in this area. We found many interesting articles in the field of Continuous bioreactor Control [2; 17; 18; 19] and even more in the field of Fed-batch bioreactor Control [20; 21; 22; 23]. However, unexpected and very interesting was the disclosure that there are very few publications dealing with the control of batch bioreactors. Most publications in this field deal with the control of dissolved oxygen.