## 1 Introduction
in 2009 used the disk diffusion method to show that volatile fennel oil has antibacterial properties against _Bacillus subtilis_, _Aspergillus niger_, and _Escherichia coli_[31]. In 2019, using the same method, [PERSON] et al. demonstrated the antimicrobial character of this oil on _Listeria innocua_ (Gram-positive) and _Pseudomonas fluorescens_[21]. Other studies carried out in 2011, 2018, and 2021 demonstrated the antimicrobial character by associating this with the components of volatile fennel oil [33; 35; 36]. The purpose of our research is to create functional foods that bring nutritional benefits to the consumer by incorporating bioactive components. In 2019, we started studying dairy products enriched with different volatile oils [37]. This research has been certified by articles, and the positive results obtained have led us to analyze as many compounds in these products as possible. For the present study, we have chosen to continue the studies carried out for kefir enriched with encapsulated volatile oils. The sensory, textural acceptability, and antioxidant capacity of the products were studied, and the results were published in 2022 [38; 39]. In this context, the present study was designed to determine the changes in the chemical composition of kefir enriched with encapsulated volatile oils by enzymatic methods and the antimicrobial activity of volatile oils. Enzyme kits from the R-Biopharm company were used. Enzyme analysis is an excellent way to identify numerous compounds in dairy products because the test kits include selected, multi-tested reagents. For the current study, we decided to use these test kits because they have many advantages, the most important being the high degree of safety. These kits are recommended by the IDF (International Dairy Federation) [40]. The analyzed compounds were lactose, D-glucose, D-galactose, acetic acid, ethanol, L-lactic acid, and L-glutamic acid. The use of volatile oils with antimicrobial capacity aims to increase the finished product's shelf life. So, using encapsulated volatile oils and cow's milk, we created three different forms of kefir. Lavender, mint, and fennel were the volatile oils that were utilized. The antimicrobial capacity of the volatile oils used was tested on several types of microorganisms that poison food products, especially dairy products. Three types of molds (_Geotrichum candidum_, _Penicillium expansum_, _Aspergillus niger_) and one Gram-negative bacterium (_Escherichia coli_) were selected. _Geotrichum candidum_, the best-known species in the genus, is a yeast-like fungus tolerant of acidic environments. Its taxonomic position was revised in 2004 by [PERSON] and [PERSON], the genus _Geotrichum_ being composed of 22 species [41]. _Geotrichum candidum_ thrives at temperatures between 5 and 38 \({}^{\circ}\)C, with an optimum temperature of 25 \({}^{\circ}\)C. It is tolerant to a wide range of pH values, with optimum values between 5.0 and 5.5. It has a low salt tolerance. Unlike yeasts, the growth of _G. candidum_ is limited by the amount of salt. Concentrations of 1% NaCl lead to slight growth suppression. NaCl concentrations of 5-6% have an inhibitory effect [42]. Blue mold disease caused by _Penicillium expansum_ is the most economically important post-harvest disease of stored fruit and vegetables. In addition to causing food spoilage, some strains of the fungus produce patulin mycotoxin [43]. Patulin is a moderately toxic mycotoxin, but according to studies, it has no carcinogenic or mutagenic effect. _Penicillium_ species have a highly evolved physiology, resulting in adaptation to a very wide range of habitats. All _Penicillium_ species can grow in food at a low pH of between 2 and 3. Most _Penicillium_ species grow at lower temperatures, and none are thermophilic [44]. _Aspergillus niger_ is a type of mold that causes \"black mold\" on the outside of certain foods, such as apricots, onions, and grapes [45].