## 1 Introduction
The liquid formulation is fed either in the form of an emulsion, suspension, or solution (aqueous or organic solvent-based). Then, the liquid feed is pumped and atomized at the entrance of the drying chamber, where it is converted into very small droplets, which form the so-called \"fog\" (droplets with a diameter in the range of 10-150 \(\upmu\)m), then microdroplets are in contact with the hot drying medium. Most commonly an inlet drying air temperature is between 150 and 200 \({}^{\circ}\)C [17; 18; 19]. Inlet temperature determines the ability of the mobile gas phase to remove the solvent by evaporation. The temperature must be as high as possible to obtain the lowest moisture content in the product while avoiding the thermal degradation and decomposition of the active components [20]. Achieving a large spray surface contact area due to the small size of generated droplets during atomization facilitates heat transfer from the hot drying gas to the atomized liquid particles. In this way, the solvent evaporates in a very short time (on average 1-20 s). Therefore, the dried particles usually do not reach the inlet temperature of the drying gas [21; 22]. Two stages of solvent evaporation and temperature profiles over time can be distinguished. During the first stage, as soon as the hot air contacts the sprayed liquid, the heat transfer causes to raise the temperature of the droplets to a constant value. With the progress of solvent evaporation, the diameter of the dried droplet decreases, and the concentration of solid substances on its surface increases. During the second stage, when the water content in the droplet reaches a critical value, a characteristic \"crust\" is forming on its surface, so that the rate of evaporation decreases rapidly [20; 23]. Then, the dried particles are transported by the drying air to the cyclone, where the powder is separated from the humid air and transferred to the product collection vessel [20]. Both the viscosity of the liquid system and the type of atomization, droplet size, and the proper manipulation of the atomization conditions affect the final size of the powder. The particle size of powders obtained by spray drying may be classified into three categories: small size (1-5 \(\upmu\)m), medium size (5-25 \(\upmu\)m), and large size (10-60 \(\upmu\)m) [24]. Depending on the size and efficiency of the dryer, it is possible to obtain the final powder from a few grams to several tons per hour [25]. As presented above, there are many factors that affect the percentage of viable probiotic bacteria remaining after the spray drying and storage. One of these is the type of protective substances added. Adding oil to a solution containing lactic acid bacteria that has been spray-dried is one of the effective methods of protecting these bacteria [26]. By adding oils, closed spaces are created, thanks to which lactic acid bacteria are less exposed to the effects of unfavorable environmental factors, such as water and H\({}^{+}\) ions. An additional benefit of oily substances is that they can only be digested by the lipase in the intestine, allowing the protected microorganisms to be released close to their optimal site of action [26]. To our knowledge, there are no reports explaining the mechanism of action of protective substances on probiotic bacteria. In this work, commercial natural plant-origin liquid oils were used to verify which of them improved the viability of _Lacticaseibacillus rhamns_ GG during the spray drying and storage for 4 weeks at refrigerated temperature and room temperature. In this study, we focused primarily on elucidation the influence of the type of liquid oils and their concentration on the viability of _Lbs. rhamns_ during both spray drying and storage at 4 \({}^{\circ}\)C and 20 \({}^{\circ}\)C (\(\pm 1\)\({}^{\circ}\)C). We also attempted to confirm and explain the protective mechanism of oils and to determine the effect of fatty acid content on LGG viability.