###### Abstract
It exhibits maximum activity at pH 4.9, thermal stability up to 60 \({}^{\circ}\)C (at pH 3.0) and a maximum activity of \(\sim\)340 FYT/mg. There are reports about production of this enzyme in _Pichia pastoris_ including its artificially modified derivative with enhanced thermal stability [22; 23; 24]. [PERSON] et al. [25] described an alternative approach to increasing the thermal stability of OPP. This enzyme was produced in the extremophile yeast _Yarrowia lipolytica_ in a non-secreted (cytoplasmic) form. Encapsulating the enzyme within the yeast cell made it stable under short-term heating at 70-80 \({}^{\circ}\)C, the temperature required for spray drying feed premixes. Producing feed enzymes in a secreted form is a standard technological approach used by all manufacturers worldwide. It allows the production of pure enzyme preparations with high activity; they are usually added to mixed folders in the course of the pelteling. This approach is advantageous for large manufacturers because it reduces the cost of logistics and warehousing. However, producing secreted enzymes has three major limitations--it requires high-quality raw matter for producing cultivation media, it requires utilisation of the cell mass as a waste and it requires high energy consumption during the vacuum concentration and spray drying stage. _Y. lipolytica_ is well known for its ability to grow on fat-containing waste, converting it to highly valuable feed protein. Therefore, cultivation of the recombinant _Y. lipolytica_ strains that produce OPP on fat-containing animal waste--as described by [PERSON] et al. [25]--would allow the simultaneous achievement of the following goals: (1) to produce a feed additive with high thermal stability (compatible with the spray drying of mixed fodder); (2) to utilise fat-containing waste without producing extra pollutants; (3) to reduce energy consumption for drying because vacuum concentration would be unnecessary; and (4) to enhance OPP stability in the stomach and intestine of productive animals, with the gradual release of the phytase as it travels in the chyme through the intestine. Taking into account the fact that [25] have reported the technology of obtaining microencapsulated phytase, including spray drying, and documented the fact of complete preservation of phytase activity at 75-80 \({}^{\circ}\)C during drying, which was not achievable when drying secretory phytase under similar conditions, the present work had an objective of demonstrating that microencapsulated phytase has a significantly higher ability to stimulate animal growth when added to feed than secretory phytase. To this end, a diet formula deficient in available phosphorous (0.15%) but with the total phosphorous including the grain phytate of 0.45% was developed to test the phytase effect on ADG in mice. The experimental design of the study included three stages. First, a recombinant _Y. lipolytica_ strain that produces OPP was cultivated on a pure swine blood sterilised by autoclaving, as described by [25]. The cell mass was then subjected to spray drying. Then the effect of the OPP-containing additive on the ADG of the mouse model was compared with a commercial phytase preparation--Ladozyme Proxi derived from _Aspergillus ficuum_ (Emzyme, Ukraine) [11]. There were also control groups: a balanced diet with adequate phosphorus (PC, 0.21% available phosphorus and 0.54% total phosphorus), a balanced diet with insufficient phosphorus (NC), the NC diet supplemented with an inorganic phosphorus up to the adequate level (1.15% available phosphorus and 1.45% total phosphorus) and the NC diet supplemented with non-recombinant _Y. lipolytica_ strain producing no phytase (mock additive) produced by cultivation on the swine blood and then subjected to spray drying. 2. Materials and Methods. The experimental group was fed the NC diet supplemented with 165 FYT/kg microencapsulated phytase, whereas the group of comparison was fed the same diet supplemented with 15,000 FYT/kg commercial secreted phytase.