## 1 Introduction  
Phylase is one of the main enzymes in the feed and poultry industry [1]. It plays the key role in splitting different kinds of phytate (myo-inositol 1,2,3,4,5,6-hexahishosphate [2]), which, being the major form of phosphorus and some essential chemical element storage in plants, makes up to 75-80% of total phosphorus in plant seeds [3]. Phylate represents a significant phosphorus share in grains and oilseeds, which cannot be assimilated because of a lack of phytase in the intestines of animals [4]. The available phosphorus demand in animal diets was evaluated as 0.35-0.45% [5]. A diet including corn (~65%) and soybean (~30%) meal possesses 0.13% available phosphorus [6]. Calculation of the data demonstrates that full release of the inorganic phosphate from the phytate in this diet can increase the available phosphorus level up to 0.48% [5]. The efficacy of free phytase decreases due to the following factors: (1) thermal denaturation when the feed is formulated; (2) degradation in the fore stomach and gizzard due to low pH and pepsin; and (3) rapid proteolysis by some proteases along with delayed releasing of the substrate (phytate) from the grain in the duodenum [6]. The _Obesumbacterium proteus_ phytase (OPP) [7] is typical to the members of the family C phytase (PhyC). Its activity reaches its maximum at pH 4.9 and keeps thermal stability up to 60 \({}^{\circ}\)C (even at pH 3.0). The phytase in the strain has a maximum activity of \(\sim\)340 phytase activity units (FYT)/mg. _Pichia pastoris_ is reported to produce the phytase and its modified derivatives with improved thermal stability [8; 9; 10]. Before, we [11] described another approach to increase the thermal resistance of OPP. The _Yarrowia lipolytica_ yeast produced the enzyme in cytosolic form. The enzyme, encapsulated inside the yeast cell, remained stable upon short heating at 70-80 \({}^{\circ}\)C, the temperature of spray drying for feed premixes. Currently, the standard technology for feed enzyme production in a secreted form widely used all over the world permits obtaining pure enzyme of a high activity. The technology is profitable for major manufacturers because of its low prime cost [12]. Nevertheless, it has some disadvantages: the method demands high-quality raw substrate, the waste disposal of the cell mass, power consumption for vacuum concentration, and spray drying of the biomass [13]. The _Y. lipolytica_ yeast is reported to assimilate different kinds of wastes and convert them into a valuable protein. Consequently, the growth of _Y. lipolytica_ recombinant strains, which could produce OPP using waste [14], could let us obtain a feed additive of high thermal stability, grow on fat-containing waste (waste and wastewater of the slaughter, ribouse, oil and fat industries), and reduce energy consumption for drying. Furthermore, it raises OPP stability in the animal's stomach and intestine, gradually releasing the phytase as the feed travels in the chyme along the intestine [14]. Animal by-products such as meat, bone meal, and poultry by-product meal are of great significance as feed ingredients in poultry feed, being not very expensive. These animal protein ingredients are good sources of amino acids with high protein levels, total digestible dry matter, and energy. A chicken intestine is rich in protein, but unfortunately it is not used as a protein source in feed additives (e.g., for fish food) [15; 16]. Poutlry by-products are valuable sources of not only proteins but also lipids. Thus, according to [PERSON] and [PERSON], the lipid amount in a chicken intestine reaches nearly 7.64%, but in the viscera it is from 13.5 to 15.5% [17]. Although the need for the use of microbial phytases in poultry feeding is of no doubt for either poultry keepers or physiologists in the field of animal nutrition, the efficiency of modern technologies for phytases use can be significantly enhanced.