## 5 Effect of the Potato Protein Isolation Method on the Properties of the Obtained Preparations  
The starch industry is investigating the possibilities of using proteins isolated from potato fruit juice (PFJ), mainly due to their promising nutritional and health-promoting properties and also non-nutritional properties [5; 51; 57]. Extracting proteins from industrial potato fruit juice (PFJ) has been the subject of research for many years. Various factors have been used to coagulate the proteins in the juice. These included increased temperature up to 120 \({}^{\circ}\)C, applied to juice with the isoelectric point pH of most potato proteins, most often in the range of 4.5-5.2 [4; 5; 10; 13; 20; 44; 61; 62; 63]; low pH in the range of 2-4, causing patatin proteins to coagulate at low temperature [25; 64]; ethanol [27; 32; 65; 66] at different pH levels; increased ionic strength in the juice with a pH of 4 and 6-7, combined with a temperature in the range of 50-80 \({}^{\circ}\)C [5; 19]. Salting out is also an important method of protein isolation. Ammonium sulfate was often used in such a process, the use of which made it possible to obtain preparations with high efficiency, but with a relatively low protein concentration [10; 32; 33; 44; 67], while the use of ferric chloride salt accelerated the precipitation of proteins from potato juice and increased the efficiency of its recovery to over 86%. This was due to the strong affinity of this salt to potato proteins. Yet another known method of obtaining native forms of potato protein is complexing them with carboxymethylcellulose. The use of CMC contributed to an increase in the solubility of the preparation and improved the stability of the emulsions formed with its participation [68]. Natural adsorbents, such as clay minerals, were also used in the process of isolating proteins from potato fruit juice (PFJ) [16; 69], enabling the reduction of the amount of glycoalkaloids in the obtained protein preparations. Studies on the extraction of patatin proteins from potato fruit juice (PFJ) have also shown that a significant difficulty in isolating these proteins is the protein-non-starch polysaccharide interactions occurring in the juice, which require pH adjustment [33]. The above-mentioned methods were described in detail in many publications in this field [4; 5; 9; 51]. The coagulated and thermally altered potato proteins did not show the most important nutritional, functional and biological/non-food importance of these proteins. These methods are currently used in industry, mainly due to the high efficiency of the isolation process (over 85%) [10]. At the same time, methods were developed to allow the isolation of potato proteins in their native form. In the late 1970s, such studies were conducted by [PERSON] and [PERSON] [70] using UF ultrafiltration (UF) to treat potato starch effluents for protein recovery. Later, other authors [3; 71] applied membrane techniques, including ultrafiltration and reverse osmosis, to obtain native protein preparations of potential food significance. The research of the above authors showed that the use of filtration membranes on an industrial scale led to their fouling by the fiber contained in the potato fruit juice (PFJ). Problems with cleaning membranes, as well as the insufficient filtration efficiency and selectivity of the method, significantly hampered the possible implementation of this technology. In the following years, thanks to intensive research, these problems were minimized by the use of the removal of the pectin fiber in the flocculation process in the presence of calcium ions [4; 19]. Other studies aimed at extracting native potato proteins from potato fruit juice (PFJ) used the expanded bed adsorption methods, allowing the concentration of proteins at a similar level as membrane techniques [14; 18; 69; 72]. As presented by [PERSON] et al.