## 1 Introduction  
Buttermilk and skim milk are characterized by a similar composition in terms of their content of lactose, casein, whey proteins, and minerals [1, 2]. However, they differ in phospholipid content. Buttermilk contains up to 10 times more phospholipids than skimmed milk [1]. The source of phospholipids in buttermilk is the milk fat globule membrane (MFGM), which surrounds and stabilizes the fat globules, which are released during the churning process [3]. Hence, buttermilk is classified as a functional food [4], and publications have cited the health benefits of the MFGM, e.g., infection prevention, cognitive improvement and brain system development, immunity and protection metabolism [5], reduction in cholesterol levels, and anticancer [2].  
Due to its structure-forming, emulsifying, and water-binding properties, buttermilk is used in the production of yogurts [6, 7, 8]; ice cream [9, 10]; as a component in mixtures for chocolate production [11]; meat marinating [12]; and to improve the flavor and texture of bakery products [13].  
The cheesemaking sector is a large part of the dairy industry [14], where innovative concepts for the enhancement of cheese quality are of concern, e.g., the implementation of probiotic bacteria and non-fat cheese production, but also sensory properties. The sensory properties, texture, viscosity, and moisture retention of cheeses can be improved using buttermilk [15] and/or the introduction of whey proteins to its composition [16]. During the cheesemaking process, it is common to increase the protein content to increase cheese yield [14, 17]. In the production of fresh and hard cheese, a high concentration of casein is recommended, while in the production of quark cheese, a high total protein concentration and a high degree of whey protein denaturation is also desirable. The protein's composition can be modulated using membrane processes or the introduction of protein concentrates into the processed milk [18]. The texture of cheeses can be affected by moisture, fat content, pH, and, importantly, the structure of the protein matrix [14]. The structure of the protein's matrix can be modified, e.g., by conducting the polymerization process of whey proteins induced by heat treatment [19; 20; 21]. The obtained whey protein aggregates are used as a thickening agent for the formulation of fermented milk [22]; for food texture design [23]; for the microencapsulation of _Lactobacillus acidophilus_, which improves their survivability in yogurts [24]; to increase the water holding capacity and reduce the syneresis of fermented milk [25]; as a fat replacement in the development of low-fat yogurts [26]; and in determining the behavior of water in cheese from buttermilk [16].  
Buttermilk and whey are considered the main by-products in the dairy industry and, despite their documented health and technological potential, are still not sufficiently utilized for the development of new products [13]. In cheesemaking, milk is primarily employed as the basic raw material, and buttermilk is usually an additive aimed at improving rheological or sensory properties [27; 28]. The replacement of milk with buttermilk in cheese production can successfully lead to a new product with improved functions and nutritional value [15].  
The aim of our research was to directly use buttermilk as a by-product of butter production and indirectly use whey as a whey protein concentrate obtained from buttermilk, allowing for the development of an innovative dairy product: fresh white cheese from buttermilk with polymerized whey proteins. The effect of the heat treatment of whey proteins on the texture, color, and gloss of buttermilk cheese was analyzed for these parameters and its possible further use as a base, e.g., for the production of curd-ripened dried cheese. We also examined how the polymerization process impacted cheese yield and composition, which will provide the necessary knowledge for future cheesemaking.