## 1 Introduction  
Modern consumers are becoming more demanding regarding food and dairy products. On the one hand, these demands are associated with the quality, attractiveness, and packaging of dairy products. On the other hand, these demands are based on the quality characteristics of the functional content of bioactive compounds and new sensory experiences. The desire of producers to satisfy these expectations is the result of innovations within the dairy industry, which consist of the introduction of new or improved versions of existing technology, e.g., membrane techniques (e.g., microfiltration and ultrafiltration), for the enzymatic conversion of lactose [1]. The microfiltration (MF) of milk is already well known [2]. Currently, the production of casein concentrate (the retentate after MF) and the rest of the components of milk (the MF filtrate) are generally directed to the production of whey protein concentrates (WPCs) and other dairy products with increased pro-health value. The base material can be composed of cow's, goat's, sheep's, or buffalo's milk, among others. Goat's milk products are well established in the dairy industry [3]. The typical composition of goat's milk is 87% water, with the remaining being a dry substance consisting of 3.8% fat, 4.3% lactose, and 3.5% protein. Additionally, goat's milk protein consists of 70% casein, 25% water-soluble whey protein, and 5% fat globule membrane proteins. Compared to cow's milk, goat's milk contains more \(\beta\)-casein fractions (0-64 g/100 mL) and less \(\alpha\)s1-casein (0-28 g/100 mL) [4, 5]. Hence, it is a valuable raw material in cheese making. Goat's milk cheese production technology involves milk being increasingly fractionated by microfiltration [6]. Then, when the casein content increases, some of the native whey proteins (serum proteins) are eliminated. The protein fraction of the serum is the permeate, which can be further concentrated by ultrafiltration (UF) [7]. In addition, the use of enzymatic processes, e.g., lactose hydrolysis and transgalactosylation, may lead to changes in lactose content [8; 9; 10]. Thus, it is possible to not only increase the protein content but also reduce or eliminate the lactose content, which leads to the formation of prebiotics, e.g., galactooligosaccharides (GOS). GOS are a mixture of galactose and glucose with the molecular structure (Gal)n-Glu. They are produced by the transgalactosylation of lactose by the enzyme \(\beta\)-galactosidase from lactose-rich products, particularly milk and whey. These stimulate the growth and development of the intestinal microflora and are prebiotics [11]. GOS are resistant to high temperatures and low pH. As reported by [PERSON] et al. [12], GOS alone, due to the presence of \(\beta\)-type bonds, are stable at 160 \({}^{\circ}\)C for 10 min at neutral pH or 120 \({}^{\circ}\)C for the same period at pH 3. At pH 2, the stability of GOS at 100 \({}^{\circ}\)C was 10 min. Therefore, an important issue that must be considered is GOS production in the raw material and maintaining it at a constant level during subsequent technological activities, e.g., heating in various temperature-time systems. The majority of reports have described the scope of GOS synthesis in the food matrix [13; 14] or the thermal stability of GOS formulations [15]. However, there are limited reports available describing the stability of GOS in goat's milk or in goat's milk with permeate, where the protein and salt system changes as compared to milk. Goat's milk in many countries is not as widespread as cow's milk. The industrial processing of goat's milk is therefore not performed. These are often the conditions of a farm and a small processing plant. However, the demand for goat's milk products is continuously increasing. This applies to the quantity and types of products. One very popular product is kefir. Kefir has a high nutritional value and has a health-promoting effect on the body [16; 17]. Many properties of kefir result from the type and amount of LAB and yeasts used.