## 1 Introduction  
Milk and dairy products are some of the most indispensable natural foods due to their altimetry wholesome benefits. They are deservedly preferred by many people around the world [1, 2, 3, 4]. The conductive effect of these products can be explained by their composition and the fact that they have an important place in the rational, prophylactic, and dietary nutrition of humans because they are vital for the maintenance of the human body [1, 5, 6].  
Yoghurt is a food obtained by the bacterial fermentation of milk and is consumed all over the world [7]. The bacteria used for yoghurt production are known as yoghurt cultures or starter cultures. _Lactobacillus bulgaricus_ and _Streptococcus thermophilus_ are the two microorganisms involved in the fermentation of yoghurt [8, 9, 10]. These bacteria provoke the fermentation of milk and produce lactic acid in order to change the yoghurt's texture and flavour. Different kinds of milk (cow's, sheep's, goat's, mare's, camel's) can be used for yoghurt production. There has been an increasing scientific and consumer interest in goat milk production in recent years [11]. Goat's milk is the most easily digestible dairy product and its amino acid composition is similar to that of human milk, which distinguishes it from other milks. It has high-quality proteins, fats, vitamins, and minerals [12, 13]. This milk can be consumed by people who cannot consume cow's milk because it does not contain as!-casein, which causes various types of intolerance to milk [10]. Goat's milk can be successfully used for yoghurt production, individually or mixed with cow's, sheep's, and mare's milk [14].  
Microbiological, physicochemical, organoleptic, and rheological characteristics are the fundamental characteristics of yoghurts that influence the acceptance and choices of consumers [8]. One way to improve the texture quality of yoghurts is to increase the dry matter content, which, in the traditional method, can be achieved via the addition of milk concentrated by evaporation, powdered milk, or protein concentrates [15; 16; 17]. The conventional method for milk concentration is evaporation [18]. It is important to note that this process has many negative effects because it can change the characteristics of thermolabile whey proteins [19]. The application of an evaporation process can reduce the product quality due to the negative effect of thermal treatment on heat-sensitive milk components [20].  
Nowadays, consumer interest is focused on natural foods produced transparently and with a minimal addition of additives. Membrane processes, such as nanofiltration, ultrafiltration (UF), microfiltration, and reverse osmosis, can allow producers to avoid the addition of milk powder and offer products with a more healthful profile [21; 22]. The separation and concentration of milk substances can be conducted via UF [23; 24; 25; 26]. This method is preferred due to its advantages--low energy costs and a novel non-thermal environmentally friendly technology that reduces the negative effects of temperature rises, such as phase changes, the denaturation of proteins, and changes in sensory properties [27; 28]. In previous our work [29], we investigated the possibility of applying UF with an UF10-PAN membrane to improve the physical, chemical, and microbiological characteristics of the yoghurts obtained.  
The aim of this experimental work was to study the fundamental properties (microbiological, physicochemical, organoleptic, and rheological) of goat yoghurts produced by UF with an UF25-PAN membrane at different concentration levels and starter cultures.