###### Abstract  
This work aimed to study the flux behavior, rejection and concentration factors, and energy demand to establish the optimal conditions during ultrafiltration of sweet buttermilk to produce ice cream. The experiments were conducted with a UF25-PAN membrane at a transmembrane pressure of 0.2, 0.35 and 0.5 MPa, and a volume reduction ratio (VRR) of 2, 3, 4, and 5. Total protein, fat, ash, and dry matter contents, phospholipid composition, and acidity of retententes and permeate were determined. The increase in the VRR led to a decrease in the permeate flux and an increase in the energy demand, rejection, and concentration factors of the main components of sweet buttermilk. The highest values of rejection and concentration factors established for fat were 98.65% and 4.93, respectively. The permeate flux and energy demand increased with the rise in the transmembrane pressure. The total phospholipids increased 2.8 times at VRR 5 compared to the initial buttermilk. The use of VRR 3 gave the best ratio between the permeate flux, the energy demand, and relatively high values of concentration and rejection factors. The use of VRR 5 will enrich the ice cream to the greatest extent to obtain a product with the highest level of biologically active substances (proteins, phospholipids, minerals). ultrafiltration; sweet buttermilk; permeate flux; energy demand; rejection factor; concentration factor; phospholipids Article  
Flux Behaviour, Rejection and Concentration Factors, Energy Demand during Ultrafiltration of Sweet Buttermilk  
[PERSON], [PERSON], [PERSON]. cholesterol, and water [7]. Depending on the type, the proteins vary between 25% and 70%. Minor proteins are also important bioactive components. Membrane technologies can be successfully used for the separation, concentration, and purification of milk components [8; 9]. They have many advantages compared to the traditional separation and concentration methods: environmental friendliness, lower power consumption, relatively low costs, yield enhancement, a better quality of the final product, and usage of room temperature during the process to keep heat-sensitive ingredients [10]. They can be successfully applied when the basic characteristics (flux, rejection, concentration, energy consumption) of the membrane process are well-known, as well as the dependence of these characteristics on some working factors. The membrane type, the transmembrane pressure, the temperature, the volumetric flow rate, the velocity, etc., affect the efficiency of the process. Therefore, it is important to know the dependence of the main characteristics of each membrane process on the working factors to choose the best conditions for the production of some food products. Membrane technologies were used to concentrate the phospholipids from butterfly [11; 12]. Microfiltration causes some performance difficulties because the size distribution of casein micelles and milk fat globule membrane fragments are very similar [13]. The authors in [14] used a combination of microfiltration and ultrafiltration to obtain whey protein concentrates from butterfly and skimmed milk. Ultrafiltration is used for the concentration of buttermilk as the ingredients were used in the production of reduced-fat washed curd cheese [15], reduced-fat Cheddar cheese [16], and reduced-fat process cheese [17]. According to the authors in [18], ultrafiltration was widely used for the concentration of various milk components. Ultrafiltration is also used to obtain a phospholipid concentrate from whey buttermilk [19]. The same authors include diafiltration to increase the phospholipid content in the concentrate. The permeate flux is an important technical characteristic of the membrane processes. Membrane fouling is a serious problem in all pressure-driven membrane processes and will reduce the flux of the membrane [20]. It limits the efficiency of the process. The fouling depends on the different components present in the feed solution (casein micelles, whey proteins, minerals, etc.) [21].