###### Abstract  
The influence of the amount of inulin addition (3%, 6%, 9%, 12% or 15% _w/w_) on the physicochemical properties of natural yogurt was analyzed. The acidity (titration; pH), texture parameters (penetration test), viscosity curves (rotational rheometer), microrheology (macroscopic viscosity index, MVI; elasticity index, EI; solid-liquid balance--SLB; multi-speckle diffusing-wave spectroscopy, MS-DWS) and physical stability (syneresis; LUMiSizer test) of yogurts were investigated. All samples were non-Newtonian pseudoplastic liquids. The sample with 15% inulin content presented an approx. 4% higher pH value (4.34), 3-fold greater MVI and almost 5-fold higher penetration force, compared to the control sample (0% of inulin). In turn, the use of inulin addition in the range of 3-15% _w/w_ resulted in a reduction of syneresis (\(p<0.05\)). A linear decrease in the values of instability indexes and sedimentation velocities was noted in the function of inulin content increase (LUMiSizer test). The application of inulin (in the range of 3-15% _w/w_) as a functional additive to yogurts significantly contributed to enhancement of their physical stability. Summing up, the possibility of obtaining natural yogurts with a high content of this prebiotic has been demonstrated, thus such products can be classified as functional foods and a health claim can be put on the label.  
functional food; fermented milk products; acidity; texture; MS-DWS method; LUMiSizer test; sedimentation phenomenon; syneresis; physical stability +
Footnote †: journal: Applied  
into the food composition (at least 3 g/100 g portions) allows the product to be labeled with a nutrition claim, and when it provides a 12 g dose per day a health claim can be used [1; 8].  
Yogurts are oil-in-water emulsion systems in which the fat phase is oil and the water phase is a solution of proteins, carbohydrates and mineral salts [9]. They are made from fresh milk and/or after reconstitution using the bacteria _Lactobacillus bulgaricus_ and _Streptococcus thermophilus_, with the possible participation of additional food ingredients and allowed additional substances [10]. Originally, the production of fermented dairy products was a way to preserve the nutrients of milk. It has been proven that consumption of these products provides a number of health benefits due to live cultures of lactic bacteria (probiotics) [11; 12]. The inclusion of various additives in the fermented milk product composition may increase their nutritional and health value [6; 10; 13; 14], and also changes their rheological and textural properties [11]. The application of 1-2% inulin addition to skimmed cow's milk enables the production of yogurt with acceptable quality and characteristics comparable to yogurt obtained from full-fat cow's milk [6].  
According to the increase in yogurt consumption [15], it is important to seek new opportunities to expand their offers to consumers and to develop products enriched with health substances at various stages of production, as well as in gastronomy or home conditions. The use of fiber as a food additive modifies its physical properties [11] and therefore it is valid to investigation the rheological properties and stability of the obtained systems using modern techniques. This paper attempts to use the multi-speckle diffusing-wave spectroscopy (MS-DWS) method ([PERSON], Toulouse, France) and the centrifugal separation analysis (CSA) method (LUMiSizer, Berlin, Germany) to identify and evaluate the intensity of physical changes in natural yogurt with the addition of a prebiotic. The purpose of the study was to assess the influence of the addition of inulin as a functional substance on the properties of natural yogurt.