## 3 Results and Discussion
### Physical Stability  
From a thermodynamic point of view, oil-in-water food emulsions are unstable systems and susceptible to various factors causing destabilization mechanisms [33]. Traditional methods for testing the physical stability of the emulsions, e.g., \"aging\" (gravitational), centrifugal (centrifugal force),  
Figure 6: Viscosity curves of yogurts with 0%, 3%, 6%, 9%, 12% or 15% _w/w_ inulin addition. Figure 7: Firmness (maximum force) and stickiness (minimum force) of yogurts with 0%, 3%, 6%, 9%, 12% or 15% _w/w_ inulin addition (a, b, c... —various letters for average values mean statistically significant differences, \(p<0.05\)). thermal (usually high temperature) and combinations thereof, are based on visual assessment and/or numerical expression of results. They do not allow detection and tracking of changes occurring during the analysis. It is possible with the application of modern methods based on the technique of scattering and light transmission through the sample, including the CSA method (centrifugal separation analysis) using the LUMiSizer dispersion analyser [34; 35]. Concerning ygourts and other dairy products, an important feature of product quality is the intensity of syneresis occurring [19; 27; 36]. The incorporation of even the smallest amount of inulin (3% _w/w_) to natural yogurt resulted in a statistically significant reduction of quantity of whey secreted during centrifugation (4000 rpm, 10 min). [PERSON] et al. [29] claimed a similar tendency. The highest degree of syneresis (42.7%) was found in control yogurt (without inulin). Larger amounts of the additive caused a further decrease in the value of this parameter (samples with 9% and 12% inulin showed the reduction by 17.6 and 25.0 percentage points, respectively). Moreover, stirred yogurt containing 15% of prebiotic presented 4.5-fold lower degree of syneresis, as against the control sample (Figure 8). A very strong correlation was found between the amount of inulin and the degree of syneresis--R\({}^{2}\) = 0.99 (Figure 8). Other scientists have also reported on the beneficial impacts of various additives on the stability of yogurts, e.g., pea fiber and concentrated whey (CW) from buffalo cheddar cheese [29; 37]. [PERSON] et al. [29] pointed out that the most stable sample turned out to be a yogurt with 1.5% pea fiber (38.86% syneresis). [PERSON] et al. [37] also determined that influence of concentrated whey (CW) content (from buffalo cheddar cheese) in buffalo milk yogurt composition on degree of its syneresis was varied. Yogurts containing 5% and 10% CW caused reduction of this indicator by approx. 15%, compared to the yogurt without the addition. However, the sample with 30% CW was characterized by over two times higher syneresis (approx. 17.1%). Regarding results of the LUMiSizer test, similarly to centrifugal test, a linear decline in stirred yogurt instability was observed with the increase in the concentration of inulin: y= -0.07x + 0.6327. Yogurts with 6% and 9% prebiotic showed lower instability indexes than the control sample (0.537)--approx. 17.5% and 35.6%, respectively. In the case of yogurt with the highest content of the preparation (15% _w/w_), this difference was double--the value of the instability index equals 0.185 (Figure 9). Additionally, the evolution of the light transmission profiles (\"fingerprints\") shows that sedimentation phenomenon occurred in each of the samples--i.e., the migration of particles to the bottom of the tube (right side of the graph, Figure 10) leading to clarification of the upper part of the sample, and thus a higher intensity of transmitted light. It was noted that yogurt without the inulin was characterized by slightly larger differences between subsequent transmission profiles, which may  
Figure 8: Degree of syneresis of yogurts with 0%, 3%, 6%, 9%, 12% or 15% _w/w_ inulin addition measured using centrifugal method (a, b, c... --various letters for average values mean statistically significant differences, \(p<0.05\)). indicate aggregation of particles.