## 3 Results and Discussion  
### Acidity  
The sensory quality, rheological properties and health values of natural yogurts as fermented products are often associated with their acidity. It depends on technological factors, type and quality of raw materials, recipe composition or storage conditions. Therefore, acidity is a measure of the production process correctness, as well as an important indicator in research on reformulation of this type of product, including yogurts [16; 19]. In accordance with the Polish standard PN-A-86061:2002 [20], typical yogurt should contain at least 0.6% lactic acid.  
The average lactic acid content in the tested samples ranged from 0.88% to 1.06% (Table 1). Along with an increase in the amount of inulin addition, total acidity decreased (Figure 1), and thus the pH value was enhanced (Table 1). The control sample (0% inulin) was not significantly different from the sample with 3% inulin concentration, both in terms of total and active acidity (pH). The total acidity of the sample with 15% inulin content was about 17% lower, and the pH was about 4% higher, compared to yogurt without the addition of inulin. Similarly, in studies of [PERSON] et al. [21] the effect of additives on the acidity of fermented products has been demonstrated. The authors studied natural yogurts (2% fat content) with the addition of cereal products. Yogurts containing goats at the level of 10% immediately after production were characterized by 7.4% (buckwheat), 11.1% (oat groat) and 34.4% (barley groat) lower acidity, compared to yogurt without an additive. In turn, [PERSON] et al. [22] did not indicate statistically significant differences between pH values for plain yogurt and yogurt with carrot, pumpkin, broccoli or red pepper (pH: 4.63-4.70).  
It was found that the application of the inulin addition caused changes in total and active acidity of natural yogurt, at the level that would not disqualify any of the samples. According to [PERSON] and Reps [23], consumers prefer natural yogurts with a pH in the range of 4.2-4.5. A lower acidity of samples with inulin 24 h after their preparation was inevitable, as the systems were \"diluted\", and adaptation and growth of lactic acid bacteria cells require adequate time. According to the subject literature, the increase in acidity occurs with the passage of storage time as a result of intensified lactic acid production by lactic bacteria with the presence of a prebiotic. [PERSON] et al. [21] proved an  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Amount of Inulin Addition (\%, \(w\)/\(w\))** & **Lactic Acid Content (g/100 g)** & **pH (-)** \\ \hline
0 & 1.06 \(\pm\) 0.02 a & 4.19 \(\pm\) 0.01 a \\
3 & 1.05 \(\pm\) 0.02 a & 4.21 \(\pm\) 0.01 a \\
6 & 1.00 \(\pm\) 0.02 b & 4.24 \(\pm\) 0.01 b \\
9 & 0.98 \(\pm\) 0.02 b,c & 4.26 \(\pm\) 0.01 b \\
12 & 0.94 \(\pm\) 0.02 c & 4.29 \(\pm\) 0.01 c \\
15 & 0.88 \(\pm\) 0.02 d & 4.34 \(\pm\) 0.01 d \\ \hline \hline \end{tabular}  
* a, b, c... --various letters for average values mean statistically significant differences (\(p<0.05\)).  
\end{table}
Table 1: Average lactic acid content and pH values of yogurts with 0%, 3%, 6%, 9%, 12% or 15% \(w\)/\(w\) inulin addition.  
increase in acidity over time in studies on natural yogurt containing 2% fat content, with an addition of various cereal products.