## 3 Results and Discussion
### Measurement of Mechanical Properties  
#### 3.2.1 Indentation of Petioles  
The results of the tests (Figure 2) show the highest resistance to indentation of the skin on the backside of the rhubarb petioles following harvest in the material treated with ozone at a rate of 10 ppm for 15 min, in the central part of the petiole--the value of indentation force amounting to 89.94 N. Ozone treatment applied at a rate of 10 ppm for 5 min did not produce an increase in the resistance of the skin to indentation, compared to the control.  
Figure 1: Water content in the samples subjected to ozone treatment and in the control sample on the consecutive dates related to storage of rhubarb petioles.  
Irrespective of the ozone treatment variants, the central section of the petiole was characterized by the highest resistance to indentation in fresh rhubarb. After the dose of 10 ppm was applied for 15 min, the mean values of the force needed to puncture the skin on the back side of rhubarb petiole were similar in all the locations tested, and application of this variant made it possible to increase resistance to indentation, compared to the control. The mean values of indentation force in the control sample amounted to 76.85 N, and in the sample exposed to ozone at a rate of 10 ppm for 15 min-88.38 N.  
The force needed to puncture the skin with \(\varphi=6\) mm indenter on the flat side of rhubarb petioles, after application of ozone in various combinations of dose and duration, compared to the control, is shown in Figure 3. The highest resistance of the skin to indentation on the flat side was found in the bottom part of the petioles in all the ozonation variants. The highest mean resistance of the skin to the indentation on the flat side of the petioles following harvest was observed for the treatment at a rate of 10 ppm for a duration of 15 min. Compared to the control (petioles not subjected to ozone treatment), resistance to indentation increased by 19%. Notably, the lowest ozone dose (10 ppm, 5 min) did not produce increased resistance to indentation of the skin on the flat side. Application of ozone treatment to improve storage stability of other raw plant materials was also reported to be effective. [PERSON] et al. [21] reported that by applying ozone treatment to field cucumbers, it was possible to prolong their usefulness for processing, as reflected by their improved mechanical properties following the storage, relative to the control sample. The fruit subjected to ozone treatment were suitable for the souring process even after 22 days. Preliminary ozonation also affected the mechanical and organoleptic properties of pickled cucumbers [22]. It was established that irrespective of the cultivar and its purpose, cucumbers subjected to ozonation presented improved organoleptic and mechanical characteristics compared to the controls and contained no hollow spaces.  
Figure 2: Mean values of the force needed to puncture the skin with \(\varphi=6\) mm indenter, on the backside of rhaponic rhubarb petioles. Differences in the results between the punching force values for individual variants are marked with different small letters, \(p<0.05\).