*2.2. The Mechanical Properties*

The mechanical parameters determine the quality of the raw material plants, which is important in terms of the storage ability. During the storage time, all ozonated sea buckthorn berries showed a higher resistance to the destructive force (Fmax) compared to non-ozonated fruits (Table 1). The mean values of indentation force in the control sample amounted to 6.84 N. To correlate moisture content with selected mechanical properties, these parameters were measured at the same time of storage (on the 1st, 4th, and 7th days). The results of the test show that water loss from cellular structures directly affects the parameters of the mechanical properties. The value of destructive force (Fmax) subjected to these conditions after the 1st day of storage was 10.1% higher compared to non-ozonated fruits. Significantly better results were recorded on the 4th and the 7th day of storage, when the value of Fmax was 36.3% and 58.6% higher, respectively, compared to the control sample (non-ozonated). The result shows that, as in the case of moisture content, the highest measured parameter was recorded on the 7th day following treatment at the ozone rate of 100 ppm for 30 min (Fmax = 9.69 N). This was likely the result of less damage to the ozonated surface of the fruit caused by microbes. Increasing the resistance of ozonated sea buckthorn berries to the destructive force has been associated with the alteration of the fruit surface due to ozone gas. Similar relationships were observed on the recorded deformation of sea buckthorn berries to the point of destruction (dl to Fmax) and destructive force (Fmax/Lmax). These parameters were significantly higher in the case of fruit exposed to gaseous ozone. The performed statistical analysis showed a significant influence of ozone treatment concentration, ozone exposure time, and storage time on the mechanical parameters (Table 1). The impact of the ozonation process on selected mechanical parameters of the plant material was also confirmed in other studies. Zardzewiały et al. [21] observed a similar effect of the ozonation process on the mechanical resistance of rhubarb in the compression test. The best results were found in the case of the material subjected to the process of ozone treatment for 15 min with a concentration of 10 ppm. Gorzelany et al. [22], using gaseous ozone for post-harvest ground cucumber, obtained a material with improved mechanical and sensory properties compared to the control. The increase in the parameters of the destructive power of ozonated apples was noted by Antos et al. [23]. Using cyclic gaseous ozone on apples, the authors observed a

similar relationship between the conditions of the ozonation process and the mechanical parameters of the stored fruit.

**Table 1.** Effect of ozone treatment concentration (OC), ozone exposure time (OE), and storage time (S) on the mechanical parameters of *Hippophae rhamnoides* L. fruit.


Note: \* n.s.—non-significant difference; Fmax—destructive force (N), dL to Fmax—deflection at the moment of destruction (mm), W to Fmax—energy required for destruction (J), Fmax/Lmax—destructive force measurement (N/mm<sup>2</sup> ). a,b,c,d,e,f,g,h,i—statistically significant differences for the effect: ozone concentration (ppm) × ozone exposure (min) × storage (days).

#### *2.3. Content of Bioactive Compounds*

The results of the present study show increased total polyphenol content after 24 h (Figure 2A). This was determined by the duration of the process and the ozone concentration. The best results were observed when the plant was exposed to ozone for 5 min with an ozone concentration of 10 ppm. In this case, the change in the total polyphenol content was 15.9% higher compared to the control sample (non-ozonated fruit). When higher ozone

concentrations of 100 ppm were used, the best results were achieved after 15 min. In this case, an increase in total polyphenol growth of more than 12.8% was found in the ozonated plant. A strong relationship with ozone dose was observed in the case of antioxidant potential (Figure 2B). The antioxidant capacity of the fruit 24 h after the ozone treatment of 100 ppm for 15 min was higher by 11.5% compared to the control sample (non-ozonated fruit). A quantity of 100 ppm of ozone significantly disturbed the fruit balance, which caused oscillation of the content of components, particularly polyphenols. Such oscillations were not observed for the antioxidant potential. Statistical analysis showed the impact of the ozone concentration and ozone exposure time on the total content of polyphenols in sea buckthorn berries (LSD0.05 = 53.07), and their antioxidant properties (LSD0.05 = 145.77). In addition, the antioxidant potential depended significantly on the time of the ozonation (LSD0.05 = 103.84) and ozone concentrations (LSD0.05 = 133.95). The increase in the concentration of secondary metabolites in ozonated sea buckthorn berries after 24 h of storage can be attributed to metabolic changes due to the occurrence of oxidative stress caused by ozone. The effect of ozone on the total polyphenol content and antioxidant potential of the ozonated fruit has been confirmed by other scientists. Rodoni et al. [24] recorded a 50% increase in the total polyphenol content compared to the non-ozonated fruit, on the sixth day after the ozone treatment, while examining the effect of 10 min ozone exposure at a dose of 10 µL L−<sup>1</sup> on tomato fruits. The increase in total polyphenol content was also observed in the case of the ozonated fruits of *Carica papaya* L. [25]. Alothman et al. [26], using ozone at a flow of 8 <sup>±</sup> 0.2 mL s−<sup>1</sup> for 0, 10, 20 and 30 min to ozonate pineapple and banana fruits, found an increase in the total polyphenols in ozonated fruit. The best results were achieved when the ozonation process was conducted for 30 min. Furthermore, the total polyphenol content of the ozonated pineapple and bananas was higher by 9.5% and 23.5%, respectively, compared to non-treated fruit. The process of ozonation under properly selected conditions may influence an increase in ascorbic acid content in the plant material [21,27]. Ozone treatment on the buckthorn berries resulted in changes in ascorbic acid content after 24 h of the conducted process (Figure 2C). However, this impact was different depending on the concentration of the ozone used and the duration of the process. The best results were obtained for 30 min using ozone at a concentration of 10 ppm. At that time, the ascorbic acid content in the tested material was higher by 12.7% compared to the control sample (non-ozonated fruit). However, when the ozone concentration was increased to 100 ppm (30 min), a significant decrease in the ascorbic acid content was observed. It is likely that the observed effects of the oxidation of ascorbic acid was the result of the activation of ascorbate oxidase being maintained by stress related to the action of ozone gas. This enzyme degrades ascorbic acid to dehydroascorbic acid [28]. The degradation of ascorbic acid under the influence of inappropriate selection of process conditions for the type of ozonated plant material was also confirmed by others. Alothman et al. [26], treating guava fruit with ozone with a flow rate of 8 <sup>±</sup> 0.2 mL s−<sup>1</sup> for 30 min, noted a decrease in ascorbic acid by 67.1% compared to fruit not exposed to this gas (control test). The increase in the content of ascorbic acid in the plant material subjected to the ozone treatment in properly selected conditions (concentration and time of ozone exposure) was also confirmed in other studies. Piechowiak et al. [15], while ozonating raspberry fruits with ozone at a concentration of 8–10 ppm for 30 min, observed an increase in ascorbic acid content in fruits subjected to the ozonation process. The performed statistical analysis showed a significant influence of the interaction of ozone concentration and ozonation time on the ascorbic acid content in sea buckthorn berries (LSD0.05 = 12.46).

**Figure 2.** The impact of ozonation process on the polyphenolic content (**A**), antioxidant activity against DPPH radical (**B**), and total ascorbic acid content (**C**) in *Hippophae rhamnoides* L. fruit (*n* = 3). Note: LSD for α = 0.05 for the impact of ozone treatment concentration (OC), ozone exposure time (OE), and the interaction between tested parameters (OC)/(OE).
