*Article* **Effect of Ozone Treatment on the Quality of Sea Buckthorn (***Hippophae rhamnoides* **L.)**

**Anita Zapałowska 1,\*, Natalia Matłok <sup>2</sup> , Miłosz Zardzewiały <sup>2</sup> , Tomasz Piechowiak <sup>3</sup> and Maciej Balawejder <sup>3</sup>**


**Abstract:** The aim of this research was to show the effect of the ozonation process on the quality of sea buckthorn (*Hippophae rhamnoides* L.). The quality of the ozonated berries of sea buckthorn was assessed. Prior to and after the ozone treatment, a number of parameters, including the mechanical properties, moisture content, microbial load, content of bioactive compounds, and composition of volatile compounds, were determined. The influence of the ozonation process on the composition of volatile compounds and mechanical properties was demonstrated. The ozonation had negligible impact on the weight and moisture of the samples immediately following the treatment. Significant differences in water content were recorded after 7 days of storage. It was shown that the highest dose of ozone (concentration and process time) amounting to 100 ppm for 30 min significantly reduced the water loss. The microbiological analyses showed the effect of ozone on the total count of aerobic bacteria, yeast, and mold. The applied process conditions resulted in the reduction of the number of aerobic bacteria colonies by 3 log cfu g−<sup>1</sup> compared to the control (non-ozonated) sample, whereas the number of yeast and mold colonies decreased by 1 log cfu g−<sup>1</sup> after the application of 100 ppm ozone gas for 30 min. As a consequence, ozone treatment enhanced the plant quality and extended plant's storage life.

**Keywords:** ozone; sea buckthorn; mechanical properties; microbial load; bioactive components

#### **1. Introduction**

The sea buckthorn (*Hippophae rhamnoides* L.) is a spiny, deciduous shrub which belongs to the oleaster family (*Elaeagnaceae Juss*.) The fruits are rich in a variety of phytochemicals with physiological properties, such as lipids, carotenoids, ascorbic acid, tocopherols, and flavonoids [1,2]. A characteristic feature of *Hippophae rhamnoides* L. is intense orange berries that densely overgrow the shoots. A peculiar smell is caused by the presence of chemical compounds such as alcohols, aldehydes, ketones, and terpenes [3]. Sea buckthorn contains carotenoids, flavonoids, phospholipids, tannins, vitamins, and macro- and microelements [4]. Due to its abundance of the biologically active compounds, the sea buckthorn is widely used as a human health-promoter. This plant is a valuable material, particularly in pharmaceutical, cosmetic, and food industries [5]. Ozone, a highly reactive triatomic form of oxygen, is a powerful agent able to degrade organic substances. Gaseous ozone, due to its properties, has been approved by the Food and Drug Administration for direct contact with food, and can have a significant impact on the quality of the fruit. Numerous studies have confirmed its beneficial effect on the quality of products, evidenced in the extension of their shelf life [6,7]. Due to ozonation, select parameters of plant materials, such as antioxidant potential, polyphenol content, or enzymatic activity, are modified. Many studies have confirmed that ozone treatment inhibits the growth of microorganisms

**Citation:** Zapałowska, A.; Matłok, N.; Zardzewiały, M.; Piechowiak, T.; Balawejder, M. Effect of Ozone Treatment on the Quality of Sea Buckthorn (*Hippophae rhamnoides* L.). *Plants* **2021**, *10*, 847. https://doi.org/ 10.3390/plants10050847

Academic Editor: Ivo Vaz de Oliveira

Received: 30 March 2021 Accepted: 19 April 2021 Published: 22 April 2021

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responsible for fruit spoilage, and reduces the loss of nutritional and sensory value of fruit during storage [8–17]. These studies have shown that the use of ozonated water is only effective in combination with blanching [8,13]. Using ozone gas is more effective, but it is necessary to change the atmosphere with other gases [9,12]. A plant composition change may occur as a result of the ozonation process [14,15]. This gas, as confirmed by numerous studies undertaken by the Environmental Protection Agency, is considerably safer than chlorine or other sanitizers [18]. In contact with solid materials, it decomposes into molecular oxygen and does not cause additional fouling.

The studies undertaken to date on the utilization of ozone treatment in fruit storage have principally concerned the effect of ozonation on the level of microbial contamination, biological activity of the fruit, and fruits' sensory characteristics [14,15,19]. However, the available data on changes in the bioactive compounds of ozonated fruits are variable and difficult to compare because they are related to different factors, forms, and concentrations of ozone, in addition to different fruits and environmental conditions in which they are produced [20].

No reports have been published about the effect of ozone treatment on the quality of sea buckthorn (*Hippophae rhamnoides* L.). To obtain factual information, we conducted a survey, the results of which are shown below. The aim of the research was to show the effect of the ozonation process on the quality of sea buckthorn (*Hippophae rhamnoides* L.).

#### **2. Results and Discussion**

## *2.1. Moisture Content*

Sea buckthorn berries consist of more than 80% water, which fluctuates greatly during storage. The moisture content depends largely on the storage conditions. Fruit stored in the same conditions should therefore maintain similar quality and strength parameters, which are dependent on the moisture content. The fruit shelf life can be influenced by the metabolic system being activated. This can be performed by adjusting the storage to proper conditions (appropriate storage temperature) or by making use of both biotic and abiotic factors (in the form of nutrition, cutting, light, water, etc.). One such factor (abiotic) is the ozone gaseous form, which may be used to slow the ripening process. The findings of our experiment show the effects on the ozone concentration and its duration on the moisture content.

The applied ozone treatment had a direct influence on the sea buckthorn shelf life. The observed changes in moisture content depended on ozone concentrations and exposure time (Figure 1). The largest water loss, which amounted to 5.20%, was observed in the control sample (0 ppm, 0 min) on the 7th day of storage, and the lowest water loss of 3.65% was recorded in the samples subjected to ozone treatment at a rate of 100 ppm, for the duration of 30 min. The performed statistical analysis showed a significant influence of ozone concentration and ozone exposure time (LSD0.05 (least significant difference) 0.47) on the moisture content of the sea buckhorn berries. This corresponds to the observation of the lowest microbial load for fruit subjected to ozonation under these conditions. It is most likely that this was caused by a decrease in the activity of microorganisms on the surface of this fruit. Ozone, as an agent that disinfects fruit surfaces, affects the activity of microorganisms and the storage process. Microbial inactivation by ozone, mainly due to the rupture of its cellular membranes, prevents damage of the fruit surface, leading to a reduction in water loss and increasing the membranes' mechanical strength. The influence of the ozonation process on the reduction of water loss in stored plant material has been also confirmed in other studies. Zardzewiały et al. [21] observed a similar effect of ozone treatment applied to rhubarb petioles. The authors recorded the lowest water loss for the exposure of 30 min with an ozone concentration of 100 ppm. The influence of the ozonation process on improving quality parameters and reducing water loss in stored ground cucumbers was also confirmed by Gorzelany et al. [22]. Antos et al. [23], using ozone on stored apples, observed a similar relationship.

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