The effect of zinc oxide solution on quality and storage life of strawberry

Introduction: Strawberry (Fragaria x ananassa) as one of the most widely consumed fruits in the world is produced in 73 countries worldwide (Muzzaffar et al., 2016). Strawberry is mainly grown in Kordestan, Golestan, Mazandaran and Gilan provinces of Iran with Kordestan, Paros, Queen Eliza, Selva, Camarosa and Pajero as the most important cultivars. Since strawberry is very perishable fruit, requires careful harvesting and handling to maintain its quality after harvesting. Considering its sensitivity to physical injuries and fungal invasion besides rapid softening and susceptibility to rots, strawberry fruit has a short postharvest life. Gray mold, caused by Botrytis cinerea is considered as the most important strawberry disease with many negative impacts on ripening, marketing and postharvest life of the fruit (Abd-Alla et al., 2011). No strawberry cultivar is resistant to gray mold. The pathogen is able to infect stored strawberries because mycelia spread from infected fruit to adjacent healthy fruit. Several technologies have been used to reduce postharvest losses and extend the storage life of strawberries. These technologies include chemical control, low temperature storage, control atmosphere packaging, essential oils and irradiation (Marjanlo et al., 2009; Maraei and Elsawy 2017). Most of these techniques could be effective for shelf life extension of strawberry fruit. However, many reports documented harmful effects of chemical fungicides on human health and environment besides causing fungi resistance. Some compounds and methods may also have adverse effect on color, flavor or texture of the fruit. Thus, in the past few decades, research has focused on the use of safe and natural preservatives. It was reported that the storage life of strawberry fruit increased by application of cumin essential oil (Marjanlo et al., 2009). Among the various alternatives, metal oxides such as magnesium oxide and zinc oxide are catching the attention of scientists worldwide. These oxides render antimicrobial activity with higher stability in comparison to organic antimicrobials. Zinc oxide nanoparticles are less toxic than other nanoparticles such as silver nanoparticles. In addition, they are safer for human beings in comparison to other metal oxides (Al-Naamani et al., 2018). Al-Naamani et al. (2018) proved the efficiency of chitosan-znic oxide nanocomposite coatings in extending the shelf life of the packed okra. The application of nanocomposite film containing 2% zinc oxide nanoparticles maintained the qualitative characteristics of Mazafati date during cold storage (Sadeghipour et al., 2019). The aim of this study was to investigate the effect of different concentrations of zinc oxide solution on quality and shelf life of strawberry fruits (camarosa cultivar) during ambient and cold storage.
Material and methods: Zinc oxide solution with brand name Eimen Jav was purchased from Eimen Novin Pushan Pak Company. “Camarosa” strawberries at commercial maturity stage were harvested from a greenhouse located in Hashtgerd, Alborz province, Iran and then transported to Agricultural Engineering Research Institute. Fruits of uniform size without any defect were selected and used for the experiments. Strawberries were dipped in the solution at different concentrations (0.31%, 0.63%, 0.94%, 1.25%) for 1, 2 and 3 minutes. Then, the fruits were air dried at room temperature, put in plastic fruit boxes and stored at ambient temperature (25 oC, 30% RH) for 6 days and cold room (4 oC, 85-90% RH) for 21 days. The sampling was done on day 0, 2, 4, and 6 through ambient temperature and on day 0, 7, 14, and 21 through cold storage. Decay percent was calculated by visual observation of each sample. Fruits with visible brown spot and softened area were regarded as decayed fruit. Texture evaluation was performed by a texturometer. Firmness values of each individual strawberry were measured at two points of the equatorial regions using a 5 mm diameter probe and 500 load cell, at 2 mm/sec-1. Total soluble solids content was measured using a refractometer, Titratable acidity (TA) was calculated by titrating of clear juice of strawberry against 0.1 N NaOH solution and the results were expressed as citric acid %. Ascorbic acid content was determined by using 2,6-dichlorophenol indophenols titration method. A five-point hedonic scale was used for conducting the sensory evaluation of the samples. A panel of 15 judges was selected to evaluate the treatments for various sensorial parameters like appearance, texture, taste, and overall acceptability. Plain water was given to the judges to rinse their mouth between the evaluations of samples. The study was conducted as a factorial experiment in randomized complete design. All determinations were carried out in triplicates and the means were separated by Duncan Multiple Range test. All statistical tests were done by SPSS ver. 22 and were meaningful at 5%.

Results and discussion: Ascorbic acid content was significantly affected by zinc oxide solution at different concentrations and immersion times. The ascorbic acid value of the control was significantly lower than the treated fruits. Ascorbic acid content decreased through the storage time as observed in all treatments and the control. Zinc oxide treated strawberries at 0.31% concentration for 3 minutes had higher firmness values and better qualitative characteristic with increased shelf life up to 2 weeks during cold storage. Additionally, this treatment reduced fungal decay up to 50% compared to the control during cold storage. Zinc oxide solution at 0.63% concentration for 3 minutes increased the shelf life of strawberry fruits up to 48 hours under ambient temperature. In this condition, fungal decay reduced up to 70% as compared to the control. Zinc oxide solution had no effect on sensory attributes of the fruits.
Conclusions: This study showed that strawberry fruits treated with zinc oxide solution (Eimen Jav solution) at recommended concentration and under storage in cold room had better quality and less decay than the control. The results introduce zinc oxide solution as a useful method for maintaining strawberry quality and extending its storage life.