بررسی اثرات آسیب مکانیکی بر خواص کیفی میوه کیوی

Kiwi, belonging to the family Actinidiaceae and genus Actinidia, is a dicotyledonous, perennial, deciduous plant that includes a number of commercially important species. Iran ranks fourth in the world in terms of kiwi production after China, Italy and New Zealand. The most common variety of kiwi grown in Iran is "Hayward" with green flesh. Kiwi has a high nutritional value because it contains a large amount of nutrients that provide many health benefits to its consumers by improving the digestive, immune and metabolic health of a person.
Due to its numerous medicinal properties, kiwi fruit has become extremely popular in the last two decades and the demand for its consumption has increased. The amount of components and the type of chemical composition of kiwi fruit depends on several factors such as genotype, weather conditions before harvesting, maturity of the fruit during harvesting and storage conditions. The fruit has a shelf life during storage.
Mechanical damage to fruit is a common cause of quality reduction and post-harvest losses during handling and storage. Bruising damage on fruits is mainly due to one or a combination of mechanical forces of pressure, impact and vibration during harvesting, transport and handling operations. Maturity stage is one of the most important factors affecting susceptibility to bruise damage for many fruits. Previous studies have shown that mature fruits are more susceptible to bruising than immature fruits. In addition, maturity at the harvest stage potentially affects the susceptibility of fruits to water loss and mechanical damage. Therefore, identifying factors that reduce mechanical damage can be a way to maintain product quality. Kiwi is a climate fruit that can be picked before it is fully matured and then it will reach full maturity over time. The purpose of this research is to investigate the effect of harvesting time and storage period on the quality properties (firmness, slouble solids content (SSC) and pH) of kiwi under mechanical damage.

Material and Methods
To conduct experiments in the first stage, enough number of Hayward kiwi was harvested from gardens in Astara city, in two intervals of 15 days. 45 semi-ripe kiwi fruits (hard texture) (T1) were picked on November 15th and 45 fully ripe kiwifruits (soft texture) (T2) were picked on November 30th in a controlled manner to avoid any mechanical damage. In order to avoid bruising during transportation, all the samples were covered by foam. In order to induce artificial bruising to the samples, each kiwi fruit was placed inside a net cloth and then hung from a height of 50 cm. Then, a metal ball with a diameter of 15 mm and a mass of 32 g was used to impact the sample. The metal ball was released in a circular path with an angle of 60 degrees to the sample and an impact force was applied to the surface of the sample. The angle of 60 was chosen by trial and error in order to prevent severe damage to the samples. Then the samples were divided into three groups and transferred to the cold room with a temperature of 0°C and a humidity of 90%. Then, the samples were taken out of the cold storage at intervals of 1, 5 and 10 days and tests related to quality properties (firmness, soluble solid content (SSC) and pH) were performed on them. The statistical design used in this research includes two independent variables: maturity stage and storage time after bruising. Analysis of the variance of the results was done with a factorial test in a completely random basic design with 15 repetitions, and then the comparison of the average of the main and reciprocal effects was done with Tukey's mean comparison test and using Minitab 18 software.
Results
The results of analysis of variance showed that the effect of the factors of harvesting time, storage period and their interaction on firmness, SSC and pH is significant at 1% level. The firmness of the fully ripe samples is much lower than the immature samples, so that even 10 days after the induction of mechanical damage, the immature samples have a harder tissue than the fully mature samples. The results of this research show that fully ripe samples are very sensitive to mechanical damage and their tissue firmness is severely affected and their quality decreases, while semi-ripe fruits are easily affected. There is no mechanical damage, and in case of damage, their quality has been maintained for a longer period of time and they are still marketable. The softening and reduction of the firmness of the fruit over time can be attributed to reasons such as the conversion of insoluble protopectin to soluble pectin and also to the reduction of hemicellulose in kiwi fruit. Also, with the increase in the storage period, the firmness of fruits has decreased, and the softening process of mature fruits has a lower slope than that of immature fruits. The slower process of decreasing firmness may be related to the higher amount of calcium in the fruits at the time of the second harvest.
The SSC of mature samples is higher than that of semi-mature samples, and the SSC of these samples increases more steeply than semi-mature samples as a result of mechanical damage with increasing storage period, which indicates this fact. It is possible that mechanical damage accelerates chemical changes in mature fruits and their deterioration process is formed quickly and their shelf life is reduced, while immature fruits have a more natural aging process even in the presence of mechanical damage, and have a better shelf life. The reason for the increase in SCC with time is attributed to the increase in the activity of sucrose phosphate synthase enzyme, which has the ability to convert starch into simple sugars such as glucose phosphate. This enzyme is activated by the hormone ethylene during the ripening process. According to the results, it is clear that with the passage of time, the acidity of the samples decreases. Also, immature samples are more acidic and their pH is lower. The noteworthy point is that the mature samples are more sensitive to the passage of time and the slope of their pH increase is greater than the immature samples. In similar studies, it has been reported that the pH level depends on the type of biochemical activities, tissue characteristics, type of organic acids and the variety of the fruit, and the increase in pH occurs due to the biochemical activities inside the fruit, which subsequently lead to substances. The acid in the fruit is converted into sugar products. On the other hand, mechanical damage intensifies the chemical activities in the fruit and accelerates the conversion of acidic substances into other products.

Conclusion
The results of this research showed that the factors of harvesting time and storage period after harvesting have a definite effect on the quality properties of the fruit. Fully matured fruits have less firmness and are vulnerable to mechanical forces and undergo chemical decay and decay over time. On the other hand, immature fruits have a firmer texture and are not easily affected by mechanical forces, and their quality properties are preserved even after being hit. However, immature fruits have a lower pH (higher acidity) and also their SSC is lower and they do not have a suitable appetite and taste for immediate consumption. Therefore, it is better to harvest kiwi fruit when it is immature to minimize the possibility of being damaged by mechanical forces in different stages of harvesting, transportation and grading, and to enter the consumption cycle with a delay until they reach full biological maturity and greed and have a good taste for the consumer.