Changes in some physical, chemical, and antioxidant characteristics of two Lemons (Citrus limon Cvs. Lisbon & Eureka) during ripening process and shelf life in the north of Iran

Introduction: The physio-chemical properties of lemon fruits are influenced by climatic conditions, growth stages and shelf life. Lemons mostly cultivated in the south of Iran and is limited in the north due to the cold condition that may be occurred every year. With the expansion of lemon cultivation in the north of Iran specially under protective condition (greenhouse), many of fruit physicochemical properties such as water percentage, taste, skin color, ripening time and bioactivity may be affected by climatic conditions, growth stages and shelf life. The nutritional value of fruit depends to amount of compounds with antioxidant properties. Today, consumers prefer consumption fruits with high bioactive compounds. The quantitatively and qualitatively of these compounds may be changed during ripening or after harvest. In general, fruit peel contains higher concentrations of antioxidants. The peel forms half of the fruit weight and is rich in natural antioxidant compounds such as phenols and flavonoids (Tumbas et al., 2010). Lemon fruit is a good source of vitamins and antioxidant compounds (Qasemi et al., 2011). Because of no comprehensive research has been done on quality changes of lemons during ripening and storage in northern conditions, Therefore, this study was carryout to evaluate the bioactive compounds and physicochemical properties of Lisbon and Eureka lemons at different harvesting times to determine the appropriate harvesting time and prevent frost injury risk.
Material and methods:In this study, the physicochemical and bioactive properties of Lisbon and Eureka lemons were evaluated during two years (2015-2016) by sampling from 17^th October to 16^th November, 10 days interval (four times) from trees and at the end of shelf life period. The location of sampling was Kotra Research Station that was located at the Citrus and Subtropical Fruits Research Center with geographical coordinates of 36 ° 42'47.7 "N 50 ° 58'25.6" E. Fruits were randomly selected from different sides of the tree (30 fruits picked from three trees). 15 fruits were evaluated at harvest time and the others were stored in common storage (7-10 ° C, 85% humidity) and evaluated 20 days after harvest. Various physico-chemical characteristics evaluated including fruit length, width, thickness; arithmetic, geometric, equivalent and harmonic diametric means; aspect ratio, sphericity, surface area, true volume, apparent volume, volume error, density, peel thickness, weight, juice and residue percentage, peel color indices (L*, a*, b*, hue angle, chroma and CCI), total soluble solid (TSS), titratable acidity (TA), technological index (TI), vitamin C, peel and pulp antioxidant capacity, peel and pulp phenolic compounds during experiment. Data analysis was performed using two-year component analysis in completely randomized design for each cultivar in three replications. Mean comparisons were made using Duncan's test and at the corresponding probability level.
Results and discussion: In this study, it was found that except for fruit volume values, other fruit physical characteristics were not significantly different in both cultivars. Eureka lemon volume error was less negative (-1.27%) than Lisbon (0.05%), indicating that Eureka lime had fruits with empty space inside or between peel and pulp. Only in Eureka, the highest fruit weight was observed on 27^th October and 17^th November. It seems that Lisbon Lemon passed through three phases of development and has reached the fourth stage. The highest water loss was obtained at 17^th October and 16^th November. Eureka variety with more outer surface, has more water loss. Juice percentage changes relational to harvest time was only significantly in Eureka lime and increased with late harvest. The highest juice percentage (33%) seen in late harvesting time. The total soluble solids in Eureka at four different harvested time and at the end of shelf life was different significantly. The amount of TA decreased during harvesting and storage. According to harvest time, the TSS/TA ratio also increased at both harvesting time and the end of shelf life. Fruit peel thickness only increased in Lisbon lemon during different harvest time. There were no significant changes observed in both cultivars during shelf life. Residue percentage and technology index increased only in Eureka during different harvesting times. Generally, the values of L*, a* and b* at the end of the shelf life were higher than the harvest time. The L* value in Lisbon and the a* value in Eureka did not change significantly over the shelf life. The amount of peel chroma, citrus color index (CCI) and total phenol increased during the shelf life. The amount of vitamin C in both cultivars was high in the 6^th November and was maintained until the end of shelf life. The amount of antioxidant capacity of the peel (50.19 to 75.72%) and pulp (0.49 to 60.92%) of Lisbon and peel (14.47 to 66.81%) of Eureka only showed significant difference at the end of shelf life. The minimum antioxidant capacity of pulp (41.47%) was related to Eureka cultivar which harvested on 5 September which was not significantly different from 17 September and 6 November (52.76 and 47.32%, respectively), while the highest value with 66.81% observed in 6 November. In general, total phenol content of both cultivars decreased during storage. In Eureka lemon, fruits had no significant difference in total phenol content of peel and pulp at different harvesting times and end of shelf life.
Conclusion: Eureka lemon fruit has an empty space in core or between the peel and the pulp that is important during processing. Lisbon completed its growth stages earlier, but both cultivars had adequate water content (33%) at the time of consumption. According to the technology index, maintains of fruit for 20 days after harvest, improves the quality of the juice. During the shelf life, the decline of peel greening was greater in Lisbon. The fruits harvested at the first harvest (17 October) had a lower CCI value at the end of the shelf life, indicating that the fruits harvested earlier would break green peel color later. The phenol content of the peel was higher than the pulp, but overall the antioxidant capacity of both cultivars was in good nutritional value. Totally based on the physicochemical traits, the fruits of Lisbon and Eureka can harvest from ‎6^th November.