Evaluation Of Nutritional Quality And Antioxidant Activity Of Eureka Lemon [Citrus Limon (L.)Burm.F.] Fruit In Different Harvest Stages

Lemon[Citrus limon?L.?Burm.f.]is one of the key cultivated species of Family Rutaceae Genus Citrus,which is only after sweet orange and mandarin.Lemon fruit is popular among consumers because of its bright color,pleasant taste and rich nutrients with vitamins,amino acids and mineral elements.In addition,lemon fruit also contain a variety of bioactive compounds,such as phenolic acids,flavonoids,carotenoids and limonoids.Among them,phenolic compounds have attracted much attention due to their strong bioactivities,including antioxidant,anti-inflammatory and antibacterial activity.Antioxidant activity is one of the most basic bioactivities,and is closely related to people's health.Lemon fruit can be harvested several times in one year and the monthly supply can meet more market demand.Eureka lemon is the lemon variety with the largest planting area and the largest output in China.How to rationally harvest and utilize it in accordance with its external and internal quality changes is a problem that needs to be solved for market expansion and an increase in economic benefits.Therefore,the Eureka lemon from Changshou,Chongqing as the material were used in this study to determine the general physicochemical indicators;the content of various nutritional quality indicators,phenolic compounds and the antioxidant activity in the pulp and peel of the lemon fruit.The growth of the lemon fruit in one year was classified into three different stages,immaturity,half-maturity and maturity respectively.Finally,the comprehensive evaluation of the fruit quality of the whole year was carried out.The main results are as follows:?1?General physicochemical indicators of lemon fruit:9 kinds of general physiological indicators of lemon fruits were measured.The highest single fruit weight was in November?177.30±25.21 g?.The yellowest fruits were in March?0.79±0.65?.The thinnest peel was in January?5.05±0.48 mm?.The minimum number of seeds was in April?12±4.36?.The highest soluble solids content was in November?8.40±0.15Brix%?.The highest juice content was in January?49.84±3.89%?.The lowest titratable acid content was in April?2.99±0.27%?and the highest total sugar content was in November?4.25±0.15%?.The physicochemical level of the fruit was the best at maturity.?2?Nutritional quality of lemon fruit:Vitamins,amino acids and mineral elements in lemon pulp and peel were detected by high performance liquid chromatography?HPLC?,flame atomic absorption spectrometry?FAAS?,graphite furnace atomic absorption spectrometry?GFAAS?and ultraviolet-visible spectroscopy?UV-vis?.Vitamin B₁,vitamin B₂,vitamin C and vitamin E were detected in lemon pulp and peel.Among the detected vitamins,the levels of vitamin C and vitamin B₂ were the highest.The total vitamin content in the pulp was the highest in April?40.98±1.00 mg/100 g?,and the total vitamin content in the peel was the highest in January?133.21±1.47mg/100 g?.The vitamin content decreased with the increase in maturity.The vitamin B₁,vitamin B₂ and vitamin C content in the peel were higher than the pulp.17 amino acids were detected in lemon pulp and peel.The highest content of amino acid was aspartic acid?Asp?,followed by glutamic acid?Glu?.The total amino acid content in the pulp was the highest in April?988.98±57.09 mg/100 g?,and the total amino acid content in the peel was the highest in June?1145.10±45.92 mg/100 g?.Whether in the pulp or in the peel,the total amino acid content decreased with the increase in maturity.The amino acid content in the peel were higher than those of the pulp.Among the 12 mineral elements detected,the highest content of the main element was K,the trace element was Fe,and the lowest content of the mineral elements was Cr.The content of K,P,S,Zn in the pulp and K,P,S in the peel both decreased with the increase in maturity.Except for P and K,the content of the remaining mineral elements in the peel were higher than those of the pulp.The nutritional quality of the fruit was the best at immaturity.?3?Phenolic compounds of lemon fruit:Ultra-high-performance liquid chromatography?UPLC?and ultraviolet-visible spectroscopy?UV-vis?were used to detect the phenolic acids,flavonoids,total phenolic content and total flavonoid content in lemon pulp and peel.Among the three types of phenolic acids determined,the highest content was esterified phenolic acid.Among 8 phenolic acids,the ferulic acid content in pulp was the highest,and the highest content in the peel was?-coumaric acid.The total content of phenolic acid in the pulp was the highest in April?269.36±33.07?g/g?and in the peel was the highest in June?769.28±16.60?g/g?.Total phenolic acid content of the peel was much higher than that of the pulp.The total content of phenolic acid in the pulp and peel decreased with the increase of maturity.The highest content of 8 kinds of flavonoids was hesperidin,followed by eriocitrin.The maximum of sum of individual flavonoids content in pulp and peel occurred in August?3098.42±161.64?g/g,9003.91±213.45?g/g?,and the sum of individual flavonoids content increased first and then decreased with the increase in maturity.The content of flavonoids in the peel far exceeded the pulp.The total phenolic content and total flavonoid content in the pulp were the highest in April?3.49±0.05 mg GAE/g,1.46±0.05 mg RE/g?,and decreased with the increase in maturity.The content of total phenolic content and total flavonoid content in the peel was higher than those of pulp.The content of phenolic compounds of the fruit was highest at half-maturity.?4?Antioxidant activity of lemon fruit:The antioxidant activity of lemon pulp and peel was detected by ABTS free radical scavenging assay,DPPH free radical scavenging assay and FRAP iron ion reduction assay.The antioxidant activity of the pulp was the strongest in April,and the antioxidant activity of the peel was higher than that of the pulp.With the increase in maturity,the ABTS value and FRAP value in the pulp decreased gradually,and the FRAP value and DPPH values in the peel decreased first and then increased.The antioxidant activity of the fruit was strongest at immaturity.Based on our comprehensive analysis,April was the best time in the immature stage,August was the best month in the half-mature stage,and January was the best in the mature stage.This study aims to provide a basis for further processing of lemon products and development of lemon resources by exploring the changes in the quality of lemon fruit throughout the year in order to achieve potential applications in the food,cosmetics and pharmaceutical industries.With the insight on the characteristics of lemon fruit at each stage and the basis of lemon fresh consumption,the ultimate realization of year-round utilization of lemons is expected.