Effects of exogenous melatonin treatment on storage quality of pear fruits during shelf life at room temperature

Abstract: 【Objective】The experiment was carried out to study the effects of melatonin at different con-centrations on the storage quality of pear fruits during shelf life at room temperature after harvest, and to screen out the optimal concentration of melatonin, so as to provide references for the application of mela-tonin in the preservation of pear fruits.【Methods】Using Xinli No. 7 as the test material, 6 groups of fruits with uniform size, consistent maturity, and without decay and mechanical damage were selected.One group was soaked in water for 30 minutes as the control, and the other 5 groups were immersed in melatonin solutions at 50, 100, 150, 200 and 250 µmol·L^-1 for 30 minutes, dried and placed on a shelf at room temperature of 20 ℃, and samples were taken every 5 days. Each sample was taken to determine the fruit weight loss rate, hardness, soluble solids content, ethylene release rate and other indicators. The hardness of the fruit was measured by GY-4 hardness tester, the soluble solid content was measured by the ATAGO PAL-1 digital sugar meter, and the ethylene release rate was measured by gas chromatogra-phy. Using a 1 mL syringe to draw air repeatedly, the sample was injected into the GC2014C gas chro-matograph for determination. The fruits stored at room temperature for 15 days were chosen to deter-mine the aroma components in the fruits. The quarter method was used to remove the core and the fruit  was cut into pieces of 0.2 cm×0.2 cm×0.2 cm and mixed evenly. 5 g of the sample was put into a 10 ml sample bottle and the internal standard 2-octanol (0.016 44 g · L^-1) 3 μL was added, which was sealed with tin foil. Heating at 40 ℃ lasted for 30 minutes, using SPME extraction head (black) to absorb aro-ma after adsorption and injection analysis. The unknown compound was searched by computer and matched with the mass spectrometry library NIST17-1, NIST17-2 and NIST17-S at the same time. Various volatile components were confirmed and the content of each component was calculated. The Micro-soft Excel 2010 was used to analyze the data, and SPSS16.0 statistical analysis software was used to ana-lyze the significance of differences.【Results】Melatonin treatment had a certain inhibitory effect on the weight loss rate of pear fruits. The weight loss rates with 100 and 150 µmol·L^-1 melatonin treatments in-creased by 4.20% and 3.75% during storage, which were significantly different from the control. At the end of storage, the hardness of the control fruit was the lowest at 3.57 kg·cm^-2, which was 24.20% lower than that before storage. The hardness of the fruits treated with 50, 100, 150, 200 and 250 µmol·L^-1 mela- tonin was significantly higher than that of the control, respectively. The soluble solid content of the con- trol fruit continued to rise during the early storage period, and reached a peak (14.17%) on the 15th day and then decreased rapidly, and it was 9.95% on the 20th day. The melatonin-treated fruits showed a con- tinuous upward trend on the 20th day, respectively, compared with that before storage, increasing by 2.05%, 1.22%, 2.08%, -0.35% and 2.63%. The ethylene release rate of the control fruits reached a peak value of 0.073 μL·kg^-1·h^-1 on the 10th day, and the ethylene release rate was 0.062 μL·kg^-1·h^-1 at the end of storage. The melatonin-treated fruits reached the peak value on the 5th day. At the end of storage, it was 74.01%, 87.05%, 85.68%, 77.18% and 80.72% lower than the control. The types and contents of aroma had an important influence on the flavor of the fruit. In this experiment, 92 kinds of aroma sub-stances were detected in the fruits with different treatments. The maximum of 40 kinds of aroma sub-stances were detected in the fruits treated with 200 µmol·L^-1, followed by the control and 50 µmol·L-1,treatment (39 types), and 100 µmol·L-¹ treatment was at least 34 types. The aroma substances commonly contained in the fruits with each treatment included Hexyl acetate, 1-Hexanol, 2-Methyl butanal, 3-Meth-yl butanal, Hexanal, Heptanal, Nonanal, Decanal, (E)-2-Hexenal, 2-Octanone, Toluene, o-Xylene, Do-decane, Tetradecane, and Longifolene. The lowest aroma content of the control fruit was 180.84 ng·g^-1, and the highest (272.59 ng · g^-1) was treated with 200 µmol · L^-1, which increased by 50.73% compared with the control. The difference in aroma content mainly came from aldehydes. The lowest one in the control was 129.03 ng·g^-1, and the highest one was in the 200 µmol·L^-1 treatment (212.51 ng·g^-1), which was 64.69% higher than the control. Melatonin treatment mainly promoted the synthesis of 1-Penten-3-ol, 2, 2, 4-Trimethyl-1,4-pentanediol diisobutyrate, 1-Heptanol, Tridecanal, α-Pinene, etc.. The synthesis of substances was inhibited, such as Octanoic acid, nonyl ester, 1-Nonanol, 2-Nonen-1-ol and 1-O-octyl- Rhamnitol. The high contents of aroma substances in the various treatments of Xinli No. 7 fruits were hexyl acetate, 1-hexanol, hexanal, trans-2-hexenal, toluene, o-xylene, and limonene, which accounted for 88.12%, 87.95%, 88.37%, 91.26%, 92.29% and 90.70% of the total aroma content of the fruits, ac-cordingly to CK, 50, 100, 150, 200, 250 µmol·L^-1 treatments.【Conclusion】Compared with the control, suitable melatonin treatment can effectively slow down the increase in weight loss rate and decrease in fruit hardness, stabilize the soluble solids content, inhibit the release of ethylene, delay the senescence of fruits, and significantly extend the shelf life of Xinli No. 7 at room temperature. At the same time, it can significantly increase the type and content of aroma substances in the fruit of‘Xinli No. 7’during shelf life at room temperature. It is generally believed that suitable exogenous melatonin treatment can im- prove the storage quality of Xinli No. 7 fruits during shelf life at room temperature.