Comprehensive Fruit Quality Evaluation

The concentration of soluble sugar in fruit was measured as described by Liu et al. (2018) (link). The sample of apple flesh was placed in a test tube, then 5 ml of distilled water was added and mixed after cutting it into chunks. The supernatant was collected after 30 min of boiling water bath. This step was repeated twice, using distilled water to adjust the volume of the solution to 10 ml. The absorbance of the solution was measured at 630 nm after adding sulfuric acid and anthrone. Six biological replicates and three technical repetitions were conducted for each treatment.
The concentration of anthocyanin was measured as described by Zheng et al. (2021) (link). The sample of fruit peel was ground with liquid nitrogen rapidly and then extracted with 1.5 ml of 1% (v/v) HCl methanol at 3°C for 30 h in darkness. The concentration of anthocyanin was measured by a multifunctional microplate reader (BioTek, USA). Six biological replicates and three technical repetitions were conducted for each treatment.
The concentration of titratable acid was measured by NaOH titration (Nie et al., 2012 (link)), and the fruit hardness was measured by a HP-230 hardness tester. The diameter of fruit was measured with a vernier caliper. Single fruit weight was measured by 1% precision electronic balance, and the average of 30 measured values was taken.