Index determination

The index of absorbance difference (I_AD) can reflect the status of fruit maturity by measuring the changes in pericarp chlorophyll content, which is calculated as the difference between the absorption at 670 nm and 720 nm within the range of 0–2.2. The value 2.2 represents green, and 0 represents complete maturity [4,15]. The I_AD value of the pericarp was determined by a DA-Meter (TR Turoni srl, Forlì, Italy).

A Color Quest XE (Hunter Lab, Reston, VA, USA) color difference meter was used to evaluate the pericarp brightness (L^*), where a^* represents “-green” to “+red” and b^* represents “-blue” to “+yellow”. Then, the chroma (C^*), hue angle (h) [16,17], and a^*/b^* were calculated.

The firmness with and without the pericarp was determined by a TA-XT Plus texture analyzer (Stable Micro-Systems Texture Technologies Corp, Scarsdale, New York, NY, USA) with a probe diameter of 8 mm, a test depth of 5 mm, and a penetration rate of 1 mm s⁻¹.

Flesh SSC values were measured using a digital, hand-held pocket refractometer PAL-1 (ATAGO, Itabashi-ku, Tokyo, Japan). The SSC was expressed in Brix at 20°C [18,19].

The sucrose, glucose, fructose, sorbitol, malic acid, quinic acid, and citric acid contents were measured by a Agilent 1100 high performance liquid chromatography (Agilent Technology, Santa Clara, CA, USA) [20]. The total sugar content was the content sum of four kinds of soluble sugars and the total acid content was the total content of four kinds of organic acids. The sugar/acid ratio was calculated from the total sugar and total acid contents.