## 2 Materials and Methods
### Determination of Indicators  
The color of peaches (L, a, b, C, H) was assessed according to the International Commission on Illumination (CIE) Delta E 2000 (CIEDE2000) color space using a colorimeter, and test times were less than three minutes. Test points were selected from most red-colored and the most light-colored parts of the fruit surface, and average values were used for data analysis. The CIEDE2000 formula was developed by members of the CIE Technical Committee, providing an improved procedure for the computation of industrial color differences [25; 26]. The formula is as follows:  
\[\Delta E=\sqrt{(\frac{\Delta L}{K_{\text{LS}}L})^{2}+(\frac{\Delta C}{K_{\text {C}}S_{\text{C}}})^{2}+(\frac{\Delta H}{K_{\text{H}}S_{H}})^{2}+R_{\text{T}}( \frac{\Delta C}{K_{\text{C}}S_{\text{C}}})(\frac{\Delta H}{K_{\text{H}}S_{H }})}, \tag{1}\]  
where \(\Delta\)E is the change in color, RT is a hue rotation term, \(\Delta\)L, \(\Delta\)C, and \(\Delta\)H are the compensation differences for neutral colors (primed values; \(L\), \(C\), \(H\)), \(S_{\text{L}}\) is the compensation for lightness, \(S_{\text{C}}\) is the compensation for chroma, \(S_{\text{H}}\) is the compensation for hue, and \(K_{\text{L}}\), \(K_{\text{C}}\), and \(K_{\text{H}}\) are constants and usually in unity.  
Based on the proportion of red area on the surface of the fresh peach, the operator divided the samples into five appearance quality grades. Consumers in the fruit market always choose fresh peaches with more red areas [27].  
Peaches were treated as a sphere for volume measurement, and diameter was estimated based on the average of height and width, as shown in Figure 1a. Peach mass was measured using an electronic scale with an error range of 0.01 g. The shape index (height/width) and density (mass/volume) were acquired by calculation.  
Figure 1: Volume, firmness, and titratable acidity measurement of peaches.  
Dielectric properties were measured using published procedures with a level of 500 mV, a test time of 1 min, a frequency of 10 kHz, and a bias voltage of 0 mV [28; 29]. The target experimental indicators were impedance (\(Z\)s) and phase angle (\(\theta\)). Measurement range was set to automatic mode, and test speed was set to fast mode [14; 30]. The experimental device is shown in Figure 2.  
The contact probe was designed and manufactured using copper as a conductive material [15]. The probe was completely inserted into the pulp of the peach. After one minute, the values on the instrument display were paused and recorded. The probe was wiped with alcohol on cotton before measuring the next sample. Avoiding areas damaged by the contact probe, four planes (with obvious pulp) were sliced from four quadrants of peaches (excluding the bottom and top) for firmness measurements, as shown in Figure 1b. Fruit firmness was determined at four equatorial positions on each fruit at 90\({}^{\circ}\)[22] after skin was carefully removed.  
Juice was extracted from the pulp of peaches and used for SSC determination. An additional 5 g of the remaining juice (without pulp) was sampled, and, after adding few drops of bromothymol blue indicator, the titration solution (0.1 mol/L NaOH) was dropped into the bottle until the juice turned from yellow to olive green, as shown in Figure 1c. The TA content of the juice was determined from the volume of the titrated solution, and the sugar-acid ratio (_SSC/TA_) was acquired by calculation.