## 2 Materials and Methods
### Calibration of Detection System and Construction of a Predictive Algorithm Model  
The following procedure was used to construct a predictive model for the vitamin C contents of the calibration set:  
Step 1--Nondestructive measurement of the reflectance of Indian jujube fruit flesh. The calibration set consisted of 120 Indian jujube samples and the light emitted by the halogen light source was parallel-polarized and bandpass-filtered before being projected onto the flesh and skin of these samples at \(\theta_{\text{i}}=\theta_{\text{B}}\). Subsequently, the NIR photodetector captured the light reflected by each sample at each wavelength. The resulting electrical voltage signal was sampled and converted by the signal processing module before being sent to a computer to compute the total reflected light intensity and Rtotal. Step 2--The vitamin C content of Indian jujube is its ascorbic acid (AA) content. The flesh of each Indian jujube fruit was juiced and filtered to produce an extract for HPLC analysis such that its AA content can be measured. The HPLC system was a Hitachi model L-2000 liquid chromatograph equipped with a Hitachi L-2420 UV-vis detector (set to 254 nm) and a Hitachi D-2500 data processor. Chromatography was performed with a LiChroCART 250-4 LiChrospher 100 RP-18e (5 \(\upmu\)m) column. The isocratic mobile phase consisted of 1.55 mL 0.5 M tetrabutylammonium hydroxide, 30 mL of methanol and 970 mL of 0.01 M potassium dihydrogen phosphate (pH 4.0). The flow rate was 1.0 mL/min [20]. The test solution was diluted to 1/5 with 1% metaphosphoric acid immediately before use and 10 \(\upmu\)L of the diluted sample was injected into the column. The chromatographic peak that corresponds to AA can be determined by comparing its retention time to that of a standard. Hence, co-chromatography was performed with a standard solution of AA and the sample. To verify the presence of interfering substances in the AA chromatographic peak, peak purity was tested by pretreating the fruit extract with 1 unit/mL of L-ascorbate oxidase at 20 \({}^{\circ}\)C. A calibration curve was prepared using standard AA solutions to determine the relationship between peak area and AA concentration. The AA concentration in the fruit was calculated in mg/100 g fresh weight (FW), according to the following equation [21]:  
\[\mathrm{AA_{F}}{=}\mathrm{AA_{E}}(10*\mathrm{W}/100+20)*100/10=\mathrm{AA_{E}}( \mathrm{W}+200), \tag{5}\]  
where \(\mathrm{AA_{F}}\) is the AA concentration in the fruit (mg/100 g FW), \(\mathrm{AA_{E}}\) is the AA concentration in the undiluted extract (mg/mL) and \(\mathrm{W}\) is the water content (%). Step 3--Calculation of the correlation coefficient of calibration (r\({}_{\mathrm{c}}^{2}\)) and root-mean-square error of calibration (RMSEC). The PLS algorithm in the SmartPLS 3 (Hamburg, Germany) software was used to calculate the r\({}_{\mathrm{c}}^{2}\) and RMSEC between the reflectance of each wavelength and the actual vitamin C content of the Indian jujube fruits. The criterion for the latent variable setting in PLS should be stop in 10\({}^{-5}\). The reflectance of each wavelength combination were then matched with the actual vitamin C values of the fruits in the calibration set to construct a predictive model for the fruits' vitamin C content. The r\({}_{\mathrm{c}}^{2}\) and RMSEC of the model for the calibration set were calculated using Equations (6) and (7), respectively.