## 2 Materials and Methods
### Spectral Acquisition  
The diffuse transmittance spectral collection device used in this study was a self-developed portable SSC detector. The principle diagram of the device is shown in Figure 1a, and the physical photo is shown in Figure 1c. An STM32 microprocessor was employed as core controller. The peripheral circuits included a spectrometer driving circuit, a halogen lamp driving circuit, a power management circuit, and communication interface circuits. The light source played a crucial role in obtaining stable spectra with the device. A halogen lamp with a power rating of only 1.5 W (997418, Welch Allyn, Skaneateles Falls, NY, USA) was selected for compatibility with the detector. This portable device featured a miniaturized, low-cost, and specialized design. It employed a miniature spectrometer (C11708 MA, Hamamatsu, Japan) with a wavelength range of 640-1050 nm. During the detection process, the device was equipped with an OLED display to show real-time predictions of SSC content. The device was powered by a rechargeable lithium battery with a capacity of 6000 mA\(\cdot\)h, supporting continuous operation for approximately 3 h. Each spectral data acquisition took less than 2 s.  
This device was not only used for collecting the spectra of kiwifruits but also for other similarly sized fruits with about 1.3 mm of thin peel. Compared with other laboratory-based spectral collection systems, this device offered significant advantages in terms of being lightweight, compact, and low-cost, which made it highly applicable for internal quality detection in fruit during the ripening period. Using this device, fruit production management became more efficient, growth trends were monitored, fertilization was guided, and rough grading on production lines was performed. It ensured the production of high-quality fruits, thereby enhancing their market competitiveness. To conduct the detection with the device, it was turned on and allowed to preheat for 5 min. Next, a polytetrafluoroethylene white reference board was placed over the probe, and the reference spectrum was collected  
Figure 1: Diffuse transmittance spectroscopy acquisition device. to calibrate the device. For spectral collection, each kiwifruit was positioned at the equatorial positions and rotated to capture spectra at three distinct equatorial locations, as shown in Figure 1b. After obtaining the spectra from these three positions, the average spectrum was computed to represent the final spectrum for each kiwifruit. This method ensured that the spectral data collected were accurate and representative of the entire fruit. As shown in Figure 2, the diffuse reflectance spectra collection device was used. The diffuse reflectance spectra were primarily collected using a USB2000+ spectrometer (Ocean optics Inc., Dunedin, FL, USA) with a wavelength range of 300-1100 nm. For the spectral collection of kiwifruit using the diffuse reflectance method, the following steps were performed:  
1. Device preparation: The device was turned on and preheated for 5 min. This step was crucial for stabilizing both the light source and the detector. 2. Reference spectrum collection: The white reference spectrum was collected using the standard reflectance white board, and simultaneously, the dark spectrum was captured by covering the detection fiber with the black cap. These reference spectra were used for subsequent spectral calibration, ensuring that the collected data were accurate and consistent. 3. Sample spectrum collection: The kiwifruit was positioned at the detection location, and spectral collection was performed at its equatorial region. To obtain comprehensive and accurate data, spectra were captured at various equatorial positions of the kiwifruit, and these locations were marked for subsequent SSC measurement. All spectral collection experiments were conducted in a dark environment to avoid interference from ambient light [24].