A novel noninvasive and cost-effective handheld detector on soluble solids content of fruits

Soluble solids content (SSC) is an important parameter for determining sweetness, maturity, and price of fruits. A fast, noninvasive, and cost-effective portable SSC detector is appreciated by fruit planters, processers, sellers, and consumers. The study developed a handheld non-destructive SSC detector which mainly consisted of a visible/near-infrared (Vis/NIR) micro-spectrometer over the wavelength of 650-1100 nm, a Raspberry Pi board, a micro-halogen lamp, a self-made "Y" shape optical fiber probe (including an incident optical fiber and a reflection optical fiber), a lithium battery, and a display. The application software developed by python language was designed to collect diffuse reflection spectra, to preprocess spectra by using Savitzky-Golay smoothing method, and to calculate the SSC of fruits based on smoothed spectra and established partial least square regression models. Kiwifruits, nectarines, and apricots, collected from several orchards, were used as fruit samples to evaluate the detection performance of the developed SSC detector. The test showed that the root-mean squares errors of the developed SSC detector operating in Vis/NIR range were 0.9%, 0.7%, and 0.8% for kiwifruits, nectarines, and apricots, respectively. The determined SSC value could be shown on the display in 2 s after triggering SSC measurement. Moreover, the detector was just about 550 g in weight. The developed novel handheld SSC detector is a convenient and fast tool for measuring SSC of fruits nondestructively.