Abstract: In this study, a specialized portable low-cost visible and near-infrared (Vis-NIR) instrument for the evaluation of grape quality was designed to meet the need for fast and non-destructive detection in the grape industry. A holographic concave diffractive grating combined with a charge-coupled device was used as the core component of this system to obtain diffuse reflectance spectral data of grape samples in the wavelength range between 400 and 1 100 nm, and the combined application of a halogen light source with low-OH Y-shaped quartz optical fiber as a reliable optical transmission medium was proposed to provide a stable Vis-NIR light source. In order to meet the requirements of different varieties with fruits of different sizes, an adjustable sample cuvette with both internal and external threads was designed. The software, written in C# based on windows operating system, was stable and could facilitate model updating. Two Vitis vinifera cultivars, Manicure Finger and Ugni Blanc, were detected using this instrument. CIE L*, a* and b* values, total solid soluble content (SSC) and total phenolic (TP) content were taken into consideration in the development of predictive models using least squares support vector machine (LS-SVM). The results showed that for both cultivars a* value, SSC and TP could be well predicted by the proposed models with a coefficient of determination for calibration (Rc 2 ) of 0.91, 0.94 and 0.90, respectively. In the external validation using 70 unknown samples, the root mean square error of prediction (RMSEP) of a* value, SSC and TP were 3.15, 1.39 °Brix and 0.24 g/kg for Manicure Finger; and 0.78, 1.56 °Brix and 0.22 g/kg for Ugni Blanc, respectively. This study has confirmed the feasibility of this Vis-NIR instrument for grape quality detection and can provide a technical basis for developing specialized Vis-NIR instruments to detect fruit and vegetable quality.

Key words: visible and near-infrared (Vis-NIR) spectroscopy, holographic concave diffractive grating, charge-coupled device, CIE L*, a* and b*, total solid soluble content, total phenolic content