## 1 Introduction  
Food products, particularly those with a high added value, are frequently subjected to stringent quality standards, which are critical for attesting to certain specific characteristics, such as geographical origin, manufacturing method, and/or producer know-how. Analytical platforms must be updated and improved on a regular basis as fraudulent methods become more sophisticated, as well as the complexity and diversity of food composition [1]. In this context, non-destructive techniques, such as spectroscopic and imaging techniques, are inserted effectively, responding to the problems of the food control field. Since these are analytical techniques that offer numerous advantages, including the non-destruction of samples, the rapidity to obtain results and the possibility of checks during the production processes, they have been studied and exploited for a long time in the agro-food sector [2]. The most commonly widespread non-destructive techniques in the food industry are indeed vis/NIR spectroscopy, NIR spectroscopy (NIRs) and image and multi/hyperspectral analysis [3]. Vis/NIR spectroscopy has been used successfully in the non-destructive evaluation of fruit and vegetable quality over the past few decades [4] and, in particular, in the grape and wine industry, e.g., to determine the optimum harvest time or to evaluate the quality parameters in works by [PERSON] et al. [5], [PERSON] et al. [6], [PERSON] et al. [7], [PERSON] et al. [8], [PERSON] et al. [9], [PERSON] et al. [10] and [PERSON] et al. [11], just to name a few. The possibility of monitoring the ripening stage of the grapes directly in the field has now become of fundamental importance both to guarantee a raw material of the highest quality and to support winemakers in their choices. Another crucial aspect is covered by sustainability. The chemical analyses normally carried out in the laboratory to evaluate the qualitative parameters of grapes require chemical reagents and also a lot of time and specialized personnel; the optical analyses in this case represent a sustainable alternative [12]. Nowadays, commercially available spectrophotometers are mostly expensive benchtop instruments that do not offer the possibility of being used outside the laboratories. The existing portable instruments offer the possibility of being used directly in the field, but also have a high cost and often a configuration that is only useful to acquire spectra and not to obtain immediate results, in terms of parameter prediction. For these reasons, the research is concentrating on maintaining the high performance of this type of optical analysis, while using simplified, portable and easy to use tools [13]. Some papers have been published on the use of portable spectrometers on vegetal matrices. [PERSON] et al. [14] used a portable vis/NIR device based on the creation of an index, which was based on two wavelength peaks (index of absorbance difference, I\({}_{\text{AD}}\)) to follow the ripening evolution of cherries; [PERSON] et al. [15] used the same device on grape berries, demonstrating that the use of I\({}_{\text{AD}}\) may be useful for monitoring technological ripeness and anthocyanin concentration. [PERSON] et al. [16] used a portable vis/NIR device implemented with an optical fiber to predict the sugar content (TSS) of kiwi fruits and [PERSON] et al. [17] used a portable vis/NIR device on apples to predict the content in TSS. Despite attempts at simplification, these non-destructive and rapid techniques are, however, characterized by an intrinsic complexity of the data collected, which requires multivariate statistical analysis techniques for interpretation. To measure fruit maturity at harvest and during the post-harvest period, various non-destructive techniques and chemometric algorithms have been developed [18]. Chemometrics is a discipline of statistics that deals with multivariate data historically derived from analytical chemistry.