## 1 Introduction  
Due to the beneficial properties of metabolites, the interest in the characterization and quantification of the metabolites--both primary and secondary--of plant-based foods is growing, and it often aims at the definition of food nutritional value, as well as of its quality and authenticity. In this context, foodomics technologies have emerged [1]. Foodomics has been defined as a new discipline that studies the food and nutrition domains through the application of advanced omics technologies for improving consumers' well-being, health and confidence [2]. Considering a large number of plant primary and secondary metabolites and their different properties, several analysis techniques have been developed for each class of compounds, which differ already starting from the extraction phase of metabolites [3]. The interest in nutritional parameters, such as sugars content, often identified as being responsible for obesity or diseases such as diabetes, determined the attention to develop analytical methods for the determination of simple sugars and polysaccharides ([PERSON] and [PERSON], 2017). Anyway, each class of metabolite, whether primary or secondary, needs a dedicated extraction and analysis method to offer the consumer as much information as possible regarding the functional values of the individual foods. _Malus domestica_ fruits, commonly known as apples, are one of the most consumed fruits worldwide, and they are generally recognized as an outstanding source of biologically active compounds, related to both functional and nutraceuticalul values ([PERSON] and [PERSON], 2017) (Figure 1). Apples have a varied and well-balanced composition; more than 84% of their weight is represented by water, in which minerals such as K, Mg, Ca and Na and trace elements such as Zn, Mn, Cu, Fe, B, F, Se and Mo are dissolved ([PERSON] and [PERSON], 2017). Proteins and lipids represent a small energetic supply contribution to _Malus domestica_ fruits, due to their relatively small concentrations ([PERSON] and [PERSON], 2017). On the contrary, the content of fibers, i.e., complex polysaccharides such as pectin, celluloses, hemicelluloses and lignin, is high in apples when compared with other fresh fruits ([PERSON] and [PERSON], 2017). Although a large variety of vitamins are present, the B complex vitamins are the most abundant. Moreover, apples are considered an important source of polyphenols, which are responsible for their well-known antioxidant properties ([PERSON] and [PERSON], 2017). Apples are commonly considered as healthy plant-based foods. They have a very low caloric impact. From a nutritional point of view, they contain a high number of sugars, balanced by the concentration of fibers ([PERSON] and [PERSON], 2017). In the last ten years, apples have been the subject of research and studies aiming to evaluate their effects on human health ([PERSON] and [PERSON], 2017). The daily intake of apples and apples-related products is often proposed in with weight control diets and has proved to have a statistically significant impact on weight reduction ([PERSON] and [PERSON], 2017). Recent epidemiological research shows how the risk of cardiovascular diseases and cancers could be reduced by the regular consumption of apples ([PERSON] and [PERSON], 2017; [PERSON], 2017), due to their phenolic acids and flavonoids, i.e., molecules with antioxidant properties ([PERSON], 2017). The nutritional value and  
Figure 1: Main primary and secondary metabolites related to functional and nutraceuticalul values of apples. health-related properties of apples depend on the bioavailability and daily intake of their nutrients and phytochemicals, as well as on their concentration [13]. Therefore, a complete evaluation of the apple's metabolites distribution is desirable. Over the years, many new _Malus domestica_ species have been produced through new grafts and hybridizations, thus obtaining apple fruits with different chemical-physical characteristics and organoleptic properties [14].