## 1 Introduction  
Polyphenols are ubiquitous plant secondary metabolites, and they are undisputedly a highly significant field of interest in human nutrition. A growing body of research over the last two decades suggests that polyphenol intake through consumption of plant foods may play a vital role in maintaining a good health status, by regulating metabolism, battling chronic degenerative diseases, and controlling cell proliferation. To date, accumulated epidemiological evidence indicates that various polyphenols may exhibit strong antioxidant and anti-inflammatory properties, which could be implicated in the prevention of cardiovascular disease, neurodegenerative disorders, cancer, etc. [1].  
Amongst several polyphenol classes, flavonoids are probably one of the most prominent ones, because they are abundant at significant amounts in many commonly consumed fruit and vegetables, but also in foods of plant origin, such as fruit juices, tea and wine [2]. The major flavonoid representative is quercetin, which occurs in plant tissues mainly as glycoside, although various types of processing may result in glycoside decomposition, releasing the aglycone [3]. Quercetin has powerful antioxidant properties, and its biological significance has been appraised by numerous investigations [4; 5]. On the other hand, quercetin is a molecule susceptible to chemical and enzymic attacks, which may result in decomposition, skeleton rearrangement, and dimerization [6]. Such modifications can inevitably lead to alteration of the biological properties of quercetin, but this does not necessarily entail loss of biological activity. Decomposition or oxidation may end up in obtaining products of inferior antioxidant potency [7], but quercetin and other structurally related flavonoids were also shown to retain antioxidant activity, despite chemical or enzymic oxidation [8; 9]. Quercetin oxidation may also result in generating compounds with enhanced biological activity [10]. Therefore, the changes in quercetin structure brought about through oxidative modification should be a subject of thorough appraisal.  
The issue of food thermal processing on flavonoids has been long before addressed, on the basis of findings which indicated that regular domestic practices (frying, boiling) could have an important impact on flavonoid content and composition of certain plant tissues [11]. Latter studies confirmed that onion boiling may provoke notable decreases in flavonoid glycosides and antioxidant activity, the extent of which was linked to glycoside structure [12]. More recent studies were in concurrence, highlighting the importance of domestic processing (frying, baking, steaming) on onion flavonoid glycosides and its nutritional consequences [13]. Polyphenol thermal degradation has also been observed in black rice flour [14] and apple juice [15] thermal processing. However, other investigations suggested that the changes traced for flavonoids in processed foods may depend on the mode of processing, yielding either increased or decreased flavonoid content [16]. Indeed, it has been supported that heat treatments may contribute to increasing polyphenolic content in vegetables, as a result of thermal destruction of cell walls and sub cellular compartments during the cooking process, which would allow for polyphenol release [17].  
Considering the above concepts, the current study aimed at assessing the effect of thermal treatment on quercetin degradation in model solutions. The phenomenon was approached by implementing a novel kinetic model, through which the effects of pH and addition of antioxidants were evaluated. The elucidation of quercetin degradation mechanism was carried out with liquid chromatography-tandem mass spectrometry (LC-MS/MS), and putative consequences on the antioxidant activity were discussed on the ground of the results from representative in vitro assays. To the best of the authors' knowledge, such a kinetic approach is, for the first time, proposed for the study of polyphenol thermal degradation.