Contribution of Phenolics and Maillard Reaction Products to the Antioxidant Capacity of Coffee Brews

Coffee is one of the most consumed beverages in the world and a rich source of antioxidants. The amounts of these antioxidants are influenced by several technological factors. Besides, antioxidants identified in coffee (chlorogenic acids, volatile and non-volatile Maillard reaction products) may contribute to the overall antioxidant capacity in different proportions. Therefore the aim of this research was to evaluate the actual contribution to the overall antioxidant capacity of coffee brews by the most relevant coffee compounds (phenolics and Maillard reaction products). To achieve this aim, radical scavenging assays were applied to assess overall antioxidant capacity. Qualitative and quantitative analysis of coffee components were carried out by chromatographic methods (HPLC-PDA, HPLC-PDA-MS and GC-MS). Model systems and the on-line ABTS-HPLC technique were used for the assessment of the contribution of coffee compounds to the overall antioxidant capacity. Firstly, the influence of: variety, roasting degree and roasting technique (conventional and torrefacto), on antioxidant compounds (phenolics and Maillard reaction products) and overall antioxidant capacity of coffee was studied. Results showed that among technological factors, torrefacto roast increased radical scavenging capacity but had no influence on the amounts and composition of phenolic compounds, which were more affected by the roasting degree and variety. Secondly, the extractability of these compounds in the most commonly used coffeemakers (espresso and filter) and the contribution of coffee antioxidants to the overall antioxidant capacity of the coffee brews were assessed. Brewing time and conditions have a great impact on the extraction of antioxidants. High water pressure during the brewing process increases the extraction speed. Nevertheless parameters like turbulence during extraction and longer contact time favour the extraction efficacy. Volatile Maillard reaction products contribute very little to the antioxidant capacity of coffee compared to high molecular weight Maillard reaction products (melanoidins) and phenolic compounds. Among 22 identified coffee phenolics, caffeoylquinic acids showed the highest contribution to the overall antioxidant capacity, whereas feruloylquinic acids and their lactones did not have any radical scavenging capacity. Moreover, the antioxidant capacity of the chlorogenic acids was highly dependent on their chemical structure. However, antioxidant capacity of chlorogenic acids accounted only for 7-16 % of the overall capacity of coffee. Additionally, to go deeper into the fate of coffee antioxidants in the human body (after ingestion/during passage through the gastrointestinal tract), in vitro faecal fermentation was carried out to study the influence of the colonic microbiota on these compounds. In vitro faecal fermentation of espresso coffee showed that chlorogenic acids are rapidly and extensively metabolized by the human colonic microflora, leading to the formation of up to 11 degradation products, which are better absorbed in the colon than their parent compounds.