Impact of heat treatment on selected vegetables: bioaccessibility of (poly)phenolic compounds after in vitro gastrointestinal digestion and colonic microbiota action, and furan occurrence

Abstract

Plant foods are the main source of dietary antioxidants, including (poly)phenolic compounds, with health properties. However, their bioaccessibility might be affected by culinary processes, as well as by gastrointestinal digestion and microbiota. Additionally, heat treatment induces the formation of volatile compounds, among them furan which has been classified as a possibly carcinogenic to humans (group 2B) by the International Agency for Research on Cancer. Therefore, the main aim of the present research was to investigate the influence of heat treatment (frying and griddling) on the antioxidant capacity, nutritional composition and (poly)phenolic compounds in selected vegetables (yellow onion (Allium cepa), Italian green pepper (Capsicum annuum) and cardoon stalks (Cynara. cardunculus L.)), as well as their bioaccessibility after a simulated gastrointestinal digestion and colonic microbiota action. Moreover, the presence of furan in heat treated vegetables in comparison with that formed during deep-frying in bread coated frozen foods was also investigated. A total of 7, 24 and 25 free and bound (poly)phenolic compounds were identified and quantified in both raw and cooked onion, green pepper and cardoon, respectively. The main (poly)phenolic compounds were quercetin glucosides (approx. 90%) in onion, quercetin and luteolin derivates (>90%) in green pepper, and chlorogenic and other phenolic acids (approx. 99%) in cardoon. Heat treatments, especially griddling process, tended to increase the total (poly)phenolic compounds in vegetables, and consequently their antioxidant capacity, since DPPH and (poly)phenols were correlated (r=0.70, p82% in cooked pepper vs 48% in raw one) and cardoon (60-67% in cooked cardoon vs 2% in raw one). After in vitro gastrointestinal digestion, both griddled green pepper and griddled cardoon still maintained the highest amount of (poly)phenolic compounds (9.447 and 41.853 µmol (poly)phenolic compounds/g dm, respectively). Gut microbiota exerted a high metabolic activity resulting in a large and rapid degradation of (poly)phenolic compounds into new metabolites, being 3-(3’- hydroxyphenyl)propionic acid by far the most abundant catabolite in all samples after 24 h of fecal incubation. Catabolic pathways for colonic microbial degradation of flavonoids and chlorogenic acids have been proposed. Griddled vegetables were the samples with the highest amount of bioaccessible (poly)phenolic compounds even after the fecal fermentation. Finally, frying process did not result in the formation of furan in vegetables, while low amount (3.5 µg kg-1 in gridlled onion) or traces of this compound were formed in vegetables during griddling. Therefore, cooked vegetables do not represent a high exposure to furan. In contrast, deep-frying process induced the formation of considerable amounts of furan in bread-coated foods (12 µg kg-1 (tuna pasties)-172 µg kg-1 (onion rings)), implying a health risk in Spanish population groups with a high consumption of frozen precooked products.