Phenolic compounds, microstructure and viscosity of onion and apple products subjected to in vitro gastrointestinal digestion

Microstructure, viscosity and their relationship with bioaccessibility of phenolic compounds in onion and apple products (untreated and HPP) and commercial quercetin supplement throughout a dynamic gastrointestinal digestion (GID) model were investigated. In non-digested (ND) samples, untreated and HPP-onion presented higher total phenolic and flavonol content (TFC-HPLC and TPC-FC) than apple counterparts. TFC-HPLC decreased throughout GID phases in all samples studied. TFC-HPLC bioaccessibility was higher in onion (similar to 17.6%) than in apple (similar to 10%) and in quercetin supplement (0.027%). HPP did not improve TFC-HPLC bioaccessibility. Throughout GID, onion and apple showed a significant decrease in both consistency (K) and apparent viscosity at 10 s(-1) but higher values were found in apple. These data agree with TFC-HPLC and TPC-FC decrease and with the lower bioaccessibilities of apple compared with onion. Food matrix had a more significant effect than HPP on TFC-HPLC bioaccessibility, which is related to the rheological behavior of the GID-phases. Industrial relevance: High-pressure processing (HPP) (400 MPa at 25 degrees C during 5 min) combined with freeze-drying enhanced significantly flavonols extractability (TFC-HPLC) in onion and apple and in some cases their bioaccessibility. Bioaccessibility of bioactive compounds in each food matrix is being required by industrials and consumers concerned to know the actual amount of bioactive compounds that are available for intestinal absorption. The change of the matrices viscosity studied throughout in vitro gastrointestinal digestion (GID) could predict the bioaccessibility of these bioactive compounds. HPP could be proposed as a strategy for increasing the extractability of bioactive compounds in vegetable derived products.