Protein-phenolic interactions in lentil and wheat crackers with onion skin phenolics: effects of processing and in vitro gastrointestinal digestion

This study aimed to evaluate the protein-phenolic interaction in functional crackers made of wheat/lentil flour with onion skin phenolics (onion skin powder: OSP, onion skin phenolic extract: OSE, or quercetin: Q) after in vitro gastrointestinal digestion. Phenolic/antioxidant recovery in crackers was lower with higher levels of phenolic addition. In vitro gastrointestinal digestion procedure was applied for crackers prepared/cooked with onion skin phenolics (functional crackers) or crackers consumed with onion skin phenolics (co-digestion). Functional crackers had similar nutritional attributes (p > 0.05), however they had lower L* values, and higher a* values. A higher concentration of OSP/OSE caused a decrease in the b* value while it was increased with the quercetin addition. Phenolic/antioxidant recovery in functional crackers was decreased by increasing the ratio of phenolic supplements. The amount of quercetin 7,4-diglucoside was lower than the theoretical value whereas the amount of quercetin was higher in functional crackers. The phenolic bioavailability index (BI_P) of co-digested crackers was higher than that of functional crackers, whereas antioxidant bioavailability index (BI_A) was mostly similar. Quercetin was only identified in functional wheat/lentil crackers with OSE. After digestion (1) TCA-precipitated peptides of the wheat crackers could not be identified, whereas that of co-digested lentil crackers was more abundant, (2) level of free amino groups of co-digested/functional crackers were lower than the control except for the co-digested sample of lentil cracker with quercetin.