Polyphenol release from protein and polysaccharide embedded plant extracts during in vitro digestion

• Phenol extracts (onion/apple) were embedded in gum acacia, modified starch and pea protein isolate.
• A two-step in vitro GI model was used to screen release from encapsulated phenols.
• Only capsules with pisane as carrier showed an enzyme-catalysed phenol release.

Encapsulated phenol extracts from industrial processing residues of onions and apples are discussed as health promoting food additives. Prerequisite for their use in human/animal nutrition is the quantitative liberation of the phenols during digestion. In the present in vitro study we, thus, determined the release rate of a variety of spray dried phenol extracts embedded in different carrier molecules. Onion and apple extracts encapsulated with either lupine/gum acacia (ratio extract to wall material: 1:2), gum acacia (1:3), pea protein isolate (1:4), or modified starch (1:3) were screened employing a two-step (gastric: pepsin, pH 2.0; intestinal: pancreatin, pH 7.5) in vitro model (pH stat). Total phenol release in the incubation solutions (solid phase extraction) was followed spectrophotometrically (Folin–Ciocalteu method); quercetin-4′-O-glucoside (onion) and phloretin-2′-O-glucoside (apple) were analysed with HPLC. Extracts spray dried in the presence of carbohydrates alone (gum acacia, modified starch) or in combination (lupine/gum acacia) exhibited an immediate, non-enzymatic phenol release (80%) due to instability in aqueous solutions. Pepsin catalysed enzymatic hydrolysis of apple extracts embedded with pea protein isolate resulted in a time-depending total phenol (max. 70% of embedded material) and phloretin-2′-O-glucoside (max. 20%) liberation; pancreatin showed no effect. Phenols were stable in gastric fluid; in the presence of pea protein isolate, phenols partly decomposed at basic pH. These results demonstrate that only spray dried phenol extracts embedded in the presence of pea protein isolate effectively release phenols under in vitro digestion conditions.