Influence of protein and leucine on in vitro accessibility and apparent permeability of phenolic compounds in blackberry-dairy blends

Abstract

There is increasing consumer interest in performance nutrition beverages that improve muscle recovery, endurance and strength, and a desired product type would be dairy-blackberry beverages. However, the effect of phenolic compounds on the digestion of milk proteins, and the effect of milk proteins on the bioaccessibility and permeability of phenolic compounds through the intestinal wall is poorly understood. In this study, dairy-blackberry beverages were made by adding milk protein concentrate and/or leucine and freeze-dried blackberry puree to skimmed milk, to formulate beverages with different protein contents and two different leucine contents. These beverages were subjected to in vitro digestion, and their protein breakdown was analysed with electrophoresis. The accessibility of flavonoids, anthocyanins and individual phenolic compounds was determined and the apparent permeability (Papp) of individual phenolic compounds through Caco-2 cells was analysed. It was found that β-lactoglobulin in samples with a higher protein content was more resistant to digestion. An increase in protein had no influence on the bioaccessibility of flavonoids as a whole but increased the bioaccessibility of the subgroup anthocyanins from 13% in water to 64% in high protein milk. The Papp of cyanidin-3-O-glucoside, gallic acid, protocatechuic acid and rutin was determined. Milk proteins improved the Papp of gallic acid and cyanidin-3-O-glucoside, to a maximum value of 4.26 and 0.36 × 10⁻⁶ cm/s but had no clear influence on protocatechuic acid and rutin. In conclusion, higher milk protein levels were associated with increased bioavailability of anthocyanins and their metabolites, which could inform formulation strategies for performance nutrition.