Digestibility, antioxidative activity and stability of plant protein-rich products after processing and formulation with polyphenol-rich juices: kale and kale–strawberry as a model

Protein–polyphenol interactions can possibly be affected by processing and modifying the product stability, the digestive fate and stability during shelf-life of proteins and polyphenols. In this study, the combined effects of the addition of a polyphenol-rich matrix (strawberry) and the preservation technology (thermal, PEF, HPP) on kale juice (K), as model for a protein-rich vegetable juice, on the techno-functional and antioxidant properties were investigated before and during in vitro digestion. While the different processing did not affect the protein and physical stability of kale juice, combination with strawberry (KS) strongly reduced protein solubility and physical stability, likely due to the protein–polyphenol interactions in a processing-dependent manner. Kale proteins were easily digestible with full digestion within 30 min gastric phase. During digestion, the antioxidant capacity of K and KS increased, likely indicative of the release of soluble, antioxidative compounds, yet it was not significantly influenced by different preservation treatments.