Valorization of Apple Peels through the Study of the Effects on the Amyloid Aggregation Process of kappa-Casein

Waste valorization represents one of the main social challenges when promoting a circular economy and environmental sustainability. Here, we evaluated the effect of the polyphenols extracted from apple peels, normally disposed of as waste, on the amyloid aggregation process of kappa-casein from bovine milk, a well-used amyloidogenic model system. The effect of the apple peel extract on protein aggregation was examined using a thioflavin T fluorescence assay, Congo red binding assay, circular dichroism, light scattering, and atomic force microscopy. We found that the phenolic extract from the peel of apples of the cultivar ``Fuji'', cultivated in Sicily (Caltavuturo, Italy), inhibited kappa-casein fibril formation in a dose-dependent way. In particular, we found that the extract significantly reduced the protein aggregation rate and inhibited the secondary structure reorganization that accompanies kappa-casein amyloid formation. Protein-aggregated species resulting from the incubation of kappa-casein in the presence of polyphenols under amyloid aggregation conditions were reduced in number and different in morphology.