Apple Waste/By-Products and Microbial Resources to Promote the Design of Added-Value Foods: A Review

Apple fruit is among the most consumed fruits in the world, both in fresh and processed forms (e.g., ready-to-eat fresh slices, juice, jam, cider, and dried slices). During apple consumption/processing, a significant amount of apple residue is discarded. These residues can also be interesting materials to exploit, particularly for direct valorization in the design of added-value foods. In fact, apple waste/by-products are rich in essential components, including sugars, proteins, dietary fibers, and phenolic compounds, as they comprise apple peels, seeds, and pulp (solid residue of juice production). In this sense, the current review paper presents an overview of the nutritional composition of apple waste/by-products, and mainly apple pomace, highlighting their application in producing value-added products through microbial biotechnology. If appropriately managed, apple by-products can generate a variety of useful compounds required in food (as well as in feed, pharmaceutics, and bioenergy). Recent strategies for the synergic use of apple waste/by-products and microbial resources such as lactic acid bacteria and yeasts are discussed. This review contributes to defining a reference framework for valorizing apple waste/by-products from a circular economy perspective through the application of bioprocesses (e.g., fermentation), mainly oriented towards designing foods with improved quality attributes.