Muffins Enriched with the Polysaccharide Fraction Residue After Isolation of Starch from Unripe Apples—Nutritional Composition, Profile of Phenolic Compounds, and Oxidation Stability

Apple by-products are a valuable raw material due to their high content of dietary fiber, minerals, and bioactive compounds, making them a promising functional ingredient in food products. The aim of this study was to evaluate the effect of adding a residue obtained from the isolation of starch from unripe apples of the Pyros and Oliwka varieties on the nutritional composition, mineral content, polyphenol and fiber levels, and color of wheat muffins. Additionally, the oxidative stability was analyzed. The results showed that the addition of the residue significantly increased the total, soluble, and insoluble fiber content, as well as the protein content. The polysaccharide fraction residue from unripe Oliwka apples had a stronger impact on enhancing the fiber content of the muffins. In contrast, muffins enriched with the polysaccharide fraction residue from unripe Pyros apples exhibited higher levels of calcium, potassium, and magnesium, while the Oliwka residue increased the contents of sodium, strontium, and iron. The addition of the polysaccharide fraction residue significantly increased the levels of chlorogenic acid, phloridzin, quercetin, and procyanidin B1. Color analysis revealed a darkening effect in the muffins after the addition of the residue, and the oxidative stability decreased with increasing levels of the polysaccharide fraction residue. This study demonstrated that apple residues obtained after starch isolation can effectively enrich muffins with nutrients and health-promoting compounds; however, their impact on oxidative stability requires further investigation.