By-Products of Plant Processing as Sustainable Sources of Antioxidants for Cereal-Based Food Fortification

Fruit and vegetable processing by-products (BP) retain much of the raw material’s bioactive fraction and are of interest as natural food antioxidants. Six plant BP (beetroot, apple, blackcurrant, aronia, blueberry, okara), the corresponding fortified gluten-free knäckebrot products and an unfortified control were characterised by total polyphenol content (TPC), DPPH, ABTS and FRAP. Differences among samples were statistically significant (one-way ANOVA, p < 0.001; Tukey HSD). Among the BP, blueberry, aronia and blackcurrant were the most active (blueberry: 377 µmol TE/g DM for FRAP); among the products, aronia knäckebrot was the highest in all four applied methods, followed by blueberry. The among-sample coefficient of variation declined 2.5- to 3.6-fold from BP to products. The dry-matter mass balance revealed systematic departure from conservative mixing at the nominal 5% incorporation: apparent recovery exceeded 100% for TPC in all six products (111–152%) but fell to 72–78% for DPPH in the three phenol-poor ones, indicating a contribution of baking-derived reducing compounds to the Folin–Ciocalteu and FRAP response. The assays ranked samples concordantly (Kendall’s W = 0.949) without being biochemically equivalent.