Rubus idaeus by-products: sustainable improvement of the antioxidant value of sourdough bread by a new end-use of exhausted seeds still containing bioactive compounds

In the quest to find effective strategies for reducing food waste impact, plant-food wastes and by-products have been considered as potential sources of ingredients to improve food quality. In this work, we explored the suitability of waste from cold pressing of Rubus ideaus seeds (WRSP) as functional ingredient in bread. Fortified bread was produced replacing wheat flour/semolina mixture with 5% or 10% WRSP. WRSP did not affect sourdough lactic acid bacteria starters development since their cell densities reached levels around 108 CFU/g at the end of fermentation. Illumina data showed the dominance of lactobacilli in the obtained doughs (82.08–88.76% of relative abundance). Concerning functional properties, our results indicated that WRSP inclusion significantly increases the content of polyphenolic compounds, particularly proanthocyanins, and the antioxidant activity of both functionalized doughs and final breads. Specifically, the substitution of 5% or 10% of the flour/semolina mixture with WRSP, as compared to the control dough, resulted in a two- to threefold increase in the total polyphenolic content and in a three- to fourfold increase in metal-reducing potential. Finally, compared to the control bread, breads enriched with WRSP exhibited enhanced redness in both the crust and crumb, and in terms of sensory qualities, they obtained higher appreciation from evaluators.