Effect of Flaxseed Press Cake (Linum usitatissimum L.) Incorporation on the Polyphenol Content, Antioxidant Potential, and Quality of Partially Fat-Replaced Cookies

Current trends in food technology include the development of reduced-fat snacks and the use of food industry by-products. This study aimed to develop a formulation for partially fat-replaced cookies enriched with flaxseed press cake (FPS). The study was conducted in three stages: (I) determination of the level of margarine replacement with maltodextrin (5–25%) to identify the most favorable level, (II) comparison of maltodextrins obtained from light-fleshed and purple-fleshed potato starch, and (III) evaluation of the effects of partial wheat flour replacement with FPS (10–30%) on the physicochemical, sensory, and antioxidant properties of the cookies. Replacing 10% of the margarine with maltodextrin with a dextrose equivalent (DE) of 17 provided the most favorable combination of physical and sensory properties among the tested levels. The incorporation of FPS resulted in a dose-dependent increase in total polyphenol content, reaching a value approximately 13.5 times higher than that of the control sample. In vitro antioxidant capacity increased by up to approximately 4.6-fold in the ABTS assay and 4.9-fold in the DPPH assay. HPLC analysis identified eight phenolic acids. Their total content increased from 2.90 mg/100 g DM in the control sample to 17.01 mg/100 g DM in B3J and 19.11 mg/100 g DM in B3F samples, containing 30% FPS and maltodextrin produced using light-fleshed and purple-fleshed potato starch, respectively. Gallic acid was the predominant phenolic acid in the enriched samples, reaching 12.47–13.82 mg/100 g DM. Samples containing maltodextrin obtained from purple-fleshed potato starch (BXF) generally had a higher total content of the identified phenolic compounds than the corresponding samples containing maltodextrin obtained from light-fleshed potato starch (BXJ). Increasing the proportion of FPS to 20–30% increased cookie hardness by up to approximately 3.1-fold and reduced the sensory scores. The most favorable balance between bioactive compound content and product quality was obtained for samples B1J and B1F, which contained 10% FPS. These findings support the use of FPS in the development of partially fat-replaced cookies with enhanced in vitro antioxidant capacity, in line with the principles of the circular economy.