Effects of Drying Method and Plant Section on Bioactive Compounds, Antioxidant Activity and Colour of Cauliflower (Brassica oleracea L. Var. botrytis L.) By-Products

Cauliflower by-products represent a valuable source of bioactive compounds that can be valorized as functional ingredients within circular food systems; however, their stability is strongly influenced by processing conditions. This study evaluated the combined effects of plant section (leaves and stems) and drying method (freeze-drying, hot-air drying at 40 °C and solar drying at approximately 30–45 °C) on the nutritional composition, pigment content, antioxidant activity and colour of cauliflower by-product flours. Proximate composition, chlorophylls, carotenoids, total polyphenols, total flavonoids, glucosinolates, sulforaphane and antioxidant activity (ABTS and DPPH assays) were determined, and colour was assessed using CIELAB parameters (L*, a*, b*, chroma, hue angle and browning index). Freeze-drying showed the highest preservation of pigments, phenolic compounds, sulforaphane and antioxidant activity, followed by hot-air drying, while solar drying resulted in the lowest retention. Leaf-derived flours consistently presented higher pigment and phenolic contents and more favorable colour attributes than stem-derived flours. Antioxidant activity was strongly associated with matrices richer in pigments and phenolics. Although leaves exhibited higher glucosinolate contents, sulforaphane levels showed only minor differences between plant sections, suggesting that stem-derived fractions may also represent a valuable raw material considering their greater biomass availability and industrial scalability. Overall, these findings demonstrate that both plant section and drying method significantly influence the techno-functional quality of cauliflower by-product flours and should be jointly considered to optimize the development of stable, functional and sustainable food ingredients.