Citrus seed oil as an agro-industrial waste: fatty acid profile, phenolic compounds, and valorization potential

Citrus species are widely cultivated fruit crops whose seeds represent an underutilized resource with considerable variation in oil content and chemical composition. This study evaluated seed morphological traits, oil content, fatty acid composition, phenolic compounds, and DPPH radical scavenging capacity of six Citrus species cultivated under identical Iranian climatic conditions, including sweet orange ( C. sinensis ), Mexican lime ( C. aurantifolia ), sweet lime ( C. limettioides ), big citron ( C. medica var. macrocarpa ), small citron ( C. medica var. medica ), and sour orange ( C. aurantium ). Significant interspecific variation was observed in seed size, weight, and oil accumulation, with oil content ranging from 25.00 ± 0.42% in sour orange to 39.65 ± 0.55% in sweet lime. Fatty acid analysis revealed linoleic (32.27 ± 0.09–39.27 ± 0.39%) and oleic acids (20.52 ± 0.03–29.36 ± 0.14%) as predominant unsaturated components, whereas palmitic (25.33 ± 0.26–28.56 ± 0.35%) and stearic acids (3.13 ± 0.10–5.62 ± 0.24%) were the main saturated fractions. Total phenolic content ranged from 2.11 ± 0.04 to 3.66 ± 0.06 mg GAE/g DW, total flavonoid content from 0.92 ± 0.02 to 1.50 ± 0.03 mg QE/g DW, and condensed tannins from 0.12 ± 0.01 to 0.23 ± 0.01 mg CE/g DW. DPPH radical scavenging capacity of seed extracts varied from 39.76 ± 0.38% to 65.86 ± 0.42%, while seed oils showed 39.67 ± 0.36% to 94.15 ± 0.55% DPPH inhibition. Multivariate analysis, including principal component and hierarchical clustering, revealed multivariate differentiation among the studied species based on the measured morphological, biochemical, and fatty acid traits. small citron, sweet lime, and sweet orange showed comparatively higher values for selected measured traits, including oil content, unsaturated fatty acid proportion, and DPPH radical-scavenging response; however, these patterns are specific to the materials and growing conditions investigated in the present study. Overall, the observed variation in oil yield, fatty acid composition, phenolic-related traits, and DPPH radical-scavenging response highlights the compositional diversity of citrus seed oils and provides a basis for further investigation of their potential utilization. Future evaluation of application-specific properties, including oxidative and storage stability, safety, nutritional efficacy, processing performance, and techno-economic feasibility, could provide the evidence needed to assess the suitability of citrus seed oils for specific food, nutraceutical, cosmetic, or industrial applications.