Citrus Peel Hydrolates as By-Products of Hydrodistillation: Volatile Characterisation and the Role of Enzymatic Pretreatment

This study comprehensively characterised the volatile composition of hydrolates obtained as by-products of the hydrodistillation of orange, mandarin, and clementine peels. Enzymatic pretreatments using pectinase, cellulase, xylanase, or their mixture were applied in purified water or citrate buffer (pH 5) prior to Clevenger hydrodistillation, and volatile profiles were analysed by HS–SPME GC–MS. Across all citrus species, hydrolates were dominated by oxygenated monoterpenes and alcohols, with α-terpineol and linalool identified as the principal constituents. Statistical analysis suggested differences in hydrolate volatile composition following enzymatic pretreatment in citrate buffer with cellulase, xylanase, or their combination (p < 0.05); notably, α-terpineol content in mandarin hydrolates nearly doubled after these treatments. Enzyme-free reflux extraction in water also led to observable changes in volatile profiles (p < 0.05), highlighting the importance of including both untreated and enzyme-free controls when evaluating enzymatic effects. The study also illustrates the distinct distribution of dominant volatiles between hydrolates with prevailing α-terpineol. These findings demonstrate the potential of enzymatic hydrodistillation for the valorisation of citrus peel by-products by enabling modulation of hydrolate volatile composition and supporting more sustainable use of citrus processing residues.