Impact of Two Pretreatment Methods on the Chemical Profile of Still-Bottom Water and the Antioxidant Activities of Hydrosol and Still-Bottom Water from Citrus × aurantium ‘Daidai’ and Citrus × aurantium L. Dried Flower Buds

Citrus × aurantium ‘Daidai’ (Citrus aurantium L. var. amara Engl.) is a sour orange cultivar whose dried flower buds are listed in the Chinese national catalogue of substances that serve both as food and as medicine, whereas the dried flower buds of the ordinary species C. × aurantium L. lack this dual recognition. Although the essential oil and hydrosol of these buds have been investigated, the still-bottom water remaining after hydrodistillation is routinely discarded and its chemical composition remains largely unexplored. In this study, we compared the chemical profiles of still-bottom water obtained from the dried flower buds of C. aurantium L. var. amara Engl. (CAVAF) and C. × aurantium L. (CALF) after two pretreatments (simple soaking vs. ultrasonic-microwave synergistic treatment) and evaluated the chemical antioxidant activities of both the hydrosol and the still-bottom water. HPLC-Q-TOF-HRMS identified 215 compounds across the four water samples. The major constituents were neohesperidin (13.18–39.66%), blumeatin (9.78–10.25%), narirutin (13.46–18.01%), and α-D-(6-O-Sinapoyl)-glucopyranosyl(1→22)-β-D-(3-O-sinapoyl)-fructofuranose (7.46–8.57%). Relative to CALF, the CAVAF samples contained a large number of annotated compounds and higher relative amounts of several flavonoids and limonoids (up to nine-fold) in the still-bottom water. Higher relative abundances of the principal flavonoid glycosides were observed after ultrasonic-microwave pretreatment compared with soaking. The hydrosol of both varieties showed only modest radical scavenging activity and weak reducing power. In contrast, the still-bottom water exhibited clear DPPH, ABTS, and hydroxyl radical scavenging, with half-maximal inhibitory concentrations ranging from 0.1901 to 4.873 mg/mL, although reducing power remained limited. These findings suggest that botanical variety and pretreatment method may influence both the metabolite profile and the in vitro radical scavenging behavior of the distillation by-products. The results supply chemical compositional support for the food-medicine homology status of CAVAF and indicate that still-bottom water is a recoverable source of potential polar antioxidants that is currently under-utilized.