Abstract: To make more efficient use of navel orange pulp resources, low-temperature superfine grinding (LSG) was applied to process whole navel oranges. Three freeze-dried powders (NRYP30, NRYP60, and NRYP90) were collected at 30, 60, and 90 min of LSG treatment. Navel orange removed yellow peel powder (NRYP) produced by ordinary grinding (OG) was used as a control. The physicochemical properties, microstructure, and functional characteristics of the four powders were systematically investigated. Compared with NRYP, a significant reduction in particle size was observed in the superfine powders (P < 0.05). As the grinding time increased, the average particle size first decreased and then increased, the color became brighter and lighter, the bulk density declined, and the flowability deteriorated. Meanwhile, water-holding capacity showed no significant change, whereas the oil-holding capacity and solubility increased, and the hygroscopicity was reduced. An elevation in the glass transition temperature was also recorded, indicating improved storage stability. Fourier transform infrared (FTIR) spectroscopy revealed that the characteristic absorption peaks of freeze-dried navel orange powder were minimally affected by superfine grinding, with no new absorption peaks observed. After LSG treatment, the contents of total phenols, total flavonoids, and ascorbic acid significantly increased (P < 0.05), with the highest levels reaching 14.72 (NRYP60), 2.25 (NRYP90), and 8.68 mg/g (NRYP90), respectively. However, the total carotene content initially increased and then declined as the grinding time prolonged. High-performance liquid chromatography (HPLC) analysis indicated that the major flavonoids in freeze-dried navel orange powder were hesperidin and narirutin, with their maximum concentrations observed in NRYP30, reaching 12 391.63 µg/g and 3 542.85 µg/g, respectively. After 60 min of LSG treatment, however, the levels of narirutin, hesperidin, and didymin were significantly reduced. Antioxidant activity initially increased and then decreased with prolonged grinding time. NRYP90 exhibited the strongest total reducing capacity. The radical scavenging rates of 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radical and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical reached the highest values in NRYP60, which were 53.79% and 37.50%, respectively, while those in NRYP90 significantly declined. In summary, LSG can improve the physicochemical properties of whole navel orange powder and promote the release of bioactive components. However, grinding time must be controlled to avoid degradation of active compounds and reduction in antioxidant activity.

Key words: freeze-dried whole navel orange powder; low-temperature superfine grinding; physicochemical properties; microstructure; bioactive components; reducing power