Surface Modification and Sugar Infusion of Freeze-Dried Hawthorn via Combined Vacuum-Ultrasound Treatment

Hawthorn (Crataegus pinnatifida Bunge) is a medicinal and edible fruit whose sour taste and dense surface structure limit its application in food industries. Conventional sugar impregnation has long processing times and uneven sugar distribution. However, the combined effects of vacuum impregnation (VI) and ultrasonic impregnation (UI) on infusion uniformity in freeze-dried hawthorn remain unclear. This study therefore aimed to elucidate the mechanisms by which combined vacuum and ultrasonic impregnation (VI-UI) affect surface structure and sugar infusion uniformity, and to test the hypothesis that VI-UI can achieve uniform sugar distribution while maintaining an intermediate sugar content. Results showed that the VI-UI treatment (195 min) disrupted cell structures and expanded intercellular spaces in hawthorn, thereby achieving a uniform sugar infusion at an intermediate sugar content of 47%. The ultrasound treatment increased surface porosity from 16.8% to 19.3%. The VI-UI treatment achieved the highest overall uniformity index (OUI = 79.65), higher than vacuum impregnation (OUI = 50.99) and ultrasound impregnation (OUI = 60.69). In conclusion, VI-UI offers a promising strategy for producing uniformly infused, high-quality preserved hawthorn products by combining the efficiency of VI with the uniformity of UI.