Vacuum and Ultrasound-Assisted Impregnation of Gala Apples with Sea Buckthorn Juice and Calcium Lactate: Functional Properties, Antioxidant Profile, and Activity of Polyphenol Oxidase and Peroxidase of Freeze-Dried Products

Vacuum impregnation (VI) and ultrasound-assisted impregnation (US) were applied to Gala apples with the aim of improving the antioxidant capacity and calcium content of freeze-dried apples. An impregnation solution was composed of 93.8% sea buckthorn (SB) juice with 0 and 4% (w/w) calcium lactate (CaL). Gala apples conventionally impregnated (CON) at 30°C for 120 min were also prepared for comparison. The VI was performed at 200, 400, and 600 mbar with 10 min of each holding and relaxation time. US treatment was performed for 10, 20, and 30 min and was continued with CON for 110, 100, and 90 min, respectively. The VI at 200 mbar and US for 30 min at 0% CaL produced freeze-dried samples with the highest antioxidant capacity, higher than that of the CON and the non-impregnated samples: fresh apple (FA) and those after dipping in 1% ascorbic acid (FA+AA). The addition of 4% CaL optimally improved the calcium content when VI at 200 mbar and US for 20 min were applied. Generally, the freeze-dried apples impregnated under these conditions had lower antioxidant capacity compared to those treated without CaL. The impregnation process (CON, VI, and US) affected the mass transfer and composition of phenolics and carotenoids in the freeze-dried apples. Additionally, polyphenol oxidase (PPO) and peroxidase (POD) activities were completely inhibited upon CON and US treatment for 30 min at 0% CaL. While at 4% CaL, US for 20 min reduced PPO activity by 19.4% as compared to FA+AA. The end products, particularly those obtained at 4% CaL, could serve as potential functional snacks for consumers with osteoporosis and calcium issues, considering their calcium content and antioxidant properties.