Ultrasound-assisted ascorbic acid solution pretreated hot-air drying improves drying characteristics and quality of...

The effective hot-air drying of foods such as jujube requires an effective green pretreatment alternative to chemical pretreatments. Jujube slices were pretreated using 5 and 10 mg mL^-1 ultrasound-assisted vitamin C (UVC) for 10, 20, and 30 min, followed by hot-air drying.

Ultrasound-assisted vitamin C pretreatment for 10, 20, and 30 min modulated the characteristics of fresh jujube slices such as water loss (from -28.25% to -25.52% after 30 min of UVC pretreatment), solid gain (from -31.68% to -26.82% after 30 min of UVC pretreatment), loss of total and reducing sugars (from 200.25 mg and 34.88 mg to 287.14 mg and 4.71 mg, respectively, after 30 min of UVC pretreatment), total soluble solids (from 76.32 ^o Brix to 82.08 ^o Brix), and water diffusivity (from 9.01 × 10^-10  m²  s^-1 to 6.71 × 10^-10  m²  s^-1 ). These characteristics were associated with altered surface morphology and improved drying characteristics. The UVC pretreatment preserved an acceptable reddish-yellow or orange-like color during hot-air drying and reduced the browning index from 26.3 optical density (OD)/g DM to 23.25 OD/g dry mass basis (DM), which was connected with reduced 5-hydroxymethylfurfural (HMF) content. On the other hand, the proportions of bioactive components such as vitamin C increased from 1.05 mg g^-1 DM to 9.02 mg g^-1 DM, phenolics increased from 12.8 mg gallic acid equivalent (GAE)/g DM to 17.5 mg GAE/g DM, flavonoids increased from 4.0 mg rutin equivalent (RE)/g DM to 4.4 mg RE/g DM, and procyanidin content increased from 2.0 mg catechin equivalents (CE)/g DM to 2.9 mg CE/g DM in UVC pretreated jujube slices, which had a positive association with increased antioxidant activity - for example, 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS) increased from IC₅₀ 22.5 mg DM/mL to 8.0 mg DM/mL, 2,2-diphenyl-1-picrylhydrazyl (DPPH) changed from IC₅₀ 36.5 mg DM/mL to 9.5 mg DM/mL, and ferric reducing antioxidant power (FRAP) increased from 2.0 mg vitamin C equivalent (VCE)/g DM to 11.9 mg VCE/g DM).

The data indicated that UVC can be used as a promising pretreatment method for improving the hot-air drying characteristics and the quality of jujube slices. © 2023 Society of Chemical Industry.