Response surface optimisation of osmotic dehydration of Chinese ginger (Zingiber officinale Roscoe) slices

Optimization of ultrasonic-assisted osmotic dehydration as a pretreatment for microwave drying of beetroot (Beta vulgaris)

The aim of this study was to optimize drying conditions during ultrasonic-assisted osmotic dehydration and subsequent microwave drying of red beetroot using the Box-Behnken design. For this purpose, ultrasonic-assisted osmotic dehydration was performed at different ultrasonic powers (50, 75, and 100 W), sonication times (20, 40, and 60 min), and salt concentrations (0%, 15%, and 30%). The subsequent drying procedures were conducted with 231, 518, and 805 W microwave power. The best condition was selected as 5.15% salt concentration, 20 min sonication time, 50 W ultrasonic power, and 716.45 W microwave power. The responses obtained under optimum conditions were determined as 68.06%, 9.54 mg GAE/g dm, 28.23, 42.66, and 3.08 for DPPH• % inhibition, total phenolic content, L*, a*, and b* values, respectively. While favorable impacts on color were detected for the applied pretreatments, the DPPH• scavenging activities of the dried beetroot were determined to be more significant after ultrasonic-assisted osmotic dehydration. Furthermore, the drying kinetics of beetroot were evaluated according to the Midilli et al. model. When the fit to the model was investigated, it was compatible at R2 > 0.90 level. As a result, the ultrasonic-assisted osmotic dehydration pretreatment performed before the microwave drying method preserved the quality characteristics of beetroot samples and was successfully applied.

Comparison of pulsed vacuum and ultrasound osmotic dehydration on drying of Chinese ginger (Zingiber officinale Roscoe): Drying characteristics, antioxidant capacity, and volatile profiles

The effects of pulsed vacuum osmotic dehydration (PVOD) and ultrasound osmotic dehydration (USOD) on drying characteristics and quality attributes of ginger were investigated. PVOD was subjected to pulsed vacuum at 13 kPa for 30 min, and USOD was subjected to ultrasound with the frequency of 40 kHz for 30 min. After PVOD and USOD treatments, the samples were then dried at intermittent microwave & air-drying oven with an output of 700 W and temperature of 60°C to the final moisture content of 0.12 g water/g d.w. The results showed PVOD and USOD treatments could improve the total phenolic contents by -1.8% to 16.4%, total flavonoid content by 7.7%-18.7%, DPPH radical scavenging by 9.5%-12.2%, and ABTS+ antioxidant activity by 17.8%-27.4%, although they prolonged the later stages drying of ginger. Besides, the PVOD- and USOD-pretreated dried samples had less brownings than the untreated-dried samples which could be attributed to the inactivation of polyphenol oxidase (PPO) and peroxidase (POD). The PPO activity was significantly reduced in the PVOD and USOD ginger, whereas POD activity was decreased in USOD ginger but increased in PVOD ginger. Moreover, PVOD pretreatment also led to a better preservation of volatile profiles and cell structure than USOD treatment. Therefore, both PVOD and USOD are effective pretreatments for drying of ginger.

Rehydration characteristics of dehydrated West African pepper (Piper guineense) leaves

The rehydration characteristics of dehydrated West African pepper leaves were investigated at hydration temperatures of 28, 60, 70, and 80°C. Four treatments were given to the leaves: blanched and sun dried, unblanched and sun dried, blanched and shade dried, and unblanched and shade dried. The hydration process of the dehydrated leaves was adequately described by the Peleg's equation. As the hydration temperature increased from 28 to 70°C, there was a significant decrease in the Peleg's constant K₁, while for most of the leaves the Peleg's constant K₂ varied with temperature. Rehydration ratio values ranged from 3.75 in blanched shade dried leaves to 4.26 in unblanched sun dried leaves with the unblanched leaves generally exhibiting higher ratios than the blanched leaves.