Abstract
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.
Highlights
The rhizome of the ginger plant (Zingiber officinale), which contains 85%–95% water, is susceptible to microbial spoilage and chemical deterioration (Mishra, Gauta, & Sharma, 2004)
To test the ultrasound osmotic dehydration (USOD) treatment, samples were placed in an ultrasonic chamber (KQ‐200DB; Kunshan Ultrasonic Apparatus Co.) for 30 min, with the operating frequency set to 40 kHz, output power set to 175 W, and intensity of 0.48 W/cm2
USOD‐ and Pulsed vacuum osmotic dehydration (PVOD)‐pretreated samples dried slightly faster than the untreated samples during the early stages of drying, but dried much more slowly than the untreated samples at the late stages of drying. This might be because more sugar was present in the USOD‐ and PVOD‐pretreated samples, and higher sugar concentrations increase the resistance to water diffusion (Mothibe et al, 2011)
Summary
The rhizome of the ginger plant (Zingiber officinale), which contains 85%–95% water, is susceptible to microbial spoilage and chemical deterioration (Mishra, Gauta, & Sharma, 2004). Several predrying treatments, including osmotic dehydration (OD) (Mandala, Anagnostaras, & Oikonomou, 2005), ultrasound (Mothibe, Zhang, Nsor‐atindana, & Wang, 2011), high hydrostatic pressure (Yucel, Alpas, & Bayindirli, 2010), and carbonic maceration (Liu et al, 2014), have been reported to reduce the initial water content and to accelerate the drying process, resulting in improved quality of the final dried products with minimal cost. Deformation of porous solid materials caused by ultrasonic waves reduces the diffusion boundary layer and increases the convective mass transfer in the fruit (Mothibe et al, 2011) Both PVOD and USOD have great potentials to increase the mass transfer of water loss and solutes gain during OD. To further explore the different effects of PVOD and USOD on intermittent microwave & air‐drying and quality attributes of ginger, we carried out our study to: (a) investigate the effects of PVOD and USOD on the intermittent microwave & air‐drying of ginger; (b) determine the effects of PVOD and USOD on dried ginger quality attributes, including color, antioxidant activity, and volatiles; and (c) explore the mechanism for PVOD and USOD affecting mass transfer characteristics and quality attributes, such as oxidase activity and microstructure
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