Optimization of microwave vacuum drying of okra and the study of the product quality

Abstract

Microwave vacuum drying (MVD) was reported to be an efficient method to dehydrate vegetables. In this study, fresh okra was dehydrated by this technique. Orthogonal design was used to optimize the drying parameters (microwave power, vacuum level, and loading density) to achieve high drying rate and product quality. Microwave power (500, 700, and 900 W), vacuum level (45, 60, and 75 kPa) and loading density (3, 4, and 5 kg/m 2 ) were set up as the treatment factors. Drying rate, energy consumption, rehydration rate, color change, total phenol, and flavonoid contents were evaluated as the responding quality parameters. The optimal drying parameters of okra were identified as 900 W of microwave power, 60 kPa of vacuum level and 5 kg/m 2 of loading density. Fresh okra was then dehydrated under MVD, hot air drying, and vacuum freezing drying. Results showed that microwave vacuum drying method was most efficient, with the drying rate as high as 2.36 ± 0.05 g/min. The contents of total phenol and flavonoid in the products of microwave vacuum drying had no significant difference from that of vacuum freeze drying and were significantly higher than that of hot air drying. Practical applicationsMVD is an efficient and energy-saving technology, compared with the conventional hot air drying or vacuum freeze drying. Meanwhile, it could retain the color, flavor, nutrients, and bioactive compounds in the product, thus it is a promising method to dehydrate vegetables and fruits. Okra is rich in nutrients and is of increasing demand worldwide. Whereas it tends to spoil even at refrigerated temperature, so that a practical drying art is essential to be developed. This study proved the feasibility and efficiency of dying okra by MVD, and worked out an optimized drying condition under MVD, which could be a guide in okra processing industry.

Influence of three drying methods on physicochemical properties of okra (Abelmoschus esculentusL.) powder

“…The dried okra exhibited TPC in the range 932.07–1,871.61 mg GAE/100 g dried sample. The TPC of fresh okra in this study corresponded to the results on the investigation of TPC of fresh okra and was higher than tomato and green bean (76.9 and 35.5 mg GAE/100 g dried sample) (Xu et al, 2018). Denaturation of polyphenols was occurred during the heating process, inhibiting its activity by the increasing activity of polyphenol oxidase that was enhanced by the increasing temperature (Zhang et al, 2017).…”

“…Thermal processing has the possibility to alter phytochemicals by affecting the integrity of the cell structure, resulting in component movement, which further caused leakage or disintegration by numerous chemical processes involving enzymes, light, and oxygen (Youssef & Mokhtar, 2014). However, prolonged use of the high temperature used during the drying process leads to the lower total phenol and flavonoid content in the product due to the decomposition of the phenolic acids and flavonoids by the long exposure to the heat (Xu et al, 2018; You et al, 2015), as shown by the results of okra drying using the microwave vacuum drying method. The phenomena of selective heating are related to microwave heating.…”

Influence of drying processes on bioactive compounds profiles, hydroxymethylfurfural, color parameters, and antioxidant activities of Tunisian eggplant (Solanum melongenaL.)

“…Kinetic models for degradation and formation of bioactive compounds were applied to relate and explain the changes in total anthocyanins, ascorbic acid, total phenols, total flavonoids, color parameters (Δ E ), HMF, enzymatic browning index, and antioxidant activities that occurred to the dried eggplant slices during drying under different drying processes (convective, infrared, ultrasonically assisted convective, freeze‐drying, and microwave drying processes). Zero, first, and second‐order kinetics models have been used to describe degradation reactions in foods and the Weibull model has been shown to fit the degradation of chemical compounds in strawberries and okra (Xu et al., 2020), sensory and ascorbic acid changes in melon (Amodio et al., 2013), and color changes in dried salak fruit (Ong et al., 2011). The cumulative distribution of the Weibull and combined distribution functions may be rewritten as follows (Amodio et al., 2013; Chouaibi, Mejri et al, 2019; Xu et al., 2020):…”

Influence of microwave vacuum drying process parameters on phytochemical properties of sohiong (Prunus nepalensis) fruit

“…Microwave vacuum drying also reduces the amount of heat required to evaporate water from food material compared to drying under atmospheric pressure (Nahimana & Zhang, 2011). Therefore, heat-sensitive food and agricultural products could be dried at relatively low temperatures due to the coupled effect of microwave and vacuum (Chauhan & Srivastava, 2009;Mousa & Farid, 2002;Xu et al, 2019).…”