## 1 Introduction  
Nowadays, rehydrated foods are very much in consumer demand because of their long shelf life without refrigeration, low contamination, volume decrease as well as ease of consumption and worldwide distribution. Rehydration is a complicated process utilized in food and is directed to bring back its original properties. The dried products are usually immersed in water during their use in food preparation. Rehydration characteristics and the retention of sensory qualities depend on structural changes after drying. Short drying time, high energy efficiency and maintaining the quality after drying are desirable. Product quality, physical appearance and rehydration time for dried products have therefore been improved to fulfill the needs of consumers. Examples of products from rehydration research are lyophilized rice [1] and vegetable and fruit powder beverages [2]. Drying technology can preserve foods by virtue of the evaporation of water and, hence, reducing the moisture content. There are several review drying techniques offered to fruits and vegetables [3] and the food processing industry [4]. Nowadays, drying is a way to add value to modern food products, not only a way to preserve foodstuffs. There are various drying techniques applied to agricultural products such as okara [5], beans [6], honeybee honey powders [7], red beetroot [8], and medical plants [9]. Each drying technique has unique advantages, making them candidates in the production of rehydrated foods. Convective drying produces dehydrated products with an extended shelf life. Heat is transferred from the surface into the food via hot air from a steam coil or fuel oils. With the circulation speed in the range of 0.5-5 m/s, the hot air will flow through the food usually prepared in the thickness of 2-6 cm and placed on a tray. If the temperature is higher than 70 \({}^{\circ}\)C, the dehydration rate may be too high. This technique is widely used for producing dehydrated vegetables and fruits because of the relatively low price and reasonable maintenance cost. However, the heating is unlikely to be uniform in various positions in the oven because of the uneven air flows. The drying characteristics of the sample are investigated, such as kiwifruits [10] and Mexican serviceberry fruit [11]. In addition to the dehydration, rehydration characteristics of green pea [12], pumpkin [13], and carrot [14] were reported. Freeze drying is a dehydration process by the sublimation of a frozen product. The operations at low temperatures reduce microbial growth or impede a spoilage reaction to preserve the final product [15]. This method is suitable for foods that are sensitive to heat loss and nutritional value, giving rise to high quality products with good recovery properties. The drying characteristics of cherry [16], stink bean powder [17], stink bean [18] and acorn [19] were investigated. In addition to dehydration, the rehydration of strawberry [20] and mussel [21] were also characterized. Freeze drying is widely used to produce commercial rehydrated foods but has a high cost compared to conventional drying [22; 23]. Microwave drying is based on dielectric heating by electromagnetic waves. In addition to a high drying rate within a short time, microwave drying tends to preserve the nutritional values of foods. It is a technology that is suitable for drying raw materials with low humidity. Microwave drying has already been applied to various products, i.e., fish [24], durian [25], instant noodles [26], _Gastrodia elata_ Blume [27], potato cubes [28], amaranth leaves [29], bitter melon [30], and sweet potato chips [31]. The major drawback of microwave drying is its nonuniform heating, particularly the overheating at the sample edges [32]. To overcome these shortcomings, microwave drying has been combined with other drying methods. Microwave vacuum drying has received much attention because of its high energy efficiency, a significant reduction in the drying time, and a superior quality of dehydrated products [33].