Removal of moisture from biological materials, popularly called drying, has numerous benefits, including ease of handling due to reduction in bulk, resulting in reduced handling costs. Moreover, drying prevents microbial growth and spoilage. Though different drying techniques share a common objective, conceptually they are different and require modification/adaptation based on the biomaterial that is dried. There have been significant scientific advancements in the past century in the field of drying of foods, fibers, and fuel. This article will provide an extensive review of various drying pretreatments and different hybrid drying techniques, including supercritical and fluidized bed concepts, microwave drying, superheated steam drying, and heat pump drying to meet tomorrow's food and energy needs.

Date fruit is a good source of bioactive compounds and natural sugar. It has the potential to be utilized as a substitute for added sugar. Although several forms of dates—such as fresh dates, date paste, and date syrup—are available in the market and used for different applications, free-flow date powder would be highly beneficial in improving shelf-life, ease of handling, and blendability with various foods prepared domestically and in industry. The objective of this study was to produce date powder in a pilot-scale spray dryer and determine its physicochemical qualities. Date powder was produced at eight processing conditions (2 carrier agents (maltodextrin (MD) and gum arabic (GA) × 2 inlet air temperature (150°C and 170°C) × 2 feedstock flow rates (25 ml/min and 40 ml/min)). To the carrier agent was added at 0.4 kg per 1.0 kg of date fruits (dry weight basis), and the feedstock to the spray dryer was prepared at 20% concentration. Date powder was obtained in all eight treatments. Color (L*a*b* values), moisture content, bulk density, wettability, solubility index, hygroscopicity, microstructure analysis (using scanning electron microscope (SEM)), and total phenolic compounds were determined for the spray-dried date powder. The physicochemical characteristics of date powder varied significantly with respect to the processing conditions. Although physical properties of date powder were significantly affected by the carrier agent, there was no difference in total phenolic compounds between date powders produced with MD and GA. An SEM study revealed that date powder produced with MD had smooth, regular-shaped spherical particles but with severe agglomeration. Date powder with GA had relatively smaller particles of irregular sphere with dented surfaces.

In this study, the fresh Moringa oleifera pods (Drumsticks) were dehydrated by microwave-assisted hot air drying (MAHD) and conventional hot air drying methods. The samples were dried at three different temperatures, viz. 50, 60, and 70°C, with and without the application of microwaves. Microwave power density of 1 W/g was used for the MAHD. The final moisture content was targeted as 13% wb. The drying curves and drying rate curves were plotted and compared. The kinetics of drying obtained experimentally were correlated with the Page equation. The constants K and N of the Page equation were determined to predict the drying kinetics for varying conditions. The quality attributes, namely, color, rehydration ratio, and volatile compounds, were analyzed and compared with that of the fresh Moringa pods. The volatile compounds were analyzed using z-Nose (an electronic nose; Electronic Sensor Technology, Newbury Park, CA) and bioactive molecules were analyzed using gas chromatography–mass spectrometry. The results showed that the MAHD method had significantly (P < 0.05) reduced the loss of volatiles during drying. Also, MAHD preserved most of the bioactive molecules when compared to the conventional hot air drying method. The samples dried at 50°C using MAHD were the best in terms of all of the quality attributes tested in this study. Also, the results established that the z-Nose can be used as a quick and inexpensive means to assess the effect of different process parameters on the aromatic quality of the product and quantitatively classify quality based on aroma.

In order to discourage the use of chemicals in raisin processing, the effect of microwave and pulsed electric field (PEF) pretreatments on the drying rate and other quality parameters like color, total soluble solids, bulk density, appearance, and market quality were compared with that of chemically pretreated raisins dried at 65°C. The untreated and pretreated samples had a statistically significant difference in drying rate (P < 0.05). The drying rate of chemically pretreated raisins was the highest when compared to others. The results showed that the PEF and microwave-treated samples had a significantly high Total Soluble Solids (TSS), along with good appearance and market quality.

Drying of pineapple by microwave-vacuum (MW-V) with osmotic dehydration (OD) as a pretreatment was studied. Central composite design was used for the independent variables sugar concentration (40, 50, and 60 °Brix), immersion time (0, 3, and 6 hours) and product temperature in the microwave cavity (50, 60, and 70°C). The dependent variables were moisture content, shrinkage, rehydration coefficient, visual quality, color, and texture. The osmotic dehydration pretreatment was also evaluated with respect to weight reduction (WR), water loss (WL), and solid gain (SG). Maximizing moisture loss was the main objective of OD pretreatment for various combinations of sugar concentration and immersion time. OD preserved the color of the samples while drying. In the complete process composed by OD and MW-V, only the influence of temperature of the product on the drying rate was statistically significant (P < 0.05), whereas OD reduced shrinkage and improved rehydration capacity. The best visual quality was obtained when the independent variables were diminished. OD also significantly (P < 0.05) affected the texture of product producing softer, less surface-hardened product compared to untreated ones.