Kebitsamang Joseph Mothibe

Study of Drying Uniformity in Pulsed Spouted Microwave–Vacuum Drying of Stem Lettuce Slices with Regard to Product Quality

Drying uniformity, shrinkage, rehydration, and textural properties were measured to evaluate the quality of pulsed spouted microwave–vacuum-dried stem lettuce slices. Drying was carried out in a 5-cm (od) vacuum drying chamber at 7–10 kPa and microwave power level of 2.4 Wg−1. Pulsed spouted microwave–vacuum-dried products were found to be more uniform compared to those obtained in a conventional rotating turntable microwave–vacuum dryer. The pulsed spouted mode also resulted in dried stem lettuce slices with low discoloration and high rehydration capacity as well as high hardness after rehydration. The total drying time required for pulsed spouted bed microwave–vacuum-dried products was approximately 60 min, reduced by 50% compared to conventional rotating turntable microwave–dried ones.

Use of Ultrasound Pretreatment in Drying of Fruits: Drying Rates, Quality Attributes, and Shelf Life Extension

Evolving advanced pretreatment techniques such as application of ultrasound prior to fruit drying have attracted significant research and development efforts due to the rising demand for high-quality dried fruits at reduced operating costs. Drying processes can be destructive to the quality of fruits due to long drying times and high temperatures in some cases. The effects of ultrasound pretreatment on quality aspects of dried fruits and on drying kinetics are reviewed. Ultrasound is known for its varying effects on different fruits; however, it has been proved to greatly increase drying rates and hence reduce the overall processing time. Ultrasound pretreatment in distilled water as the media is an interesting concept when low-calorie dried fruits are required.

Microwave-Assisted Pulse-Spouted Vacuum Drying of Apple Cubes

Microwave-assisted pulse-spouted vacuum drying (MPSVD) of apple cubes was examined in a laboratory-scale apparatus. Aside from the drying time, structural and textural properties of the dried cube were measured. Results are compared with alternative drying techniques developed earlier in our laboratory. These include microwave-spouted bed drying (MSBD), microwave vacuum drying (MVD), and conventional vacuum drying (VD). Comparison is made in terms of the key quality parameters, viz. color, texture, apparent density, rehydration property, and sensory evaluation. Over the range of operating conditions tested, MPSVD apple cubes had the best color and significantly highest sensory evaluation score.

Effects of Ultrasound and Microwave Pretreatments of Apple Before Spouted Bed Drying on Rate of Dehydration and Physical Properties

Pretreatments of fruits before drying have been proposed as alternative way to improve the quality of dried fruits and reduce drying time. In the present study, the effects of different pretreatments (ultrasound and microwave) on apple cubes prior to spouted bed drying were investigated. Ultrasound pretreatment resulted in a great loss of sugars (25.7% for 15 min) compared to microwave pretreatment, where little rise in sugar content was observed (1.6% at 300 W). Ultrasound pretreatment improved the drying rate more than microwave pretreatment. Nevertheless, microwave-pretreated dried apples were harder and had lower water activity than ultrasound-pretreated ones. Low-frequency nuclear magnetic resonance (LF-NMR) analysis was used to study the water state and revealed that all pretreatments promoted redistribution of water among apple cell compartments and showed that most of the water lost was from the vacuoles. If dried low-calorie fruits are required then ultrasound pretreatment would be preferred.

Experimental Investigation and Mechanism Analysis on Microwave Freeze Drying of Stem Lettuce Cubes in a Circular Conduit

Stem lettuces are an important vegetable due to its nutritional characteristics. Stem lettuce was used as the model in this investigation. The ice-melting and freezing point temperature as well as distribution and migration of unfrozen water in stem lettuce cubes during the microwave freeze drying (MFD) step were studied using differential scanning calorimetry (DSC) and low-field pulsed nuclear magnetic resonance (LF-NMR). MFD of stem lettuce cubes was carried out in three circular conduits with different diameters (40, 55, and 70 mm) at a microwave frequency of 2,450 MHz. The total drying time for MFD was almost the same for all circular conduits; that is, approximately 4.5 h. This was less than the 7.0 h required for conventional radiation freeze drying (FD) in a 40-mm-diameter circular conduit and far less than the 20 h required for FD in a flat slab model. The microwave energy supplied and specimen temperature profiles during MFD were strongly influenced by the tissues, unfreezable water content, moisture distribution, and ionic characteristics of the samples. It was also found that the MFD process involving the conduits can be divided into three steps based on the amount and distribution of unfrozen water in the frozen samples and how heat is supplied in order to prevent corona discharge.

In vitro hypoglycemic and cholesterol lowering effects of dietary fiber prepared from cocoa (Theobroma cacao L.) shells

Three dietary fiber (DF) powders; soluble dietary fiber (SDF), insoluble dietary fiber (IDF) and total dietary fiber (TDF) were prepared from cocoa bean shells (CBS) by enzymatic treatment. These DFs were evaluated for their effects on glucose adsorption, glucose diffusion, starch hydrolysis, cholesterol binding, sodium cholate binding and oil binding capacities using in vitro model systems by simulating gastric intestinal conditions. The results showed that SDF generally exhibited significantly (p < 0.05) higher glucose adsorption capacity (GAC), α-amylase inhibition activity, cholesterol and sodium cholate binding capacity, but less significant (>0.05) glucose dialysis retardation index (GDRI) and oil binding capacity, when compared with IDF and TDF which both showed similar effects. Moreover, it was discovered that the three CBS dietary fiber powders contained intrinsic antioxidants (phenolic compounds). The study suggested that CBS could be an alternative cheap source of DF with additional benefits. Thus, CBS fibers could be incorporated as low calorie bulk ingredients in high-fiber diet to reduce calorie and cholesterol levels and control blood glucose level.