|Abstract:||Drying is one of the oldest and most complex unit operations in the food industry. Four billion tons per year of food are produced for human consumption worldwide, and about 20% of the products are dehydrated. Hot-air drying is the most widely used drying method for preserving fruits and vegetables. It is, however, a time-consuming and energy-intensive process. Elevated temperatures and long drying times produce undesirable quality changes in the final products. Numerous efforts have focused on improving drying efficiency and product quality by developing new drying technologies and new pretreatments for the raw materials. However, few researchers have evaluated the combined effects of different pretreatments and operational drying conditions, making it difficult to replicate their results on a commercial manufacturing scale. Since the drying conditions (air velocity, humidity, and temperature) are variable within the same location, the reports have shown inconsistent results in the drying kinetics of fruits and vegetables.
Researchers have studied the drying kinetics of fruits and vegetables with various drying temperatures, air velocities, sample sizes, and pretreatments, but very few attempts have looked into the effects of the intake air humidity on the drying kinetics, nor did many groups evaluate their hot-air drying results over a significant range of temperatures, air velocities, or intake-air humidities. Furthermore, the previous studies often used only one food type to evaluate their drying process. During drying experiments, the nature of the sample is important for evaluating and explaining the drying behavior and kinetics. The overall goal of this study was to develop a new hot-air dryer platform capable of modifying drying parameters such as the temperature, velocity, and relative humidity of the intake air for the purpose of evaluating the drying of food samples with different microstructures. The effects of ultrasonication as a pretreatment were also evaluated.
In the first study, the use of ultrasonication as a pretreatment for the enhancement of the hot-air drying of selected fruits and vegetables and the hydration of barley grains was evaluated. The design of the ultrasonication pretreatment took into consideration its potential inclusion in a typical industrial line. The ultrasound pretreatment was shown to reduce the thermogravimetric analyzer (TGA) drying time by up to 22%, yielding products with lower equilibrium moisture content (which can improve shelf-life), and increasing the hydration rate and equilibrium moisture content of hydrated grains.
The second study was undertaken to examine changes in the drying and drying rate curves, moisture diffusivity, and microstructures of fresh and spoiled plant tissues. Currently, there is no information available for understanding the differences between the drying of edible tissues and that of food waste materials. Significantly higher yeast and mold counts (log CFU/g) were found in the spoiled tissues, causing tissue softening and deterioration. The TGA drying curves showed that spoiled plant tissue dried significantly faster (up to 30%) than its fresh counterpart, with a higher effective moisture diffusivity. The results demonstrated that drying spoiled plant tissues would be less time consuming and potentially less energy intensive than drying fresh ones.
In the third study, the effects were evaluated of hot-air drying (AD), freeze drying (FD) and refractance-window drying (RWD) on the color (L*, a*, b*, and E), glass transition, specific heat, and surface morphology of 4 fruits and 3 vegetables. The quality retention in the dried fruits and vegetables was found to be product and drying-method specific. The FD products exhibited better quality retention than the other drying methods. The AD fruits and vegetables displayed significantly less quality retention in most of the quality indexes measured in the study. The AD kale and spinach had similar but darker colors than the control, indicating chlorophyll degradation. RWD also yielded relatively good product quality.
In the fourth study, we developed a new single-channel hot-air drying platform with the capacity to control air velocity, relative humidity, and temperature to evaluate the drying kinetics of apples, strawberries, and chickpeas, with and without an ultrasound pretreatment. Four thin-layer models were used to examine the drying data. Changes in selected quality attributes were evaluated to examine the effects of drying under different hot-air conditions. The results showed that an ultrasound pretreatment enhanced the drying of apples, as shown by a shorter drying time, higher drying rate, higher moisture-diffusivity value, and less quality degradation. The thin-layer models provided a good fit for the drying data. The effects of the conditions on the drying kinetics were product-type dependent. Relative humidity played an important role as well.
In summary, the newly developed single-channel hot-air dryer platform established an important research tool to accurately replicate the drying and atmospheric conditions normally encountered in a commercial manufacturing facility. This research tool allows accurate control of drying temperature, air velocity, and relative humidity of the intake air, and the evaluation of the drying kinetics of different products. This equipment can be used to create drying guidelines for specific products by establishing the highest final-product quality and the highest drying efficiency at specific drying parameters. Furthermore, drying conditions can be recommended to commercial manufacturers depending on their location, the time of year, and the desired quality of the final product.
In addition, the information obtained during the design, fabrication, and test runs of the single-channel hot-air dryer was used to develop a multichannel hot-air dryer platform, which can dry up to 8 samples at the same time, with 4 independent sets of drying parameters.