Zhenfeng Li

Microwave Drying of Corn (Zea mays L. ssp.) for the Seed Industry

Microwave drying of corn (Zea mays L. ssp.) was conducted in this study and its germination was tested after drying. The objective of the test was to determine an optimum microwave drying method for corn seeds to achieve maximum germination and minimum drying time, as well as to obtain the maximum temperature that can achieve the best results without reducing the viability. The different methods used for drying included constant temperature drying at 30, 40, and 50°C; intermittent power microwave drying; and fixed power microwave drying at 2, 3, 3.5, and 4 W/g. The drying was started with an initial moisture content of 33% (wb) and ended with a final moisture content of 16% (wb). In the constant temperature method, the microwave power was automatically controlled to keep the temperature constant throughout the drying period. A control cycle of 20 s was used for intermittent drying in which the microwave generator was on for 12 s and off for 8 s throughout the drying process. In fixed power microwave drying, the power was constant throughout the process. The germination tests were conducted on all samples with the rolled blotting paper method. The minimum drying time was reached with 4 W/g fixed power where 0% germination resulted. The optimum method for drying of corn seeds for maintaining a high germination rate was 4 W/g intermittent power microwave drying, which took 110 min to dry from 33% moisture content to 16% (wb). The germination percentage of 4 W/g intermittent power was 93.3%. The maximum permissible temperature of corn seeds when subjected to intermittent microwave power resulting in maximum germination was 67°C.

Control of Microwave Drying Process Through Aroma Monitoring

Aroma variation during food processing is of great concern tofood industry. The obstacles for an online aroma monitoring and control system are the speed and convenience of the aroma detection. In this study an ultra fast gas chromatograph (zNose™; 7100 Fast GC Analyzer, Electronic Sensor Technology, Newbury Park, CA), was employed to detect food aroma. A real-time aroma monitoring system to control a microwave drying process was designed. Detected aroma signals were analyzed with a fuzzy logic algorithm to dynamically determine drying temperatures during the drying process. An automatic phase controller was used to adjust the microwave power level to meet the temperature requirement. Carrot and apple were used as samples to test the system. Based on the fuzzy-controlled temperature profiles, simple linear control methods were further developed to imitate fuzzy logic control where the assistance of zNose™ was unnecessary. Results have shown that the newly developed control strategies can improve the quality of products undergoing microwave drying in terms of aroma retention.

Control of microwave drying process through aroma monitoring

Carrot cubes were dried in a microwave oven and the aroma was monitored and controlled. An electronic nose was used to detect carrot aroma. Drying temperatures were first set to 50°C, 60°C, 70°C, and 80°C. Six typical peaks were recorded every 4 minutes for all the three replicates. Color, aroma and sensory evaluation were performed after drying. An untrained taste panel was included in the sensory evaluation. Samples could be dried in a short time at high temperatures but the interior of some cubes was burnt. Drying time at low temperatures was much longer and much more aroma was lost. For fixed temperatures the best results were achieved at 60°C. A fuzzy logic control system was then designed and employed to control the whole drying process, using carrot aroma peaks as variables. Based on a fuzzy logic control profile, a simple linear control method was developed. It was shown that the carrot color and aroma were intact with these new control strategies and less time and power were consumed.

Speed Control in Microwave Drying Of Apple

Microwave drying of apple at constant temperatures of 50 – 80 °C follows typical drying curves. The middle stage of the drying process shows a faster drying speed and accelerated moisture evaporation. Meanwhile, more flavors are lost, surface color is degraded, and charring often occurs. To improve the drying effects, drying curves were controlled and changed in this study. The drying curve was linearized by automatically varying drying temperatures in the middle stages. A microwave drying system with the ability of temperature, power, and moisture control was developed for this purpose. The controlled drying process therefore led to an optimized temperature profile. To simplify the drying methods, apples were further dried with the obtained temperature profile, while drying curves were online monitored but not controlled. It was proved that slowing down the drying speed in the middle stage could improve the product quality on color, flavor, and overall appearance, while the drying time and energy consumption were still acceptable.

Optical Temperature and Power Control in Microwave Drying

A microwave drying system, which can automatically and continuously adjust microwave power, control sample temperature and measure sample mass, was developed and used in apple drying. At three drying temperatures, i.e., 75oC, 65oC, 55oC, applied microwave power was recorded and analyzed, and a relationship of the power with respect to drying rates was built. With this relationship, apples were further dried at the same three temperatures but using variable power during drying process based on the online obtained drying rates. A better temperature control was achieved and less energy was consumed with this method. Depending on the resulted power profiles, a linear and a three-step power control methods were further developed to simplify the control system. With the developed methods, mass measurements were no longer required and power control could be easily implemented, while temperature control and energy consumption were still acceptable.

Design of a real time aroma monitoring and control system

The purpose of this project was to design a real time aroma monitoring system to control microwave drying processes. Aroma profiles were measured with an ultra fast gas chromatograph (zNoseTM, EST) and expressed as retention time and peak area. These signals were then analyzed using a fuzzy logic algorithm to determine drying temperatures. Phase control was used to adjust the microwave power level. Carrot and apple samples were used to test the system. The system was shown to have the capacity to successfully fulfill the desired control functions and lead to better retention of aromatic compounds in the dried materials.

Assessment of a Chinese Spirit Using Electronic Sensory Tools

The flavors of six Chinese spirits were evaluated using a zNose®, a fast GC analyzer. The first derivative profile, retention time, and peak area were used to discriminant samples at various grades. Principle component analysis (PCA) and canonical discriminant analysis (CDA) were adopted for data analyses, where 100% classification was achieved. A network analyzer was also included in this study to attempt the possibility of estimating the concentration of alcohol by dielectric properties. Dielectric constant and loss factor of 0-100% ethanol-water mixtures in 10% interval and 42%, 50%, 53%, 62%, 64% ethanol-water mixtures, as well as spirits samples were measured and compared.