Quality analysis of ultrasonically dried strawberry and comparison to conventional drying methods

Conventional, hot air drying is one of the most commonly used drying methods in the food industry for its simplicity, high productivity, and low capital cost. However, hot air drying has many limitations surrounding energy requirements, processing efficiency, and costs coupled with significant food quality degradation (i.e., physical, chemical, and nutritional properties) compared to the starting food material. Novel and alternative drying technologies which do not use thermal energy are thought to have the potential to reduce negative quality changes. Ultrasonic energy represents a novel and alternative technology for drying to improve the resulting food quality due to its non-thermal approach. Ultrasound is a type of acoustic energy ranging between frequencies of approximately 16 kHz and 100 MHz, which is just above the audible frequency range of the human ear. An ultrasonic transducer is used to convert electrical energy into mechanical energy via air vibrations. Ultrasonic waves are characterized by mechanical properties that are capable of achieving a processing goal, such as moisture removal, without applying thermal energy. This research is an investigation of drying of strawberries using a novel ultrasonic drying method and compares the quality outcomes to hot air drying (60ºC) and freeze-drying. Various chemical and physical properties as well as stability parameters of dried strawberries are measured. Chemical and nutritional properties measured include total phenolics, antioxidant capacity, and ascorbic acid. Physical properties measured include color changes, browning and rehydration ratio. Stability parameters measured include glass transition temperature and water activity. Additionally, drying curves and drying rate curves were generated to compare ultrasound drying and hot air drying in terms of total drying time, drying rate, and final moisture content. The results demonstrate that ultrasound drying accelerates the drying process and reduces drying time of strawberries compared to hot air drying. Hot air drying time was 135 minutes while ultrasound drying time was approximately 30 minutes; ultrasound reduced the drying time of strawberries by approximately 78%. In terms of physical quality, ultrasound resulted in a higher rehydration ratio (3.07) than hot air drying (2.84); US resulted in higher total color change (20.1) compared to hot air dried total color change (15.9). In terms of chemical and nutritional qualities, the results signify that all three drying methods result in some degree of loss compared to fresh strawberries. Freeze-drying results in the highest retention of total phenolics (2237 mg GAE per 100 g DM), antioxidant capacity (percent inhibition of DDPH radical at 3 concentrations), and ascorbic acid (102.8 mg AA per 100 g FW). Between hot air drying and ultrasound drying, there were no significant differences observed in retention of total phenolics (HAD: 1764 mg GAE per 100 g DM; US: 1793 mg GAE per 100 g DM), antioxidant capacity, and ascorbic acid (HAD: 79.7 mg AA per 100 g FW; US: 79.3 mg per 100 g FW) of strawberries. Lastly, ultrasound drying resulted in strawberries with a higher final moisture content (9.07% wb; 0.099 db), higher water activity (0.4077), and lower but not significantly different glass transition temperature (-5.130ºC) compared to hot air drying (MC 7.43% wb, 0.08 db; aw:0.3648; Tg: -1.047ºC). Overall, the ultrasound drying technology herein exemplifies promising potential for accelerating drying without thermal energy input and without contributing to additional quality degradation compared to conventional, hot air drying. However, limitations and inconsistencies are encountered due to the novel nature and rudimentary design of the technology and must be overcome for future adoption of the methodology and technology. The major shortcoming in this research is related to the use of wet-mode ultrasonic transducer box to dry food which is a dry-mode operation. As a result, the ultrasound transducer box was often overheated, and drying had to be stopped to allow the transducer to cool. Furthermore, suitability and compatibility of solid food samples must be considered in the future to interpret results of efficiency, productivity, and overall food quality using ultrasound. While ultrasound clearly demonstrates enhancement to drying i.e., accelerates drying and reduces drying time, further development of design features is needed to improve ultrasound drying to result in consistency and continuity. Process and technological optimization is first recommended in order to interpret the effects of US on quality of strawberries in this research and to interpolate the quality results from this research to other food products. A continuous and consistent technology is perceived as a better reflection of ultrasound efficiency, potential for quality improvement, and feasibility for industrial scale-up.U of I OnlyAuthor requested U of Illinois access only (OA after 2yrs) in Vireo ETD syste