Microwave Heating of Oils and Oil-Rich Food Products

Xu Zhou

doi:10.7273/000006292

Microwave heating is an efficient method for delivering energy to foods. Since the first invention of microwave ovens in 1947, microwave heating units for domestic and industrial food applications have been powered by magnetrons. But microwave ovens powered by magnetrons have several drawbacks, including inability to control microwave frequency and phase, high operation voltage, and short service time. Over the past decade, solid-state (SS) microwave generators based on semiconductor transistors have been developed for the telecommunication industry. This research studied the unique advantages and control features of SS generators for effective implementation in the food industry.We developed a method to measure and compare frequency spectrum of microwaves from magnetrons and SS generators and studied the influence of frequency spectrum on heating pattern and energy coupling. Results show that the microwave peak frequency and bandwidth of magnetrons varied depending on the food and food position, and varied between individual ovens of the same model, leading to uncertainly in heating patterns of foods. In contrast, the SS generator provided microwaves not only at the set frequency but also within a narrow band, regardless of food. The stable and narrow frequency spectrum of the SS generator could support a single standing wave pattern in multi-mode cavities, and consequently predictable heating patterns in foods. Moreover, adjusting SS frequencies minimized power reflection, and using complementary frequencies improved heating uniformity. It is important to effectively heat vegetable oils in several food applications, such as frying and cooking. Traditional heating methods, including steam boilers powered by coal, fuel, or natural gas, contribute significantly to carbon emissions. Transitioning to electrified heating methods, such as microwave heating, is imperative to reduce CO2 emissions. Thus, another main objective of this research was to systematically study microwave heating of oils and oil-rich foods. First, our experiments and computer simulation show that under the same microwave conditions, vegetable oils heat more rapidly than water due to a stronger electric field in the oils. Second, we measured the dielectric properties of peanut oil, engine oil, and glycerol, and evaluated the influence of moisture and double bonds on microwave heating and dielectric property. Results show that the glycerol part in vegetable oil is the major contributing factor for microwave heating. Furthermore, we studied the influence of microwave heating at two frequencies (2.45 and 5.85 GHz) in frying and post-frying heating on oil reduction in French fries. Results show that post-frying was the key to oil reduction. Compared to 2.45 GHz, microwaves at 5.8 GHz could produce French fries with lower oil content. This dissertation provides guidance for designing microwave heating systems based on SS generators and supports their implementation in food processing operations. Also, this dissertation bridges the knowledge gap regarding microwave heating of oils and demonstrates opportunities for utilizing microwaves in electrifying industrial heating of vegetable oils.

Microwave Heating of Oils and Oil-Rich Food Products

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