Research On The Mechanism Of Wheat Germ Stabilization By Microwave Treatment And Its Application

There is abundant resource of wheat germ (WG) in China. However, because of its high endogenous enzyme activity and rich unsaturated fatty acids, it tends to get acidificated and deteriorated rapidly, which seriously limits its development in deep processing industry. Microwave radiation has advantages of fast heating and less destruction of nutrients. Studies have confirmed that microwave stabilization of WG is better than the conventional heating methods. But the mechanism for microwave stabilization is not yet quite clear, non-thermal biological effect of microwave is also the focus of academic debate. This paper first analyzed the influence of microwave radiation on enzymatic characteristics and dynamics of WG lipase (LA) and lipoxygenase (LOX) based on an elaborate survey of WG resource distribution and quality difference in China, using a microwave reactor with simultaneous cooling and precise temperature control system, examining for possible non-thermal effect of microwave radiation on enzymes of WG. Then, a self-made continuous microwave processing device is used to study the effect of microwave on WG stabilization and its impact on WG quality. This paper intends to provide a theoretical basis for the application of microwave technology in WG stabilization. The main research procedures and conclusions are summarized as follows:1. Based on an acknowledgement of the basic status of China's WG resource distribution, commercial WG samples were collected from 9 domestic large-scale wheat flour enterprises of different provinces, nutritional compositions were determined and quality differences were analyzed. The result shows that the average acid value of commercial WG is 22.80mgKOH/g, the highest is up to 60.4mgKOH/g in summer. The average water content of WG is 12.10%, protein content is 33.07%, crude fat is 11.12%,α-VE content is 0.23%, total colonies is 2.65x104 cfu/g. WG resources are mainly concentrated in Shandong, Henan and Hebei province, et al.2. Two enzyme activity determination methods were set up, a copper soap colorimetric method to faster determine LA activity of WG, and a modified spectrophotometric method to faster determine the activity of WG LOX. By the first method, the optimum conditions of LA determination in the experiment were:adjusted WG water activity to 0.84, added 0.75ml olive oil to per gram WG, incubation for 4h at 55℃, finally the amount of free fatty acids produced was determined by copper soap colorimetric. By the second method, the optimum conditions of assaying LOX activity were:a mixed solution of 2.0 mL 0.1 M phosphate buffered solution (pH=6.5) and 200μL substrate solution (the concentration of linoleic acid was 1.25-2.53mM and Tween-20 was 0.08-0.12%(w/v)). Detection wavelength was 234 nm at 25℃3. The influence of microwave radiation on protein structure and activity of WG LA was analyzed, using a microwave reactor with a simultaneous cooling and precise temperature control system. Possible conformation change at different temperatures induced by high-field microwave radiation was detected by CD and fluorescence spectroscopy. The result shows that microwave radiation at 10-23℃has no effect on LA activity and its spatial structure, indicating that microwave radiation has no non-thermal effect in this temperature range; Microwave radiation at 60℃can completely inactivate LA activity within 20s (150W/5mL). The content of a-helix decreases while P-sheet content increases, indicating a change of LA secondary structure. Fluorescence spectroscopy suggests that the microenvironment of Tyr and Trp in LA changed. Compared with conventional heating, microwave radiation can inactivate LA activity at a lower temperature (45℃) within a shorter time (20s), and led to a greater change in LA spatial structure. Microwave radiation can achieve a fast-passive LA activity effect at low-temperature.4. The influence of microwave radiation on enzymatic characteristics of crude LOX from WG was analyzed, using a microwave reactor with a simultaneous cooling and precise temperature control system. The result shows that microwave radiation at 10-23℃has no effect on LOX activity or its kinetic parameters, which indicats microwave radiation in this temperature range has no non-thermal effect; Microwave radiation at 60℃can completely inactivate LOX activity within 20s of 150W/5mL, and lead to a change in enzyme kinetic parameters, while it remains 20% of its full activity after conventional heat treatment for 30 min at the same temperature. The above result further confirms that the effect of microwave radiation for LOX inactivation is better than conventional heating method.5. By using an improved design of continuous microwave stabilization equipment, the optimal microwave stabilization process for WG was studied, and the mechanism of WG microwave stabilization was preliminarily revealed. When WG on-line processing capacity is 20kg/h, the optimal conditions for microwave stabilization of WG are: microwave power 4kW, processing time 5min and air volume 60 Nm3/h. In such condition, LA activity in WG decreases to only 16.11% of that of control group. After 60 days enhanced storage, the minimum acid value of microwave stabilized WG only increases by 6.56%, and maintains a original sensory quality. The main mechanism for microwave stabilization is to lower the water activity of WG and inactivate majority of LA activity, thus effectively inhibit lipid enzyme reaction. On the other hand, a certain intensity of microwave radiation can completely inactivate LOX, avoiding the enzymatic oxidation of unsaturated fatty acid. In addition, microwave radiation also kills part of the microorganisms corruption, and because of the decline in water activity of WG, the proliferation of microorganisms are under control. Thus, the rancidity process of WG slows down.6. The impact of microwave radiation on WG quality was analyzed, including WG microstructure and starch pasting characteristics during microwave processing. Scanning electron microscopy and micro-viscometer were used. The results show that the majority starch granules in microwave treated WG were still intact observed by SEM graph, accompanied by a clear outline. However, with the microwave radiation intensity increasing, melt-like films appeared indicating that some oil spilled and starch gelatinized, because of the original space structure changes between fat, protein and starch induced by microwave radiation; The gelatinization temperature of microwave treated WG rose to 59.3-65.4℃. Depending on their extent of pre-gelatinization, different WG samples gave significantly different peak viscosity (349-466BU) and break down values (69-87BU). The fatty acids composition and content of microwave treated wheat germ were the same as untreated wheat germ, the VE content did not change significantly as well.