Investigation On The Growth And Stability Of Heat Induced Rice Starch Radicals Treated By Microwave

Microwave heating technology is widely applied in various field of food processingindustry. The behavior of starch in microwave field has become a main subject ofinvestigation in many academic study and application research. Up until now, a fundamentalamount of researches have been conducted concerning starch granule shapes, gelatinizationand other physical chemical properties during microwave heating. Analyzing changes instarch material after microwave treatment through the angle of radicals will help develop theapplied technique of microwave food processing. It also may help us better understand themechanism behind the property changes in starch during microwave heating and will becontributive to the establishment of evaluation system of microwave processing. In this paper,we selected rice starch as the object. The quantitative development, components and evolvingsituations of radicals formed in rice starch treated with microwave were investigated. Wediscussed the type of starch radicals and their chemical structure evolvement both undertreatment and during storage. Also we compared the relative quantity of radicals aftermicrowave treatment and oven heating under the same temperature, aiming to search for themajor mechanism of radical generation progress under microwave irradiation. The mainresults of the study were listed as follows:(1) The response of rice starch radicals in microwave electromagnetic field andcomponent analysisThe radicals generated in microwave field were determined using electron paramagneticresonance (EPR). The result indicated that starch radicals were carbon centered. The EPRspectra of starch radicals treated by microwave showed that the quantity of radicals increasedwhile increasing the power and treating time. Meanwhile, starch of a much lower wateractivity (0.4) produced more radicals after microwave heating and the change of microwavepower had more impact on the quantity of radicals. By using simulation software, three maincomponents of rice starch radicals were identified and separated from the signal. Thecontribution of each component changed while increasing power and prolonging treatingtime.(2) Microwave effect on the propagation and annihilation of components in rice starchradicalsThe relative quantity and content contribution of radicals during heating and storagewere assessed. Experiments were conducted to evaluate the stability and annihilating patternof the radicals. The results suggested that most of the radicals were C1centered with a smallamount of C6centered radicals. Under constant power of microwave, some of the starchmolecule might dehydrate and C1centered could transform into another chemical structure.The C6centered radicals were stable with long half-life period. The C1centered radicals wereless stable after double bound formation between C2-C3. The initial EPR signal appeared tobecome stronger right after treatment, but slowly decreased after one day.(3) Effect of microwave on chemical bonds of rice starch radicalsFourier transform infrared(FT-IR) spectroscopy showed that after microwave irradiation, no changes occurred in chemical groups in starch. Raman spectroscopy indicated thatvibrational changes occurred in the starch glycosidic bond, glucose ring skeleton and otherchemical bonds during treatment. The results suggested C1and C6centered radicals formingin starch and the chemical structure transformation of C1centered radicals.(4) The study of microwave overheated rice starch radicalsUsing thermo gravimetric analyzer(TGA), we defined the microwave treated starchsample with weight loss less than50%during250℃~350℃as overheated starch. From thephotos took by scanning electron microscope(SEM), granule properties of rice starch indifferent stages of microwave process were examined. A large amount of granules deformedseverely with some transformed into a structure of layers and pieces in overheated starch.Information of functional groups provided by FT-IR spectrometer suggested a small amountof carbonyl group were produced in the overheated starch. EPR result proved the radical wasstill carbon centered. The signal intensity of overheated starch was two orders of magnitudelarger than that of non-overheated starch. Using simulation software, a new component wasfound in the radical signals. During storage, the annihilation progress went through a veryshort period fast decreasing, then a period of steady growth and finally the stage of slowdecrease.(5) Comparison of microwave and oven induced rice starch radicals under constanttemperatureThe single-mode microwave equipment was programmed to reach and maintain a certaintemperature in order to analyze the effect of microwave heating and conductive heating on thequantity of starch radicals. when treated in microwave oven, results indicated the samplecould be uniformly irradiated when the vibration frequency was set above50%. Underconstant temperature of120℃, few radicals were produced after1h of conductive heating.Above100℃,1h of microwave treatment induced much stronger radical signal than1h ofconductive heating under constant temperature.Microwave heating could induce stable radicals in starch with low water activity. Therewere components with different chemical structures in starch radicals. Some of thecomponents had the ability to transfer into each other. The quantity and structure of theradicals were decided by microwave power, heating time and water activity of the starch.Under consistent temperature, microwave heating might generate more starch radicals thanconductive heating.