Studies On The Dynamics Of Infrared Drying And Its Effects On Rice Storage Quality

The total production of rice in China exceeded 1,700 million ton in 2014,which need to be dried immediately and stored safely to avoid the loss in postharvest operations.However,the conventional hot and ambient air drying could not dry the rice with high drying efficiency and the dried rice was easily be aged and deteriorated during storage.The improvement drying efficiency and storage stability of rice represents an area of great concerns for rice industry.As a novel drying technology for food materials,Infrared(IR)drying has a promising potential to achieve high drying rate,energy saving and improvement in the quality of dried rice.IR could provide the potential to be used as an effective method to inactivate lipase,reduce free fatty acids and improve the storage stability of rice.Therefore,based on the characteristics of IR drying,this study investigated the effect of IR heating and tempering treatments on milling quality and storage characteristics of rice under accelerated conditions and the application of novel IR drum dryer,which can provide the theoretical basis and technical references for rice industry.1.Characteristics of infrared drying for rough rice and its effects on milling quality of rice during storage Freshly harvested medium grain rice cultivar M206 with initial moisture content of 25.03±0.21%(d.b.)were used for conducting this study.Rice samples were separated into three parts and respectively dried by using infrared drying(IRD),hot air drying(HAD)and ambient air drying(AAD)methods for comparison.For IR drying,rice samples were heated to temperature of 60℃under radiation intensity of 4685 W·m-2,followed by 4 h tempering and natural cooling.For hot air drying,the rice was dried to final moisture content after 3 passes of drying.Every pass includes a 20-min drying with 43℃hot air and a 4-h tempering process.For ambient air drying,the rice was consistent dried with ambient air for 18 h till target moisture content.All the dried samples were divided into three portions,which were respectively used as rough,brown and white rice for storage at 35±1℃with relative humidity of 65±3%for ten months.The drying characteristics and milling quality of rice were measured to evaluate the quality of white rice dried by IRD method.Free fatty acid,peroxide value and iodine value were determined to detect any obvious degradation of lipids in rough and brown rice during storage.Since the physicochemical properties have strong relationship with rice cooking and eating quality,the differences among the rice samples dried by IRD,HAD and AAD in the physicochemical characteristics and cooking quality during storage were investigated as well.The results showed that high heating and drying rates of rice were achieved under IR heating.It took only 58 s to heat rough rice to temperature of 60℃with corresponding moisture removal of 2.17 percentage points during IR heating.The total moisture removal after natural cooling reached to 3.37 percentage points without additional energy input.During the initial period of drying with consistant drying rate,the average drying rate of IRD was 1.68×10-2 H2O/(kg rice·min),which was 21 and 186 times of the drying rate of HAD(9.04×10-5 kg H2O/(kg rice·min).and AAD(8.13×10-4kg H2O/(kg rice·min)).Moreover,the kinetic model and balance calculation equation was proposed based on the heat and mass transfer balance analysis of the thin layer of IRD.Compared with the milling quality of rice dried with AAD,the whiteness of the rice dried with IRD was increased with 3.99%,and the head rice yield and total rice yield showed no significant differences(p>0.05).After 4 months of storage,the decrease values of the white rice whiteness that milled from the rough and brown rice dried with IRD were reduced 82.2%and 47.9%as compared with those rice dried with AAD.2.Studies on dynamics and optimization of IR drum drying for rough rice A novel IR drum dryer was developed based on the drying characteristics of IR drying and physical properties of rice.Longer resident time and lower loading weight of rice could increase the heating rate of rice.The drum rotating speed showed no significant effect on drying rate of rice.The response surface methodology was used to conduct the optimization of the operational parameters of IR laboratory scale drum dryer.The optimal resident time for rice drying was 59.5 s.the corresponding optimal power of IR emitter,loading weight of rice and rotating speed of drum were 1694 W,450 g and 10.7 rpm,respectively.Under this technical condition,the rice temperature was dried to 60±2℃.Simultaneously,the dryer could achieve the higher moisture removal and uniformity with lower energy consumption.Moreover,the dried rice showed similar milling quality as that dried with AAD.Therefore,the optimal drying parameters could be reference for scaling up the IR drum dryer to implement continuous drying in rice industry.3.Effects of IRD on the changes in lipid and volatile components of rice during storage and its related mechanismEffective inactivation of lipase and consequent improvement in the long-term storage stability of rough and brown rice was achieved under accelerated storage condition.For rough rice,the free fatty acid(FFA)concentration of the stored rough rice dried by IRD was 5.12% after 10-month storage,compared to 6.8 months for HAD and 5.1 months for AAD dried samples.The cooresponding FFA concentration of IR dried rice was 6.91%at month 10,same concentration as the brown rice stored for 7.4 and 5.2 months dried by HAD and AAD,respectively.Compared with AAD,the FFA concentration of the rough and brown rice dried with IRD and stored for 4 months were reduced by 59.5%and 54.7%,respectively.Due to the limited inhition effect of lipoxygenase,the content of peroxides showed a significant increase trend with flucturations and the unsaturated degree of lipid were decreased during storage.Moreover,the varieties of aroma volatiles of rice kept diminishing while the varieties of stale volatiles kept increasing during storage.IRD inhibited the degradation of rice lipid and delayed the generation of stale volatiles like aldehydes and ketones.Compared with the rice dried with AAD,the increment of relative content of hexanal in the total amount of volatiles components of the stored rough and brown rice that dried with IRD were reduced by 18.0%and 22.5%.Therefore,infrared heating resulted in an effective inactivation of lipase,reduced the lipid hydrolysis,achieved consequent improvement in the lipid stability during storage,and significantly extended the period of safe storage.4.Effects of IRD on the changes in main physicochemical characteristics of rice during storage and its related mechanismThe infrared drying showed positive effects on the color,microstructure,gelatinization and pasting characteristics of stored brown rice.Due to the inhibition effect of high temperature on the degradation of rice lipid,the amount of oxidation products that leached into endosperm was reduced and the yellowing of stored rice was achieved by IRD.The L*value of the samples dried with IRD was higher than that of samples dried with AAD.After 4 months of storage,the increase in yellowness index of the stored rough,brown and white rice that dried with IRD was reduced by 26.3%、47.9%and 26.2%of the increase in that value for AAD dried rice.From the observation of the inner microstructure of rice kernels,it is showed that the protein and starch granules was more clustered,the cell walls in the endosperm were more obscure,and the structure was more compact distributed in rice samples stored after 10 months of storage than those without storage.Comparatively,the changes in microstructure of the rice dried by IRD was less than that dried by HAD and AAD.The results of gelatinization and pasting characteristics indicates that IRD decreased the temperature range and enthalpy of gelatinization,and the peak and breakdown viscosities of the rice samples.These changes might because of the annealing and denaturation effect of IRD on starches and proteins,and its inhibition effect on the activity of those enzymes related with the microstructure changes in rice.Therefore,the IRD treatment could improve the stability of the abformationed physicochemical characteristics of rice during storage.5.Studies on the effects of IRD on the changes in cooking quality and textural characteristics of rice during storageDuring storage,with the increase of connections and network in rice starch and protein regions,more water could be absorbed and held in rice kernels.Improved water absorption ability increased the volume of starch granules and consistently enhanced water-holding capacity.Therefore,the water uptake ratio(WUR)and volume expansion ratio(VER)of all samples slightly increased in the first four months of storage.Then there was a considerable increase in the WUR and VER till month seven then constant values were obtained till the end of the storage period.There was a decreasing trend in solid loss percentage(SLP)for all samples.Comparatively,the rice dried by IRD was better maintained than the samples dried by AAD,especially during the initial four months.The hardness values of rice cooked from all rice samples were stable in the first four months and increased in the following storage period.However,the adhesiveness of the rice cooked from the rice dried by HAD and AAD decreased more than those dried with IRD.Comparing the different storage forms of rice,the hardness of the cooked rice of stored white rice samples increased less than rough and brown rice.