Preparation Of Lightly Milled Rice By Superheated Steam And Its Physicochemical Properties

Rice is the staple food for half of the world's population.Reducing the milling degree of rice can greatly improve rice nutrition and save a large amount of food,which in line with the current situation of China.The indica rice and japonica rice that widely cultivated in south and north of China respectively were used as the research objects.The objective of this paper is to develop a nutritious and stable rice product with low milling degree,namely lightly milled rice(LMR),by using superheated steam(SS)technology.It is expected that this LMR product will partly replace well milled rice to alleviate the problem of food shortages and nutrient losses.To achieve the above-mentioned objective,three research topics were studied in this paper.Firstly,the effect of aleurone layer on the cooking behavior of rice and related mechanism were studied,and LMR was prepared on the basis of the mechanism.Secondly,a SS and hot air hyphenated equipment in laboratory scale was designed and manufactured,and a stable LMR product was produced by deactivating the peroxidase in LMR using SS generated from the equipment.Finally,the effects of SS deactivation treatment on the eating,storage and nutritional properties of LMR were evaluated.The effect of heat-moisture treatment on the swelling and rheological properties of starch was also studied as a theoretical guide to improve the process of SS deactivation treatment.The main results of the above three research topics are as follows.Aleurone layer slightly affected rice cooking quality(optimum cooking time,water absorption,volume expansion ratio and total solids loss)while remarkably affected rice texture(hardness and adhesiveness)and peak viscosity.Histological investigation showed that channels were formed in aleurone layer during cooking.The channels facilitated the penetration of water,which could explain why aleurone layer exhibited slight effects on rice cooking quality.On the other hand,thick cell walls and thermally stable aleurone grains were widely distributed in aleurone layer.Leached components accumulated on them and formed a reinforced coated film on rice surface during cooking,which may be a possible mechanism accounting for the remarkable effect of aleurone layer on rice texture.The SS and hot air hyphenated equipment is mainly composed of personal computer,electric steam generator,air compressor,parallel superheaters,processing chamber,PID temperature controller,thermocouple thermometers and flowmeter.The SS generated by the equipment can effectively deactivate the peroxidase in LMR.In terms of the efficiency of deactivating the peroxidase,SS is significantly higher than that of isothermal hot air.In addition,SS deactivation treatment did not cause a significant loss of moisture in the rice grain.Although the SS generated by this equipment resulted in a slight gelatinization on the surface of rice grain,it did not cause significant loss of moisture and fissures of the grain.SS deactivation treatment did not significantly change the natural physicochemical properties of LMR including appearance(whiteness of uncooked rice and morphology of cooked rice),cooking quality(water absorption,volume expansion ratio and total solids loss),and texture profile(hardness,adhesiveness chewiness and cohesiveness).SS deactivation treatment effectively prolongs the shelf life of LMR,which is better than low temperature(4 °C)storage.SS deactivation treatment could effectively inhibit the lipid deterioration of LMR by suppressing the oxidation of unsaturated fatty acids and by reducing the acid value,peroxide value and carbonyl value.The changes of pasting properties,including peak viscosity,trough viscosity,final viscosity,setback and pasting temperature were also retarded by SS deactivation treatment.The grain appearance was well preserved whereas the micro-mechanical behavior of starchy endosperm was interestingly changed(intercellular cleavage shifted to intracellular cleavage)after six months of storage,which can not be prevented by low temperature,vacuum packaging or SS deactivation treatment.SS deactivation treatment did not cause the loss of non-starch nutrients(fat,protein,ash and dietary fibre)or promote the oxidation of unsaturated fatty acids in LMR,but significantly increased the total phenolic content and antioxidant activity of LMR.Compared with well milled rice,LMR prepared by SS had a higher antioxidant activity,higher contents of non-starch nutrients and phenolics,and slower starch digestion rate.SS deactivation treatment is actually a short-time heat-moisture treatment.The effects of heat-moisture treatment on the swelling and rheological properties of four starches from different biological sources(rice,corn,wheat and potato)were investigated,and the results showed that heat-moisture treatment inhibited the swelling and gelatinization of starch granules and decreased the shear recovery of starch gel independent of the biological source of starch.Heat-moisture treatment significantly affected the critical strain,flow point,ratio of elastic components,shear-thinning behavior and consistency of starch gel and the resuts depended on the biological source of starch,suggestting that the physicochemical properties of starch in LMR could be significantly changed by SS if the treatment time was prolonged.Therefore,hightemperature and short-time process should be a preferred choice in the preparation of LMR by SS.Compared with well milled rice,the LMR product perepated by SS deactication treatment had similar cooking and eating quality,higher grain yield(7% higher than well milled rice),and higher nutritional value.The contents of fat,protein,ash,total dietary fiber,total phenolic acid,total antioxidant activity,slowly digestible starch and resistant starch of LMR product were 3.04,1.08,2.44,1.54,5.6,3.0(or 4.7),1.40 and 1.49 times of those of well milled rice respectively.In addition,the LMR product also had a good shelf life.In summary,SS deactivation treatment could be considered a suitable method for the stabilization of LMR and the resultant LMR product may be a potential alternative to well milled rice.