Study On The Regulation Of Rice Starch Digestibility By Radio Frequency Treatment

Radio frequency（RF）heating technology as a novel physical processing technology has the advantages of high heating rate,volumetric heating,and deep penetration depth.It is considered as a new modification technology of starch with potential application.An important reason associated with utilization of rice as staple food is its rich in starch,which satisfies the basic energy and nutritional needs of most of the world population.However,excessive and long-term consumption of such grains can lead to elevated blood sugar levels that is associated with the diabetes,obesity and cardiovascular diseases.Therefore,how to reduce the digestibility of rice starch and its products has become a current research hotspot.In this paper,different kinds of rice and rice starch were treated by RF heating technology.The effects of RF treatment on the multi-scale structure of rice starch were systematically studied.The relationship between the multi-scale structure and digestibility of rice starch after RF treatment was elucidated.Moreover,the effect and mechanism of RF treatment and protein complexation on the digestibility of rice starch was investigated.In order to provide theoretical guidance and technical support for the design and production of rice starch-based foods with specific properties and desirable nutritional function.The main research contents and results are as follows:（1）The effects of RF treatment on the microstructure,physicochemical properties,and digestibility of different rice varieties（indica,japonica and waxy rice）were investigated.Microstructure analysis exhibited that RF treatment caused starch gelatinization and protein denaturation in rice grains with increasing treatment time.After RF treatment,indica and japonica rice remained the A-type crystals with the formation of amylose-lipid complex,while the crystalline structure of waxy rice was disrupted.RF treatment enhanced the T_o,T_p and T_c,increased the slowly digestible starch（SDS）and resistant starch（RS）contents in indica and japonica rice,thereby reducing the digestibility.Compared with the control,the peak viscosity（PV）of indica and japonica rice decreased with the increase of RF treatment time,while the PV of waxy rice increased with the increase of RF treatment time.The storage modulus（G’）and loss modulus（G’’）of all samples after RF treatment obviously increased compared to the control.In addition,it was found that the heating rate of different forms of rice（granule and powder）was different during RF treatment,resulting in certain differences in the structure of the samples after RF treatment.Therefore,it is necessary to further analyze the effects of RF treatment on the physicochemical properties and digestibility of rice flour.（2）Based on the above research,the effects of RF treatment on the physicochemical properties and digestibility of different varieties of rice flour were studied.The results showed that RF treatment caused the aggregation of starch granules,resulting in the increase of particle size.X-ray diffraction（XRD）and Fourier transform infrared spectroscopy（FTIR）analysis showed that RF treatment promoted the formation of starch-lipid complexes in indica and japonica rice flour,and improved the short-range ordered structure of waxy rice flour.These structural changes could affect the interaction between rice flour and water,thus influencing the gelatinization and rheological properties of rice flour.In vitro digestibility results showed that the resistant starch content of indica,japonica and waxy rice flour increased from 39.04%to 42.02%,39.95%to 45.12%and 36.26%to 43.61%after RF treatment,respectively.（3）In order to further illustrate the regulation mechanism of RF heating technology on the digestibility of rice starch,the effects of RF treatment on the multi-scale structure and digestibility of rice starch（indica,japonica,and waxy rice starch）were evaluated.Scanning electron microscopy analysis indicated that RF treatment for 10 min had no effects on the starch granules,while RF treatment for 20 min caused the granular surface erosions and morphology alteration.XRD analysis showed that RF treatment did not alter the A-type crystal form of starch,but disrupted the relative crystallinity.FTIR analysis exhibited that RF treatment promoted the formation of starch-lipid complexes in indica and japonica rice starch and enhanced the short-range ordered structure of waxy rice starch.Small-angle X-ray scattering analysis presented that RF treatment enhanced the thickness of amorphous lamella and reduced the thickness of crystalline lamella of indica rice starch and japonica rice starch,while inducing an an increase in crystalline lamella and a decrease in amorphous lamella of waxy rice starch.Differential scanning calorimetry results suggested that starch orders with a higher thermal stability formed after RF treatment,thereby reducing the accessibility of starch granules to the enzyme.The amylose content of indica rice starch increased after RF treatment.RF treatment caused the starch chains breakage,which increased the A chain content and decreased the B1chain content,thus reducing its molecular weight.The resistant starch content of indica,japonica,and waxy rice starch after RF treatment increased from 38.02%,33.29%and 31.13%to 49.35%,43.65%and 42.92%,respectively.（4）Furthermore,in order to explore the effect of the interaction between starch and protein in starch-based foods on starch digestibility,the effects of RF treatment combined with proteins（whey protein isolate（WPI）,soybean protein isolate（SPI）and pea protein isolate（PPI））on the digestibility of waxy rice starch（WRS）were investigated,and compared with traditional hot water treatment.The results showed that the rapidly digestible starch（RDS）content of WRS decreased and the RS content increased after RF treatment with protein addition.Protein acts as a physical barrier during the digestion of starch.The starch-protein complex formed by RF treatment presented a typical three-dimensional network structure due to the faster heating speed and shorter time consumption,while the complex prepared by hot water treatment exhibited a sheet.During RF treatment,starch and protein mainly interacted through non-valent bonds.WRS-WPI showed stronger hydrogen bond and hydrophobic interaction,WRS-PPI mainly showed hydrophobic interaction,while the WRS-SPI showed weaker hydrogen bond,hydrophobic and electrostatic interactions.Low field nuclear magnetic resonance analysis exhibited that RF treatment enhanced the content of weakly bound water and reduced the content of free water in complexes,thus reduced the accessibility of enzyme to starch.（5）Based on the fact that RF treatment combined with protein can reduce the digestibility of WRS,the effect of different amounts of Ca Cl₂ on the structure and digestibility of WRS-protein complexes during RF treatment were investigated.The addition of appropriate Ca²⁺content could increase the short-range ordered structure and lamella structure,and form a more compact and uniform microstructure.The addition of Ca²⁺reduced the free water content in complexes and increased the content of bound water and weakly bound water.Synergetic Ca²⁺-RF treatment could significantly reduced the RDS content and enhanced the RS content of complexes.The RS content increased with the increase of Ca²⁺content（0～2%）.Compared with the 31.24%RS of RF-WRS,the RS content of RF-WRS-WPI-2%,RF-WRS-SPI-2%and RF-WRS-PPI-2%was 51.05%,52.82%and 55.93%,respectively.