Effects Of Degree Of Milling On Texture,in Vitro Digestion And Fermentation Properties Of Rice

Rice is one of the most important cereals in the world.It is often processed into white rice for consumption.However,the higher the degree of milling(DOM),the greater the loss of nutrients and the higher postprandial glycemic response.In addition,high processing costs,high processing energy consumption and high carbon emissions are all problems derived from the processing.Therefore,reasonably shortening the milling time and reducing the DOM is an effective measure to improve the yield of rice,reduce the loss and waste during processing and retain the nutrients in the rice bran to a greater extent.In this research,rice samples with different milling times were prepared and the corresponding DOMs were calculated.After cooking,the texture properties and the(in vitro)small intestine digestion properties of cooked rice were investigated.The digestive residues were collected to understand their effects on the gut microbiota composition and metabolite levels.This study will help to provide a theoretical basis for shorting the milling time and reducing the DOM to produce rice with a balance of palatability and nutrition.The results were listed as follows:(1)The analysis of the cooking properties,the starch content and the structure of leached materials were used to reveal the mechanism for the influence of DOM on the texture properties of cooked rice.The results showed that different DOMs altered the texture properties of cooked rice.The degree of starch gelatinization(DSG)and the fine structure of the leached starch were the determining factors.The hardness of rice was mainly determined by DSG and the structure of leached amylose(SLAm).The higher the DOM,the greater the structural damage to the rice kernel,the lower the content of nonstarch hydrophilic biomolecules,and the easier the water was absorbed by the starch granules.And most of the starch granules dissolved then broke up to leach out,resulting in higher DSG,hence lower hardness.In addition,this study found that an increase in the proportion of shorter chains(corresponding to degree of polymerization)in leached amylose caused a reduction in rice hardness.The adhesiveness of rice was mainly determined by the content of leached starch(CLS)and the structure of leached amylopectin(SLAp).The higher the starch content(especially the amylopectin content)in the leached materials,the greater the adhesiveness.Meanwhile,the high proportion of long chains in the leached amylopectin had a greater adhesiveness.Compared to brown rice(MD 0s),5s-milled rice(MD 5s)showed a significant decrease in hardness and a significant increase in adhesiveness,which indicated that MD 5s has higher palatability.(2)The CPS(Combination of parallel and sequential)digestion kinetics model was used to parameterize the digestion process,then dynamically presented the microstructural and components distribution changes to explain the mechanism of the effects of DOM on the digestion properties of cooked rice.The results showed that with the increase of DOM the microstructure and starch digestibility of the cooked rice(powder)significantly changed.Two starch fractions with different digestibility constants,i.e.,rapidly digestible starch fraction(RDF)and slowly digestible starch fraction(SDF)were present in rice digestion.RDF were largely consisted of the fully gelatinized and swelled starch,while SDF were contributed by the 1)relatively intact starch granules,2)protein matrix encapsulated starch and 3)starch-protein binary complex.Resistant starch(RS)was mainly proposed to be the cell wall encapsulated starch originated from the bran layer(RS type Ⅰ)and the amylose-lipid complex(RS type Ⅴ).In comparison to white rice,MD 5s may be representative of having a higher RS content and lower digestibility,second only to brown rice.(3)An in vitro anaerobic fermentation model was developed to investigate the fermentation of digestive residues from rice at different DOMs in the faeces of healthy people.The results showed that rice with different DOMs had different effects on the gut microbiota composition,and there was the possibility that gut microbiota could distinguished and utilized different fermentation substrates.WC-MD 5s played a positive role in maintaining the diversity and homogeneity of the microbiota,possibly due to differences in digestive residue composition and main components(such as starch)utilization.WC-MD 5s had the highest starch utilization,the degree of order and fluorescence intensity change after fermentation.Rice with different DOMs were utilized in different ways and to the extent by the gut microbiota,which may affect the production of metabolites.WC-MD 0s and WC-MD 5s promoted the growth of more acid-producing genus such as Phascolarctobacterium and Blautia,etc.For the rice with different DOMs,the differences of utilization by the gut microbiota determined the differences in the production of short chain fatty acids(SCFAs).WC-MD 0s produced the highest SCFAs amount,followed by WC-MD 5s and the lowest by WC-MD 60s.