Research On The Eating Quality Improvement And Digestibility Of Extruded Rice

Extruded rice（ER）is a rice-shaped product prepared by extrusion technology.The nutritional value of ER could be higher than native rice by designing the nutritional components.Because of the high temperature,high pressure and high shear force during extrusion,the cooking time of ER is shorter than native rice,but this may also lead to the high cooking loss,poor rice grain formability,soft taste and high digestibility of ER,which impaired the eating quality of ER.The physicochemical properties of materials and the interaction between macromolecules during extrusion may have determined the eating quality of ER.Therefore,pre-fermentation treatment（PFT）and addition of Glycerin monostearate（GMS）were used to investigate the effect of physicochemical properties of materials and the interaction of starch and GMS during extrusion on the eating quality of ER,and the digestibility of improved extruded rice were evaluated.The main results were as follows:（1）PFT significantly changed the physicochemical properties of rice flour.The protein,lipid and ash contents of rice flour decreased from 8.36%,0.33%and 0.49%to 4.55%,0.12%and 0.18%,respectively.The total starch and amylose contents increased from 90.39%and 17.32%to 94.99%and 19.82%,respectively.The structure of starch in the rice flour were changed,too.The amorphous regions of starch granule were partially hydrolyzed,the relative crystallinity and the short-range order increased.The weight-average molecular weight（Mw）of starch molecules decreased from 4.94×10⁸ g/mol to 2.46×10⁸ g/mol,and the proportion of short chains,long chains and ultra-long chains of amylopectin increased from28.97%,9.29%and 0.88%to 30.54%,10.44%and 1.23%,respectively.During fermentation,the microbial composition changed,the lactic acid concentration increased to 20.17 mg/mL,and the pH of the fermentation broth decreased from6.33 to 4.42.In addition,soluble protein and reducing sugar increased from 0.18mg/mL and 0 mg/mL to 1.77 mg/mL and 0.3 mg/mL,respectively.PFT significantly improved the eating qualities of ER,decreased the cooking loss of ER from 11.04%to 5.42%,improved the formability of rice grains,and improved the taste of ER（hardness and adhesiveness increased from 1386 g and 33g sec to 1750 g and 48 g.sec,respectively）.The improved eating qualities of ER may be due to the reduction of protein,lipid,ash,and the increased content of amylose,long chains and ultra-long chains of amylopectin,which promoted the interaction between starch molecules in the ER and resulted it forming a more homogeneous,denser and more stable gel structure.（2）The chemical compositions of ER did not change during extrusion,but the structure of starch was changed significantly.The crystalline structure of starch was destroyed and the relative crystallinity decreased from 28.12%to 7.28%after extrusion.The starch molecules were degraded,Mw decreased from 14.45×10⁷g/mol to 4.22×10⁷ g/mol.The destruction of starch structure may have weakened the interaction between starch molecules,so that the gel structure of ER was weakened（G′and G′′decreased,Tanδdecreased）and the hardness and adhesiveness of ER decreased.In addition,the destruction of starch structure has exposed more hydroxyl groups of starch to combine with water molecules,which increased the water solubility of extruded rice flour（the water solubility increased from 11.17%to 41.05%at 100℃）and the cooking loss of ER.The eating qualities of ER were significantly improved with the addition of0.5～1.5%GMS,the formability of rice grains was improved,the cooking loss decreased from 10.13%to 3.87%,hardness increased from 948 g to 1570 g,and adhesiveness decreased from 52 g·sec to 24 g·sec.The improved eating qualities may be related to the formation of starch-GMS complexes during extrusion.With the addition of GMS（0～1.5%）,starch and GMS formed V-type crystalline structure,the relative crystallinity increased from 7.28%to 15.85%,and the complex index increased from 2.63%to 69.09%.The water absorption and water solubility of extruded rice flour were limited with the formation of starch-GMS complexes（At100°C,the water absorption of extruded rice flour decreased from 853.53%to522.15%,and the water solubility decreased from 41.05%to 10.22%）.The gel structure was enhanced（G’and G’’improved）with the form of starch-GMS complexes.（3）Both PFT and addition of GMS could lower the digestibility of ER.The RDS,C∞and k of starch in pre-fermented ER decreased from 62.06%,91.98%and4.90×10^-2 to 59.48%,85.65%and 4.48×10^-2,and the RS increased from 7.67%to10.11%.The decreased digestibility of pre-fermented ER may be related to the decrease of protein,lipid and ash content and the increase of amylose content,and the increase of the proportion of long chains and ultra-long chains in amylopectin,which enhanced the interaction of starch molecules,therefore a more uniform and denser gel structure of ER was formed.The addition of 0.5～1.5%GMS significantly lowered the digestibility of starch in ER.RDS,C∞,k values decreased from 62.06%,91.98%and 4.90%to 53.61%,81.61%and 3.95%,and RS increased from 7.67%to 18.25%with the addition of GMS.The decreased digestibility of starch in ER may be related to the formation of starch-lipid complexes that restrict the binding between enzymes and starch.