Study On The Influence Of Whey Protein Isolate On The Digestibility Of Starch And Its Mechanism

Starch is the major nutrients and energy sources of human diet.The consumption of food rich in rapidly digestible starch(RDS)leads to higher glycemic index,which is associated with the development of diet-related illnesses such as cardiovascular disease,type II diabetes and obesity.On the other hand,slowly digestible starch(SDS)and resistant starch(RS)have been recommended to control and prevent these syndromes.Proteins are another major nutrient of human diet and commonly present in starchy food.Interactions between protein and starch influence the physicochemical properties and digestibility of starchy foods.These interactions further affect postprandial glycemic response.This study provided a better understanding for digestibility of whey protein isolates(WPI)on corn starch(CS)and the formation mechanism of cooking and baking system.Moreover,the consequence of their interactions on the rheological behaviors,thermal properties,short-range structure and morphological characteristics were evaluated.Meanwhile,the in vitro digestibility of corn steamed cake and muffin were analyzed in a real system.The results will allow for the development of higher natural protein in starchy foods.And it will provide theoretical guidance for designing food formulations of low digestibility and high nutrition.The main contents and results are as follows:Firstly,with the combination of CS and WPI,the binary blending system was constructed by cooking and baking condition.In the case of high protein ratio blends(1:0.25,1:0.45),the digestibility was significantly reduced.The RDS content of cooked starch decreased by 4.15% and 7.80%,while the RS increased by 8.31% and 8.82%,respectively.Similarly,with the increase of protein ratio in baked starch,the RDS decreased by 4.18% and 11.2%,the RS increased by 5.51% and 7.51%,respectively.In baking condition,starch digestibility and protein digestibility is lower than that in cooking condition.Fourier transform infrared spectroscopy(FTIR)analysis indicated that there was no covalent interactions between CS and WPI.More orderly structure was formed with increasing ratio of WPI.Moreover,protein-starch interaction resulted in forming encapsulation starch granules and acting as a barrier for starch digestibility as shown by confocal laser scanning microscopy(CLSM).The gelatinizing process of starch granules during heating was restricted by protein portion in the system,while retrogradation process was strengthened as observed by differential scanning calorimetry(DSC)and rheological behaviors.WPI seemed to act as an inactive filler to prevent amylose rearrangement and weaken the corn gels as showed by rheological measurements.Secondly,the mechanisms of starch digestibility affected by protein were explored on the bases of physical barrier effect?enzymatic reaction and their interactions.Pepsin treatment of CS-WPI significantly increased starch digestion,as indicated by an increased content(2.37%~8.43%)of RDS.Protein had a certain physical barrier effect on starch.Enzyme activity test showed that the hydrolyzed protein inhibited the activity of amyloglucosidase.The shortrange structure of starch in FTIR was favored by hydrolyzed WPI.The molecular weight of polypeptides influenced CS-WPI interaction.An increased number of small and medium molecular polypeptides were more likely to interact with starch,primarily through hydrophobic interaction.Studies on rheological properties showed that the G' and G'' exhibited a sudden drop with increasing urea concentration,indicating the interaction force was mainly through hydrogen bonds.Protein retarded the digestive evolution of starch molecules and reduced the starch hydrolysis as determined by HPLC-SEC.Finally,the binary blending system of corn flour and WPI was constructed by cooking and baking.The effect of WPI on the digestible properties of a simple system(corn flour and WPI)and a multiple system(steamed cake and muffin)were investigated.The RDS in cooked system decreased by 5.11%~17.70%,and reduced by 9.02%~18.84% in baked system.WPI may interacted with native corn protein combined on the surface of starch granules,which increased the viscosity of the system,and improved the retrogradation properties of corn flour based food.Moreover,WPI fortification significantly reduced the digestibility of corn steamed cake and muffin,and did not affect their original textural characteristics.