Effect Of Oat ?-Glucan On Protein Digestion In Vitro

Dietary fiber,as one of the seven major nutrients in the digestive system,plays an important role in health.It has become an indispensable food in modern healthy diet.Oat ?-glucan is a kind of soluble dietary fiber which exists in the cell wall of endosperm and aleurone layer of oat.It has the significantly beneficial physiological functions of antihyperlipidemic,hypoglycemic,enhance immunity,maintain the intestinal micro-ecological environment and others.However,when people ingested nutrients such as oat ?-glucan and protein at the same time,whether the impact of oat ?-glucan on protein digestion and how the impact of the study is inconclusive.In this paper,casein(animal-derived protein)and soybean protein isolate(plant protein)were selected as the model proteins,and with the different concentrations of 0%,0.5%,1%,2%,4%of oat ?-glucan(purity of 80.13%)on the digestion of protein was studied.The conclusions were summarized as follow:(1)The total protein digestibility(PDT)of casein and soy protein isolates decreased significantly(P<0.05)when the addition amount of oat ?-glucan was increased from 0%to 4%.Under the conditions of gastrointestinal digestion time of 30min,60min and 120min in vitro,the PDT of casein and soy protein isolate decreased by 6.21%,3.71%,3.41%and 3.68%,3.29%,3.09%,respectively.The results showed that the PDT decreased with the prolongation of digestion time,and the effect of oat?-glucan on protein digestibility was decreased.Overall,the effect of oat?-glucan on the total protein digesti-bility of casein protein digesti-bility(89.15%-96.66%)was stronger than that of soy protein isolate(94.28%-99.05%).(2)When oat ?-glucan was added from 0%to 4%,the content of peptides and total free amino acids in the digestive juice of stomach and intestine were significantly decreased(P<0.05).The decreased degree of peptides and total free amino acids in the digestive juice of stomach and intestine was reduced with the prolongation of digestion time(30min,60min,120min),indicating that the influence of peptides and total free amino acids in the solution decreased with the time of digestion.Overall,the content of peptides in the digestive juice of gastrointestinal tract was higher than that of total free amino acid,which indicated that the main process of protein digestion in vitro was the first step.(3)The viscosity of the protein gastric digestive juice and intestinal digestive juice was significantly increased(P<0.05)with oat ?-glucan increased from 0%to 4%when the shear rate was 100 s-1 in the advanced rotary rheometer.The effect of oat ?-glucan on viscosity of gastric digestive juice was greater than that of intestinal digestive juice.The elasticity modulus G' and viscous modulus G" of the digestive juice of stomach and intestine all increased with the increase of the oscillation frequency.With the increasing of the oscillation frequency,when the addition amount of oat P-glucan was 0%,0.5%,G' is slightly larger than G",the elasticity component of the system is dominant;when the addition amount of oat ?-glucan is 1%and 2%,G" is slightly larger than G',the viscous component of the system is dominant.The results showed that with the increase of oat P-glucan concentration,the viscosity of the whole digestion system increased,which hindered the interaction between protease and protein could lead to the decrease of protein digestibility.(4)The average size of gastrointestinal digestion products of casein and soy protein isolate increased significantly(P<0.05)when oat P-glucan was added from 0%to 4%.Maybe it is related to oat ?-glucan interacted with protein and forming a large-particle polymer.The increase of average size is that the stomach digestion stage larger than the intestinal digestion stage,which indicated that the interaction of oat?-glucan and protein mainly occurred in the stage of stomach digestion.At the same digestion time,the PDI value in the digestive juice of stomach and intestine gradually increased and the homogeneity of the system decreased with the addition of oat ?-glucan.The peak values of volume partical size distribution of protein and stomach digested products were all shifted to the left,and the peak of the digestion products of the protein was significantly smaller than that of the digestive digestion products.PDI values were between 0.13 and 0.43,which indicated that the distribution of casein gastrointestinal digestion products was relatively concentrated in different digestion time(30min,60min,120min),and the particle size distribution of the system was homogeneous.There were many peaks in the particle size distribution of the digested products of soybean protein isolate,and the PDI values were between 0.23 and 0.62,indicating that the particle size distribution of the system particle distribution is not uniform.(5)Effects of oat ?-glucan on protein digestive enzyme activities of casein and soy protein isolate were studied by adding no oat ?-glucan(only protein and protease)as control.It was found that with the increase of the amount of oat ?-glucan,the inhibitory effect on protease activity was increased.With the prolongation of the digestion time(30min,60min,120min),the inhibition rate of oat P-glucan on protease(pepsin,trypsin and chymotrypsin)decreased.In addition,the protease activity of oat?-glucan was inhi-bited in the following conditions:no addition of protease>the addition of casein>the addition of soy protein isolate.The decrease of protease activity of no addition of protease was much larger than that in adding casein and soy protein isolate,which indicated that the effect of different amount of oat ?-glucan on casein and soy protein isolate in gastrointestinal digestion was inhibited by inhibiting protease activity in turn affects protein digestion.(6)The effects of oat ?-glucan on protease activity was studied to investigate the influences of oat ?-glucan on protein digestion.When the addition of oat ?-glucan was increased from 0%to 4%,pepsin and trypsin occured fluorescence quenching,the peak of fluorescence intensity decreased and the crest of the curve shifted slightly to the right,indicating that the microenvironment of the tryptophan residue changed,resulting in a decrease in the hydrophobicity near the residue and a change in the tertiary conformation of the protease.Through calculation we can see that the quenching mechanism of oat ?-glucan to pepsin and trypsin endogenous fluorescence is static quenching.With the prolongation of the digestion time,the binding constant of pepsin,trypsin KS and the number of binding sites n decreased gradually,indicating that oat ?-glucan and protease binding capacity was weakened,resulting in the effect of oat?-glucan on protein digestion is reduced.The number of binding sites n is between 1 and 1.5,indicating that about an oat ?-glucan molecule is bound to a protease.The number of pepsin binding site n>trypsin binding site n told that the binding capacity of ?-glucan to pepsin was greater than that of trypsin,and it was found that the effect of oat?-glucan on protein digestion was greater in the stomach digestion stage than in the intestine digestion stage.In conclusion,different amounts of oat ?-glucan significantly inhibited the digestion of protein(casein/soy protein isolate)in vitro,and the mechanism is related to the complex formed by oat ?-glucan and protease.The effect of oat ?-glucan on digestion of protein in vitro was decreased with the prolongation of digestion time,and the influence on the digestion of stomach was greater than that of intestine digestion.The effect of oat ?-glucan on digestion of casein was larger than that of soy protein isolate.