Effects And Potential Mechanisms Of Dietary Fibers On Lipid Digestion Of Protein-stabilized Emulsions

Emulsion-based delivery systems can encapsulate,protect,and delivery various lipophilic components,such as liposoluble vitamins,carotenoids,phytosterols,and flavonoids.The bioaccessibility and bioavailability of these bioactive components usually depends on the rate and extent of the lipid digestion process of emulsion.The impact of dietary fibers on lipid digestion within the gastrointestinal tract depends on their molecular and physicochemical properties.In this dissertation,considering the digestion parameters(molar concentration of titration solution,ionic strength,concentration of lipase and bile salts,temperature,rotation speed,etc.)we standardized the single-and multiple-stage in vitro GIT digestion models based on pH-stat method.Then three dietary fibers with different electrostatic characteristics(cationic chitosan,anionic alginate,and neutral locust bean gum)were chosen.The particle size,?-potential,apparent viscosity,and microstructure were measured at different simulated physiological stages(initial,mouth,stomach,and intestine phase).Assuming that each tricylglycerol molecule would release two free fatty acid molecules,we calculated the ratio of FFAs released from the initial emulsion sample.After that,the kinetic parameters were calculated from the dynamic FFAs releasing profile using LOS analysis.The results were presented as follows:1.The stock solution of chitosan,alginate,and locust bean gum was added into the ?-lactoglobulin stabilized emulsion,respectively.The polysaccharide concentration in the secondary emulsion was maintained at 0,0.1,0.2,0.4 wt%.The lipid digestion duration was set at 2 h(37?),and the sodium and calcium ion concentration in the final digestion fluids was adjusted to 150 mM and 10 mM,respectively.The lipase concentration was maintained at 1.6 mg/mL,and the bile salts at 5 mg/mL.The mean particle size,PSD,?-potential,and rheological parameters were measured before and after the lipid digestion process.The results showed that the influence of dietary fiber on lipid digestion within simulated small intestine depends on polysaccharide type and concentration.In particular,the addition of chitosan(0.1-0.4 wt%)increased the mean particle size from 0.206 ?m(control group)to about 170-240 ?m(secondary emulsion);the influence of alginate to particle size was not obvious at relatively lower concentrations(0.1-0.2 wt%),but alginate increased the particle size to 54 ?m at high concentration(0.4 wt%).Locust bean gum induced flocculation(d43 ? 1-11 ?m),and the mean particle size depended on the polysaccharide concentration.According to the PSD,?-potential,and confocal microscopic images,we could assume that:(1)low concentration of alginate and locust bean gum promoted droplet flocculation by depletion force and electrostatic interactions with charged species in the gastrointestinal fluids,which was not strong enough to resist the mechanical shear force or enzymolysis within GIT;(2)at relatively high dietary fiber levels(0.4 wt%),the alginate molecules bound cationic calcium ions and formed calcium alginate hydrogels,which can encapsulate part of the lipid droplets;(3)large and compact aggregates were formed between chitosan molecules(0.1-0.4 wt%)and anionic lipid droplets by bridging mechanism.2.By recording the volume of sodium hydroxide(0.1 M)used to titrate the FFAs released during lipid digestion,one can calculate the ratio of hydrolyzed TAG molecules and get the real-time releasing profile of FFAs.At relatively low dietary fiber concentrations(0.1 and 0.2 wt%),the initial rate of free fatty acid release decreased in the following order: control ? LBG ? alginate > chitosan.However,the final lipid digestion rate and extent(? 82%-88%)was not significantly affected(P > 0.05).At the highest dietary fiber level(0.4 wt%),the initial rate of lipid digestion and the final amount of free fatty acids produced decreased in the following order: control ? LBG(? 83%-87%)> chitosan(? 72%)> alginate(? 60%).3.To analyze the digestion data using LOS method,13 sampling points were chosen from the FFAs releasing profile within the first hour.During the first 10 min of lipid digestion,8 points of sampled;after the lipid digestion proceeded further,the sampling points were chosen every 10 min.The results showed that the kinetic mechanisms of dietary fiber regulating the lipid digestion process were different when various types and concentrations of polysaccharides were used.The reactions could be classified into into two species according to the number of pseudo-first order reaction stages(i.e.,single-or two-phase reaction).4.To maintain the polysaccharide concentration at the same level,the concentration of dietary fiber in the secondary emulsion used for multiple-stage GIT model was improved to 1.6 wt%.In this way,the initial polysaccharide concentration at the beginning of lipid digestion was 0.4 wt%.The results showed that:(1)the final lipid digestion rate of alginate within the single-stage GIT model was approximately 60%,whereas more TAG molecules were digested(? 83%)when the multiple-stage GIT model was used;(2)the lipid digestion rate of chitosan group within the single-stage GIT model was about 72%,and was improved to about 83% using the multiple-stage GIT model;(3)there was no significant difference in control group and LBG group when different GIT models were applied.By comparing the effect of single-and multiple-stage GIT digestion model,we presume that pretreatment of the emulsions with artificial saliva and gastric fluids improved the lipid digestion rate and extent by altering the total surface area of lipid exposed to the digestive enzymes,or by altering the nature of the interfacial layer separating the lipid phase from the digestive enzymes.In summery,one can control the lipid digestion rate and extent by structural design approaches: by changing the type and concentration of dietary fiber,the particle size,?-potential,microstructure,and apparent viscosity would also be changed,which means the lipid digestion process would be manipulated.The single-and multiple-stage GIT models used in this study could provide an efficient tool for screening functional emulsion-based delivery systems in either feed or food industry.