| The formation and application of protein/polysaccharide complexes are of great concern in food research area. Soy proteins are high quality protein which can be used as alternatives to animal proteins, thus, it is very valuable to develop food products utilizing soy proteins. In this thesis, soy proteins(SP)/gum arabic(GA) mixture was extensively investigated to get more insights into the complex formation, and electrostatic interactions between soy proteins and gum arabic. After researching the functional properties related to microencapsulation, SP/GA complexation was used as a tool of microencapsulation successfully. The major results are as follows:(1) Phase behavior when SP/GA complex formed. In this part, an overview of the effect of each factor on SP/GA mixture phase behavior was given. Phase boundaries(bionodals and spinodals) were determined by changing mixing order, mixing concentration and mixing ratio firstly, and then SP/GA/water ternary phase diagrams were conducted. Results showed that phase separation occurred at low polymer concentrations, and a drop-shaped and asymmetric two-phase region was anchored in the water-rich corner. NaCl had great effect on phase boundaries that the biggest two-phase region was appeared at 100 mmol/L NaCl, while the phase separation was suppressed completely at 250 mmol/L NaCl. Thus, there was no two-phase region any more. In conclusion, phase separation was induced by electrostatic interaction, and insoluble complexes could not be formed under the condition with high NaCl concentrations due to salt screening effect. pH also affected phase behavior greatly. Compared with the phase diagrams at pH 3.0, the two-phase region at pH 4.0 was found to be smaller revealing that phase separation occurred at higher polymer concentrations. It was because net charges for soy proteins were smaller at pH 4.0.(2) Effect of SP/GA net charge ratio on complex formation. In this part, turbidity titration for SP/GA mixtures at different mass ratio was conducted and the critical pH values(pHφ1) where insoluble complexes began appearing were determined. The corresponding pHφ1 values for the mixtures at SP/GA mass ratios of 1:4, 1:2, 1:1, 2:1, 4:1 and 8:1 were 2.75, 3.15, 3.55, 4.10, 4.55 and 4.65, respectively. Then, the electromobility of soy proteins and gum arabic at the pH range between 4.5 and 2.3 were measured, and charge densities(ZN) for soy proteins and gum arabic at different pH values were calculated using soft particle theory. Further analysis indicated that the product of SP/GA mass ratio(ρ) and SP/GA charge density ratio was approximate 1 at any pHφ1 values. It was revealed that the number of charges for the two polymers was equal when SP/GA insoluble complexes began to be formed.(3) Thermodynamic characterization when SP/GA complex formed. In this part, thermodynamic properties when SP/GA insoluble complexes(pH 3.0) and soluble complexes(pH 5.6) formed were mainly studied. At pH 3.0, when soy proteins were titrated into gum arabic(SP-to-GA titration), phase separation occurred at a SP/GA mass ratio of 0.42. It was equal to the SP/GA mass ratio(0.36) where the charges for soy proteins and gum arabic were the same at pH 3.0. ITC experiments displayed an exothermic process at pH 3.0, and the maximum enthalpy changes(ΔH) was-0.70 ± 0.02 cal/(g protein). At pH 5.6, ΔH was-0.10 ± 0.01cal/(g protein). It meant that electrostatic interactions also existed at pH 5.6. However, only soluble complexes could be formed and no phase separation could occur at any SP/GA mass ratio. Reverse titration at pH 3.0(GA-to-SP titration) showed that turbidities increased rapidly with the addition of gum arabic, indicating that phase separation could occur when a small amount of gum arabic was in soy protein solutions. Thus,there was no such charge relationship for soy proteins and gum arabic as mentioned previously. ITC result also showed exothermic process but a much higher ΔH(-1.40 ± 0.01 cal/(g polysaccharide)) than that for SP-to-GA titration. Effect of NaCl concentration on the complexing behavior by SP-to-GA titration at pH 3.0 was studied at last. ΔHs and binding isotherms changed monotonically with the increase of salt concentration from 0 to 200 mmol/L.(4) Fuctional properties of SP/GA complexes. The functional properties of SP/GA complexes were investigated and compared with those of heat-treated soy proteins(85-SP and 95-SP)/GA complexes and gelatin/GA complexes, in order to clarify the relationship between the functional properties of complexes and microencapsulation. The complexes were prepared at a 1:1 protein/polysaccharide mixing ratio and different pH values(pHφ1、pHopt and pHpd). The results indicated that the emulsion activity for SP/GA system was greater than that for gelatin/GA system. Interfacial tension had negatively correlation to emulsion activity, indicating that surface activity with oil droplet for complexes determined the emulsion activity. Emulsion stability for protein/GA mixture was greater at pHφ1 and pHopt relative to that at pHpd. Frequency sweeping results showed that SP/GA complexes were structured with a characteristic length scale of ~ 40 nm, which was much larger than that of other complexes. It meant that the structure of SP/GA complexes was less tighter. Temperature sweeping(40-10 ℃) results illustrated that the structure was much tighter for gelatin/GA complexes when temperatures were lower. However, characteristic length for SP/GA complexes was almost the same during the cooling process.(5) Applications of SP/GA complexes in microencapsulation. At last, application of SP/GA mixtures in flaxseed oil microencapsulation was investigated. The effects of SP/GA mixing ratio(1:2, 1:1 and 2:1) and pH(2.80, 3.15 and 3.75) on complex preparation were studied firstly. The results showed that the highest complex yields were achieved at the corresponding pHopt for any SP/GA mixing ratio, especially at SP/GA = 1:1, pH 3.15 and SP/GA = 2:1, pH 3.75. They were 81.2 ± 2.0%, 88.1 ± 0.6%, respectively. The SP:GA ratio in the complexes increased with pH increasing, suggesting that SP/GA complex formation was monitored by electrostatic interactions. Thereafter, the microencapsulation of flaxseed oil was detected in accordance with the results of the SP/GA complexation, that is the optimal condition for microencapsulation was corresponded to the condition where complex yield was the highest. Under the conditions of SP/GA = 1:1, pH 3.15, the microencapsulation yield and total yield reached 81.5 ± 0.1% and 81.7 ± 0.4%, respectively; while under the conditions of SP/GA = 2:1, pH 3.75, the microencapsulation yield and total yield reached 77.4 ± 3.7% and 86.7 ± 2.4%, respectively. Microscopic morphology revealed that the formation of a biopolymer shell around the oil droplets was achieved at specific conditions. The results of antioxidation experiment showed that oxidative rate for flaxseed oil was greatly reduced after microencapsulation under certain conditions. In other words, formed microcapsules had a role in inhibiting flaxseed oil oxidation. |
PDF Full Text Request