| Complex coacervation reaction is a self-assembled phenomenon that always happened among two orabove two kinds of macromolecules driven by electrostatic interactions. Through the application ofdifferent extents of reaction products, we could obtain series of complex coacervates with variable size,special surface capability or excellent emulsification. Such complex coacervates will confer great potentialsin industries like food, chemical engineering, pharmaceutical and textile. However, due to the largedifference among raw materials and big difficult in controlling the parameters, its process ofindustrialization is always slow.The current paper was focused on an important application field of complex coacervation-microencapsulation. The formation mechanism of complex coacervates was been investigated to disclosethe variable reaction phases suitable for preparation of nano-capsule and microcapsule. Furthermore, thecorresponding capsule type products were specifically fabricated with jasmine essence as core material;also their capsules’ characteristics were explored. The main contents are as follows:The gelatin-gum arabic system was chosen as raw materials of the reaction. Firstly, the turbidity titrationcurve at variable mixing ratios (2:1，1:1，1:2，w/w) for gelatin-gum arabic was obtained via drop-wise additionof acetic acid and then tested the absorbance values at600nm. Through analysis of the curve, it was found thatthis system could form soluble complexes and insoluble complexes by pH adjustment. In correspondence, thiscurve had the possibility to form nano-capsule and microcapsule.The complex coacervates and corresponding microcapsules at the mixing ratio of1:1and the same pHrange were investigated to find their relationship in each other, for the purpose of simplifying microcapsulepreparation optimization processing. Comparisons on morphology, yield and internal characteristic(anti-mechanical force capability-heat-resistance) were performed. Finally, a simplified method was proposed,in which the zeta-potential measurement helps to establish the approximate mixing ratios of wall materials; theturbidity analysis facilitates the finding of suitable pH corresponded to the highest turbidity to make the perfectmicrocapsule. According to this method, a new combination of wall materials named gelatin and sodiumcarboxymethyl cellulose was provided and verified by morphology.The dynamic-temperature rheological analysis was applied to investigate the cooling process ofcomplex coacervation happened between gelatin and two variety of gum arabic from different types of tree(A. Senegal: AA; A. Seyal: BA). It was found that the systems composed of different gum arabic conferredgreat variance on the property of viscoelasticity. In particular, the transition temperature from viscosity toelasticity was much higher for AA type complexes than BA type complexes. As the elasticity representedthe solid-like capability, such rheological curve could be applied to guide the preparation of microcapsuleby adjusting the cooling conditions. Therefore, the cheaper BA type gum arabic could own a similarmorphology with AA type. The specific cooling conditions are20-10°C for AA and5-0°C for BA。The coacervates of gelatin and gum arabic with three mixing ratios (2:1,1:1and1:2, w/w) wereprepared in order to elucidate their pH-dependent complexation mechanism. Firstly, the analysis ofmorphology and size distribution showed that with pH decline, the coacervates became larger for themixing ratio of1:1and1:2; whereas, the trend went oppositely as to the mixing ratio of2:1. Through thecomposition analysis of coacervates, such transition pattern was found to be consistent with the conversionrate of gum arabic. Coacervates prepared by the mixing ratio of2:1were chosen to further investigate theformation mechanism at the molecular level. During the complexation process with pH decrease, gelatinmolecules experienced a conformational change from a flexible pattern to an ordered PPII helix. On theother hand, gum arabic went through a transition from partly ordered PPII helix to a more compactstructure, called PPI helix. Such characteristics of conformational transition were also similar for thesystems with mixing ratios of1:1and1:2. Therefore, it was presumed that the change of coacervates size was due to the combined reasons including the conformational transition and the reacted amount of gumarabic.The isothermal titration calorimetry was used to analyze the binding isotherm for complexcoacervation. The curve verified again the existence of two phase products, including soluble complexesand insoluble complexes. Also, the curves at different temperatures showed that hydrogen bond played animportant role for structure formation. Specifically, the low temperature facilitated more hydrogen bondproduced. Considering the limit electrostatic interactions existed for soluble complexes, the correspondingnano-capsules could be strengthened by hydrogen bond through reacting just at room temperature, withoutthe need of heating.The colloidal behavior of soluble complexes was investigated using turbidity titration (600nm) withthe help of an in situ acidification-glucono-δ-lactone. The results showed that, with the decrease of mixingratio, the corresponding pH range for soluble complexes became a little larger. Furthermore, the addition ofNaCl and urea indicated the interactions between macromolecules were not only electrostatic interactions,but also hydrophobic interactions. Through the analysis of particle distribution, pH4.80under the mixingratio of1:1was chosen as the favorable condition for the nano-particles preparation. Subsequently,corresponding nano-capsules with jasmine essential oil entrapped were prepared. Based on the parametersof general size, polydispersity index (PDI), zeta potential and load, the best nano-capsules weresuccessfully prepared (r=69.56nm, PDI=0.135and zeta potial=-14.80mV) The optimized conditions wereselected as follows: total concentration0.5%, jasmine essence concentration0.5%, emulsifier concentration0.5%, emulsifier composition Span80and Tween80(1:1), cross-linker transglutaminase (0.25%). Theresults of transmission electron morphology were consistent with the particle size distribution, furtherverifying the encapsulation capability of nano-capsules.In order to obtain heat-resistant jasmine essence nano-capsules, the hardening process via cross-linkertransglutaminase was investigated. Their heat-resistance capability against80°C water bath was evaluated bysize, PDI and zeta potential. The results showed that the nanocapsules cross-linked at alkaline conditionsconferred a better stability. Specifically, the nano-capsules processed at pH7for hardening process could endurethe incubation at80°C for7h. The final characteristics are as follows: size=101.89nm, PDI=0.106, zetapotential=-9.45mV. Such particles stability were still better than the original value of un-hardenednano-capsules (zeta potentiall=-8.67mV). Based on the particle size transition rules among nano-capsules’formation and heat effect, it signified the macromolecules’ absorption, cross-linkage and heat-unfolding were themain reasons for size variation.During the analysis of volatile compounds for jasmine essence nano-capsules hardened at pH7undervariable heating time, five characteristic flavor compounds (linalool, methyl anthranilate, cis-jasmone,dihydromyrcenol, benzyl acetate, acetic acid phenethyl ester, cinnamic acid methyl ester and indole) for jasmineessence were found gradually decreased along the extension of time. According to these results, it could beconcluded that the nano-capsules containing jasmine essence were able to resist the heat processing (80°C)below5hours. Furthermore, the thermal analysis was applied to explore the capability of jasmine essencenano-capsules against dry heating effect. The results found the nano-capsules processed at pH7,8displayed abetter heat-resistant capability than pH6. After15min heating at200°C, their total weight lose was only lessthan20%. |
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