Emulsifying Properties And Mechanisms Of Low-methoxylated Citrus Pectins Prepared With Alkalescent Processes

Pectin is a plant polysaccharide composed of D-glucuronic acid(d-Gal A),D-(d-Glc A),and partially methylated glucuronic acid,and is present in all plants.According to the extracted sources materials,pectin can be divided into apple pectin,beet pectin and citrus pectin.Pectin can be divided as high-methoxylated citrus(HMCP)(degree of esterification more than 50%)and low-methoxylated citrus(LMCP)(degree of esterification below 50%),depended on the degree of esterification(DE)of pectin.Pectin with different degrees of esterification were obtained by employing different extraction methods.Among them,pectin extracted from citrus peel by using acid method is mostly high-methoxylated pectin(HMP).Citrus pectin with different esterification degree has different emulsifying and gelling properties.Appropriate methods can be used to prepare LMCP,but LMCP obtained by using the traditional method has some drawbacks,such as small molecular weight and low viscosity.Thus traditional method is not suitable to prepare the larger molecular weight and high viscosity pectin.This paper proposed a new method for preparing LMCP without lowering raw material--HMCP molecular weight(M_W).Besides it also analyzed the factors that affected LMCP emulsion stability and the mechanism of calcium lactate promoted LMCP emulsion stability.The results are following:1.HMCP solution pH(regulation with ammonia),H₂O₂ addition and reaction temperature affected the degree of esterification(DE)significantly.The modified citrus pectin with different DE could be prepared by increasing HMCP solution pH and adding different concentration of H₂O₂.LMCP could be prepared by adjusting HMCP solution pH to 9 and adding26.5 m M H₂O₂.Through the analysis of the reaction conditions,it showed that reaction temperature affected the esterification degree,yield and M_W of modified pectin significantly.The reaction temperature increased,the DE of modified citrus pectin increased,but its yield and M_W decreased.2.It product LMCP when added ammonium acetate to HMCP solution.The results showed that modified pectin DE decreased with increasing ammonium acetate concentration but its degree of amidation(DA)increased with increasing ammonium acetate concentration.Ammonium acetate concentration had little effect on the modified pectin yield.The DE and M_W of modified pectin reduced when raised the reaction temperature.The DA of modified pectin increased with increasing reaction temperature.Extended reaction time,the DE of modified pectin decreased too,but reaction time did not have significantly affected on modified pectin yield and its M_W.In order to obtain LMCP,the reaction factors were 20%ammonium acetate,reaction time 24 h and reaction temperature 20?.In this condition,it got LMCP with its DE 43.00±3.10%and its M_W3.97×10⁵.3.It could produce LMCP when changed HMCP solution to weak alkaline conditions using sodium hydroxide.It product modified citrus pectin(MCP)when changed HMCP solution from 6.00 to 8.00 and kept different temperature(45??65?)for different time(3h?7h).It showed that the DE of modified pectin was decreased with increasing HMCP solution pH and it had the same tendency when increased reaction temperature and extended reaction time.Solution pH,reaction temperature and reaction time had significantly affected on MCP yield.It had low yield when increased HMCP solution pH and increased reaction temperature.It also reduced MCP yield when prolonged the reaction time.HMCP solution pH,reaction temperature and reaction time also had affected modified pectin M_W.Modified pectin M_W was decreased when increased reaction temperature and reaction time at alkaline condition.In the condition:HMCP solution pH 8.00,reaction time 24 h and the reaction temperature 40?,It got LMCP with its DE 37.50±2.50%,M_W 1.50×10⁵ and its M_W was a third of the M_W of HMCP.4.An emulsion was prepared by homogenizing 30%rapeseed oil with 70%(wt/wt)aqueous solution.The emulsion viscosity increased when the concentrations of calcium lactate and LMCP(DE 43.00%)in the emulsion were increased.A stable emulsion was obtained when the concentrations of LMP and calcium lactate were 1.00%and 9.00 m M at pH 3.00,respectively.Analyzed the relationship of emulsion stability with emulsion particle size distribution,it knew that the stable emulsion particle size distribution was between 10?m and 100?m.Thus,particle size distribution affected emulsion stability.There was no correlation between emulsion stability and emulsion zeta potential.Optical microscopy and laser scanning confocal microscopy revealed that the stable emulsion was a Pickering emulsion.In the Pickering emulsion,LMCP with calcium formed micro gels that were absorbed on the oil surface.5.The effect of two kinds of calcium salt(calcium lactate and calcium chloride)on pectin emulsion stability was studied.Four kind polysaccharide:polygalacturonic acid(PG),LMCP-1,LMCP-2 and LMCP-3 were chose as emulsifiers.The results showed that LMCP-1,LMCP-2 and LMCP-3 with calcium lactate could form the stable emulsion.LMCP-3 with calcium chloride also formed the stable emulsion.Calcium lactate promoted emulsion stability.The solution viscosity of LMCP emulsion in the presence of calcium lactate was larger than that in the presence of calcium chloride.Microscopic images showed that the stable emulsion formed with pectin and calcium ions is Pickering emulsion.Pectin with cross-linking agent calcium salt adsorbed on the surface of oil droplets.Cryo-SEM images showed that LMCP together with calcium ions as the small globular structures adsorbed on the surface of the oil droplets.The density of small particles adsorbed on the surface of the oil droplets which formed with calcium lactate was larger than that formed with calcium chloride.There were empty spaces on the surface of oil droplets in the LMCP emulsion that formed with calcium chloride.The negative ion of calcium lactates as the target could guide LMCP adsorbed on oil droplets.