Concentration-Induced Gelation Of Ca²⁺-Containing Sodium Alginate And High Methoxy Pectin Systems And Its Application

Sodium alginate （SA） and high methoxy pectin （HMP） are water-soluble polysaccharides with naturaland abundant sources, and they are widely applied in the food and pharmaceutical industries due to theirgel-forming properties. SA gels are usually produced by Ca²⁺gelling methods （CGMs）, and the gelation ofHMP is also affected by Ca²⁺. However, the gelling mechanism of SA with Ca²⁺still needs to be improved,and the acting mechanism of Ca²⁺in the gelation of HMP is still unclear. Furthermore, the commonly usedCGMs are not appropriate for the investigation of the Ca²⁺-involved gelling processes （CIGPes） of SA andHMP as the methods either fail to control the gelling process or introduce other materials besides thesubjects into the investigated systems. Therefore, in order to better understand the Ca²⁺-involved gellingmechanisms of SA and HMP as well as to broaden their application range, a new method namedconcentration-induced Ca²⁺gelling method （CICGM） was developed in the study, and the CIGPes of SAand HMP were investigated using the gelling method from the aspects beneficial to practical applications.Firstly, a concentration-induced Ca²⁺gelling system （CICGS） of SA was developed using two SAshaving different viscosity （low-viscosity alginate （LA） and medium-viscosity alginate （MA）） obtained fromthe same brown algae, and their chain interactions during the concentration-induced Ca²⁺gelling process（CICGP） were investigated rheologically. The main structural differences between LA and MA were foundto be their molecular weight distributions according to the analysis results of gel permeationchromatography and1H nuclear magnetic resonance. As shown by the rheological investigation results,1%（w/v） of SA and4μmol/mL of CaCl2were the appropriate initial conditions for preparing a SA CICGSwhich can be converted from a solution state with good flow properties to a gel state of uniform meshstructure during water evaporation. Furthermore, the Ca²⁺-containing aqueous systems composed of LAand MA mixed at ratios of0:4,1:3,2:2,3:1and4:0（weight basis） showed significantly different flowproperties and concentration-induced Ca²⁺gelling behavior, indicating that the molecular weightdistributions of SAs have important influence on their molecular chain interactions mediated by Ca²⁺.Secondly, using the CICGP of SA and its “egg-box” gelling mechanism as references, the effect ofCa²⁺on the gelation of HMP was investigated by the CICGM. The aqueous system composed of1%（w/v）of HMP （75.6%of degree of methyl esterification） and4μmol/mL of CaCl2formed a gel whose strength（storage modulus G′≈1140000Pa） was higher than that of the referenced SA system during waterevaporation. This result confirms that Ca²⁺participates in the gelation of HMP, and also suggests that thegelation of HMP likely involves other factors besides the participation of Ca²⁺. Compared with the SAaqueous system, the flow behavior of the HMP aqueous system was weakened in a greater extent by thesame amount of Ca²⁺addition （flow index n decreased by29%, consistency index k increased by1226%,and loss angle δ reduced by30°）, indicating that the Ca²⁺likely participates in the gelling process of HMPthrough a non-binding interaction with the HMP molecules. Furthermore, the Ca²⁺-free aqueous system ofHMP could not form gels in the water evaporation although33.7%of its δ value was reduced by the waterevaporation. However, the δ value of the HMP aqueous system was reduced from62°to29°due to theexistence of Ca²⁺. These results indicate that Ca²⁺plays a role of triggering and accelerating in the gelationof HMP, and the function of Ca²⁺was achieved through an acting way different from its acting way in thegelation of SA as supported by the absence of an optimal ratio of HMP-Ca²⁺interactions and the high freeCa²⁺content in the HMP gel formed after large amount of water was evaporated in comparison with the SAsystem.Thirdly, the effect of Ca²⁺on the flow properties （charactered by n and k）, physical state properties（charactered by δ） and concentration-induced Ca²⁺gelling properties （charactered by storage modulus G′and loss modulus G″） of the aqueous systems composed of mixed SA and HMP （A/P） at weight-basis ratios of3:1,2:2and1:3（labelled as A3P1, A2P2and A1P3, respectively） were investigated basingunderstanding the gelation of Ca²⁺-containing aqueous systems of SA and HMP during water evaporation.The existence of Ca²⁺changed the linear relationship between the n, k and δ values of the A/P systems andtheir A/P ratios. Specifically, due to the existence of Ca²⁺, the n values of the A1P3, A3P1and A2P2decreased orderly while their k values increase orderly, and the δ values of the A2P2and A1P3werereduced below45°. During water evaporation, the Ca²⁺-containing aqueous systems of both A3P1andA1P3（labelled as CaA3P1and CaA1P3, respectively） showed a CICGP while the Ca²⁺-containing aqueoussystem of A2P2（CaA2P2） kept in a non-gel state all the time. These results manifest that the rheologicalproperties of the A/P aqueous systems are distinctly different from those of the aqueous systems of SA orHMP alone due to the existence of Ca²⁺.Finally, the CICGM was combined with spray drying method to prepare a concentration-induced Ca²⁺gel （CICG）, and the gels were applied as a masked agent of raw taste for starch and an antioxidative andcontrol-release carrier for conjugated linoleic acid （CLA）. The CICG of SA in the starch-SA gel microbeadswas observed to be coating on the surface of the starch granules using scanning electron microscopy.Compared with the raw material waxy corn starch, the starch-SA gel microbeads showed much weaker rawtaste, rough feeling and unpleasant throat feeling in milk, and kept the original flavor and smoothmouthfeel of the milk. This comparison result indicates that the CICGP of SA can be applied as a maskedagent of raw taste for starch to facilitate the direct application of cereal raw starch as slowly digestiblestarch. In addition, the CICG formed by the CaA3P1showed digestion stability relatively appropriate fordeveloping small intestine-target release carriers. Octenyl succinic anhydride modified starch （OSA starch）paste （1%, w/v） and CLA mixed at a volume ratio of25:1could form a stable emulsion, and the freeamylose in the paste could form stable complexes with CLA during the emulsifying operation, so CLA canbe introduced into the aqueous systems of SA and HMP through the emulsifying and stabilizing effects ofthe OSA starch on it. CaA3P1and OSA starch-CLA emuolsion mixed at a volume ratio of4:1could formCLA carriers through spray drying due to the existence of Ca²⁺. After the same accelerated oxidationtreatment, the peroxide value （POV） of the CLA without carrier protection was10.3times of its originalPOV while the POV of the CLA in the carriers was1.6times of its original POV, indicating that the CICGcarriers have a good antioxidative protection effect on CLA. Besides, the CICG carriers released44（±1.8）%and50（±2.9）%of CLA in simulated stomach and small intestine conditions, repectively, suggesting thatthe small intestine-target release properties of the carriers for CLA need to be improved.Briefly, the following conclusions can be drawn based on the above investigated contents and results:1. CICGM is simple and easy for controlling the gelling process;2. the SA-Ca²⁺interactions can beregulated through adjusting the mixing ratios of the commercial SAs of different viscosity to produce SAproducts with desirable application properties;3. the gelling properties of HMP shown in the concentratingprocess of its Ca²⁺-containing aqueous systems can be used to produce new Ca²⁺-related pectin gels;4. theCaA2P2, which can not form gels in water evaporation but has the weakest flow properties and thestrongest solid state properties, can be applied as a heat-stable thickener in the food industry, and theCaA3P1and CaA1P3having good flow properties and concentration-induced Ca²⁺gelling properties canbe applied as gelling agents in the food and pharmaceutical industries;5. the CICGM combined with spraydrying, and the CICGPs of SA and HMP have good practical application prospects in the field of food andmedicine.