Study On The Cold Gelation Between Pectin Of Premna Microphylla Turcz With Soy Protein Isolate And The Preparation Of Edible Films

Premna microphylla Turcz,also known as‘‘Dou Fu Chai'',belongs to the family Verbenaceae.The pectin AOP is a pectin extracted from Premna microphylla Turcz by ammonium oxalate extraction method.This paper aims to study the sodium ion(Na~+)-induced gelation of AOP,the cold gelation between AOP and soybean protein isolate(SPI)and the effect of Na~+on the cold gelation,as well as the preparation of edible films by using the cold gelation between AOP and SPI.The experimental results are as follows:(1)The Na~+-induced gelation of AOP was studied by gel hardness test,thermodynamics analysis and gelation forces analysis.When the Na~+concentration was higher than 0.4 M,solid AOP gel could be formed.In addition,there was an optimal Na~+concentration(0.6 M),which enabled AOP to form the gel with the strongest structure and the best solid-like character.When the Na~+concentration was higher than0.6 M,the strong aggregation of AOP would reduce the order and strength of gel structure,thus weakening the gel properties.Na~+induced gelation of AOP was mainly caused by the electrostatic shielding effect and hydrogen bonds.When sufficient Na~+was added to AOP,the expansion degree of AOP chains decreases due to the electrostatic shielding effect of Na~+,and the pectin chains got close to each other.Subsequently,hydrogen bonds were generated between pectin chains,thus forming a gel.(2)The cold gelation between AOP and SPI as well as the effect of sodium ion(Na~+)were studied by texture test,rheological analysis,microstructure observation,thermodynamics study and gelling forces analysis.The results showed that AOP could cold gelate with SPI at room temperature.Besides,there was an optimal complex ratio(OEP 0.3%w/w,SPI 2%w/w)for the two biopolymers to form a gel with the best gel strength and the highest solid-like character.Addition of low content of Na~+(10 m M and 20 m M)could improve the gel hardness,water holding capacity(WHC),thermal stability and crystallinity of the composite gels.However,higher content of Na~+(30m M and 40 m M)would reduce the gel hardness,WHC and thermal stability,and lead to the formation of new amorphous structure with poor crystallinity.Gelling forces analysis revealed that hydrogen bonds and hydrophobic interaction were the main interaction forces in the composite gels.In addition,low content of Na~+could promote the interaction between AOP and SPI,while higher content of Na~+would weaken the gelling forces of the two components by electrostatic shielding.(3)According to the conclusion that Na~+could enhance the gel strength of AOP-SPI composite gel,edible films were prepared by AOP,SPI and Na~+.In addition,gallic acid(GA)were added in the preparation of the films to improve the fresh-keeping ability of the films.The properties of the films were characterized by physical properties tests,mechanical properties tests,microstructure observation,antioxidant and antimicrobial activity study.The results showed that AOP-SPI-Na-GA1%film showed better mechanical properties and film structure.The addition of GA could improve the functional properties of AOP-SPI-Na membrane,due to the presence of carboxyl and phenolic hydroxyl groups in GA molecules which could interact with AOP and SPI by hydrogen bonds.Furthermore,the addition of GA enhanced the antioxidant activity of AOP-SPI-Na film and enhanced the antimicrobial activity of the film against two common pathogens,Staphylococcus aureus and Escherichia coli,in a concentration-dependent manner.