Ethanol-induced High-methoxyl Apple Pectin Gels: Gelation Mechanism And Rheological Properties

As a multifunctional heteropolysaccharide,pectin is widely used in various food systems due to its unique physical and chemical properties and physiological activities.Among them,the most widely application is as a gelling agent to improve the rheological properties of food.Most of the pectin in natural foods is high methoxyl pectin,which is currently the main product supplied by domestic pectin manufacturers.At present,most of the research on pectin gels is based on the improvement of traditional gelation methods.The gelling system is mainly based on acidic food matrix,low concentration of calcium or high sugar conditions,which cannot fully meet the needs in the field of food processing.Therefore,it is of great significance to develop a new technology of high methoxyl pectin gel that is different from the acid-pectin gel.This research used high methoxyl apple pectin as the object and used ethanol to induce gelation to form a new type of ethanol-induced high methoxyl apple pectin gel.In order to reveal the gelation mechanism and rheological properties of ethanol-induced high methoxyl apple pectin gel,this research made a preliminary discussion on the microstructure evolution,rheological characterstic and gelation properties.The main conclusions are as follows:(1)The transition process of"solution-sol-gel"was investigated.The results showed that the mix system was pseudoplastic fluid at the solution and sol stage,and the pseudoplasticity became more obvious with the increase of ethanol content.Once the critical gelation point was exceeded,the non-linear rheological properties of ethanol and gel no longer had a positive correlation in the gelled system.During the"sol-gel"transition process,the dependence of the moduli on frequency decreased with the increase of ethanol and the system tended to be more stable.The microstructural characterization of the ethanol-induced high methoxyl apple pectin gel system showed that the gelation transition didn't involve the change in chemical bond,but was a purely physical process.SAXS results indicated that with the increase of ethanol,the pectin molecular chains began to entangle with each other,associate and form local crosslinking.The molecular clusters gradually agglomerated,expanded and linked with larger fractal dimension,eventually,the elastic network formed.In addition,the discontinuity of the pore size distribution became more obvious.The results of Cryo-scanning electron microscopy provided objective evidence for judging the gelation process.As the amount of ethanol changed from little to high,the network pore size changed from large and dispersed to agglomerated and dense with filaments.The three-dimensional skeleton of the gel gradually expanded and became uniform.(2)The results of ethanol-induced high methoxyl apple pectin gel system under thermal cycling showed that the effect of rheological properties was related to the ethanol content.Under the same thermal treatment,if the content of ethanol was higher,the elastic and viscous moduli of the gel would be higher with stronger thermal stability.When the temperature was in the range of 25?60°C,E_33.3 and E_37.5 were thermally reversible,and the samples were always in a gel state,while the first thermal cycle treatment was irreversible for E_28.6.The elastic and viscous moduli of ethanol-induced gel both showed frequency dependent with excellent Power Law result.After the thermal treatment,the frequency dependence was reduced,however,increasing the cycling of thermal treatment didn't contribute to the frequency stability.The LAOS results indicated that ethanol-induced gels with higher ethanol content after thermal cycling would change from LAOS type III(weak strain overshoot behavior)to LAOS type I(strain thinning behavior),and the ability to resist external damage under large strains was also enhanced.Quantitative analysis of Chebyshev coefficients showed that ethanol-induced gel systems were strain stiffening.Gels without thermal cycling presented a shear-thickening to shear-thinning transition with increasing strain,however,only shear thinning behavior was observed after thermal cycling.(3)The turbidity and water holding capacity of ethanol-induced high methoxyl apple pectin gel increased with larger amount of ethanol,but the thermal stability didn't change.The results of texture profile analysis showed that the ethanol-induced high methoxyl apple pectin gel had good mechanical properties.The hardness and cohesiveness of the gel presented a clear upward trend with the increase of ethanol content,but the springness of the gel didn't change significantly.The resilience had a prominent advantage when the ethanol content was 33.3%.With the delay of storage time,the hardness,cohesiveness,springness and resilience of the gel all increased.The headspace-gas chromatography experiments of isoamyl acetate showed that the release of flavor substances in the ethanol-induced high methoxyl apple pectin gel system was affected by many factors.It is necessary to consider both the restriction effect of the gel structure on flavor substances and the solubility of the ethanol solvent for non-polar small molecules.