Effect Of High-voltage Electric Field On Model Food Freezing Behaviors

High-voltage electric field(HVEF)has shown great potential in food processing for energy saving,low cost,and its significant bio-electromagnectic effects.Most importantly,it can be applied to those processing units operated in air,which exactly meet the demand of food refrigeration.Thus numerous studies have been carried out to investigate the effect and mechanism of HVEF on freezing and it is found that HVEF can induce polarization as well as realignment of water molecules,resulting in the premature of ice nuclei.However,there still exist some problems to be solved,such as dielectric breakdown and poor reproducibility of the results which restrict the industrial application of HVEF-assisted freezing.Prior to the freezing experiments,a new electrode system was designed to make sure the system can operate at high electric field strength with no partial discharges or sparking.And then this system was used to conduct subsequent experiments including three parts.Firstly,pure water and dimethyl carbonate(DMC)were selected as the representative of polar solvents and nonpolar solvents,respectively,to verify the universality of polarization-realignment mechanism.Secondly,sucrose and four amino acids were introduced into water to make non-electrolyte solutions of gradient concentration and electrolyte solutions of different electronegativity,respectively,for the purpose of finding the interaction between HVEF,solutes and solvent and their combined effect on freezing behaviors.Finally,the effects of HVEF on the freezing properties and quality of semi-solid food were studied by using agarose gel as model.The main research results are as follows:1.HVEF-assisted freezing was proved to be effective on reducing the supercooling degree of water and promoting the total freezing rate.And this effect enhanced with the increase of electric field intensity,which was significant(p<0.05)especially when the external voltage was above 6 kV(with an initial internal electric field intensity>1.49×10~5 V/m).Under 6 kV,the supercooling degree of water reduced by 86.4%and the total freezing rate increased by 22.47%compared with the control.The reproducibility of results was about only 30%at 3 kV,and increased up to 80%when the applied voltage was above 6 kV.No significant change was observed in DMC,indicating that solvent molecules possessing a lager dipole moment will gain better enhancement in HVEF-assisted freezing.2.The initial internal electric field intensity increased with the decrease of relative permittivity of aqueous due to the introduction of solutes.The influence rule of HVEF acting on sucrose solutions and amino acid solutions was the same with water,and the differences mainly depended on the concentration and electronegativity of solutions.For sucrose solution with a low concentration of 5%,the polarization of water molecules induced by HVEFs was the predominance effect on freezing behavior,while those with higher concentration mainly depended on the hydration of sucrose molecules.For amino acid solutions,electrophoresis would take place due to the electrostatic attraction.The concentration of the ions and charges in the bulk liquid would decrease to varying degrees,resulting in a potential difference at the solid-liquid interface which allowed to promote or retard the phase transition period.3.The surface tension and contact angle of frozen-thawed liquid decreased after treated by HVEF,suggesting that HVEF-assisted freezing could lower the surface energy.Meanwhile,HVEF was capable of enhance the degree of ionization and polarization of the solution,with an increase of the conductivity and a shift of pH value towards higher acidity or alkalinity.4.HVEF-assisted agarose gel freezing allowed a significant reduction of the supercooling degree and total freezing time(p<0.05).Numerous ice crystals with fine particle sizes and uniform distribution were obtained.Some reinforcement was detected in the gel strength by increasing the electric field intensity,but not significant to rescue the structural damage caused by freezing and thawing.Combined with the freezing behaviors of liquid model food,the application of HVEF to the freezing process of real food system has great potential to increase the freezing rate and maintain the food nutrition as well as sensory quality.