Research On Moisture Absorption Characteristics Of Egg White Peptide Powders And Effects On Their Quality Characteristics

This study was granted by the key projects in the National Science & Technology Pillar Program during the Twelfth Five-Year Plan Period(2012BAD33B03),mainly focused on studying on the hygroscopicity of egg white peptide(EWP)powders,water dynamics distribution and migration during the moisture absorption process and effects of moisture absorption on quality characteristics of EWP powders.This paper revealed the hygroscopicity and moisture absorption laws of EWP powders and physicochemical properties of EWP powders varieties after moisture absorption.Moreover,these may be able to lay a theoretical foundation for later researches on stable storage of EWPs and the synergistic reactions of functional peptides in polar microenvironment.We have innovatively carried out four study contents.The summarizations of them are listed below.(1)In the first study content,the hygroscopicities of EWP powders with different molecular weights of less than 1 k Da,1~3 k Da,3~10 k Da and more than 10 k Da were studied by the Saturated Salt Solution in static weight difference method.One Variable Quadratic Equation models have been constructed to conduct the moisture absorption of EWP powders.The regression analysis results illustrated they are simple,but with good fitting degrees.Additionally,the moisture absorption isotherms of EWP powders were drawn.Based on them,we have calculated the moisture absorption rate equations,initial moisture absorption rates and critical relative humidity(CRH)of EWP powders.We found that the EWP powder of more than 10 k Da had the maximum moisture initial speed of 0.1051 g/m L;The EWP powder with less than 1 k Da molecular weight had the maximum absorption rate and strongest absorption capacity.And its theoretically equilibrium moisture content is 31.5%;The 1~3 k Da EWP powder had the lowest CRH values of 61.57%,and the storage relative humidity(RH)of EWP powders should not be higher than this value.(2)In the second study content,three oligopeptides,DHTKE,MPDAHL and FFGFN derived and identified from EWP with the strongest absorption capacity,were investigated.With the helps of the software of Peptide Analyzer and reversed phase HPLC,we have got the information about net charge distributions of the three oligopeptides.It was found that the isoelectric points of the three oligopeptides were all in the acidic p H range,and the order of hydrophilic capacity was DHTKE > MPDAHL > FFGFN.Moisture sorption kinetics curves and isotherms of the three oligopeptides were obtained by using the water Dynamic Vapor Sorption.We found that the DHTKE powder obtained by freeze-drying after solid phase synthesis presented as hydrated crystal structure,DHTKE·x H2 O,and the internal combined site with moisture was not saturated.Additionally,it is very difficult to detach the absorbed moisture combined with DHTKE by means of physically reducing the RH.While MPDAHL and FFGFN peptide powders are mainly in amorphous structure,their amorphous structure will be irreversibly collapse during the moisture absorption process,and will release the capillary water into external environment.(3)In the third study content,the water dynamics and migration and distribution of three egg white oligopeptides,DHTKE,MPDAHL and FFGFN at 25% RH and 75% RH were investigated by low field Nuclear Magnatic Resonance.It was found that there were four water populations with different free degrees distributing in the three oligopeptides during moisture absorption process,and the most obvious change in the three oligopeptides was combined water with the shortest T2 relaxation time.It was proved that there were exchanged water fractions in all of the three oligopeptides by deuteroxide leaching experiments.In addition,the origin of each T2 relaxation signal peak was inferred.The shortest T2 relaxation signal was rooted in the combined water and a part of hydrogen protons in polar groups of sample.The next slower T2 relaxation signal peak was rooted in capillary water or interstitial water and hydrogen protons in non polar groups of sample.The fractions with T2 relaxation time more than 1000 ms are free water populations.(4)In the fourth study content,the physicochemical properties of DHTKE,MPDAHL and FFGFN were compared to analyze their quality characteristics after respectively exposed in 75% RH and 25% RH for 30 h.After moisture absorption,it was found that there was no new chemical substance generating in samples,but the p H values were significantly(P<0.05)increased illustrating the stability of sample solutions decreased.Moreover,the rate of DPPH free radical scavenging of three oligopeptide solutions also decreased significantly(P<0.05),which indicates moisture absorption has an adverse effect on bioactivity of EWP.In the Fourier transform Mid Infrared detection,the number and strength of-OH group at 3600~3300 cm-1 were both increased after moisture absorption.And the groups stretching absorption around 1000 cm-1,such like C-C and C-H appeared red-shift phenomena,which revealed that these groups have become unstable after moisture absorption.The results of Differential Scanning Calorimetry showed that as the moisture absorption capacity of DHTKE was strong,the massive absorbed moisture will cause hot-melt of sample disappeared and make the sample gasifying easier.After moisture absorption,the crystal structure of MPDAHL will change and its thermal melting point will increase.While the physical space structure of FFGFN powder will change slightly and its melting temperature point will decrease after moisture absorption of.In a word,moisture absorption will result in the change of the spatial structure of the egg white oligopeptides and will affect samples' sensitivity to heating.In addition,absorbed moisture increases the fluorescence of the hydrophobic peptide FFGFN in high concentration solution and interferes with the detection of MIR and DSC.