| In modern food industry many food emulsions need to be frozen to prolong the shelf life,or as a necessary part during the producting for some special food.But when frozen emulsions are thawed,they are sometimes inevitable partly broken down and oil off eventually,which generates negative quality attributes,such as an undesirable appearance,texture,or flavor.Therefore,it is very important and significant to explore the mechanism of freeze-thaw stability of food emulsion system,and use various methods and strategies to improve food emulsion freeze-thaw stability,in order to extend the application of food emulsion and develop functional food.In this paper,freeze-thaw stability of Pickering emulsion stabilized by soy protein isolate(SPI)heat-aggregated particles was investigated,in order to reveal the freeze-thaw stability mechanism of emulsions from the aspects of particle properties,emulsion structure and water phase composition.First,the freeze-thaw stability of emulsions stabilized by SPI,whey protein isolate(WPI)and their heat-aggregated particles as well as sodium caseinate(NaCN)was studied.The results show that,as for SPI and WPI,heating treatment(95 ?,15 min;70 ?,30 min)could lead to particle size and surface hydrophobicity to increase,and the intraparticle interactive forces to be enhanced.Compared to untreated proteins(SPI and WPI),emulsions stabilized by heataggregated particles(HSPI and HWPI)have better stability against coalescence and creaming.And HSPI even exceeds NaCN to exhibit superior stability against creaming,due to the strong steric stability of the Pickering emulsion and the formation of the gel network structure.Next,the effects of protein concentration(c)and oil fraction(?)on the freeze-thaw stability of emulsion stabilized by SPI and Pickering emulsion stabilized by HSPI were investigated.The results showed that the increase of protein concentration(0.25~4.0%)and oil phase ratio(0.1~0.5)would lead to the increase of apparent viscosity of the emulsion and the enhancement of the structure of gel network,so these emulsions were very stability against creaming.Especially if c?1.0% or ??0.4,obvious stratification would not appear after three freeze-thaw cycles.In addition,the crystallization of aqueous phase promoted the interaction among the droplets,and enhanced the apparent viscosity and gel strength of emulsions.Finally,the effects of ionic strength on SPI heat-aggregated particles and the freeze-thaw stability of Pickering emulsion stabilized by them in the three stages of emulsion preparation were studied.The results showed that the emulsions were completely broken down after three freeze-thaw cycles without NaCl,but the emulsions were relatively stable with 100~500 mM NaCl.With the increase of ionic strength,the freezing point of the emulsions decreased.The apparent viscosity of the emulsions and the strength of the elastic gel network decreased,if NaCl was added before the heat treatment.The apparent viscosity,the strength of the elastic gel network and the creaming index increased,if NaCl was added before emulsification.The creaming index increased,if NaCl was added after emulsification.Therefore,the presence of NaCl would significantly improve the freeze-thaw stability of the emulsion,but excessive NaCl will lead to emulsion droplet flocculation,which is not conducive to the stability of the emulsion.In summary,due to the high steric hindrance stability of the Pickering emulsion and the structure of the gel network,the emulsions stabilized by protein heat-aggregated particles exhibit excellent freeze-thaw stability,and they can be further improved by optimizing the protein concentration,oil phase ratio and ionic strength.It is of great value and significance for the great potential of food-grade Pickering emulsions as well as developing and researching new functional foods. |
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