The Effect Of Chitosan And Its Composite Coating On Egg Preservation

As an excellent source of high quality protein,minerals,and vitamins,eggs have been consumed in a daily diet all over theworld.Although the eggshells have been regarded as a good natural protective barrier,there are many micro-pores on them.Furthermore,some micro-cracks on eggshells are easily produced during transport and storage due to their fragilenature.Besides the loss of moisture and escaping of CO₂,the penetration of microbial from the quantities of tiny pores on shell can accelerate the quality deterioration of eggs resulting economic loss.Therefore,coating the egges with barrier films is an effective method to prolong the shelflife of eggs.Because of the excellent properties of film forming,barrier,antibacterialproperties and safety,chitosan have been regarded as an excellent coating material for food preservation.In this paper,the effects of coating agents based on chitosan（CS）was studedon egg preservation,including the influencing factorssuch as the number of coating layers,the concentration of coating solution,the coating time of the second coating,thelayer-by-layer coating of CS/Sodium Carboxymethyl cellulose（CMC）and the CS/Nano-Agcomposite coating.The main contents and results are as follows:（1）The effects of the number of coating layers and the concentration of coating solution of chitosan coatingon egg preservation.Eggs coated by 0.5%（w/v）chitosan for singletime（CS1）,two times（CS2）andthreetimes（CS3）,respectively,were storedtogether with the untreated eggs（CK1）,eggs washedby water（CK2）and eggs treated by 1%acetic acid（CK3）at 25?C and 80%relative humidity（RH）for 36d.Thesurface micro-morphologies of eggshells were observed,and the qualities indice of eggs such as weight loss,Haugh unit,yolk index,pH of albumen were measuredregularly.The results showed that because of the similar coating thickness,eggs in CS2 and CS3 groups presented similar and the best qualities during storage.Eggs in CS1 and CK3 also showed the similar quality owing to their similar microscopic appearancesof eggshells.In general,the shelflife of CS2 and CS3 was at least 15 d longer than that ofCS1 and CK3,and at least 20 d longer than that ofCK1 and CK2.Based on the above results,in addition to the CS2 which was coated two times using 0.5%（w/v）chitosan solution at day 0,another twogroups were also prepared,including CS2-30 which was coated by 0.5%（w/v）chitosan solution for the first time at day 0 and the second time at day 30,and CS-1%which was coated by 1%（w/v）chitosan solution forone time at day 0.And the untreated group（CK1）and water washed group（CK2）were set as control groups.Aftertreatment,the five groups were stored at 25?C and 70%RH for 72 d while the indiceof eggs were measured regularly.The results presented that CS2 and CS-1%presented the similar thickness of coating layer.However,the coating layer of CS2 was morecompact andstablethan CS-1%,leading tobetter qualities of CS2.Therefore,the egg shelflife of CS2 were 40 d longer than the two control groups.It indicated that two times coating could increasethe thickness of the coating layer significantly,and then the barrierpropertiescould be improved,thus enhancing thepreservation effects on eggs.Althoughthe thickness of coating layer between CS2 and CS-1%was similar,CS2 showed the more dense and stable coating structure.Therefore the preservationeffectof CS2was more durable.However,the quality of the eggs in CS2-30 was lower than that in CS2,which indicated that there was no obvious effect to improve the protection function of coatings withthe second coatingat day 30.（2）The effect of second coating time on egg preservation was studied.Eggs coated by 0.5%（w/v）chitosan for singletime and two times at day 0,designating as CS1and CS2,respectively.And the other groups were coated by 0.5%（w/v）chitosan for the first time at day 0,then the second time at day 8,16,and 24,respectively,designating as CS2-8,CS2-16 and CS2-24.The four groups were storedtogether with CK1 andCK2at25?C and 70%RH for 72 d,whilethe indice were measured regularly.Result showed that after the second coating at day 8,the increasingrate of weight loss,the decreasing rate of yolk index and Haugh unit CS2-8were all slowed down.As a result,CS2-8 and CS2 showed the similar preservative performance among the groups.The shelflifes of these two groups were16 d longer than CK1 and CK2,and at least8 d longer than CS1.After the second coating at day 16,the increasing of weight lossof CS2-16 was inhibited.However,no significantly changes onother indice were observed.In addition,CS2-4showed no changes on all the indiceafter the second coating at day 24,which presented the similar qualities with those of CS1.Therefore,the secondcoating could inhibite the deteriationof eggs,with the pre-condition thatthe second coating should performed in the early storage when the quality of eggs was not go down significantly.（3）The effects of layer-by-layer coating by CSand CMC wereinvestigated on egg preservation.Eggs were coated by 0.5%（w/v）CS and 0.5%（w/v）CMC alternatelyfor one,two and three times,and weredesignated as CS/CMC-1,CS/CMC-2 and CS/CMC-3,respectively.Theywere storedtogether with CS2 andthe controls CK1and CK2at 25?C and 70%RH for 94 d,and the quality indice were measured regularly.Results showed that the thickness of coating layer was signigificaly increased by layer-by-layer coating for two times.Therefore,CS/CMC-2 showed comparably higher value of yolk index and Haugh unit,as well as lower and stable pH of albumen,compared to other groups.Theshelflife of CS/CMC-2 were at least 26 d longer than that ofthe control groups,and at least 8 d longer than that ofCS2.However,CS/CMC-3presented similar coating thickness with CS/CMC-2,resulting similar egg qualitieswithCS/CMC-2.（4）The effects of CS/nano-Agcomposite coatings on egg preservation were investigated.Nano-Agparticles prepared using theextract of citrus peels were added to CSsolutions with the weight ratios of 0.5%,1%,and 2%,separately,to prepare the compositecoating solution.Eggs coated by the above compositecoatingsolutions separately,were designated as CS/Ag0.5,CS/Ag1,and CS/Ag2,accordingly.They were stored with CS2 and thecontrols CK1and CK2 at 25?C,70%RH for 94 dwith the indicemeasuredregularly.Results showed thataddition of nano-Ag could improve the structuralstability and the antibacterial activity of chitosan coating.However,extra nano-Ag could reslute in self-aggregation,which would possibly decrease the barrier properties of the coating.Therefore,the coating containing 1%nano-Ag showed the best preservation effects on eggs.Eggs in CS/Ag1 group showed the highest yolk index,Haugh unit value and a stable pH of egg white,while thetotal count plate of internal contentwas also well controlled.As a result,the shelflife of CS/Ag1 was at least38d longer than that of CS2,and 56d longer than that of CK1 and CK2.