Preparation And Characteristics Of Diacylglycerols And Its Interfacial Crystallization, Stabilization Research On Water-in-oil Emulsions

Diacyl glycerols(DAG)are structural lipids of substitution of hydroxyl groups through two fatty acid residues on the glycerol skeleton.This molecular structure has the properties of emulsification and lubrication,as well as the advantages of safety,nutrition and good workability.DAG can regulate blood lipids,lose weight,relieve diabetes,prevent arteriosclerosis,decrease the burden of the intestine etc.,and is therefore widely used as a functional oil substitute.In this paper,Noval DAG rich in medium-chain fatty acid diacylglycerols(MCD)and medium-and longchain fatty acid diacylglycerols(MLCD)were prepared via the enzymatic esterification of monoacylglycerols(MAG)with caprylic acid.The physicochemical properties,thermodynamic properties,crystal form and morcrostructure of DAGs were characterized.The W/O emulsions were prepared using DAG,and the interfacial crystallization and stability of DAGs based W/O emulsions were studied.The main results are as follows:High purity diacylglycerols(DAG)rich in medium-chain fatty acid diacylglycerols(MCD)and medium-and longchain fatty acid diacylglycerols(MLCD)were prepared via the enzymatic esterification of monoacylglycerols(MAG)with caprylic acid followed by molecular distillation(MD),solvent fraction and low-temperature centrifugation.The content of DAG in the crude product was 44.8 ±0.1%,under the selected esterification conditions,which were MAGs/caprylic acid mole ratio of 1:3,reaction temperature of 65 ?,reaction time of 30 min and enzyme load of 5 wt%.Subsequently,the one-step MD and solvent fraction in methanol/ethanol increased the DAG content to 61.3 ± 0.8%.Eventually,the product containing 86.6 ± 0.6% of DAG with 39.3 ± 1.3% of MCD and 47.3 ± 0.6% of MLCD was obtained by the methanol crystallization at 0 °C with a water content of 9wt% and a 1:3 ratio of glycerides/methanol(v/v)followed by the centrifugation separation(11000 × g)at 0 °C.Diacylglycerols(DAG)rich in medium-chain fatty acids and medium-and long-chain fatty acids were prepared to study its effect on the crystallization ofhydrogenated palm oil(HPO)in water-in-oil(W/O)emulsions.For comparison,two commonly used surfactants were used;glycerol monostearate(GMS)and polyglycerol polyricinoleate(PGPR).DAG-HPO emulsion showed a melting and crystallization onset temperature and enthalpy between the emulsion stabilized by GMS and PGPR,and DAG-HPO emulsion had less ? crystal form than GMS-HPO emulsion.Microstructure of the emulsion observed by polarized light microscopy indicated that DAG could form interfacial Pickering crystals around water droplets and could promote the surface-inactive HPO nucleation at the oil-water interface.We propose this effect is due to the molecular similarity between the fatty acids of the prepared DAGs and the HPO,facilitating acyl-acyl interaction.This effect was also observed in the GMS stabilized emulsion but was absent when using PGPR.Therefore,the emulsifying properties of DAG and the interfacial crystallization may accounted for the smaller surface-average diameter and the increased hardness of emulsion when compared to the hardness of the PGPR emulsion.The contact angle of2% DAG in 20% HPO was larger than 90°,which was favored for acting as an effective stabilizer to tailor the texture of W/O emulsion by altering the pattern of crystallization.In this study,emulsions with DAG were prepared(structural lipid)via using two different methods(DAG pre-crystallization followed by emulsification and DAG pre-emulsification followed by quench crystallization).The effects of continuous phase DAG crystals on water-in-oil coagulation,flocculation,sedimentation behavior and storage stability were studied.The results showed that the crystals size of DAG in the continuous oil phase decrease gradually with the increase of shear rate in the pre-crystallization process,and the precrystallized emulsion tends to be stable.It is obvious that DAG has good interfacial crystallization and stability for pos-tcrystallization emulsions.When storage time is less than 4 days,the phenomenon of layered oil release in emulsion is not obvious,showing a more stable state.After 10 days,the droplet size gradually increased and the emulsion showed an obvious stratification.The particle size distribution curve showed that the pre-crystallization emulsion under the shear rate of 10000 rpm and thepost-crystallization emulsion had similar stabilization.The interfacial crystals in the post-crystallization system and DAG crystals of continuous phase in the pre-crystallization system were observed by polarizing light microscope.The above results illustrated that the emulsions prepared by DAG pre-emulsification followed by quench crystallization were more stable than DAG pre-crystallization followed by emulsification.