Effect Of Lipid On Solube Characteristic Of Milk Protein Concentrate

Milk protein concentrate, as a functional milk ingredient with high protein content and low lactose content, has been widely used in food industry. However, its application is limited for the poor solubility, especially after storage. Lipids in milk introduce positive effects into dairy products, such as the excellent flavor, abundant nutrition and special texture. However, they might cause lipid oxidation and reduce the solubility of milk powder because the interaction between proteins could produces protein aggregations. Milk lipids would release to the particle surface even with very low content in the milk powder, which extends the wetting time when dissolved in water. The oxidation of lipids could also reduce protein solubility by crosslinking proteins. This study was focus on the effect of lipid on the protein solube characteristic of milk protein concentrate, which lacks of intensive study now.Firstly, we established two different model systems using different lipids after analysis the lipid composites in milk as well as their melting thermograms. The first system studied the effects on the solube characteristic of milk powder during storage at 45 ℃ for 30 days caused by saturated milk lipid simulations with different melting points using methyl laurate, methyl myristate and methyl palmitate.. In the second system, unsaturated fatty acids was studied, i.e., ricinoleic acid and oleic acid. when stored at 35 ℃ for 60 days. The lipid release was well studied by gas chromatography(GC), confocal laser scanning micrograph(CLSM) and low field nuclear magnetic resonance(LF-NMR). The changes in the powder solubility under different conditions was studied and the mechanism was explored to explain solubility reductions.The lipids composites of MPC powder surface after spray drying was affected by the melting point of lipids. There was less content of the lipid with lower melting point if with same saturation. The release of unsaturated fatty acids onto the surface was easier comparing to the saturated lipids with similar melting point. During storage, the phenomenon of lipid release was more significantly observed when identified by the CLSM images and the change of relaxation time(T2), though the lipid contents on the powder surface didn’t changed remarkably in GC detection.Different lipids also affect the solube characteristic of protein. The protein solubility during first ten minutes when dissolved in water was influenced by lipid types, surface lipid content, inner lipid content and mobility of lipid. For saturated lipid, the lipid with higher surface lipid content dissolved slower at first; the dissolution speed turned slower for the lipid with higher inner lipid content. The most different dissolution was observed for MPC powder obtaining lipid with highest melting point and poor mobility. For the second system, oleic acid decrease the dissolution speed of powder, while ricinoleic acid accelerated the speed. Unsaturated lipids also promoted single protein components’ release. The solube characteristic was affected by lipid. Saturated lipid with higher melting point could most effectively promote the solubility reduction. For unsaturated lipid, oleic acid had no obvious effect on the protein solubility though it was slightly oxidized, while ricinoleic acid caused positive effect on the solubility.The storage temperature was very important to protein solubility. Higher temperature would accelerate protein solubility decrease. The interaction between caseins caused insoluble aggregates formation during storage, which decreased the protein solubility. The main driving forces for protein aggregation were in the prefer orders of hydrogen bond > disulfide bond > hydrophobic interaction.In conclusion, the effect of lipid on protein solube characteristic is a double-edged sword. Lipids with different melting points, saturation degrees, compositions and structures played different roles on MPC solube characteristic. Unsaturated fatty acid might improve the solubility. The research could provide a new perspective for MPC application in microcapsule and improvement of MPC solubility in long term storage.