Preparation And Some Properties Of Caseinate-Hydrolyzed Gelatin Composites

Caseinate, as one of the most important proteins in milk, has good functional properties and higher nutritional value. Gelatin is the denatured product of collagen, and has ability to be as thickening agents and to form thermo-reversible gels. Owing to their extensive sources, low cost, fine properties and high nutrition, they are widely used in food industry.In the present study, trypsin (EC3.4.21.4) was used to obtain gelatin hydrolysate with a limited hydrolysis degree of2%. Then microbial transglutaminase (TGase, EC2.3.2.13) was applied to crosslink caseinate and hydrolyzed bovine gelatin to prepare modified proteins. The content of free amino groups and4-hydroxyproline in the prepared composites were determined and used as an index to select optimal transglutaminase addition, reaction temperature and time via single factor experiments. Required composites were all prepared in the optized crosslinking conditions. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis qualitatively indicated that crosslinking reaction occurred during composites preparation. Assaying of4-hydroxyproline content of the prepared composites indicated that gelatin or hydrolyzed gelatin was conjugated into the composites. Caseinate and crosslinked caseinate were used as two controls, some related functional properties of the modified composites were evaluated. The study is aimed to investigate the impacts of different ratios on the functional properties of the composites, and one more to investigate the impacts of limited hydrolysis on crosslinking.Limited gelatin hydrolysate (DH about2%) was hydrolyzed in trypsin addition of100U/g protein for3h at room temperature. The optimal reaction conditions (i.e. TGase addition, reaction temperature and reaction time) were investigated based on caseinate-hydrolyzed gelatin composite (4:1)(i.e. caseinate crosslinked with bovine gelatin hydrolysate in the ratio of4:1). Extensive crosslinking would occur with high TGase addition, leading to a higher viscosity and making some deterioration of functional properties. Temperatutre is also important to crosslinking, TGase tends to get inactive in high temperature, while lower temperature is bad for reaction rate. The optimal reaction conditions were fixed as TGase addition of10U/g protein, reaction temperature of45℃, and reaction time of4h. Caseinate-hydrolyzed gelatin composites in ratio of2:1and6:1, caseinate-gelatin composites (i.e. caseinate crosslinked with bovine gelatin) in ratio of2:1,4:1and6:1were prepared in the above mentioned conditions.4-Hyp content of the prepared composites were measured as an index of the crosslinking degree (4-Hyp cpntent of native gelatin was142.9g/kg protein.) The results of SDS-PAGE indicated that hydrolysis reduces the molecular mass of the original gelatin; while TGase crosslinking forms the biopolymers with higher molecular mass than97.4kDa which left in the top of the stacking gel. Different content of4-hydroxyproline of the composites and FT-IR spectra analysis also elucidated that TGase could catalyze mingle crosslinking between caseinate and bovine gelatin hydrolysate or bovine gelatin. The analyzed functional properties of the composites show that the crosslinking decreased in vitro digestibility, emulsifying activity and emulsifying stability except for the long-storage stability; while increased surface hydrophobicity, water holding capacity, apparent viscosity, viscoelasticity and hardness of the gels. Additionaly, the isoelectric point of the crosslinked composite tends to a more acid pH. Caseinate-hydrolyzed gelatin composites show better solubility in each pH value, while caseinate-gelatin composites show slightly lower solubility.Thermogravimetric analysis indicated that in the range of20to450℃, the extent of mass loss of the four samples were in a descending order of caseinate-hydrolyzed gelatin composite (4:1), caseinate-gelatin composite (4:1), caseinate and crosslinked caseinate, which is74.94、72.60、70.34, and69.45%, respectively. While for enthalpy change, the order is caseinate-gelatin composite (4:1), caseinate-hydrolyzed gelatin composite (4:1), crosslinked caseinate and caseinate, which is3303,1614,713and194J/g, respectively.The present results evidenced that the prepared caseinate-hydrolyzed gelatin composites, for their better solubility, emulsifying and rheological properties, could be used as a potential food ingredient.