Artifact In Rheological Tests Caused By The Gelatin Adsorption Layer At The Interface

With its good gelling properties and surface activities,gelatin has a wide application market in the food and pharmaceutical industries.The linear viscoelasticity of gelatin can be analyzed and characterized by oscillating shear rheological experiments to provide guidances for its industrial application.When it is above the gelation temperature,the gelatin is in a sol state.In the small-oscillation shear rheology experiment,the loss modulus is greater than the storage modulus and the loss tangent is greater than 1,showing a liquid response;when it is below the gelation temperature,gelatin is in a gel state,the loss modulus is less than the storage modulus,and the loss tangent is less than 1,showing a solid response.This paper studies the aqueous gelatin solution with a mass fraction of 6 wt%.The main content and results of this paper are as follows:1.Through a variety of combined test conditions,the 6 wt% gelatin solution was subjected to a constant temperature dynamic rheological test.It was found that at a non-gelling temperature(40 ?),when using the lower plate test system to test,the 6 wt% gelatin solution showed a solid response.While using the concentric cylinders,it showed a normal liquid response.In the test conditions that exhibit a solid response,changes in the shape of the geometry,the material of the geometry,and the type of sealing oil used to prevent solvent evaporation can cause changes in the complex modulus and complex viscosity.2.The inverted bottle test of gelatin solution shows that the samples in the temperature range of 34 ? to 40 ? are in a fluid state,and the solid response of the lower plate test system does not meet the actual state of the solution.Through the temperature gradient measurement experiment of the upper and lower plates and the rheological test experiment under the condition of no temperature difference test,it was found that the temperature gradient between the upper and lower plates is not enough to cause the gelatin solution to gel,and the possibility of measuring the solid state response due to gelatin gelation is ruled out.3.By exploring the effect of adding sodium dodecyl sulfate(SDS)and different p H on the rheological test of gelatin solution,the hypothesis that interfacial adsorption is responsible for the solid response of the rheological test of gelatin solution is verified.When enough SDS is added,the test results of the sample can be restored from the solid response to the liquid response.Changing the p H of the solution changes the gelatin molecules from positively charged to negatively charged,and the test result changes from a solid response to a liquid response.Finally,through theoretical deduction,the contribution of the interface adsorption layer to the test signal of the rheometer from the largest to the smallest for the geometry of different shapes is parallel plate,cone plate,and concentric cylinders,which confirms the actual test results.This study strongly recommends that more attention should be paid to interface effects when measuring low-viscosity protein solutions.