LAB 7: POLYSACCHARIDE RHEOLOGY

Transcription

1 Dana Shintani 11/15/11 A02 T PM LAB 7: POLYSACCHARIDE RHEOLOGY I. PURPOSE/OBJECTIVE: The purpose of this experiment is to measure the viscosity of different mixtures by using a viscometer and to gain a better understanding of the functional properties of polysaccharides. II. INTRODUCTION: Functional properties of polysaccharides are the physical and chemical properties that determine how useful a polysaccharide is during processing, storage, preparation and consumption. This is relevant to the food industry because polysaccharides act as thickening agents, stabilizers and gel formers. A thickener increases the viscosity of a food such as pie filling and a stabilizer helps maintain the dispersion of two or more immiscible phases such as in an emulsion. Gel forming contributes to texture in a jam or jelly. Some important food polysaccharides include starch, cellulose, gums, pectin, and seaweed extracts. The functional properties are thus based on their interactions with water and their intermolecular interactions. Evaluations of these functional properties are frequently accomplished by rheological measurements. Measurements of viscous, plastic and elastic characteristics of solutions and gels of polysaccharides help define unique characteristics of the polysaccharides. Rheology is a branch of physics that deals with the deformation and flow of materials. In food systems deformation and flow are major factors that affect the texture of foods. For example, the texture of celery is dominated by is crispness; these properties can be defined in terms of elasticity, viscosity and plasticity. III. PROCEDURE:

2 The procedure followed for the experiment is found in Principles of Food Composition Laboratory Manual (2011) Experiment 7, Polysaccharide Rheology, pages IV. DATA/RESULT: Table 1: Viscometry Data- spindle number, RPM, 2 dial readings for corn syrup, xanthan gum and cornstarch. Figure 1: Plot of the scale readings vs. RPM for corn syrup, xanthan gum and cornstarch. (See Table 1).

3 Table 2: Viscometry Data- RPM, average of readings, calculated viscosity and log viscosity values for corn syrup, xanthan gum and cornstarch. Figure 2: Plot of the viscosity vs. RPM for corn syrup, xanthan gum and cornstarch. (See Table 2).

4 Figure 3: Plot of the log of the viscosity vs. RPM for corn syrup, xanthan gum and cornstarch. (See Table 2). Table 3: Observations of HMP and LMP. V. CALCULATIONS: Viscosity Calculation: Average Viscometer reading for RPM 6=1.0 Conversion Factor=200 Viscosity=1.0*200=200 mpa.s

5 VI. DISCUSSION: The xanthan gum is a psedoplastic solution, the cornstarch is a dilatant solution and the corn syrup is a newtoian solution. These classifications agree with the observations of the fluids by stirring with a spoon. A Dilatant fluid s viscosity increases as the stress and the rate of strain increases, when cornstarch was stirred with a spoon, the faster it was stirred the thicker it seemed. A Newtonian fluid s shear stress is linearly proportional to the shear rate, when corn syrup was stirred the viscosity seemed to be constant. A Pseudoplastic fluid s viscosity decreases as the stress and rate of strain increases, when xanthan gum was stirred, the faster is was stirred the thinner the solution appeared. The ph of 3.3 on the formation of the HMP gel made it more solid. This data agrees with the theory that gelation of HMP takes place at a ph below 3.5. HMP forms gels when hydrophilic solutes are present due to dehydration. An addition of acid protonates the carboxyl groups of non esterified galaturonic acid subunits reducing the repulsion between the chains. For the LMP, CaCl2 made it the most solid while AlCl3 was semi solid and NaCl remained a liquid. The predications agree with the experimental results. LMP requires cations such as calcium for gel formation as cations reduce negative charge repulsion between chains to form a cross-linked network. VII. CONCLUSIONS: This method and instrument was useful in the experiment because it gave accurate results. The advantages of the instrument were that it gave actual data to compare to the manual stirring of the solutions. From the experiment, we were able to learn how to test the viscosity of different solutions and evaluate the functional properties of polysaccharides. No suggestions for improvement. VIII. QUESTIONS: 1. The ph of 3.3 made the pectin gel into a solid. The data agreed with what was predicted from theory. HMP forms gels when hydrophilic solutes are present due to dehydration. An addition of acid protonates the carboxyl groups of nonesterified galaturonic acid subunits reducing the repulsion between the chains.

6 2. For the LMP, CaCl2 made it the most solid while AlCl3 was semi solid and NaCl remained a liquid. The predications agree with the experimental results. LMP requires cations such as calcium for gel formation as cations reduce negative charge repulsion between chains to form a cross-linked network. 3. The xanthan gum is a psedoplastic solution, the cornstarch is a dilatant solution and the corn syrup is a newtoian solution. These classifications agree with the observations of the fluids by stirring with a spoon. 4. A Dilatant fluid s viscosity increases as the stress and the rate of strain increases, when cornstarch was stirred with a spoon, the faster it was stirred the thicker it seemed. A Newtonian fluid s shear stress is linearly proportional to the shear rate, when corn syrup was stirred the viscosity seemed to be constant. A Pseudoplastic fluid s viscosity decreases as the stress and rate of strain increases, when xanthan gum was stirred, the faster is was stirred the thinner the solution appeared. 5. Xanthan gum became thinner as the solution was stirred faster in comparison to corn starch whose viscosity remained constant. It is beneficial because it can be used as a food thickening agent.