104 Food Research Laboratory
New York State Agricultural Experiment Station, Geneva, New York 14456
(315) 787-2266 (Campus: 122-266)
E-mail:mar2@cornell.edu
Isaac Newton (~ 1860) was the first to formulate a mathematical description of the resistance of a fluid to deform or flow when a stress is applied. The resistance was described as the viscosity, and mathematically defined as the shear stress divided by the rate of shear strain. From that time until Couette developed the first rotational viscometer (~ 1890), viscosity was measured using stress driven (gravity) flow. Many techniques which use this principle, such as flow cups, u-tubes and capillaries are still very popular.
For many years, because of synchronous motors, only controlled strain rate instruments were available. The first controlled stress instrument capable of continuous rotation was developed by Davis, Deer and Warburton (1968 J. Sci. Instr. 2, I, 933-6) at the London School of Pharmacy. This instrument used an air turbine drive and an air bearing. In the early 1970's, a second generation of instruments was developed using induction motor drive to avoid the problems associated with the air turbine.
It is generally recognized that fundamental rheological parameters, such as apparent viscosity (Pa. s) and modulus of rigidity (Pa) are useful properties in product development and that simple empirical tests cannot provide reliable data on complex food products.
Researchers and Product Development Scientists mention the lack of data on the viscosity or viscoelastic properties of food products at different temperatures or on the contribution to viscosity or rigidity by ingredients in food products, such as mixtures of gums, proteins and pectins.
We can provide quantitative answers to many of the questions on rheological behavior of foods that are either liquids, semi-solids or that undergo a phase transition (e.g., sol-to-gel). These studies can be done either as a service project in which the identity of the product ingredients is not known to us or a research study in which we have knowledge of the ingredients.
In our laboratory, we have a Carri-Med CSL2 100 controlled stress rheometer, a Haake RV2 viscometer, and several glass capillary viscometers. In addition we have a Voland Texture Analyzer.
This state-of-the-art controlled stress rheometer is being used for obtaining shear rate-shear stress, creep compliance, and small amplitude oscillatory data; the first test provides data on viscous liquids, while the latter two provide data on viscoelastic materials. Data from the CSL2 100 rheometer are obtained under the control of as well as analyzed by a Gateway 2000 4SX-33 computer. Tests can be conducted over a wide temperature range (-5 to 95 deg.C).
The Carri-Med CSL2 100 Rheometer is a product of T.A. Instruments
The Haake RV-2 is a direct drive, concentric cylinder, controlled strain rheometer. This intrument has been a workhorse for the lab. With the many available geometries it is very useful in measuring the viscometric properties of many food materials.
A mixer device attached to a Haake RV2 viscometer has been developed to obtain shear rate-shear stress and yield stress data on food suspensions. The data reduction is done using software developed in our laboratory.
A pressure chamber was reciently developed to allow testing of food materials with overpressure. With this system temperatures of 133 degrees C and pressures of 36 psi can be reached. This lets us test foods at process conditions.
The Voland Stevens is a plunger type analyser that can be used to give force to distance information on solid or semi-solid foods.
We have determined the role of composition in the rheological behavior of several foods:
In particular, we can provide data on the rheological behavior during sol-to-gel transition and on the relative contribution of individual components (e.g., starches and proteins) in a food to the overall rigidity modulus of the food. These data will enable you in choosing food gums that are appropriate and in overall improvement of your products.
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