Is Your Food Isotropic or Anisotropic?
The Difference Can Affect Your Texture Measurement
By Dr. Malcolm Bourne, New York State Agricultural Experiment Station and Institute of Food Science
Cornell University, Geneva, New York

The words used in the title may be new to some readers so we begin by defining them.

Isotropy' (noun) or 'isotropic' (adjective) means that the material displays the same physical properties with the same values when measured along axes in different directions.
'Anisotropy' (noun) or 'anisotropic' (adjective) means that the material displays different physical properties and/or different values of properties when measured along axes in different directions.

A book is an excellent example of an anisotropic body. If it is grasped firmly at the top and bottom edges (axis 1) and pulled, it will be found to have a high tensile strength. It if is grasped at the two sides (axis 2) and pulled, it will again exhibit a high tensile strength. However, if it is grasped by the covers (axis 3) and pulled, the tensile strength will be almost zero because the covers will simply open.

Some foods are isotropic whereas other foods are anisotropic. For isotropic foods it does not matter in which direction it is tested, but for anisotropic foods it is essential always to test the food from the same direction.

Most gels and gelled foods are isotropic. Potato flesh is isotropic.

Some examples of anisotropic foods are:

  • Meat and other fibrous foods. Cutting across the fibers will give a different texture reading than when the same blade is used to cut between the fibers.
  • Cooked lasagna appears to be softer when the cutting blade is oriented parallel to the direction in which it is issued from the extruder than when the blade is oriented perpendicular to the direction of extrusion.
  • Some cheeses are anisotropic. For example, the method of manufacture of Tilsit cheese incorporates lentil-shaped holes in the cheese. This cheese requires a higher force to compress when the long axis of the holes lie parallel to the compressing platen than when the short axis is parallel to the platen.
  • Surimi has a flaky structure. A lower force is required to compress or cut surimi when the flakes are parallel to the blade then when they are perpendicular to the blade.

When making preliminary tests to specify the test protocol on your product, you should try testing the product along all three axes. If the instrument reading is the same with each of the three orientations, then the product is probably isotropic, and it can be presented to the test machine in any direction. If there is a difference in results between the three axes, then the product is anisotropic and the test protocol should specify exactly the orientation of the text specimen every time it is presented to the machine in order to eliminate this source of variation in the measurement.

This article is copywrited by Dr. Malcolm Bourne, and is presented at this website by permission of the author.

Also by Dr. Bourne: The Perils and Pitfalls of the Puncture Principal
Dr. Bourne's new book is now available! Please check it out by following this link.