Thursday, February 18, 2016

Gooey Sweet Rheology

Foods always make for fun and interesting rheology experiments. Witness the ongoing (and incorrect) characterization of ketchup as thixotropic [1]. Talking about viscoelastic and other complex rheological behaviors of polymeric materials really won't grab the attention of too many people, even those with a technical background. But mention the unusual flow properties of food and suddenly everyone perks up.

And so it is with a new research paper (open access with registration until 3/29/16) on the rheology of caramels and how it changes with the recipe.

It's a nice little piece of work, the most surprising result being that even though there are 6 ingredients [2] in the recipe, the rheology can be simplified using standard techniques that greatly reduce the number of variable that we need to be concerned with.

What I really didn't like about the work however, was the comparison of caramel to rubber, or specifically to what they call "Ferry 'type VII'" materials. Can I see a show of hands of how many people know what that means? Anyone? Anyone? Yep, only the SOR (Society of Rheology) members got it.

John Ferry wrote a book, Viscoelastic Properties of Polymers which is canonic. I can't imagine anyone being serious about rheology and not having read the book - it is that good. In it, Ferry shows some "ideal" rheological curves for 8 different types of non-Newtonian systems, ranging from dilute solutions of polymers all the way to highly crystalline systems. One such plot is this:
Storage modulus curves from J.D. Ferry
which shows the curves, designated by Roman numerals, which is where the 'type VII' came from.

But no one ever calls something a Ferry "type x" [3] material. It's not wrong, it's not incorrect, it's just improper and shows a newness to the field. And so to call a caramel as a rubber is also improper. It may have some elastic behavior that mimics rubber, but it is not a rubber. A rubber has an entirely different chemical structure that I won't go into today.

But worse yet is think of this characterization of caramel as a "rubber" falling into the wrong hands, such as those of The Food Babe. She already made name for herself by associating an ingredient in Subway's bread with also being used in yoga mats, so imagine what could happen in a case like this where the researchers already have made the connection to rubber.

When caramels are outlawed, only outlaws will have caramels. But at least we have a good understanding of how to prepare them.

[1] Well, maybe incorrect isn't the correct word, as ketchup is thixotropic, but that characteristic isn't what makes it so hard to get it out of the bottle. It's the yield stress that drives the us nuts.

[2] Or maybe you can call it 4 since 2 of them were held constant. Apparently the researchers are not familiar with Designed Experiments for formulations or similar types of analysis.

[3] Or should that be Ferry 'type X', since they are all Roman numerals?

Previous Years

February 18, 2013 - Viscoelasticity in the Bathroom

February 18, 2011 - How to (not) Become a Polymer Engineer

February 18, 2010 - A Concept Kitchen


Anonymous said...

As a Mid-Atlantic exile of a certain age from the Upper Midwest, I wonder if a couple of my younger coworkers ignorance of Ferry's book is due more to its 1980 publication date or Wisconsin's geographical distance. One is our rheometer operator who is a bachelor chemist, but has taken a number of short courses; the other is a newly minted Chem. Eng. Ph.D. from a Mid-Atlantic university whose thesis work was in polymer melt processing.

John said...

I'd be curious as to what book the Ph.D. used for studying viscoelasticity.

Anonymous said...

Rakesh Gupta,'Polymer and Composite Rheology,' 2nd Ed. (2000)