Rheology at the dinner table
Evan Zabawski | TLT From the Editor September 2016
A popular condiment explains non-Newtonian flow.
One must simply cause the ketchup to shear and it will flow readily.
© Can Stock Photo Inc. / OxfordSquare
WE HAVE ALL STRUGGLED AT ONE TIME OR ANOTHER to get the right amount of ketchup out of a glass bottle. Ketchup never pours readily—and sometimes pours too quickly—and each witness to the struggle offers a different technique to get it right.
Ketchup’s origins stem from a seasoned sauce made from fermented or pickled fish thought to have been brought by traders from Vietnam to Southeast China. Around the late 17th or early 18th Century, British traders sought to recreate the sauce at home, evidenced by a recipe published in 1732 for Ketchup in Paste by Richard Bradley, which referenced “Bencoulin [a British settlement on Sumatra] in the East Indies” as its origin. British recipes did not call for a tomato base and instead used walnuts, anchovies, oysters or Jane Austen’s purported favorite: mushrooms.
The first known recipe calling for tomatoes wasn’t until 1812, but it was still missing vinegar and sugar. Ketchup’s popularity rose due to its nearly year-long shelf life, but the challenge was year-round preservation of the tomato pulp obtained during a short growing season. A variety of preservatives were employed, including coal tar and sodium benzoate. By 1896 a study deemed that 90% of commercial ketchups contained ingredients that posed a health hazard, and Dr. Harvey Washington Wiley spearheaded efforts against their use.
Dr. Wiley believed high-quality ingredients and proper handling negated the need for such preservatives, and he partnered with a ketchup manufacturer whose recipe called for ripe, red tomatoes (which had higher levels of the natural preservative pectin) and much higher levels of vinegar (which also reduced spoilage). His partner’s name was Henry J. Heinz, who began producing ketchup in 1876 and developed a preservative-free recipe in 1906.
A concoction of tomato paste, vinegar, sugar and spices is not inherently thick, so thixotropic xanthan gum is added, which gives ketchup its non-Newtonian properties. (A Newtonian fluid is one whose viscosity does not change relative to time, flow or stress, and time-dependent non-Newtonian fluids are called thixotropic.) Non-Newtonian fluids, whose viscosity is time independent, are largely shear thickening (like Oobleck) or shear thinning (like paint).
These properties prevent the ketchup from flowing off of a hamburger but also are what makes it so difficult to get out of a bottle. There is no trick to it; one must simply cause the ketchup to shear and it will flow readily. Some people have taken the literal approach by inserted a knife up the neck of the bottle, which works best when the bottle is full. In 1983 Heinz introduced the squeeze bottle, completely negating this effort, since forced flow through an orifice creates the necessary stress for the viscosity to decrease. Funnily enough, it took until 2002 before they designed the bottle to be stored upside down so that partially consumed bottles did not require shaking to get the ketchup toward the cap.
Returning to our struggle with a glass bottle, others resort to thumping the bottom of the bottle. Though this does occasionally produce positive results, it can be difficult to control and can produce an excessive glop. This excess of ketchup is a result of achieving the yield stress, a point at which the viscosity decreases by a factor of 1,000.
For best results, firmly tap the sweet spot of the bottle with two fingers (index and middle). Just where is that sweet spot you ask? On a bottle of Heinz it is where the 57 is embossed, a secret that Heinz’s Website claims only 11% of people know, so now you have joined a reasonably exclusive club.
Ketchup displays thixotropic (it will eventually flow to the bottom of the bottle), shear-thinning (squeeze bottle) and yield-stress (big glop) properties, making it a great example for learning about rheology.
Evan Zabawski, CLS, is a senior reliability specialist in Calgary, Alberta, Canada. You can reach him at evan.zabawski@gmail.com.