Science & Technology

History and How It’s Made: The Gloppy Chemistry of Ketchup

Ketchup Bottles

Satisfyingly sweet and tangy, ketchup — or catsup — is a staple in many kitchens around the world. Originally a thin, fermented fish-based sauce from the Far East, the idea (not the recipe) was brought back to Europe from Southeast Asia by the British in the 18th century. It then went through several iterations — cockles and muscles, oysters, anchovies, cucumbers, mushrooms — before becoming the thick, tomato-based sauce so many of us love to slather onto our food today. So, what’s in our favorite condiment, and why is it so hard to get out of the bottle?

Ketchup starts with ripe red tomatoes that are washed, sorted, and chopped. At this point, some manufacturers choose to heat the tomatoes to 200°F (90°C). This is called a “hot break” and it deactivates the enzymes in the tomato that would otherwise break down its pectin. Pectin is a polysaccharide that cements together the cell walls of the tomato, and it helps to determine the thickness of the ketchup. Next, the tomatoes are sent to pulping machines where they are stripped of their seeds, skins, and stems.

The pulp and juice mixture is then sent to a cooking tank and boiled for 30 to 45 minutes. This is when water is evaporated to both thicken the sauce and kill off pathogenic microorganisms. During the cooking process, the tomato pulp is mixed with various spices and seasonings. Most of the ingredients are added early in the cooking process but volatile spice oils and vinegar are mixed in later to avoid too much evaporation. The acidic vinegar brings the final pH value of the ketchup down to around 3.9, a pH value that’s hostile to most microorganisms. So why doesn’t ketchup taste acidic? Because it contains (a lot of) sugar. Which also happens to be a natural preservative that dehydrates food and gets rid of more microorganisms.

Ketchup Horizontal

After the cooking phase, the remaining fibers and particles are removed by filtration. This leaves air bubbles in the ketchup, so it’s de-aerated to prevent bacterial growth and discoloration before it’s packaged. The containers — bottles, cans, packets — are then cooled to prevent flavor loss, labeled, and packed. The entire process takes two to three hours.

Ketchup is famously hard to pour, and it owes this behavior to an important additive. Left to its own devices, the sauce would be pretty thin and watery because the tomato pulp that gives it consistency is sieved out, so commercial ketchup makers generally add a small amount of thickener to their recipes. Xanthan gum is the most popular one because its molecular structure gives it an impressive ability to hold lots of water at very low concentrations, but it comes with a side effect: it also turns ketchup into a non-Newtonian, shear-thinning fluid. This means that the viscosity of ketchup decreases — it flows more freely — when it’s shaken, pressed, or otherwise put under stress.

Pouring Ketchup

Xanthan gum is a polysaccharide produced by the fermentation of simple sugars in vegetables by the bacterium Xanthomonas campestris. A molecule of the gum consists of a very long chain of sugar atoms that is straight and stiff like a rod. It also has a negative electrical charge, which attracts and binds water. When two long-charged rods get too close, they strongly repel each other. When the number of rods in the solution — their concentration — is high enough, they end up positioning themselves at right angles to each other, and they also get slightly entangled. This more or less immobilizes them and traps the water molecules in their box-like matrix. It’s the formation of this network that marks ketchup’s transition from a liquid to soft solid.

A sufficiently large shear force — created by shaking or hitting the bottle — provides the energy needed to remobilize the rods, forcing them to slide past one another. They release the water molecules, allowing the ketchup to flow. The amount of force needed to create this flow is what scientists refer to as “yield stress.” The larger the force applied, the faster the rods will move; therefore, the faster the ketchup will flow and the thinner it will seem. When you stop shaking the bottle and the shear forces subside, the network of Xanthan molecules reforms, trapping the water again and making the sauce thicker.

If this all seems complicated, that’s because it is. In fact, there’s a whole branch of science devoted to explaining this type of flow behavior called rheology.[1] The science of rheology has enabled scientists to determine the best way to get ketchup out of a glass bottle[2] and to come up with a more efficient, squeezable plastic one. Heinz itself has thoughtfully positioned the “57 Varieties” label on the neck of the bottle to help you achieve the perfect pour.[3]

Hotdog With Ketchup

Tomato ketchup did not start off with a healthy reputation. Back when it was invented, its popularity was partly due to the fact that it could be kept for several months, but that was not easy to accomplish. The tomato-growing season was short, and makers of ketchup had to find a way of preserving tomato pulp so that they could make it all year round. They did their best with iffy substances such as boric acid, salicylic acid, formalin, and benzoic acid. This resulted in a yellowish concoction to which coal tar was added to turn it red.

Even then, makers handled and stored the pulp so badly that their ketchup commonly contained contaminants like dead bacteria, spores, and mold. It developed a reputation as being “filthy, decomposed and putrid,” something that could cause “many cases of debility and consumption.”[4]

Enter the American pure food movement and its most ardent advocates, Dr. Harvey Washington Wiley, chief chemist in Teddy Roosevelt’s Agriculture Department and a fierce foe of artificial preservatives, and the like-minded American entrepreneur, Henry Heinz. Heinz had started producing ketchup in 1876 and, at a time when nobody cared, was doing his best to make his products as pure as possible. Together, the two men successfully fought for the passage of the Pure Food and Drug Act of 1906. At the same time, Heinz found a way of avoiding artificial preservatives altogether with a recipe that contained more salt and twice as much sugar and vinegar as his rivals’.[5]

Gourmet Hamburger

Heinz’s ketchup was also noticeably thicker because he used costlier, fresh tomatoes that contain more pectin than tomatoes that are old or overripe. A massive public relations blitz persuaded consumers that they would much rather have Heinz’s wholesome ketchup than the cheaper, unhealthier condiment of his rivals. The rest is history.

Thanks to Henry Heinz, tomato ketchup is no longer a serious health hazard. If it can be said to have any health benefits, these can all be credited to one important ingredient: lycopene. Lycopene is a carotenoid found in yellow, orange, and red fruits and vegetables, and it gives ketchup its red color. It is one of the most powerful and effective dietary sources of antioxidants that we know of[6], and ketchup is one of the most concentrated sources of lycopene that you can buy.

According to research, the lycopene in tomato products can:

Fresh Ketchup

For the best health results, eat organic ketchup, which has been shown to contain around three times as much lycopene as non-organic brands.[18] As a rule of thumb, the darker red the ketchup, the higher the lycopene levels.

That said, although it is low-fat and low-calorie, ketchup contains way too much salt and sugar. A 20-ounce bottle of Heinz ketchup contains about 33 tablespoons of sugar! Ketchup is normally eaten in too small amounts to be considered an effective health food. But ketchup’s acidity makes it a good cleaning agent, so should you find an old bottle lingering in the back of your fridge, you can use what’s left to clean your silverware, your cast iron pots, and even your dog.


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