Squeezing tomato ketchup in a bowl to observe flow behaviour

Science of tomato ketchup

It’s probably very controversial what I’m about to say, but it’s just my humble opinion: ketchup ruins hamburgers & french fries. Like truly ruins them. It’s such a disappointment to bite into a perfectly fine juicy hamburger and then discover it’s literally covered in ketchup. And I find I waste a good french fry on the ketchup. That said, the science of ketchup is very interesting!

Nowadays ketchup is a staple in so many restaurants & homes. Most of us know ketchup as a sweet tomato sauce, ideal for dipping or dressing. However, ketchup started out as a more general condiment, which didn’t even necessarily contain tomatoes. It seems that the British discovered a fermented flavoursome condiment in China centuries ago. Back home, they tried to replicate it and over time, it developed into the (tomato) ketchup we now know.


Proof for ketchup having its origins in Asian sauces, can still be found nowadays. In the Netherlands there’s a quite common sauce called ketjap, which is an Indonesian soy sauce. The name, when pronounced, is very similar to that of ketchup!

close up of red tasty tom tomatoes
Fresh grown tomatoes, a core ingredient of ketchup!

The ingredients of ketchup

The core ingredient of current day ketchup, or tomato ketchup, are tomatoes. The amount varies per brand and type. Tomatoes are washed and ground finely to create a smooth consistency. Along the way, some of the water is removed from the tomatoes to thicken up the sauce. Since tomatoes contain mostly water, about 95% of a tomato is water, the weight of the initial tomatoes might be higher than the actual weight of the tomatoes in the final ketchup. For instance, 148g of tomatoes are used to make 100g of a specific Heinz tomato ketchup (Feb-2018, Albert Heijn store website).

You might think that ketchup is a good way to use up old, low quality tomatoes. However, often the opposite is the case. Tomatoes for ketchup can be harvested at their prime since the time to get from harvest to production facility is often very short. They don’t have to take into account travel of the tomatoes to different stores, etc. Big tomato ketchup brands often have their own contracts with tomato growers to grow tomatoes specifically for their ketchup.

Apart from tomatoes, there’s more in ketchup, else it would have just been a tomato paste or sauce. The main two other ingredients tend to be vinegar & sugar. Both of these are essential to ensure the tomatoes don’t spoil and that the ketchup can be held for long enough. You will also commonly find salt and spices/herbs in your ketchup.

french toast with ketchup
Just one of many ways to enjoy ketchup: with French Toast!

Ketchup is a way to preserve tomatoes

Fresh tomatoes, unlike apples for instance, can’t be stored for very long periods of time. If you’ve ever stored tomatoes at home you will know they are prone to softening. Once the skin has softened these areas are more prone to mould growth causing the tomato to spoil. Since tomatoes don’t grow the whole year that results in seasons with plenty of tomatoes and seasons with only very little. Canning tomatoes is a way to preserve them year round, but so is making ketchup!

Lower water activity helps to preserves ketchup

As we discussed before, a food with a low water activity is less prone to spoilage by micro organisms. Once the water activity is low enough, bacteria & moulds won’t be able to grow in the food anymore.

The water activity of a food is a measure for the amount of available water in the food. A low water activity (close to 0) means there is little available water. A high value (close to 1) means there’s a lot of available water. Pathogenic micro organisms (those that make you sick) tend to require a high water activity for proper growth, often >0,9.

What causes the lower water activity of ketchup?

The water activity of a fresh tomato is very close to one, it contains so much water. Ketchup though has a water activity closer to 0.93 thanks to the addition of both sugar (about 20w%) and salt (only about 1w%) to the tomatoes. This is still high enough for most micro organisms to grow. A lot more sugar or salt would have been necessary to stop all growth which would make the tomato ketchup pretty unappealing. However, food safety often depends on the hurdle theory, putting several hurdles in place that each slow down spoilage and that together will prevent spoilage.

green and various types of red tomatoes
Tomatoes are slightly acidic by themselves.

Lowering the pH-value

The next and major hurdle is the low pH-value of ketchup. A low pH-value means that a food is acidic. Tomatoes by themselves are already acidic. Vinegar though has a pH-value quite a bit lower than that, bringing the final pH-value of the ketchup down to about 3,9, although the exact value will depend on the brand and type. At this pH-value a lot of micro organisms do not grow anymore, a major hurdle for preserving the ketchup.

So why doesn’t ketchup taste that acidic? That is because of that sugar. The sweetness of the sugar hides the acidity of the vinegar.

How ketchup is made

Ketchup starts with the tomatoes, which are sorted and washed. The process from that moment on varies slightly per manufacturer but consists of a few core steps.

First of all, the tomato has to be broken down into smaller pieces. This can be done through crushing, or by using a homogenizer which breaks down all the bits and pieces in a tomato. The tomato may also be peeled. Peeling can be done with the help of steam. A short hot steam treatment of the tomato will loosen the skin and make it easy to remove.

The tomatoes need to undergo at least one heat treatment step. A sufficiently warm heat treatment can also be called a ‘hot break’ and it deactivates any enzymes in the tomato. These enzymes, which are naturally present, could otherwise break down pectin. Pectin is a large colloid molecule which helps to thicken the tomato ketchup, just as it does for pumpkin puree. If it is all broken down during processing, the ketchup will be a lot more watery.

The other heat treatment may be a cooking process. During this process the tomato paste is mixed with the vinegar, sugar and other ingredients and cooked together to form the desired consistency. During the cooking process water may be evaporated to concentrate the ketchup and possible micro organisms can be killed. The hot break process and cooking process do not necessarily have to be one and the same process.

Instead of starting out with the fresh tomatoes, ketchup manufacturers my also buy already made tomato paste/puree and convert that into ketchup. In that case the manufacturer really only has to blend the paste with its own ingredients to make the ketchup recipe, cook it and fill it into packaging! The video below shows an example of such a process. Notice especially how thick the tomato paste at the start is, this requires very specific equipment to even be pumped into the process.

Colour of ketchup

Ketchup does not need any additional colourants to be red. Using bright red tomatoes will give a bright red ketchup. That red colour comes from a molecule called lycopene which is part of tomatoes and is what makes them red.

For ketchup processing it is recommended not to use iron processing equipment for risk of losing the red colour. Also, the exposure to oxygen should be limited to prevent colour loss and from turning the tomato brown.

Squeezing tomato ketchup in a bowl to observe flow behaviour
Ketchup being pushed out of a tube package.

The flow of ketchup is special

If you’ve ever eaten ketchup you will just know that it doesn’t come out of its bottle by itself, even if you turn it upside down. Pretty strange though isn’t it? Diced tomatoes and tomato juice would flow out all by themselves immediately. We find it so normal though this special behaviour that you might have never even considered it.

Ketchup is a so-called non-Newtonian fluid. A Newtonian fluid, such as water, has a constant viscosity, or ‘thickness’ independent of whether it’s shaken, stirred, pressed, etc. It only changes with temperature. For a non-Newtonian fluid that is not the case so its viscosity (flow behaviour) changes depending on how you apply stress such as shaking or mixing. Ketchup does not flow when you place the bottle upside down, however, if you shake it, it will start to flow! The ketchup literally becomes thinner because of it, this is what we call shear thinning.

The field that studies this type of behaviour in food is called food rheology and is a perfect example of the field of food physics.

Thickening ketchup

As we discussed in the processing section, pectin plays an important role in thickening ketchup. If all the pectin is broken down, the ketchup will become watery. The use of low quality tomatoes, or those that have already started spoiling will also result in a more watery ketchup. This can be overcome by adding in thickeners again such as starch (from corn for instance) or gums (similar to those in ice cream).


A chemical genetic roadmap to improved tomato flavor, Denise Tieman et. al, Science  27 Jan 2017: Vol. 355, Issue 6323, pp. 391-394, DOI: 10.1126/science.aal1556

pH-value of ketchup, link

How was ketchup invented, National Geographic, 2014, link

The seven kinds of catsup (ketchup) you’ll meet in a 19th century cookbook, Slices of Blue Sky, link; great article digging into various different catsups, most of them not tomato

Water activity values of select food ingredients and products, Schmidt, S.J., link, p.412

NPCS Board of consultants & engineers, The Complete Book on on Tomato & Tomato Products Manufacturing (Cultivation & Processing)(2nd Revised Edition), 2017, chapter 10, link

Featherstone, S., A Complete Course in Canning and Related Processes: Volume 3 Processing Procedures for Canned Food Products, 2015 chapter 5, link

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