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 severa
l 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 .
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.
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.