Homemade tomato paste, made from peeled and deseeded tomato purée simmered on low heat, usually reaches a thick, spoon-coating consistency in 3-5 hours. Timing depends on how watery the tomatoes are and how wide the pot is.
Why does it take so long?
Most of a tomato's weight is water, and reaching paste consistency means evaporating most of that off. A slow simmer on low heat lets the water evaporate in a controlled way while giving the tomato's sugars time to caramelise and deepen the flavour.
Wide pot or narrow pot?
A wide, shallow pot or baking tray gets there much faster than a narrow, deep pot, since a larger surface area speeds up evaporation; in a wide pan the time can drop to 3 hours, while a narrow pot can stretch it to 5-6 hours.
Made in the oven, timing is similarly 3-4 hours, at around 120-140°C; the mixture should be stirred every 30 minutes, folding the darker edges back into the centre.
What needs watching during simmering?
Paste simmering on the stove needs frequent stirring — the risk of catching on the bottom rises as it thickens, and without stirring it can burn. Stirring should get more frequent in the last hour, with the heat kept low throughout.
How do you know the consistency is right?
A spoonful placed on a flat plate is done once it doesn't weep any water around the edges and holds the spoon's mark. At this point the colour also shifts from deep red to something closer to brown.
When should salt go in?
Salt is usually added in the last 30 minutes of simmering; added at the start, it concentrates along with the water and the paste can end up saltier than intended. About a tablespoon of salt per kilogram of tomatoes is a reasonable starting point, adjusted to taste.
Is there a way to speed it up?
Draining the tomatoes through a sieve before simmering, removing some of the excess liquid, can cut total time by 1-1.5 hours. That step also reduces the fibre content, though, so anyone who prefers the traditional dense texture tends to skip this shortcut.
How should it be transferred to jars once done?
Once it reaches the right consistency, the paste should go into clean jars while still hot, topped with a thin layer of olive oil to seal it off from air and slow down mould. Stored this way, it keeps in the fridge for a few months, or longer in the freezer if divided into small portions.