How is opened tomato paste stored?

An opened jar goes into the refrigerator, its surface is smoothed and its lid is closed tightly. We cannot write here how many days it will keep; that information is on the jar's own label, because once a package has been opened, the storage conditions and, where needed, a time limit for consumption must be stated on the label.

Why does spoilage always start at the surface?

The surface is the only part of the jar that touches air, and mold shows up there first. Tomato paste is an acidic product — the communiqué in force sets the pH of tomato paste at no less than 3.9 and no more than 4.6 — but acidity does not remove the contact with air at the surface.

So most of storing comes down to a single job: making the surface smaller and reducing its contact with air. After each use, smoothing the paste with the back of a spoon, scraping down what is left on the sides and closing the lid tightly is the most concrete thing that can be done inside the jar.

How long the jar will take to finish is part of this equation too. A large jar you rarely use keeps the same surface in contact with air for weeks; a small jar that runs out quickly never lives through that stretch. Choosing the package size according to how fast you use it is a decision that comes before any storage technique.

The jar, or another container?

Do not leave paste sold in a metal can in its can after opening; the guideline says that once canned foods have been opened and used, the remaining part should be kept in a suitable container in the refrigerator. The same guideline adds that the packaging of purchased packaged foods should not be used for another purpose.

The kind of container you transfer it to matters as well: acidic foods are advised not to be prepared or cooked in aluminum and copper containers. A glass or food-grade plastic container is enough if its lid closes tightly. Take the paste with a clean spoon every time; dipping a used spoon back into the jar stands directly against the rules for preventing cross-contamination.

How many days does it keep?

There is no single answer to this question, because the products are not the same: salt content, packaging and production method change from jar to jar. That is exactly why the labelling legislation counts special storage and use conditions among the mandatory information, and asks for a separate period to be written for after the package is opened where this is needed.

Looking at the label on the day you first open the jar is a shorter job than guessing later on. Writing the date you opened it on the lid finishes the job; the period on the label only helps if the starting point is known.

If the label gives no period for after opening, what is left is observation: the color, smell and texture of the surface. A darkening color, or a layer that dries and hardens on the surface, is not on its own a sign of spoilage; mold, a sour smell and bulging are the signs to watch for.

If you see mold or bulging

When you see mold on the surface, taking off only that part is not considered a safe solution; getting rid of the jar is the sounder decision. The same holds for the packaging itself: the guideline says that products that have bulged, whose packaging is damaged or that have leaked should not be bought and should never be used.

This check can also be done in the store, before the jar is opened. While reaching for the shelf, looking at whether the lid sits flat and whether there is a trace of leakage at the side of the jar takes a few seconds.

What can you do today?

Buy a jar sized to the pace you use it; if you cook once a month, a large package produces waste, not savings. If you have bought a large jar, you can portion part of it into small containers and freeze it — the guideline says foods to be frozen should be divided into meal-sized amounts and packed so that no air gets in, and that containers should not be filled to the brim for purée-like foods. You can start writing the opening date on the lid today as well.