How do vegetables stay fresh in jars for so long?

Question

Example: Yesterday I bought some Salsa which contains tomatoes, how does a jar of tomatoes stay fresh and safe to eat for months in store shelves where actual tomatoes only last a week or two?

In: Chemistry

Answer 1

So, things going bad is (mostly!) caused by bacteria getting to it. You stop or slow things going bad by removing and/or preventing bacteria from getting from it. In the case of tomatoes in salsa, this usually means:

1) the tomatoes/salsa were, at one point, cooked at a very high heat. This kills the bacteria already present on the tomatoes and gives us a “clean” starting point.

2) we now want to alter the pH. Tomatoes themselves are mildly acidic; to discourage bacteria that “eat” tomatoes, we need to make the food more acidic than most of them will tolerate. This is usually done by adding vinegar or lemon juice, but chili peppers will also contribute.

3) certain preservatives are inherently anti-bacterial, and get added to food to slow down bacteria progress. Lactic acid and benzoic acid are both popular ones for food, but there are dozens. They help stymie any trace bacteria that happens to get in.

4) the container itself. Cans/tins are best for storing long life food, but even a regular screw top glass jar is absolutely airtight when closed properly. Cheap plastic won’t work, as if tends to be slightly porous, but high grade plastic works. Airtight means no or little foreign bacteria getting into your salsa

5) putting your salsa in the fridge – which most brands will recommend – adds an additional layer of protection: most chemical reactions and bacteria activity slows down or stops below a certain temperature.

All of the above is how salsa will stay fresh for a long while. Note that pretty much all foods use some combination of these five options – pre-cooking/pasteurising, pH control, preservatives, airtight packaging and refrigeration – to prevent foods decaying too fast, but most foods can’t utilise all of them without the food being damaged or degraded in some way by the process. Hence why we can store some foods for extremely long times, and others don’t cope with it well.