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I am asking about a method or a trick to measure (or guess) how acidic is food. Please don't suggest any expensive tool; this will be for home usage only.

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Litmus paper is quite inexpensive. Edmund Scientific has 100 strips for US$1.95.

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  • Where can I find this? On supermarkets?
    – mmonem
    Commented Sep 11, 2010 at 20:27
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    Does it harm the food or I have to use it on a sample of the food?
    – mmonem
    Commented Sep 12, 2010 at 9:29
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    @mmonem: My wife, a chemist, doesn't think there should be any problem putting it directly into the food. They require very small samples, so using it on a sample of a food should work as well. Dipping a spoon in, and rubbing the paper across the back of the spoon should be fine. The only problem I could see is if you food has a deep color (tomato sauce, blackberries, etc). It might be hard to tell the color of the strip through the food coloring.
    – KeithB
    Commented Sep 12, 2010 at 13:51
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    @mmonem: If you're in the US, you can order them at the link I gave. Otherwise just search in Google Shopping for litmus paper. Here is a page which gives instructions more making pH test paper of various types yourself.
    – Dennis Williamson
    Commented Sep 12, 2010 at 21:41
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    It's unfortunate that even Edmund Scientific describes that as "litmus paper" - litmus is really a specific pH indicator, that classic red/blue acid/base test that you might've used in your high school chemistry class.
    – Cascabel ♦
    Commented Sep 13, 2010 at 13:26
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Some less precise methods using common ingredients:

Anthocyanin-containing foods (red cabbage, blueberries...) tend to turn blue in an alkaline, red in an acidic environment. Turmeric goes more lemon-yellow in acidic, more orange in very alkaline environments.

Baking soda can indicate strong acidity, it will activate and foam.

If there are green vegetables in a dish, their cooked colour gives you a hint about the cooking liquid (effect is not instant!)- if their color tends to go in the direction of olive drab, you are quite acidic; if they are unnaturally green your cooking liquid is rather alkaline.

Also, browning behaviour, especially of protein rich foods, gives you a hint - if it is hard to brown, your marinade might be just too acidic.

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