The canning technique - water bath canning or pressure canning – is determined by the acidity (pH) of the food to prevent undesirable microorganisms (bacteria, mould and yeast). Below is a description of pH, then we will look at how to test pH at home, and then share a chart of the pH of fresh fruits and vegetables with you – but all of this is not essential knowledge for home canners, as reputable preserving recipes for water bath canning and pressure canning will have used scientific methods to check this for you.
The definition of pH is the “potential of hydrogen” – it is the amount of acidity (or alkalinity) in a food (solution). pH has a numerical scale from 0 to 14.
pH Scale: 0 = strongly acidic, 7 = neutral, 14 = strongly basic (alkaline)

For home food preservation:
pH ≤4.6 = food is high acidity (suitable for water bath canning)
pH ≥4.6 = food is low acidity (suitable for pressure canning)
Note: The pH scale is logarithmic, which means that each whole pH value below 7 is 10 times more acidic than one whole pH value above i.e. pH 2 is 10 x the acidity of pH 3, and 100 x the acidity of pH 4. This is important because if your water bath canned food is "a little over" the safe pH for water bath canning, it may be a big difference in "real life" and jeopardise the safety of your preserved food.
High acid food has a pH of 4.6 or less
Acidic food is processed in a water bath canner. Acidic food can be naturally acidic (like most fruits) or have acid added to vegetables to create pickles, chutney, relish and salsa. Tomatoes and figs also require acidification before canning, as some varieties have a pH slightly above 4.6. Acids added to food include vinegar, lemon juice or citric acid.
Low acid food has a pH above 4.6
Low acid food must be processed in a pressure canner. Meat and vegetables are two examples of low acid food suitable for pressure canning.
Boiling water temperatures will not destroy Clostridium botulinum spores – the acidity (pH) can stop them multiplying and prevent the production of neurotoxins if the pH is 4.6 or lower. The canning process (temperature + processing time) can then remove any other microorganisms that cause spoilage.
When testing pH for canning, it must be done correctly and accurately. We will discuss how to do this next.
Author: Megan Radaich          
Image Credit: https://www.ibchem.com/IB16/08.31.htm 
Publication: www.foodpreserving.org

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