Random Observations on pH

This post has very little substantive content, but it's something I have observed a couple of times before and thought was interesting.  Most fruit is acidic enough to be processed in a boiling-water bath (BWB) canner (i.e., pH of 4.6 or lower).  Foods that are low in acid or basic can be BWB processed if enough acid--generally vinegar--is added to drop the pH below the threshold at which the bacteria that causes botulism can grow.  This is how pickled vegetables are made safe for BWB canning (though just tossing some acid into a jar isn't satisfactory; it has to be enough to lower the pH of both the solids and liquids to no greater than 4.6).

I have pH test strips and sometimes check the level of things I'm making--though this can be difficult if the food in question is strongly-colored (e.g., sauerkraut made with red cabbage).  Recently I canned pears.  As is often the case, I had some left over that didn't fit into a jar for processing, so I tossed them into a container and put them in the refrigerator along with most of the leftover syrup.  A week later, I checked the pH of the syrup out of curiosity.

As can be seen, the pH was 5.5-6.0.  This is not acidic enough, but the pear-to-syrup ratio in the container was much lower than in the processed jars.  For comparison, the pH of the water from my kitchen faucet is fairly neutral:

The few peach slices in the container acidified the syrup, but only to a point: this has implications for lightly-packed canning jars.  The fruit slices themselves will be acid, but if there aren't enough, perhaps the syrup will not be.