Q: I have a jar of Kalamata olives in my fridge, that seem to have developed a white scum (for lack of a better word!).  What is it, and are they still safe to eat?

Kalamata and other olives may release some of the olive oil contained in the fruit into the pickling brine.  The low temperature of the fridge will cause the oil to thicken and turn opaque so that it appears to be a scum on the surface of the liquid.  It may be a bit more noticeable in Kalamata olives because the brine is dark colored, which provides contrast.  It is also possible that some protein from the olives may rise to the surface and form a light skin, as may  additives used in the manufacture.  Any of these may be what you are seeing.

Try this - remove a bit of the brine and its scum into a small dish and let it warm to room temperature.  If the scum is oil, it will turn more liquid and will change from white to transparent.  It may also coalesce into globs.  If that happens, and if the expiry date on the jar hasn't passed, the olives are probably safe to eat.

A very thin skin on the liquid is likely from protein or additives, and probably harmless. Alternately, the scum may be mold, in which case it may be stringy, like mother of vinegar, or fluffy, like the growth you see on those lost containers of leftovers from the back of the fridge.  If that is the case, the olives are likely bad.

All that said, I still stick by the motto When in Doubt, Throw it Out!