Why do potatoes cook more slowly in a stew? I recently made a stew, not for the first time either, and I've tossed in a few potatoes, cut into cubes. It took about an hour and a half until they were soft. The stew was simmering the whole time.

In contrast, if I put potatoes into water and parboil them, even if I add salt to the water (or things like peppercorns or some herbs), they will be soft in about 15 to 30 minutes.

Why does that happen?
This won't apply to all stews, but if there's something acidic in the liquid (such as tomatoes or vinegar), some items will not soften or break down as quickly.

I know this includes both potatoes, onions, pasta and rice, but I would assume that other starchy vegetables would have similar behavior.

But there can be other factors in play, too.  Many people cook potatoes to mash in a fair quantity of boiling liquid, while many stews are simmered.  As stews often have liquids other than water, we avoid taking them to a full boil for a few reasons:


they make circulate inefficiently, leading to scorching on the bottom of the pot.
if the liquid is viscous, it will create bubbles that pop and make a mess
it cooks off aromatic compounds ... and anything you smell while cooking is something that's no longer in the finished dish.