Why are my potatoes hard in a chilli, curry or stew?
Acid stops potato pectin dissolving, so a potato that would soften in twenty minutes in plain water can still be firm after two hours in tomatoes or wine. What to do about it.
- Difficulty
- beginner
- Time
- 5 min
- Read
- 2 min
Short answer
Tomatoes, wine, vinegar, lemon and cider all hold the pectin between potato cells together, and no amount of extra simmering will break it down. Cook the potatoes separately in plain salted water until nearly done and add them to the dish for the last ten minutes, or add the acidic ingredient only after the potatoes are soft.
This catches people who have cooked potatoes successfully for years, because the failure looks like undercooking and responds to none of the things that fix undercooking. It is the same effect that keeps apple pieces intact in a cider stew, and the same reason a tomato-heavy bean pot never quite gets there.
Step by step
- Recognise the signature.The potato is uniformly firm, not soft outside and hard inside. It has been in the pot far longer than a potato should need. The dish contains tinned tomatoes, tomato purée, wine, cider, vinegar, lemon or a lot of tamarind. The rest of the dish is done.
- Know what is happening.Potato cells are cemented together by pectin. In near-neutral water, heat breaks that pectin down and the cells slide apart, which is softening. Below roughly pH 4.5 the pectin chains stay cross-linked and resist that breakdown almost indefinitely. Calcium, which tomatoes also supply, reinforces the effect.
- Fix the batch in front of you.Lift the potatoes out, boil them in plain salted water until a skewer goes in easily, and return them. If that is impractical, dilute the pot with stock or water and accept a much longer cook and a thinner dish.
- Sequence properly next time.Put the potatoes in with the stock or water and get them most of the way to soft before the tomatoes, wine or vinegar go in. Ten minutes of head start is usually enough.
- Or par-boil as standard for potato-in-tomato dishes.For a chilli, a Bombay potato, a tomato-based fish stew or a potato and chickpea curry, boiling the potatoes separately to just-done is not a workaround — it is the normal way those dishes are built, and it also stops the potato clouding the sauce with released starch.
- Use the effect deliberately where it helps.A splash of vinegar in the water when boiling waxy potatoes for a salad is the same chemistry working for you: the pieces hold their edges instead of crumbling.
Tips
- The threshold is not a cliff. A single tin of tomatoes in a large pot of stock may only slow things down; a reduced tomato base with wine in it will stop softening altogether.
- Bicarbonate of soda raises the pH and will let them soften, but it also dulls the flavour of a tomato sauce and pushes it towards soapy. Use it as a last resort and sparingly.
- The same rule governs carrots, beetroot and beans in an acidic pot, though potatoes show it most obviously.
Questions people ask
Does this apply to a slow cooker too?
Yes, and worse, because a slow cooker never gets hot enough to force the issue. Potatoes going into an acidic slow-cooker dish should be par-boiled or cut small.
Why do potatoes in a plain beef stew go soft fine?
Because stock is close to neutral. It is only the acidic additions that lock the pectin.