Q: Why does my cold brew taste sour even though I steeped it for 24 hours?

If you steeped for 24 hours and it's still sour, the grind is probably too coarse. Coarse grounds need more time, but beyond 18–20 hours you start pulling bitter compounds that mask the sourness rather than fixing it. Try a medium-fine grind (like table salt) and keep the steep to 14–16 hours.

Q: Can I use hot water to speed up cold brew when traveling?

You can, but it changes the flavor. Hot water extracts faster, so you'll get a more bitter, sharper cup that can read as sour. If you're in a hurry, use room-temperature water and accept a longer wait—or switch to a hot brew method instead.