According to the Apollo Program Summary Report, page 6-14:
In zero gravity, when the food packages were rehydrated with water containing gas bubbles, the bubbles could not be removed from the food. The hydrogen gas separator used on lunar flights did not successfully remove the gas from the water on every occasion. Gas bubbles in the food and water contributed to intestinal problems experienced by the crewmen during the last two missions.
Are gas bubbles still a problem in space food? What measures are taken to abate the problem?
Related:
- Spinning in space to manage gas? (no answer, more specific than this question)
- Can you enjoy a cold carbonated drink in space? (OrganicMarble's comment there ought to be made into a proper answer for that question)