In the first edition of Andrew Chaikin's 1994 book, "A Man on the Moon, there is a paragraph in chapter 1 which talks about troubles the Apollo 1 testing crew were going through.
Chaikin states that on January 27, 1967 around 1 pm just after testing had started, Grissom smelled something like sour milk. It took the crew about an hour to find the source of the smell, and the testing continued.
Here is the specific excerpt from the book:
Today's simulated countdown was nothing new; it wasn't considered dangerous-The Saturn booster was not fueled-or even difficult. But there was trouble almost from the time Grissom and his crew climbed into the command module cabin, around 1 pm. First there was an unidentified odor in the breathing oxygen that reminded Grissom of sour milk; that alone held up the test for an hour. Finally the problem was solved, and at 2:45 P.M. the pad crew installed the command module's heavy, two-piece hatch and sealed it shut.
As you can see, Chaikin never really mentioned what exactly was causing the smell or how they fixed it.
However, I am curious about this. Does anyone know what exactly was causing the smell that Grissom smelled, and how they fixed the problem?