Making Greek Yogurt Part 3 -- What can Go Wrang and What to Do

My initial heating of the milk got too hot – over 180 (F).  Taste the milk.  It will probably taste scalded or off in some way.  The milk has been ruined, and you need to start over.  This is an important reminder that an electronic thermometer that can beep when your batch goes over a specific temperature is so important.
Run out of time?  If a stubborn batch just won’t thicken up, you can put the pot into the fridge and start again later. Gently re-warm up to 110° (NOT back up to 180°) and continue. (Food safety hint – putting a very large hot pot into your fridge will warm up your fridge and possibly spoil other food. Cool off any warm pot by placing it in a sink full of cold water or ice water before putting it into the fridge.) If it still doesn’t thicken the next day, check your starter, add more starter, or get another container of fresh yogurt from the store to add fresh starter.
While brewing the yogurt, the temperature went below 90 (F).  No Problem.  The yogurt culture (starter and whatever has brewed in the meantime) has gone dormant at the lower temperature.  Gently re-heat to 110 (F) and the culture will re-activate.  Continue holding at 110 (F) until finished.
While brewing the yogurt, the temperature went over 120 (F).  This is a problem.  Temperatures over 120 (F) start to kill off the yogurt culture.  Cool to 110 (F), and add new starter.  Stir the new starter GENTLY into the mixture, and continue to cook.
How exact are the temperature requirements?  a few degrees over or under is usually not a problem.  For the initial heating to go over 180 (F) or 185(F), especially for a longer time, is probably the most critical temperature, and the most serious cause of a ruined batch.  A batch that goes over 120 (F) can still be saved.  Bring the temperature down to at least 115 (F) or so, and see if the yogurt continues to develop.  If not, the batch is NOT ruined, but the starter may have been killed off.  Gently stir in a fresh starter, and continue.  After this fix, the total active cultures in the batch might be decreased, so don’t use this batch as a starter in subsequent batches.