From here: https://cooking.stackexchange.com/a/13533/6168
Overly sour yogurt is a sign of inconsistent inoculation
From: http://www.thefreedictionary.com/inoculation
The act or an instance of inoculating, especially the introduction of an antigenic substance or vaccine into the body to produce immunity to a specific disease.
What is that quote talking about? How can "inconsistent inoculation" be prevented?
In this case, inoculation refers to the introduction of yoghurt bacteria (lactic acid bacteria) that transform the milk into yoghurt.
If anything goes wrong - from unclean conditions that let other bacteria grow to unfavourable temperatures that inhibit the proper growth of your desired lactobacillae - you speak of improper inoculation.
The effects on taste and possibly food-safety have been discussed in your linked posts.
So work in a clean environment, use a good strain of bacteria and maintain the recommended temperature range to prevent this.
