Is it because of additives that yoghurt keeps for a long time without spoiling?

Answer

No. According to the Ministry, the legislation does not permit preservatives to be added to yoghurt in the first place. The real reasons for a long shelf life are heat treatment, the natural protection provided as lactic acid bacteria lower the pH of the medium, controlled fermentation, the cold chain and advanced packaging technologies. Home-made yoghurt spoils sooner because these conditions cannot be controlled to the same degree at home.