by Gutted on Fri Nov 20, 2015 11:26 am
It's a myth propagated by the culture providers so that they can keep selling cultures and maintain their market. Bacteria reproduce, it's what they do, if they suddenly died or became weak then all bacteria would perish which does not happen.....They thrive when given what they require, suitable conditions ie food and an appropriate temperature for the particular bacteria.
What can happen is that they can evolve and adapt to their circumstances and they can become affected by bacteriophages which are viruses which target bacteria which can change them. This is a big concern amongst food manufactures.
For these reasons, it might be beneficial to begin afresh after a certain amount of cycles to ensure that what you are culturing is what you intend to culture and not some mutant.
I tend to go to about 10 generations before restarting. That can be about 20 weeks or so. Not really a big investment. Be aware that using store yoghurt will be much weaker than your own cultured yoghurt and so will take quite a lot longer to reach finished. However once you have done your own to full strength, it should culture much quicker for all further generations from that source.
Therefore do not worry about it, just set a limit and get it somewhere close.
Long ago I used to use supermarket yoghurt in my early days of yoghurt making buy stopped because I wanted to be certain of the bacteria and strain used in my yoghurt. I now tend to use probiotic capsules as starters but they take a very long time in comparison with my yoghurt to reach a finished fully set state. A capsule will take about 16 hours whereas a previous batch of my yoghurt can complete in around 4 hours but that has recently increased to more like 6 hours as my yoghurt maker temperature has lowered by about 3-4C.