Fermentation

I've been having issues with making milk kefir. Mostly, actually, fear of drinking it after fermentation. Sometimes I notice this foam on the top, and after straining, I don't know how to describe it and don't have any photographs.

When my kefir is done, it smells almost identical to sourdough starter. Is this normal? Does this mean its safe? It is different to commercial kefir, which I know is more of a thin yoghurt.