It is very safe. Just cover it with a towel and an elastic band to keep bugs out. In order for a culture to culture it must be kept at an optimal temp for it to grow. Each culture requires a different temp. Yoghurt needs a little warmer temp, kefir and buttermilk require room temp. It may take longer or shorter to culture depending on how warm or cool your house is. You will know it's ready by how thick it is. If you are using heavy cream it will be thick like sour cream, but not super thick. The creame fraiche is on the top and the bottom will be a little more liquidy. Skim the thick cream of the top for sour cream and use the rest for buttermilk, or make butter and buttermilk.