Kefir bacteria and yeast. Scanning electron micrograph (SEM) of Lactobacillus bacteria and yeast from kefir, a fermented milk beverage containing beneficial yeast as well as probiotic bacteria. Lactic acid bacteria produce lactic acid from the milk sugar lactose. The acid coagulates the milk. Yeasts produce carbon dioxide and a low concentration of ethanol (alcohol). Kefir thus tastes somewhat like an acidic carbonated drink. Magnification: x2500 when printed at 10 centimetres wide.