Kefir Fermenter: Set for Use with Jar (Regular Mouth) You Already Have This device designed for use with "regular mouth" Mason Jar you have. Recommended volumes of jars: 0.25 - 0.5 L or 8 oz - 17 oz. Place kefir grains in container and install in jar. Save money on shipping and if you already have suitable glass jar. Kefir grains live in plastic container (BPA free, FDA approved). Thus you don't need strainer and pouring from one glass to another. Simple!