For subsequent batches, I use homemade leftover yogurt. (chefdehome.com)
However, Asus has claimed the error has been rectified in subsequent batches of the Prime. (goodereader.com)
Once my yogurt was made I could use that as the starter for subsequent batches as your recipe suggests but the tartness developed as soon as I began making yogurt with yogurt instead of the powdered culture. (davidlebovitz.com)