Sake lees or Sake Kasu (酒粕) is basically the leftover by-product from the suspended solids after sake is fermented and refined.
Larger sake producers extract the sake from the lees by machine and the kasu comes out in thin dry sheets called itakasu (板粕). Meanwhile, smaller producers press their sake by hand and this method yields kasu that is moist and chunky called namakasu (生粕).
You can purchase sake lees all year round, but the fresh sake lees are available only in the winter, between February and March when sake production takes place.
You can find sake kasu (sake lees) at Japanese grocery stores (I buy mine at Nijiya) or local sake brewery if there is any (Sequoia Sake Brewery in San Francisco gives out on Saturday – thank you for the info, Janet!).
