A semi-solid filter cake waste product from potato processing is used as a growth medium for producing single-cell protein. A pre-fermenting yeast, such as Endomycopsis fibuliger, is introduced into the growth medium to produce amylase for breaking down into glucose the starch naturally present in the potato waste medium. A primary fermenting yeast, such as Candida utilis, is introduced to the potato waste medium and uses the glucose for growth.