The loss of mannitol, as a representative of water-soluble secondary metabolites, during blanching of mushrooms (Agaricus bisporus) was studied. Evacuated mushrooms of different sizes were blanched at temperatures ranging from 50 up to 100 degrees C. The decrease of the mushroom mass and the increase of the mannitol concentration in the blanching fluid was measured as a function of blanching time and temperature. A logistic equation was used to describe the mechanism of shrinkage. The mannitol concentration in both fluid and mushroom increased during the first stage of shrinkage. Implications for industrial processing are suggested.