A method for thermally treating a product in order to have the product obtain the desired product properties, comprising the following steps: (a) determining the water activity value which, for carrying out the thermal treatment, is required to have the product obtain the desired properties; (b) contacting the product with water vapour, at an increased temperature and an associated saturated vapour pressure (p*), while the temperature on the surface of the product is 80-260.degree. C.; (c) setting a vapour pressure (p) in step (b) such that p/p* is equal to the water activity value determined in step (a); and (d) maintaining for a time duration (t) the process conditions mentioned in steps (b) and (c) for obtaining the product with the desired product properties.