<P> The cooker works by trapping the steam produced from boiling the cooking liquid inside the vessel . This causes internal pressure and temperature to rise quickly . After use, the steam is slowly released so that the vessel can be opened safely . </P> <P> In 1679, French physicist Denis Papin, better known for his studies on steam, invented the steam digester in an attempt to reduce the cooking time of food . His airtight cooker used steam pressure to raise the water's boiling point, thus cooking food more quickly . In 1681, Papin presented his invention to the Royal Society of London, but the Society's members treated his invention as a scientific study . They granted him permission to become a member of the Society afterwards . </P> <P> In 1864, Georg Gutbrod of Stuttgart began manufacturing pressure cookers made of tinned cast iron . </P> <P> In 1918, Spain granted a patent for the pressure cooker to Jose Alix Martínez from Zaragoza . Martínez named it the olla exprés, literally "express cooking pot", under patent number 71143 in the Boletín Oficial de la Propiedad Industrial . In 1924, the first pressure cooking pot recipe book was published, written by José Alix and titled "360 fórmulas de cocina Para guisar con la' olla expres"', or 360 recipes for cooking with a pressure cooker . </P>

Why is it easier to cook in a pressure cooker