A novel easy-to-use force-displacement tester was developed from which an index of cake strength was obtained by measuring the peak force required to cause the powder cake to fail. The tester was applied to investigate the effect of temperature, relative humidity, exposure time and glass transition on the cake strength of SMP. Exposing SMP to high relative humidities of 76 and 100% and temperatures in the range of 10-40 degrees C showed an increase in water sorption and cake strength over 24 h exposure. A strong relationship between lactose glass transition and cake strength was demonstrated. For the higher temperature condition of 40 degrees C, it was shown that the cake strength reached a maximum value and then decreased over time during a 1-week exposure time. This decrease may be due to lactose crystallisation. Crystallised lactose does not have the sticky characteristic of amorphous lactose and inhibits the development of sinter bridge formation. (C) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved..