There are two commonly used definitions of enzyme activity currently in use [10] and students should be made aware of both. The most commonly used definition of activity is the “standard enzyme unit” (U), normally defined as the number of micromoles of substrate degraded or micromoles product formed per minute, under standard assay conditions (of e.g. temperature and pH). In this case, it is technically more straightforward to measure product (inorganic phosphate) levels produced rather than levels of substrate (phytic acid) degraded. The standard definition of 1.0 U of phytase activity therefore is the amount of enzyme that catalyzes the production of 1.0 μmol inorganic phosphate per minute under standard assay conditions.