Phytase is an enzyme that catalyzes the release of phosphorus from phytate. The sources of phytase with respect of swine nutrition are: endogenous phytase produced in the small intestine, microbial phytase produced in the large intestine, intrinsic plant phytase derived from feedstuffs, and exogenous microbial phytase added to the diet (Humer et al., 2015). The endogenous phytase activity is negligible in swine and the intrinsic phytase activity in feedstuffs is variable, with corn and soybean typically containing minor phytase activity (Eeckhout and De Paepe, 1994). Consequently, only 20 to 30% of phosphorus bound to phytate is released by the action of these phytase sources (Adeola and Cowieson, 2011).
The addition of exogenous microbial phytase to swine diets is a common practice to efficiently and economically enhance phosphorus release from phytate (Selle and Ravindran, 2008). The effects of exogenous microbial phytase follow a curve of diminishing returns, with most of the beneficial effects generated within the dose of phytase necessary to destroy 30 to 40% of the dietary phytate and proportionately lower effects thereafter (Cowieson et al., 2017).
Exogenous microbial phytases are typically derived from bacteria or fungi, such as Escherichia coli, Aspergillus niger, Peniophora lycii, and Buttiauxella spp. (Selle and Ravindran, 2008). These microbial phytases are divided into 3- and 6-phytases according to site of action on phytate, and into first or new generation depending on generation of development. All commercially available microbial phytases for swine are classified as acidic phytases, with optimal activity at pH of 2.5 to 5.5 (Humer et al., 2015). Table 1 presents the characteristics of some of the current commercial phytase sources for swine.
Phytase activity is expressed as phytase units (FTUs or FYTs). One FTU is officially the amount of phytase required to liberate 1 mmol of inorganic phosphate per minute from 0.0051 mol/L sodium phytate at pH 5.5 and temperature of 37°C (AOAC, 2000).