The stability of phytase under storage and during feed processing determines the ultimate value of phytase as much as its efficacy. There are many factors influencing phytase stability, including thermostability, coating, storage form, storage temperature, storage duration, and feed processing (Table 1).
Thermostability and coating
Phytase is susceptible to denaturation by excessive temperature during storage and feed processing. Phytase thermostability can be achieved through coating application to provide protection to phytase or through genetic modification to make phytase intrinsically thermostable. Heat-stable phytase is able to withstand high temperatures under storage and application of heat during pelleting compared to non-heat-stable phytase (Slominski et al., 2007).
Moreover, coating also provides protection to phytase against environmental insults. Coated phytase is able to counteract some of the adverse effects of premix components, high storage temperature, and long storage duration, compared to uncoated phytase (Sulabo et al., 2011).
Phytase can be stored in pure form or in a mixture with vitamins or vitamin and trace minerals. Phytase activity is lost to greater extent in premixes containing vitamins and trace minerals than in premixes containing only vitamins (Sulabo et al., 2011; De Jong et al., 2016). The interaction of phytase with premix components seems to affect phytase stability, with inorganic trace minerals appointed as the most likely components to interact with phytase (Shurson et al., 2011). Storage of phytase in pure form is the best means to optimize phytase stability and minimize loss of phytase activity during storage (Sulabo et al., 2011; De Jong et al., 2016).
Phytase is exposed to varied temperatures and humidity during storage depending on location and season. Storage under conditions of high temperature and humidity, i.e. at 99°F and 75% humidity, considerably reduces phytase activity (Yang et al., 2007; Sulabo et al., 2011). Freeze storage at -4°F also reduces phytase activity (De Jong et al., 2016). In general, storage at room temperature (73°F) or 39 to 73°F at low humidity is ideal to optimize phytase stability and maximize phytase activity during storage (Sulabo et al., 2011; De Jong et al., 2016).
Phytase is stored for varying lengths of time depending on inclusion rate and feed mill volume. Phytase activity gradually decreases with an increase in storage duration, but both storage form and storage temperature influence the rate of degradation during storage (Sulabo et al., 2011; De Jong et al., 2016). In general, storage of phytase for less than 90 to 120 d in pure form or less than 60 d in a premix optimizes phytase stability (De Jong et al., 2016).
The most commonly adopted feed process in swine diets that affects phytase stability is pelleting. Pelleting conditions vary depending on equipment and diet, but normally consist of conditioning temperatures ranging from 149 to 203°F. Phytase activity gradually decreases with an increase in conditioning temperature above 149°F, even with use of heat-stable phytase (De Jong et al., 2017). Alternatively, post-pelleting application of liquid phytase onto pellets is one strategy to maintain phytase stability (Gonçalves et al., 2016).