As the number of commercial phytase feed enzymes available on the market continues to expand, so too does the range of differing characteristics such phytases exhibit. Although a positive result of ongoing positive research and development, such variety makes direct comparisons difficult. This challenge is particularly acute when assessing phytases for use in superdosing strategies where phytases are typically applied at three to four times the standard dose with the aim of eliminating the anti-nutrient effects of phytate.

Intrinsic phytase thermostability

What is needed, therefore, is a fresh approach to phytase comparison when considering superdosing, and it would appear that a greater understanding of these critical enzyme characteristics -- along with their potential interactions -- is perhaps the best place to start. In fact, such thinking has evolved steadily since several papers were presented at the second International Phytase Summit, held in Rome during December 2012.

The first of these characteristics, which can have a substantial impact on phytase efficacy and directly affect performance response in the pig, is thermostability. If a phytase exhibits greater intrinsic thermostability, it can make it unnecessary to use coating technologies when developing a commercial product capable of surviving typical pelleting temperatures.

At present, little research data is available on the impact such coatings may have in potentially delaying the release of a phytase within the stomach. However, when the target is to drive phytate as close to elimination as possible, every additional second available for phytate destruction in the stomach is of value.

Optimized stomach activity

There are also important differences in how phytases behave once active within the gastrointestinal tract. Phytate is most easily degraded in the acidic environment of the stomach, so the activity of phytase enzymes within the range pH 2.0-4.0 is another key indicator of potential efficacy that needs to be understood.

Unfortunately, at present the standard point at which phytase enzyme efficacy is evaluated is pH 5.5, and as the graph in Figure 1 clearly shows, this bears little relevance to activity at the lower pH values. If the information available to the end-user is limited to phytase activity levels at pH 5.5, then any comparison has the potential to be hugely misleading, and may be one explanation for the sometimes wide variation in results seen between phytases when superdosing.

Continued phytate breakdown

However, perhaps the most important characteristic is the ability of a phytase to maintain high rates of activity at lower phytate concentrations. This has several important implications.

The first is that the phytase will act sooner when concentrations of soluble phytate in the stomach rise following a meal, and will continue to act for longer as phytate is broken down by the phytase and concentrations eventually begin to drop. The difference this makes can be clearly seen in Figure 2, where the upper blue line represents the characteristic of a phytase exhibiting much greater activity at low phytate concentrations.

Such differences in activity are less important at the standard level of phytase dosing, where only 50-60 percent of the P locked up in dietary phytate needs to be released to increase available P (AvP) by 0.10-0.13 percent. However, at superdosing levels, when phytase activity much achieve as much as 80-85 percent breakdown of total phytate to eliminate its negative effects on nutrient digestion, phytase activity at low phytate concentrations is critically important.

In fact, given that some phytate is insoluble (unavailable), achieving 85 percent phytate breakdown may require available stomach phytate concentrations to be reduced to 0.05 percent or below. For many commercial phytase products, this may not even be possible, since activity would simply cease before concentrations get this low.

Rapid phytate elimination

The other key implication revolves around the ability of a phytase to effectively "neutralize" phytate as it becomes available, but before it reaches a concentration at which it has negative effects on the digestion of other nutrients. Regardless of whether this increase in soluble phytate is due to ingestion of feed, or the result of more insoluble phytate being released as the level of soluble phytate in the stomach drops, the impact is the same.

Figure 3 depicts an example of how the soluble phytate levels in the stomach might increase over time for the same diet dosed with two different phytases. The point at which negative effects on digestion appear is represented by the red line.

A phytase able to reach maximum activity at lower phytate concentrations (the lower, dark blue line), and so reach a rate of activity that matches the rate of phytate release (the point at which the line plateaus) below the red line, will be highly effective. The threshold for phytate anti-nutrient effects will simply not be reached.

For a phytase that requires a much greater phytate concentration to achieve the same rate of activity (the upper, light green line), the result is likely to be much less effective. In this situation, the phytate concentration would be allowed to rise above the threshold at which anti-nutrient effects impact animal performance, regardless of how high a dose rate is applied.

High-dose phytase suitability

What has become clear is that not all phytases capable of delivering 0.10-0.13 percent AvP, or even those targeting 0.15-0.17 percent AvP release, exhibit the characteristics necessary to achieve reliable near elimination of phytate, even if used at the high doses typical of superdosing. Comparing phytases for use in high-dose strategies on the same basis as those used to simply deliver increased AvP is therefore inappropriate, and could well lead to the selection of phytases that are simply not capable of reliably delivering superdosing benefits.

The abilities of a phytase to be highly active in the stomach, rapidly degrade phytate as it solubilizes and continue that breakdown until very little phytate is left are all essential to the removal of phytate as an anti-nutrient. Data on phytase characteristics such as thermostability, pH profile and, critically, the rate of activity at low phytate concentrations is also therefore essential if end-users are to make direct, meaningful comparisons between the products on offer.