That exogenous (added) phytase has become a staple ingredient in most pig and poultry diets worldwide is hardly surprising. It is rather the more perplexing issue of having to pick the 'right' phytase from a myriad of available products, all with more or less similar claims. Clearly, this is quite confusing, but when heat resistance is the criterion, the choice is easier.
Not only does phytase help reduce environmental pollution by increasing the digestibility of the scarcely digestible plant-derived phosphorus, but it also improves feed efficiency, animal growth, and most importantly profitability of the animal production. Thus, the use of phytase is no longer a question in the minds of most professionals.
Without doubt, the technical and scientific literature is replete with efficiency reports for every single commercial product; each examining all aspects concerning the commercial application of a phytase in pig and poultry diets. Yet, they all tend to point to the fact that most modern products are quite effective when it comes to releasing vegetable phosphorus. The glaring gap in the literature is in the area of heat stability, and especially when it comes to side-by-side comparisons among different products - especially when it is more than two products that are compared. This is what is missing today, and it is the last piece of the puzzle that commercial nutritionists are called to complete, so far with incomplete data.
Natural ingredients, such as cereals that make up the majority of any pig and poultry diet, contain a certain (low) level of ‘natural’ phytase (Table 1). This phytase is activated when the kernel germinates to release bound phosphorus for the energy needs of the new plant. Exogenous phytase mimics this process in the gastrointestinal tract by releasing this bound phosphorus making it thus available to the animal.
Being similar, both natural and added phytase share another common feature: they are both extremely sensitive to exposure to high temperatures, such as those encountered during pelleting, for example. This has to do with their very own nature, in that being proteins, they are heat sensitive (only a few proteins are naturally heat resistant). Thus, exposure to excessive heat reduces their efficacy and as such they release less phosphorus. Because of this, a high recovery (or resistance) to heat is a desired feature in commercial phytase products. To make exogenous phytase heat-stable, its formulation (enzyme and carrier) and coating processes require a certain degree of sophistication and technology that today is patented. Yet, not all commercial products enjoy the same efficacy in being made heat resistant.
A study was conducted at the Danish Technical Institute in Kolding aiming to compare five commercial phytase products in terms of heat stability. These products are described in Table 2. Clearly, with these five, the vast majority of commercial application of added phytase in pig diets is covered in a global approach.
For the trial, a single batch of a typical diet was prepared. This was based on wheat and soybean meal. Afterwards, sub-batches of 300 kg were mixed with one of the enzyme products, while one sub-batch was kept without any added enzyme as a negative control. The common initial batch of feed ensured that any inherent inaccuracies in feed preparation and mixing would not affect the trial. In addition, the common ingredients ensured that any biological variation in natural phytase would not be an issue in this trial. The double dosage of enzyme ensure that the contributions from natural phytase were minimised, and that the samples collected contained enough added phytase for an accurate analytical determination at the laboratory.Each diet was then subjected into different temperature and time variables, while being in the conditioner compartment of the pelleting machine. The holding times were 30, 60, 90, and 120 seconds, whereas temperature was 75, 85, 90, and 95SDgrC. The negative control diet that was not pelleted was held at 20SDgrC. All in all, for each product, 16 samples were collected, one for each time × temperature combination.
Results confirmed that natural phytase is indeed extremely sensitive to heat exposure (Figure 1). From 364 units/kg in the negative control it dropped linearly with increasing temperature, reaching zero activity for the diet held at 90SDgrC for 60 seconds. Obviously, there is little that can be done for this, apart from ignoring natural phytase contributions in pelleted diets.
Results concerning the added enzyme were equally revealing (Figure 2). One of the products (Quantum XT 2500) was virtually non-resistant to heat as its recovery was just 3% when exposed to 95SDgrC for 120 seconds. Another product (Finase) from the same supplier was more stable (almost 60% recovery), meaning the two products are formulated and coated differently. This alone gives an indication of the extreme variability in quality characteristics that exists today in the market.
The most stable product was Ronozyme NP CT 10.000, which even at the most extreme conditions tested (95SDgrC for 120 seconds), showed an almost complete recovery (98%). This proves that added phytase destruction during pelleting is not a fact that has to be accepted as inevitable, when new technological advancements can make phytase heat resistant. All other tested phytase products were intermediate in their resistance to heat exposure, with values ranging around the 60% mark, implying a similar (older) formulation and coating process used by several manufacturers.
Similar results has been seen for some of the tested phytases in a publication from the Danish Pig Research Center (See Box).
Clearly from this trial, it would be safe to assume that for pelleted feeds, not all commercial phytase products should be considered equal. There are products that should be avoided, as they are clearly not intended for being exposed to thermal processing. In contrast, the majority of commercial products give satisfactory, yet not complete, resistance during pelleting.
The issue of percentage recovery is not only a matter of how much phosphorus will be released once the animal ingests the feed. Clearly, a lower recovery rate will provide less phytase to the animal, which in turn will release less vegetable phosphorus, reducing thus animal performance and overall profitability. This issue can be resolved by increasing the dosage of the added phytase. And here is where real economics should be applied. When phytase products are compared, their heat stability index should be part of the equation, as an apparently less expensive product to buy may be at the end the most expensive (or less effective) to use.
Heat stability phytase and xylanase
The heat stability of two phytase and four xylanase products was investigated, and the results demonstrated that enzyme activity dropped under the highest conditioning temperatures. For the phytase products, Ronozyme NP and Phyzyme XP TPT, and for the xylanase products, Ronoxyme WX, Econase XT 25 and Danisco Xylanase 8000 G, more than 80% of the enzyme activity was preserved up to 95°C. All these products were comparable in terms of heat stability. Porzyme 9302 stood out by being less heat stable; at 90°C xylanase activity was reduced to half of the original level.
Conditioning and pelleting processes at a feed mill normally do not exceed 95°C and should therefore not cause problems to the activity of the studied enzymes except for Porzyme 9302.
Enzyme activity in this study demonstrates only heat stability and not the effect in the pig. To ensure that the feed contains the declared levels of enzymes, it is possible to analyse this for phytase, whereas for xylanase it is a matter of trust. The effect of Ronozyme WX and Porzyme 9302 is currently being investigated in different dosages in finisher feed.
The heat stability of the products in this study was studied in a trial facility at the Technological Institute in Kolding where it was possible to control temperature and duration of conditioning and pelleting. The feeds with the enzyme products were conditioned for 30 seconds in the temperature interval 80-100°C prior to pelleting.
The study was financially supported by the Pig Levy Fund and the EU and the Rural Development Programme under the Danish Ministry of Food, Agriculture and Fisheries and has project ID DSP09/10/58. (Dorthe K. Rasmussen)