Pelleting can potentially lead to an extra loss of phytase activity.
The pelleting process: factors influencing phytase recovery
The heat susceptibility of a phytase depends on its intrinsic heat stability. However many factors will have an impact during pelleting and these need to be considered when evaluating the heat stability of a phytase.
BY LODE NOLLET, MSC, PHD, GLOBAL PRODUCT MANAGER, ENZYMES AT HUVEPHARMA
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uring pelleting, the feed mixture is heated in the conditioner by adding steam. Conditioning has several aims: it improves pellet quality, it reduces power consumption during compression in the
die and it increases the hygienic status of the feed. Dry steam is injected at 1.5 to 2 bar and at 127°C to 134°C. The conditioning temperature may vary from 65°C to 90°C, and conditioning times vary from short (30 sec.) to extended (3 to 4 minutes). Afterwards, the conditioned feed is com- pressed in a die to form the pellet. The transfer of the mash feed through the die holes generates friction, which can po- tentially lead to an extra loss of phytase activity. After die compression, the pellets are cooled (and dried) in a vertical countercurrent cooler. The final temperature should be around room temperature, and the moisture content should be below 13%.
▶ ALL ABOUT FEED | Volume 28, No. 9, 2020
Processing factors affecting enzyme recovery The following factors have been shown to significantly affect the enzyme activity in the feed pellet.
Conditioning temperature and time Enzyme inactivation increases with temperature and resi- dence time in the conditioner. Adding enzymes in feed mills using a hygieniser or extended conditioning will result in rather low enzyme recovery.
Moisture content of the mash feed Dry heat has less effect than wet heat, thus low-quality steam (oversaturated steam containing water droplets) negatively impacts enzyme stability.
Diameter and length of the die Die friction will increase temperature at the pellet surface, so small pellets (2 mm) tend to have lower recovery than large (5 mm) pellets. The L/D ratio (length to diameter ratio) of a die is therefore an important parameter: the higher this value, the more friction heat and the less recovery is to be expected.
Feed composition Minerals and fibre negatively affect the die throughput as they increase friction. However an increase in free fat percent- age results in higher throughput as fat is a lubricant. A recent trial conducted at Ghent University in Belgium investigated
PHOTO: DOMNITSKY
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