Introduction:
In numerous countries across the globe, enzymes are commonly incorporated into the feed of monogastric animals. This practice stems from the recognized benefits enzymes provide in enhancing the productivity of these animals. However, proving the presence of enzymes in commercial feed remains challenging, prompting extensive research efforts. Enzymes, being proteins, are highly sensitive to feed processing conditions. Unlike other feed proteins, which function as amino acids, enzymes lose functionality if they undergo irreversible denaturation during processing. Therefore, it is crucial to measure enzyme activity within compound feed. This paper focuses not on existing methods for measuring enzyme activity but rather on describing the characteristics of feed enzymes. It explores the differences in thermal stability among enzymes sourced from various origins, the impact of enzyme-feed matrix interactions, potential solutions to mitigate these effects, data from feed processing trials, and future trends.
Most pig and poultry feeds require some level of processing. Some feeds are made into pellets, a process involving steaming the feed mix before extruding it into pellets under pressure. Pelleting increases nutrient density, improves feed storage properties, and reduces microbial content. Typically, the pelleting temperature ranges from 65 to 90 degrees Celsius. Such high temperatures can degrade heat-sensitive nutrients, including enzymes.
In recent years, concerns over feed-borne pathogens and the desire for better granulation quality have led producers to increase the temperature, duration, and pressure of feed processing. Some even resort to secondary processes like double pelleting or puffing. Enhanced processing poses challenges for enzyme stability. Strategies to address this include adding liquid enzymes post-cooling to avoid heat exposure and using hydrophobic coatings or more heat-tolerant enzymes.
Despite these efforts, published data on enzyme activity retention rates in feed remains limited. Ensuring enzyme stability is vital for manufacturers, necessitating lab evaluations before product sale. Since 1993, several studies have been documented, highlighting the importance of both in vitro and in vivo enzyme activity assessments.
Phytase:
Phytase, accounting for approximately 20% of commercial enzyme preparations, has garnered significant attention regarding its thermal stability. This interest arises partly because many plant-based feed components contain phytic acid, which is poorly absorbed due to its complex structure. Additionally, monogastric animals exhibit low endogenous phytase activity. Furthermore, plants containing phytic acid also possess substantial phytase levels, complicating phosphorus digestion issues and contributing to environmental phosphorus pollution. This pollution has become a major constraint in intensive livestock farming regions.
Phytase exhibits a broad range of sources and varying properties. Liu et al. (1998) reviewed literature up to 1998, revealing optimal activity temperatures ranging from 45 to 77 degrees Celsius among bacterial, fungal, yeast, and plant-derived phytases, with a notable difference of 32 degrees Celsius. Dvorakova et al. (1997) described phytase isolated from Aspergillus niger, active between 25 and 65 degrees Celsius with an optimal temperature of 55 degrees Celsius. After incubation at 60 degrees Celsius for 10 minutes, its activity decreased by 5%, while at 80 degrees Celsius, initial activity dropped by 80%. In the quest for thermally stable enzymes, Wyss et al. (1998) studied the thermal denaturation of purified phytase from A. fumigatus and A. niger. Both sources denatured below 55 degrees Celsius. However, raising the temperature to 90 degrees Celsius caused the phytase from A. fumigatus to refold into an active conformation, whereas A. niger’s phytase remained inactive. Certain heat-resistant phytase variants are expected to enter commercial markets soon.
The inactivation of enzymes in solution doesn’t imply identical behavior in feed, as feed components protect enzymes from steam or high temperatures. Simons et al. (1990) added phytase to “general swine feed†preheated to 50 or 65 degrees Celsius before pelleting. At 50 degrees Celsius, particle temperatures reached 78 or 81 degrees Celsius without reducing enzyme activity, but at 65 degrees Celsius, reaching 84 or 87 degrees Celsius, enzyme activity decreased by 17% or 54%. Gibson (1995) added three plant acidase preparations to a wheat-based diet and pelletized at 65 to 95 degrees Celsius. Two preparations were inactivated at 65 degrees Celsius, while one retained significant activity above 85 degrees Celsius. Wyss et al. (1998) also added phytase from Aspergillus fumigatus and Aspergillus niger to commercial feed before pelleting at 75 or 85 degrees Celsius. At 75 degrees Celsius, both enzymes showed similar activity, but at 85 degrees Celsius, A. niger phytase retained more activity than A. fumigatus, aligning with their denaturing kinetics study. Eeckhout et al. (1995) added commercial phytase preparations to feeds and observed a 50% to 65% loss in activity at granulation temperatures of 69 to 74 degrees Celsius.
Inactivation impacts not only exogenous microbial enzymes but also natural feed ingredient enzymes. Gibson (1995) noted that granulation above 85 degrees Celsius significantly reduced endogenous phytase activity in wheat. Eeckhout and dePaepe (1994) reported that wheat bran, rich in phytase, exhibited only 56% activity in granulated samples compared to ungranulated ones. Jongbloed and Kemme (1990) found that granulation near 80 degrees Celsius reduced phytase activity in pig feed, whether formulated with high or low phytase activity ingredients. Their subsequent experiments confirmed that granulation reduced phosphorus absorption rates in high-phytase feeds, consistent with endogenous phytase inactivation.
Research institutions and the feed industry are increasingly focused on phytase stability due to rising processing temperatures and growing demand for improved phosphorus absorption. Methods such as encapsulation or granulation offer protection against thermal damage. Alternatively, developing thermostable enzymes or restoring activity post-denaturation are promising avenues. However, none of these methods effectively shield endogenous enzymes in feed ingredients from high temperatures.
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