INTRODUCTION
In numerous countries worldwide, enzymes are frequently incorporated into the feed of monogastric animals. This is primarily due to the proven enhancement of production performance in these animals when such enzymes are used. Despite this, proving the presence of enzymes in commercial feed remains challenging, prompting extensive studies over the years. Enzymes, being proteins, are highly sensitive to feed processing conditions, just like other feed proteins. However, unlike other proteins, enzymes typically function at the level of individual amino acids, meaning their structure doesn’t need to remain intact. Consequently, during feed processing, if an enzyme undergoes irreversible denaturation, it will cease to function. Thus, detecting enzyme activity in compound feed becomes crucial. This paper focuses not on existing enzyme activity measurement methods but rather on the characteristics of feed enzymes, including differences in thermal stability among enzymes from various sources, interactions with the feed matrix, issues arising from these interactions, solutions to mitigate such effects, data from feed processing trials, and future trends.
Most pig and poultry feeds require some form of processing. Some types of feed are granulated, a process involving tempering the feed mixture with steam followed by extrusion into pellets under pressure. Granulation increases nutrient density, improves feed storage properties, and reduces microbial load. Typically, granulation temperatures range from 65 to 90 degrees Celsius (Gibson, 1995), temperatures that can degrade heat-sensitive nutrients, including enzymes.
In recent years, heightened concerns over feed-borne pathogens and feed quality have led producers to increase processing temperatures, times, and pressures, sometimes even applying secondary granulation or puffing. Enhanced processing presents significant challenges for enzyme stability. Various strategies have been employed to address this issue, such as adding liquid enzymes post-cooling or using hydrophobic coatings to protect enzymes during processing. While post-granulation enzyme addition is possible, most feed enzymes are introduced pre-processing. Using heat-resistant enzymes or protective coatings can help mitigate the impact of heat treatments.
Despite these efforts, data on enzyme activity retention rates in feed remain limited (Chesson, 1993). Ensuring enzyme stability is vital for feed manufacturers, necessitating rigorous laboratory evaluations before enzyme products are sold. Since 1993, numerous studies have been published in journals or conference proceedings, with additional findings appearing in critical scientific literature. While in vitro enzyme activity measurements—whether in solution or within feed—are essential, recent studies underscore the importance of verifying these results with in vivo effects.
PHYTASE
Phytase, accounting for approximately 20% of commercial enzyme preparations, has garnered substantial attention regarding its thermal stability (Bedford and Schulze, 1998). This interest stems from the fact that many plant-based feed ingredients contain phytic acid, which is poorly absorbed due to its complex chemical structure (Cheryan, 1980; Eeckhout and dePaepe, 1994; Ravindran et al., 1995). Additionally, monogastric animals often exhibit low endogenous phytase activity (Pallauf and Rimbach, 1997). Complicating matters further, plants containing phytic acid also possess significant amounts of phytase, intertwining nutritional phosphorus digestion challenges with environmental concerns related to phosphorus accumulation in soils, which can lead to pollution and hinder intensive livestock production.
Phytase exhibits a broad range of sources and varying properties. Liu et al. (1998) reviewed literature up to 1998, finding that bacterial, fungal, yeast, and plant-derived phytases exhibited optimal activity temperatures ranging from 45 to 77 degrees Celsius, with a difference of up to 32 degrees Celsius. Dvorakova et al. (1997) described phytase isolated from Aspergillus niger, noting its activity range of 25-65 degrees Celsius, with an optimal temperature of 55 degrees Celsius. Incubation at 60 degrees Celsius for 10 minutes reduced initial activity by 5%, while at 80 degrees Celsius, activity dropped by 80%. In the quest for thermostable enzymes, Wyss et al. (1998) studied the thermal denaturation of purified phytase isolated from A. fumigatus and A. niger. Both sources denatured below 55 degrees Celsius. However, raising the temperature to 90 degrees Celsius caused A. fumigatus phytase to refold into an active configuration, whereas A. niger phytase remained inactive. Thermostable phytase variants are expected to enter commercial markets soon.
The inactivation of enzymes in solution does not necessarily indicate similar behavior in feed, as feed components can shield enzymes from steam or high temperatures (Chesson, 1993). Pelleted feed phytase activity measurements provide valuable insights into enzyme inactivation levels. Simons et al. (1990) added phytase to "general swine feed" preheated to 50 or 65 degrees Celsius before pelleting. Heating to 50 degrees Celsius resulted in particle temperatures of 78 or 81 degrees Celsius, without reducing enzyme activity. At 65 degrees Celsius, particle temperatures reached 84 or 87 degrees Celsius, causing enzyme activity losses of 17% or 54%. Gibson (1995) added three phytase preparations to a wheat-based diet and pelleted them at 65-95 degrees Celsius. Two preparations were inactivated at 65 degrees Celsius, while only one retained significant activity above 85 degrees Celsius. Similarly, Wyss et al. (1998) added phytase isolated from A. fumigatus and A. niger to commercial feed pre-pelleting at 75 or 85 degrees Celsius. At 75 degrees Celsius, both enzymes retained comparable activities, but at 85 degrees Celsius, A. niger phytase activity surpassed that of A. fumigatus, aligning with their denaturation kinetics findings. Eeckhout et al. (1995) added commercial phytase preparations to feeds, observing 50-65% loss of activity at granulation temperatures of 69-74 degrees Celsius.
Enzyme inactivation impacts not only exogenously added enzymes but also endogenous feed ingredients' native enzymes. Gibson (1995) noted that granulation above 85 degrees Celsius inactivated much of the endogenous phytase activity in wheat. Eeckhout and dePaepe (1994) reported that wheat bran is rich in phytase, yet granulated samples exhibited only 56% of the ungranulated samples’ activity. Jongbloed and Kemme (1990) found that granulation near 80 degrees Celsius reduced phytase activity in pig feed, regardless of ingredient phytase content. Their subsequent phosphorus absorption experiments confirmed reduced phosphorus uptake in feeds rich in endogenous phytase.
Research institutions and the feed industry are increasingly focused on phytase stability due to rising processing temperatures and growing nutritional and environmental demands for phosphorus absorption. Encapsulation or granulation of exogenous enzymes offers protection against thermal damage. A more fundamental approach might involve identifying thermostable enzymes or refolding denatured ones. Unfortunately, none of these methods can prevent the destruction of endogenous enzymes in feed ingredients by high temperatures.
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