In 1975, the US feed industry first applied microbial enzymes as an additive to compound feeds. In the 1980s, enzyme preparations were commonly used in foreign compound feeds, and in the early 1990s, they were introduced into China. Enzyme preparations are biological products produced by microorganisms. Feed enzyme preparations include a single enzyme preparation and a composite enzyme preparation. The composite enzyme preparation can be divided into wheat, barley and corn-soybean-type diets according to the type of diet. Generally, in the granulation process of 60-65 ° C or less, the enzyme preparation treated by the stable carrier can maintain about 80% activity, and some specially coated enzyme preparations can maintain high activity below 75 ° C. . However, considering the inactivation of Salmonella, the granulation temperature generally reaches above 90 ° C, at which time the most stable enzyme preparation will also become ineffective. Therefore, in China, the heat resistance requirements of enzyme preparation products are much higher than those in foreign countries. Whether the heat resistance of the enzyme preparation can be improved is one of the key points for further promotion of feed enzyme preparations in China.
Factors that may affect the heat resistance of the enzyme preparation:
(1) Bacterial species, feed enzyme preparations are biological products produced by fermentation of microorganisms such as bacteria, yeasts and fungi, and the enzymes produced by fermentation of different strains have different heat resistance properties. Most of the enzyme strains currently used in the feed industry come from fungi and rarely from bacteria. Bacterial enzyme preparations have more advantages than fungal enzyme preparations. Such as bacterial xylanase, derived from Bacillus subtilis, near neutral pH, thermal stability is better than fungal xylanase, not sensitive to xylanase inhibitors, high activity to insoluble xylan;
(2) Coating technology and granulation production process, using coating technology and granulation production process can improve the heat resistance of the enzyme. However, the use of coating treatment to prevent the destruction of the enzyme preparation will have a great negative impact on its bioavailability. The reason for the different bioavailability of granular phytase and coated phytase is that the coated phytase is released more slowly in the gastrointestinal tract of animals;
(3) Carriers, different carriers of the enzyme preparation may have an effect on the heat resistance of the enzyme. The ideal carrier should help the enzyme and the nutrients in the feed to reduce the inhibition of nutrient small molecules or endogenous enzymes, can change the optimal pH value to the ideal value, is not conducive to microbial growth, and does not produce immune response. And blood coagulation reactions. As for which carrier to choose, it is necessary to consider various factors such as enzyme activity and cost.
However, no matter what kind of bacteria or process is used, the enzyme preparation has a certain limit to the high temperature tolerance.
Since the enzyme is a biocatalyst, it is sensitive to temperature like other proteins. Therefore, the feed processing process has an important influence on the activity of the enzyme preparation. The optimum temperature of the general enzyme is between 35 and 40 ° C, and the maximum temperature is not more than 50 ° C, but the temperature during the granulation process can reach 120 to 150 ° C, or higher, accompanied by high humidity (causing a higher feed Under the conditions of the water activity, high pressure (change the spatial multidimensional structure of the enzyme protein), the activity of most enzyme preparations will be lost. In general, non-starch polysaccharide enzyme (NSP enzyme), xylanase is more stable than glucanase; phytase is more susceptible to temperature than NSP enzyme.
1 granulating
1.1 Common conditions for granulation
The granulation process is roughly classified into three types, namely dry granulation, steam granulation and secondary granulation. Domestic pellet feed production uses steam granulation. At present, the commonly used granulation conditions are: granulation temperature is 77~88 °C, and quenching and tempering time is 1-2 min. In recent years, some feed mills have produced feed products with high health indicators and no pathogens (especially no Salmonella). There has been a trend to increase the granulation temperature for more efficient sterilization. The granulation temperature commonly used for these feeds is generally specified above 85 ° C, which is the indicated temperature for the effective killing of Salmonella. At present, Western Europe has begun to adopt a secondary granulation process with a granulation temperature of 90 °C. The actual production measurement results show that the granulation technique can be effectively sterilized.
1.2 The effect of granulation on the thermal stability of enzyme preparations
The temperature and humidity involved in the granulation process (tempering and squeezing) are easy to deactivate the specific protein with catalytic activity, and the degree of loss of enzyme activity is obviously affected by the type of enzyme preparation. The effects include two aspects: (1) when the temperature rises, the reaction speed also accelerates; (2) as the temperature rises, causing some hydrophobic bonds in the enzyme protein molecule to break, thereby changing the conformation of the molecule and losing the enzyme activity. The untreated β-glucanase has a survival rate of only 10% in the feed after granulation at 70 ° C; the β-glucanase produced by Trichoderma is tempered for 30 s at a temperature of 75 ° C, which survives. The rate was 64%, and the survival rate was only 19% after 90s granulation; the activity of amylase was significantly decreased at 80 °C. The phytase decreased by more than 50% after granulation at 70-90 °C.
It should be noted that the humidity at the time of granulation also has a great influence on the activity of the enzyme preparation. At a certain temperature, the relationship between the water content and the water activity of the feed compound enzyme additive and the compound feed is expressed by the adsorption isotherm of water. Although this relationship is not a linear relationship, the general trend is that the higher the moisture content of the sample, the greater the water activity. At higher water activities, the denaturation of the enzyme protein is significantly enhanced. For example, when the moisture content of the sample is reduced to 10%, the lipase starts to be inactivated until the temperature is raised to 60 ° C. When the moisture content is increased to 23%, significant deactivation occurs at room temperature. For most enzyme preparations, reducing the water activity to below 0.3 at a near neutral pH and at a lower temperature prevents deterioration due to enzyme protein denaturation and microbial growth, thereby maintaining high enzyme activity.
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