Corn is the most important source of energy in feed. It is called the king of feed and contains about 70% starch in its grain composition. Therefore, improving the utilization of corn starch is the key to improving the efficiency of weight gain and feed efficiency in livestock and poultry farming. Since there is a considerable proportion of the anti-acid and anti-enzyme crystal structure in the starch granules, which is not conducive to the digestion and absorption of the animal, the crystal structure must be disintegrated (ie, gelatinized) to be fully hydrolyzed by the enzyme to increase the digestibility. The starch gelatinization degree obtained by different processing methods is different, and the energy consumption is also different. Different animals need different degrees of starch gelatinization and with the increase of starch gelatinization degree, the energy consumption is also correspondingly increased. Therefore, how much is the gelatinization degree of corn suitable?
1 Extrusion of corn
Corn puffing treatment belongs to the category of feed moist heat processing. Puffing has the functions of disinfecting, sterilizing, increasing crude protein, increasing total energy, and greatly improving the gelatinization degree of starch. In fact, the main purpose of corn puffing processing is to increase the gelatinization degree of corn starch.
1.1 Characteristics of starch
All plant starches are composed of glucose molecular units, and corn starch is no exception. The starch granules are composed of amylose and amylopectin in parallel. The amylose molecules are arranged neatly, and the amylopectin molecules are arranged irregularly. Usually, amylose and amylopectin are overlapped and overlapped with each other, and there are many voids between them. Like an empty mesh bag, the glucose polymerization long bond is a network wire, and the hydrogen bond between the dehydrated glucose molecules is a network segment. Raw starch is insoluble in water. Once it is added with water and heated, it will swell. The molecular structure becomes unstable with increasing temperature, resulting in the final molecular structure disintegrating due to hydrogen bond cleavage. This is the gelatinization of starch. Different plant starches and different concentrations of starch solution have different gelatinization temperatures (see Table 1).
1.2 The degree of gelatinization of extruded puffed corn
It can be seen from the above starch characteristics that gelatinization of starch occurs under certain conditions of moisture and temperature. The starch gelatinization degree obtained by different processing methods is different. Different processing methods such as baking, bursting, and swelling can also increase the gelatinization degree (α value) of corn starch. The specific values are shown in Table 2.
It can be seen from Table 2 that the degree of gelatinization of the extruded puffed corn starch is the highest. It is feasible to achieve 100% gelatinization of corn starch in production practice, but productivity will have a certain degree of influence.
1.3 Working mechanism of extruded puffed corn
Corn puffing is a starch gelatinization process under the combined action of moisture, heat, mechanical shear, friction, enthalpy and pressure difference. When corn flour is mixed with steam and water, the starch granules begin to absorb water. When the cavity is expanded, the rapidly rising temperature and the enthalpy of the spiral blade accelerate the water absorption of the mesh-like starch granules, and the crystal structure begins to disintegrate, and the hydrogen bond breaks and swells. The starch granules begin to rupture and become a viscous melt. At the exit of the extruder, due to the sudden pressure drop, the vapor (moisture) is instantaneously lost, causing a large amount of expanded starch granules to disintegrate and starch gelatinization. High temperature, high pressure and mechanical shearing make extrusion expansion more complete than starch gelatinization by other processing methods. Generally, the degree of gelatinization can reach 80% to 100%. Compared with the conventional cooking process, the cell wall of the plant can be broken. The starch chain is shorter, which increases the digestibility more effectively. There are many factors affecting corn puffing, mainly water, puffing temperature, puffing pressure difference and intracavity mechanical shearing force, which are also several factors that can be controlled in the current puffing production.
At present, corn extrusion is divided into dry and wet methods. Many users think that adding water is a wet method, and adding water is a dry method. Others think that it is a wet method to add steam or water from the expansion chamber. Puffing, in fact, this is a misunderstanding. The so-called wet method refers to the steam pre-tempering and then puffing. The dry method is without steam pre-tempering and direct expansion. Even if water is added, it is dry. In general, wet production is more efficient than dry production, but steam boilers are required, and investment is larger than dry method. In the production of puffed corn, whether it is dry or wet, depending on the user's specific conditions and product requirements.
1.4 The role of corn puffing
The digestive organs of young animals, especially early weaned piglets, have not matured and the digestive enzyme activity is very low. Studies have shown that piglets have insufficient amylase secretion within 42 days after birth, and the digestive enzyme activity is regressed due to weaning stress. Diarrhea is often caused by indigestion of starch, which affects production performance. When the corn is puffed, the starch is gelatinized, the crystal structure of the starch is irreversibly destroyed, and the water absorbs and swells rapidly in the small intestine of the animal, which greatly increases the area of action and penetration of the amylase, and increases the hydrolysis rate and digestion degree of the starch. At the same time, gelatinized starch greatly increases the sensitivity of the amylase and makes it act more rapidly. In addition, gelatinized starch can also stimulate the production of lactic acid in the stomach of young animals, which can prevent the production of pathogenic microorganisms, thereby reducing and eliminating squatting in piglets.
For aquatic animals, the effect of gelatinized starch is also very significant. The utilization rate of raw starch of rainbow trout is only 20-24%, while that of cooked starch is 52-70%. The digestibility of cooked starch is 96%. The starch is 38%. The gelatinized starch in the aquatic feed also enhances the binding properties of the feed and improves its stability in water.
It is precisely because of the above reasons that gelatinized starch is widely used in young livestock materials, special feeds and aquatic feeds, and extrusion and puffing has become an important means of starch gelatinization. In fact, not only does corn need to be puffed in these feeds, but other grains used as energy feeds need to be puffed.
2, the determination method of corn gelatinization degree
For puffed corn for feed, the most important thing is to require ripening (that is, the degree of gelatinization). As for puffing, it is the inevitable result of cracking of starch granules and flashing of water. Generally, the bulking degree is characterized by the bulk density of the material. Therefore, the expanded corn has two requirements, the degree of ripening and the degree of expansion, which are measured by the degree of starch gelatinization and the bulk density of the material. The degree of starch gelatinization is determined by the amyloglucosidase method, and the bulk density of the material can be measured by a bulk density meter. The degree of ripening and the degree of expansion are interrelated. The degree of ripening is not necessarily high, and the degree of ripening is high. For most feed enterprises, the conditions for measuring the degree of gelatinization are not available, but the bulk density is easily measured, and the degree of ripeness reflected by the bulk density is also relatively accurate. Therefore, the bulk density has become an important indicator for the evaluation of puffed corn by feed enterprises.
3, different products require different degrees of expansion
It is generally believed that the gelatinization of cereal starch helps the enzyme to contact the glycosidic bond to promote digestion, and the starch in the raw material is always pasted as a principle for producing high quality pelleted feed. Experimental studies have shown that the improvement of corn starch gelatinization rate is positively related to the breeding efficiency, especially the suckling pig feed. So how high is the degree of gelatinization? Some people in the feed industry think that more than 70%, some think that more than 80%, and some think that more than 90% is good. At present, there is no unified understanding, but it is generally believed that it should reach more than 85%. Wu Mengqian (1995) reported that in order to obtain a better feeding effect, the gelatinization degree of corn starch should be above 73%, and the level of about 91% is better. The effect of different gelatinized cooked corn flour on the growth performance of 28-day-old weaned piglets was tested. It is known from the test results that the daily weight gain of the pre-conservation period (0-14 days after weaning) is significantly increased with the corn starch alpha value increasing from 28.6% to 70.2% or 91% (P < 0.05 =, with the corn starch alpha value). Feed efficiency was also significantly improved from 28.6% to 91% (P < 0.01 =; however, there was no significant difference in feed intake between the 4 groups).
At present, there is no uniform standard for the puffing degree of puffed corn. According to our experience, the puffing degree of corn is summarized as follows for producing different products:
According to the bulk density of the final product (after drying and cooling, 2mm sieve plate crushing), the expanded corn can be divided into three types:
1) Low puffing products. The bulk density is > 0.5kg / l, generally low temperature puffing, 80 ~ 120 & ordm; C or so, the finished product has higher moisture, the gelatinization degree can be 60 ~ 80%, can be used in the late milking piglets, can also be used for multi-dimensional and enzyme preparation coating Process.
2) Medium puffing products. The weight is 0.3 ~ 0.5 kg / l, the temperature is 100 ~ 150 & ordm; C, the finished product moisture is 8 ~ 10%, the gelatinization degree can be more than 90%, and it is used for special animal feed such as suckling pig, scorpion, fox and leeches. , aquatic feed.
3) High puffing products. The bulk density is 0.1 ~ 0.3 kg / l, the temperature is 140 ~ 170 & ordm; C and higher, the finished product moisture is 4 ~ 8 %, can be completely gelatinized, generally using dry puffing, used in the carrier of composite phospholipid powder, and foundry industry , paint industry.
4. At present, the application of domestic expanded corn is mainly in the following aspects:
1) Suckling pig feed. The piglet contains 60% or more of the corn component. Ideally, all the corn components are puffed, but the production cost will increase sharply. Conventionally, half of the corn components are moderately expanded and then granulated with other components (the soybean meal is also puffed). The gelatinization degree of the starch in the formulation is generally 60 to 80%. Of course, all the corn components can be puffed at a low level to achieve the same effect, and the efficiency of the puffing production process will be relatively high. Puffed corn is used in many domestic piglet feed products.
2) Special animal feed. In recent years, special animal breeding such as scorpion, fox and leeches has developed rapidly in China, mainly in the northeast and north China. In order to ensure the digestion and absorption rate of starchy substances, the traditional way of feeding special animals is to feed the ingredients steamed, which is inefficient and cannot be raised on a large scale. When puffed corn is used, it is only necessary to soak the material for 30 minutes before feeding, saving a lot of manpower and material resources and improving efficiency. Nowadays, the industry basically uses puffed corn to make materials, such as Bowei, Jiaxing, Hualong, and the Academy of Agricultural Sciences, but it requires more than 90% of the puffed corn gelatinization, and the product requires fine powder, compared with the piglet. The expanded corn used is slightly higher.
3) Aquatic feed. Mainly used for shrimp and squid, such as Fuzhou hippocampus.
4) Other industrial raw materials. The corn flour is highly expanded and used as a composite phospholipid powder carrier (the company has a special article), or the corn is peeled and de-emerged and highly expanded to produce different degrees of α-starch, which can be used in the food industry, foundry industry and coating industry. Wait. Due to the controllable friction shear in extrusion, different starch degradation products can be produced. Compared with the general drum-dried α-starch, the extruded starch has a lower viscosity and a wider range.