Grinding, mixing, conditioning and their influence on starch modification and pellet quality
Figure 1: Presentation of the factors affecting the pellet quality
Traditionally the creation of formulae and the selection of raw materials have the major influence of 40 %, followed by the particle size amounting to 20 % and by steam conditioning. 15% are influenced by the die specification and 5 % by cooling and drying. Expander treatment (HTST-principle "high temperature - short time") has an increased effect in comparison to steam conditioning and the 15 % influence of the pellet die is very clearly reduced.
For this reason a number of processes can be used for improving the physical and nutritional physiological pellet quality. The selection of raw components and the formula optimization provide the basis for further processing. In the following article the process stages grinding and conditioning are to be presented by the example of the effect on starch in feedstuff.
Factor raw material / starch
The modification of starch is one of many processing possibilities for increasing feed quality. Due to their nutritional composition grain fruits are classified as energy supplying feedstuff. Not only the energetic potential provided by the starch content determines the utility value. Ingredients such as the content of fibres, albumen, fat and mineral substances, the nutritional quality and the acceptance of the feedstuff are also important influencing factors. Due to its high amylopectin content starch is almost insoluble in cold water (hydrophobic).
Increasing the digestibility of starch in the feed mixture is the main target for increasing the fodder value. Starch is mainly digested in the small intestine. Starch is decomposed into glucose for being absorbed in the following. This process is simplified and made more effective by using highly modified starch components in feedstuff.
Factor particle size
In animal production a coarse particle size is of major importance for the development of the digestive system, for growth and performance. It is also of importance to produce hard pellets and structurized feed with low fines ratios for avoiding physical and nutritional physiological losses of nutrients.
A homogeneous grain size distribution (problem of demixing) with an optimal surface structure of the feed components is preferred in feed mixtures, see figure 2. By grinding the grain components of compound feed mixtures the starch is made easily accessible and thus can be modified more easily. A corresponding grinding process providing an adapted and homogeneous grain size distribution has a positive effect on starch modification.
Figure 2: Homogeneous granular size structure also for fine grinding degree d50 with 0.75 mm (wheat, maize) on the roller mill
Figure 3: Influence of particle size (d50) on starch modification at 70°C and abt. 18% moisture
Figure 4: Principle of grinding by crushing rollers for a homogeneous grain size distribution and 2-stage crushing roller mill
Figure 5: Particle size distribution with a grinding stage of (d50 maize) 1.3 mm for achieving a homogeneous grain structure in feed mixtures
Figure 6: Inhomogeneous particle size distribution when grinding maize corn by a hammer mill
Conditioning and the effect of heat and moisture on the product are achieved by adding water and saturated steam. Besides the nutritional physiological advantages of hydrothermal treatment moisture and temperature help to gelatinise raw starch. This ensures the optimum condition of the intra-particulate structure of the starch matrix. It increases the machine efficiency in the processing steps which follow; the best possible and product-adapted conditioning thus achieves high starch modifications. The regression analyses made during the conditioning tests at the KAHL pilot plant (see figure 7) showed a starch modification which was by 2.8 % higher with an increased moisture of 1 % (Stiller 2016).
Figure 7: Ideal influence of moisture on starch modification at the same temperature and with the same particle size
Bad processing by inhomogeneous hydrothermal effect (inhomogeneous structure, see figure 6) damages the nutrients and will then have a negative impact on animal performance and animal health. The Maillard reaction is intensified by higher temperature when homogeneous moisture is missing, this will lead to a loss of amino acids in combination with sugar. This improper conditioning leads to a loss of digestible nutrients and a reduced energy value. A further disadvantage is the enhanced effect of tensides as described by Hasenhuettl and Hartel (2008). During gelatinisation the tensides form complexes of emulsifying agents with starch molecules which leads to a reduced water-binding capacity. The surface becomes lipophilic and prevents the penetration of moisture into the starch grain, this also impairs starch modification. A reduced starch modification cannot be compensated by an extended retention time. Resulting from the processing parameters an increased starch modification cannot be achieved after a certain retention time (Stiller 2016) as can be seen representative in Figure 8 in sub-test 13.
In this case Goh (2016) gives a maximum conditioning time of 60 - 70 seconds, the initial moisture should be about 18 % for conditioning. The compound feed industry is faced with a conflict between optimum pelleting capacity and optimum treatment. The preferred moisture content ranges from 14 % to 17 %. While an expander can easily cope with this product moisture, it is far too high for a conventional pellet press. The nutritive value of the raw components is limited by low moistures and is also reduced by bad pellet qualities produced by the presses and the following cooling and drying.
Figure 9: Construction of a conditioner
The conditioner must provide a sufficient moisture quantity from condensed steam for effectively cooking raw starch. If necessary, the moisture content is increased by means of direct water addition. Gelatinisation and dextrinisation are the typical result of a combination of moisture, heat, mechanical energy and pressure. The machine configuration of material addition, pressure, shaft rotation, number of paddles and paddle angle has been optimized for the conditioners. In steam conditioners functioning at atmospheric pressure, the gelatinisation of starch is achieved, whereas expanders ensure even the desired dextrinisation of starch. Besides the technical-physical settings which condition a feed mixture, the biological parameters have, as already mentioned, a decisive influence. Attention should be paid to the fact that the gelatinisation temperature varies according to the respective starch type of the respective cereal. Simple starch modification can be explained in four phases and is shown in figure 10.
Figure 10: Process of starch modification (modified according to GOH 2016)
Phase I: In case of a hydrophobic cell wall cold water adheres to the surface of raw starch. Amylose and amylopectin are non-swellable in cold water and thus cause this condition.
Phase II: By adding warm water/moisture amylose and amylopectin become permeable to water (hydrophilic). Water is stored on the surface and it starts to swell.
Phase III: When exposed to heat starch can physically bind a lot of water. An even better accumulation of water and swelling is achieved when the starch grains burst/break. After a longer lasting temperature effect the starch grains dissolve and gelatinise.
Phase IV: Shearing and friction further increase the temperature effect, the hydrogen bridge bonds of the water molecules resolve themselves, continue to swell and completely form starch paste. There is a positive correlation between increasing temperature and the water absorption of starch grains. During this process amylose shows a more pronounced swelling capacity. The crude starches of cereals can thus absorb up to ten times their own weight in water.
The most intensive conditioning treatment is provided by the expander (see Fig.11); the so-called pressure conditioning is an optimized conditioning process for compound feed and individual components. The expander consists of a thick-walled mixing pipe with exchangeable wear inserts. The single-mounted shaft has dosing, mixing and kneading elements. A hydraulically adjustable cone and the pipe end form the annular gap. Maximum pressure is abt. 40 bar, operating temperatures at the expander head are between 90 and 140°C depending on the specific energy input and the intended use of the product. At the expander outlet the pressure is reduced to atmospheric pressure, the material is expanded and part of the water which has been added evaporates (flash effect, expansion).
Subsequent drying is not necessary, although higher processing moistures can be applied for pre-conditioning and in the expander.
Figure 12: Surface of starch (on the left) in its untreated state, (on the right) after the flash effect
By means of adjusting the cone during operation, the pressure, the intensity of kneading, the product heating, and the energy consumption can be controlled and programmed continuously and instantaneously. In this way the process can be adapted to a change in raw materials and recipes. The expander ensures an increase of the pelleting capacity at a smooth press operation. The pelleting press is practically only needed for shaping, as the binding properties of the feed mixtures are already activated in the expander. For this reason the importance of the pelleting die for the pellet quality within the process is reduced correspondingly. The flexibility of the press is increased due to the reduced lengths of the effective bores which are required for compaction of the product. Various formulae can be processed with one die without die exchange. Pellet hardness and fines can be controlled by varying the input of mechanical energy and the feed mixtures can be ground with a higher grinding degree. In this way the required structural advantages for animal nutrition are taken into consideration. An improved PDI (Pellet-Härte-Index/index of pellet hardness) is achieved; this has also been confirmed by practical evaluations. Muramatsu et al. (2013) indicate this PDI advantage by a 26-31 % increase and with reference to a moisture addition of 2.1 % they indicate an increase of the piled weight from 817 g/ kg to 901 g/ kg owing to the expansion process.
Further advantages of the expander treatment are an improved pellet quality, even when using components which are difficult to process, the addition of high liquid quantities, the inactivation of harmful substances, the elimination of salmonellae and the improvement of the nutritive value. The reduction of the production costs within the process underlines the economic efficiency. In case of immediate pelleting of the expanded product the specific electrical energy consumption in the pelleting press is reduced considerably so that the energy consumption of expander and pelleting press together is not much higher than in the case of pelleting only.
The processes used - thermal and hydrothermal - in combination with mechanical stress, grinding, mixing, conditioning and pelleting provide different results with regard to starch modification and pellet qualities. The result is influenced by the type of cereal, its condition and origin; this must be considered in the process stages. The considerably increased degree of starch modification achieved by the hydrothermal pressure processes should be the subject of a benefit analysis of all treatment processes from grinding to the final product and the costs incurred should be beneficial to the total process. A process optimization ensures beneficial energy efficiency. The conditioning times and conditioning temperature should be checked, also for ensuring a careful treatment of the feed mixtures during temperature treatment. The expander treatment is a solution for reducing the conditioning times. A short temperature increase to 90°C within abt. 1 - 2 seconds takes place in the expander; with a retention time of abt. 5 seconds a pressure adjusted expansion temperature of 110°C results. The correlated combination of the physical parameters such as pressure, temperature and moisture save the nutrient qualities and even improve the nutritional physiological digestibility. Pathogenic germs are effectively eliminated and the animal health is thus increased. Procedurally, the pelleting capacity and pellet quality are increased as well.
In addition to the temperature, the moisture content is an essential factor. In a physical sense water contained in feedstuff as well as in other biological materials fulfils the task and the function of a solvent. Moisture is necessary for making the starch grains swell and gelatinise. The target is to pay due attention to the relevant parameters in connection with the different starch types in feed formulae and for steam and moisture management, for being able to effectively cook raw starch and for hydrolysing starch.
Starch modification allows a better utilization by young animals such as piglets whose natural enzyme system is not yet sufficiently developed. Also with regard to animals having a short digestive system such as fish, cats or dogs, it is essential to be well informed of the energy carrier starch and its modification. As cattle and horses are not able to microbially digest all starches as best as possible, starch modification is an advantage also in this case. In pigs and poultry, the biological indicators have improved. Besides, a more efficient digestibility of starch relieves the metabolism, which has a positive influence on the animals' health (excretion in faeces and urine). The more efficient digestibility of starch relieves the metabolism and has, among other things, a positive influence on health. Thus more energy is used and the speed of digestion is increased simultaneously. This effect may include consequences for formula optimization.
Hydrothermal starch modification is essential in modern animal nutrition, technical and economic investments are indispensable in targeted and product-dependent application.
GOH (2016): Role of proper steam conditioning – feed pelleting efficiency & feeding value. ASIAN FEED MAGAZINE, Edition January/ March
HASENHUETTL and HARTEL (2008): Food emulsifiers and their applications. Springer Verlag
MURAMATSU et al. (2013): Impact of Particle Size, Thermal Processing, Fat Inclusion and Moisture Addition on Pellet Quality and Protein Solubility of Broiler Feeds. Journal of Agricultural Science and Technology A 3, 1017-1028.
STILLER (2016): Dependence of starch modification on retention time, temperature, moisture, and particle size of the grain Bachelor thesis at the Humboldt University Berlin, Faculty of Life Sciences, Unit Feed and Feed Additives, Department Safety in the Food Chain, Federal Institute for Risk Assessment, Berlin
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Article made possible through the contribution of Amandus Kahl