Maintaining Ingredient Quality in Extruded Feeds

Extrusion processing using a combination of moisture, pressure, temperature and mechanical shear, is been used in the feed industry. It results in physical and chemical changes such as ingredient particle size reduction, starch gelatinization and inactivation of enzymes. Mild extrusion processing usually enhances the digestibility of plant proteins.

Fishmeal production is not expanding worldwide; therefore, more plant meals will have to be formulated into fish feeds to accommodate expected increases in fish feed production. Plant meals contain starch, which must be cooked to make it digestible to fish. Extrusion processing gelatinizes starch and improves the digestion of starch.

Extrusion processing can increase the nutritional value of canola meal, rapeseed, peas and soybean meal. As higher amounts of plant meals are formulated into fish feeds, the bioavailability of nutrients, especially bioavailability of minerals, will be of increasing concern because plant meals contain lower amounts of minerals compared with fishmeal.

The effect of extrusion processing on mineral availability for fish is not known. Thus, the raw ingredient formulation, selection of process equipment, and processing conditions are independent regions of control that may be exercised in the extrusion cooking of aquafeed. Although the control regions are independent, they are interrelated to the point that discussion of one must include the other.

Raw material utilization and cost effective formulation are key operational factors. The ability to alter processing conditions and raw material formulations to keep formulation costs at a minimum while maintaining high quality standards and minimum operating costs is a challenge for every processor.

Within certain limits set by a nutritionist, the extrusion cooking process can produce a wide range of products. In general, during the extrusion cooking of cereal grain and protein blends, the moistened granular or floury materials are converted into dough. The starchy components gelatinize, resulting in a substantial uptake of moisture and an increase in dough viscosity. Some protein constituents may impact elasticity properties that are characteristic of hydrated and developed glutinous dough. Other proteinaceous materials, those with low protein solubility such as meat meal or fishmeal, may contribute less to the adhesive and stretchable functional properties.

Ingredient selection has a tremendous impact on final product texture, uniformity, extrudability, nutritional quality, economic viability and ability to accept oil during coating or flavoring process. The common components of a recipe include starch, protein, fat, and fibre. An understanding of each component and how the extrusion process is affected is critical to forming an approach for effective diagnostics and troubleshooting.

One of the main categories of components found within many of the extruded products is the carbohydrates. The primary type is starch, a complex carbohydrate. The starch is usually sourced from either the cereal grains or from tubers. The cooking process has a pronounced effect upon the starch. In the raw state, starch has a granular nature and exists as a distinct particle (or granule) with very strong internal attractions between the various portions of the starch molecule within the granule. This is also the condition of the starch as it enters the extruder barrel. This condition is commonly referred to as ‘raw’ or ‘native’ starch.

Starch may be gelatinized at different moisture levels. For example, when boiling starch, moisture levels as high as 90 percent w/w are used, but in the extruder barrel, much lower moisture levels are used. However, the extrusion cooking process is seldom operated with an excess of water. The total operating moisture is typically w ≈ 15 to 30 percent w/w. In the extruder, the complete rupture of the starch granule is brought about by the combination of the moisture, the heat, the pressure and most importantly the mechanical shear. This process will typically take between 10 to 15 seconds.

Within the extruder, moisture is required to allow starch to gelatinize into a fluid mass, permitting it to pass through the die opening at the discharge of the extruder. As the material discharges from the die, the moisture level should be sufficiently high to retain its fluidity, but low enough to ensure that the starch will stiffen up (as a result of the inherent moisture and temperature losses, which occur at the die). As a result, moisture levels in the range of 15 to 30 percent w/w are typical. Excessively low moisture limits the lubricating effect as the product is conveyed along the barrel, causing high energy consumption. Water content also allows the expanded product to remain soft, permitting the cell structure to puff (and subsequently collapse).

The gelatinization of starch is affected by the conditions of heat and moisture during cooking. Additional cooking of gelatinized starch increases the viscosity and the surface tension of the gel sufficiently to cause the material to become so thick it cannot be poured from an open container. This condition is referred to as retrogradation. Starch, when cooked, can be puffed or expanded to a remarkable degree. If a comparison between the diameters of the expanded product to the diameter of the die orifice is used to express the degree of expansion, then starch can be expanded by a factor of up to five.

The next most important category of components is the proteins. The extrusion process has been found to provide sufficient cooking to denature proteins, but because of the short retention time, does little damage to the nutritional value of the heat-sensitive amino acids. The denaturation of protein is a phenomenon very similar to the gelatinization of starch. In the presence of heat and moisture the grains hydrate and swell. The action of the shear encountered within the extruder barrel leads to the rupture of the membrane and the disentanglement of the molecules. The shear also leads to the alignment and stretching of these molecules. Due to these changes the formulation becomes a plasticized, fluid mass. As the mass begins to cool cross-linking of the molecules into a three-dimensional structure begins to occur, leading to a rigid physical form.

As a result of denaturation, protein may undergo one or more of the following changes:

When more severe cooking takes place, the protein is not merely denatured, but is hardened beyond that stage to a very tough, horn-like condition. Under specific conditions, severe cooking can damage certain amino acids, rendering them unavailable to animal nutrition.

Proteins can be classified as plant and vegetable sources or as animal and marine sources. Vegetable or plant proteins are largely water-soluble and therefore possess very functional properties during extrusion. The functionality or water-soluble properties of plant proteins can be measured with several laboratory tests. The primary test for potential functionality is the measurement of protein dispersibility index (PDI). The PDI is a means of comparing the solubility of a protein in water, and is widely used in the soybean processing industry. A PDI of 100 indicates total solubility.

During the milling or extraction steps to refine a plant protein for use as an ingredient in extruded products, there are often one or more heating steps which affect the PDI value. These heating or drying operations are usually very mild and do not significantly lower PDI values. A PDI value of greater than 40 will have significant functionality during extrusion, reasonable binding, and some expansion potential. Extremely high PDI values (>80) may actually be so functional that, at high levels in a recipe, may contribute to a stickiness or tackiness when hydrated that eventually results in unstable extrusion conditions.

Proteins of animal or marine origin may be subjected to higher temperatures during manufacturing. Higher process temperatures are employed for many reasons including improved extraction and separation from fat and water components, and adequate pasteurization. Where high temperatures have been employed over an extended time period, the resulting protein solubility is quite low and these proteins may be essentially inert during the extrusion process. Inert means that the protein will not contribute to binding or expansion, but may actually reduce expansion. This is in part due to the presence of significant levels of minerals and fat components, but mainly due to the denatured (non-soluble) structure of the protein. The high temperature processing of ingredients will be reflected in low PDI values and dark colors.

Animal proteins are supplied to the extrusion system in a fresh (un-cooked or lightly cooked) or spray-dried form that will have significant solubility and functionality. Protein solubility is an indication of the degree of denaturation of protein ingredients. Denaturation does not necessarily impact protein digestibility. Denaturation does impact extrusion functionality and usually occurs in a temperature range of 55-70 °C.

Extrusion does not seem to adversely affect fats and oils. Studies have shown little or no changes in the free fatty acid levels, nor any indication of rancidity due to heat oxidation of the fat. Proper levels of fat are important in the cooking process. Fat is a lubricant, allowing product to ease through the screw(s) and barrel of the extruder with less resistance. Too much fat retards product expansion and the degree of cook, making a denser product. More retention time in the barrel, together with higher temperatures, in most instances will tolerate levels of fat in excess of 12 to 15 percent w/w.

Conversely if the fat is bound, such as in a coarsely ground or whole oil seed, then significantly higher levels of fat may be tolerated. Almost all ingredients contain some level of oil or other lipid constituents. Oils or derivatives of various fats such as lecithin or mono and diglycerides are often added to recipes to impart specific emulsifying or textural properties. The presence of oil and similar ingredients will act as a lubricant in the extruder screw. Fat addition reduces specific mechanical energy inputs. At lower inclusion rates, lipids can disrupt cell structure and texture by affecting plasticity and viscosity. In most recipes, the addition of lipids will begin to affect expansion and product durability at levels of less than 7 percent (total crude fat). If internal fat levels exceed 12 percent (total crude fat), distinct shapes may not be possible. At moderate inclusion levels, fats will tend to yield large cell sizes and thick cell walls in the extrudate.

Materials with a high fibre content show an increase in bulk density after expansion, when the product densities are based upon uniform grinds of feed and expanded product. The presence of the fibre particles appears to provide a nucleation site for bubble formation during the puffing process. At low inclusion levels (less than 5 %), fibrous ingredients may not have a noticeable impact on extruded products. Particle size of the fibre is important and if smaller than 400 microns, the fibre may actually increase expansion and reduce bulk density of the extrudate. Large particles of fibre in a recipe usually result in a coarse, fuzzy product surface appearance after extrusion. If the particle size is less than 50 microns, there is less effect on expansion even at higher levels in the recipe. Very fine fibre particles create an extremely small cell structure in the product after extrusion. Insoluble fibre remains nearly inert during extrusion and the individual fibre particulates can serve as nucleating sites during the expansion process at the die. More soluble forms of fibre have less contribution to reduced expansion even at high inclusion levels. Several studies have indicated that extrusion can increase fibre solubility. The extent of this conversion depends on processing conditions.

The Phase Transition Analyzer (PTA) Instrument measures the glass and melt transition temperature of ingredients which are a complex mix of biopolymers. Knowing the glass and melt transition temperatures of the ingredients or ingredient mix helps assess s the suitability of the raw materials for extrusion and how the properties of that recipe will be affected by the extrusion temperatures and moistures.

The particle size of the raw materials will affect the texture and uniformity of the final product. The extrusion cooking process can utilize a broad spectrum of ingredient particle sizes. It is desirable, but not necessarily essential that particles be of uniform size and density to prevent segregation during mixing and transport prior to extrusion. Most importantly, a uniform particle size promotes uniform moisture uptake and cooking during extrusion which prevents hard, partially cooked particles in the final product.

When whole grains are received into the manufacturing facility, they should be pre-ground to pass through an opening of 1,000 micron or larger prior to mixing. The final formulation is then ground just prior to extrusion to achieve the desired final particle size. When die openings are three millimeter in diameter or larger, it is common for this final grinding step to be through a screen having 1.2 mm openings. With die openings smaller than 3 mm in diameter, the maximum particle size should be one-third the die opening. Smaller ingredient article size results in smaller cell structure of the extrudate.

Raw materials are selected primarily based on their nutritional and functional contributions. Secondly, economics enters into the selection process. Many recipes are formulated based on least cost formulation software programs. Thirdly, the availability of the raw material becomes a factor.

When purchasing or selecting raw materials, establish a specification range based on desirable characteristics. This range of specifications should include the proximate analysis and other known critical qualities. However, some desirable characteristics are only vaguely recognised and no satisfactory test exists as yet to monitor quality in a reliable manner. There exist variabilities within a raw material due to influences such as the variety, growing season, and post-harvest handling or processing of grains. Different types of grains, legumes, and variations within animal or marine protein sources are reflected in the processability of raw materials. Many problems can be avoided by developing historical databases that record raw material characteristics that correlate with good processing. Establishing a sample library of acceptable and unacceptable raw materials may be especially useful in maintaining a smooth running extruder and troubleshooting future challenges.