By Warren G. Dominy
High-temperature, short-time (HTST) extrusion cooking can change the nutritional value or the functionality of feed ingredients. Changes in starch, dietary fiber, mono- or disaccharides, proteins and vitamins may be either beneficial or deleterious.
The nutritional value of a protein depends on its digestibility as well as the level and availability of essential amino acids. The nutritional value of a vegetable protein is usually enhanced if the protein is mildly heat-treated, to inactivate antinutritional factors. The extrusion cooking process has been demonstrated to be suitable for the inactivation of protease inhibitors and other antinutritional factors.
Enzymes such as lipase and lipoxygenase in feeds and feed ingredients may cause deleterious reactions that can contribute to off-flavors during storage if not inactivated through extrusion cooking. Enzyme inactivation during extrusion cooking contributes to the stability of a feed and its acceptability after extended storage. In general, enzyme inactivation increases as extrusion temperature is increased.
Either over- or under-heating of soy protein is detrimental to protein quality and decreases amino acid availability. Over-processed extruded ingredients may exhibit off-flavor, cause textural changes, loss of functional properties, nutritional losses of amino acid loss through Maillard reaction or oxidation and reduced protein digestibility through cross linking of protein. These processing changes may also be escalated when the ingredients have already been heat processed, as for example feeds with 44% or 48% soy, or genetically altered ingredients, such as trypsin inhibitor-free soybeans.
Carbohydrates are the most abundant class of organic compounds. Carbohydrates are important feed ingredients that give the extruded product functionality by serving as binding agents, viscosity builders, suspending agents and emulsifiers.
Carbohydrates can be classified into two general groups: digestible and indigestible. Digestible carbohydrates include: starch, sucrose, lactose and some oligosaccharides, as well as free monosaccharides. Indigestible carbohydrates include: oligosaccharides in leguminous seeds and cell wall polysaccharide (dietary fiber) like cellulose, hemicellulose and lignin.
Ingredients that are the major sources of digestible carbohydrates (starch) include wheat, corn, rice, barley, oats, sorghum, other grains and their by-products, such as wheat midds, wheat bran, corn meal, rice bran, etc.
Extrusion cooking of raw starch or cereal grain products results in the gelatinization of starch and therefore improved digestibility. The degree of gelatinization is one important determinant for the rate of enzymatic starch hydrolysis and intestinal absorption. Differences between various processes where the product is more or less gelatinized are presumed to result from variations in physicochemical properties, including starch granular structure, viscosity, solubility, macromolecule modifications and lipid complex formation.
Fiber usage in the extrusion process is often limited by its effect on product expansion. Dietary fiber is made up of cellulose, hemicellulose and lignin, and is usually defined as a polysaccharide plus lignin that is not digested.
In the extrusion process, thermal treatments can change dietary fiber content and composition with physiological effects. During the extrusion process, fiber content and composition in high fiber formulations increases in soluble fiber by approximately 3% and carbohydrate analysis increases by 4% to 5%. Redistribution of insoluble to soluble fiber in extruded wheat flour has been reported from 40% in raw flour to 50% to 75% soluble in extruded flour. Whole wheat flour exhibits no change. While mild or moderate extrusion conditions do not significantly change dietary fiber content, it does solubilize some fiber components. Under more severe conditions, the dietary fiber content tends to increase, mainly due to enzyme resistant starch fraction. The lack of resistant starch in an extruded product may be due to the rapid cooling of the product as it exits the die, which would prevent the crystalline alignment of the starch molecules forming the resistant starch.
While the modifying effect of extrusion on the main ingredients is desired, its secondary effect on vitamins is damaging.
Most vitamins are easily destroyed by temperature, oxygen, moisture and light. As well as temperature, moisture, and oxygen, there are additional parameters in extrusion cooking that influence the stability of vitamins: raw materials, pressure, flow rate, screw speed, energy input, die open area, shear, redox reactions, drying temperatures and duration.
Increasing flow rate improves retention for vitamins B1, B6, and B12, although the proportionally increased pressure to shorter residence time in the extruder barrel is thought to be a predominant influence. Folic acid follows the same pattern as the other B vitamins to 90 kg/h where increased input results in detrimental effects.
Higher water content in the extrudate improves vitamin retention of all the B vitamins by reducing temperature and extrudate residence time in the extruder barrel. Specific mechanical energy (SME) input has been negatively correlated with vitamin retention.
The vitamins most sensitive to the extrusion process are A, E, C, B1, and folic acid, where as, the other vitamins of the B-complex (B2, B6, B12, niacin, Ca-pantothenate and biotin) are very stable. Vitamin E alcohol, menadione sodium bisulfite complex (MSBC), ascorbic acid and coated ascorbic acid are the least stable vitamins. These vitamins can lose 20% of their activity at the lowest expander temperature and shortest residency time. At the expander’s highest temperatures and longest resident times, these same vitamins lose at least 65% of their original activity. Vitamins in the beadlet form of Vitamin D, vitamin E acetate, B12, ascorbyl phosphate, and choline chloride have been shown to have only slight losses when processed through an expander.