24 Feed Processing: Effects on Nutrient Degradation and Digestibility A.F.B Van der Poel,1 E Prestløkken2 and J.O Goelema3 Wageningen University, Animal Nutrition Group, Marijkeweg 40, 6709 PG ˆ Wageningen, The Netherlands; 2Felleskjøpet Forutvikling, Department of Animal and Aquacultural Sciences, Agricultural University of Norway, PO Box ˚ 5003, N-1432 As, Norway; 3Pre-Mervo, PO Box 40248, 3504 AA Utrecht, The Netherlands Introduction The dynamics of nutrient degradation in the reticulorumen and of nutrient digestion in the intestines are major determinants of the utilization of diet ingredients by ruminants These dynamics of nutrient digestion should be known and controlled to improve ruminal and total tract digestibility and to optimize production and composition of milk and meat A careful and appropriate selection of concentrate ingredients to meet the required supplementation of the forage could fulfil this objective Moreover, the processing of feeds can be used to manipulate the nutrient degradation characteristics in the rumen and the site of nutrient digestion, being a helpful tool to optimize ruminant diets Amongst other nutrients, protein and starch are important diet constituents for ruminant diets Protein appearing in the small intestine of the ruminant originates from dietary protein escaping microbial degradation in the rumen and from protein synthesized by microbes in the rumen Dietary starch is either degraded to volatile fatty acids in the rumen yielding energy for synthesis of microbial protein, or digested as glucose in the small intestine (see Chapters and 10) The quality and the content of protein and starch may greatly affect the nutritional responses to the diet Protein and starch account for a considerable part of the diet costs and a balanced supply of protein and total carbohydrates is important to minimize output of nitrogen in faeces and urine Thus, optimizing the supply of these nutrients by processing can be important to maximize the financial income and to minimize the environmental impact of ruminant production For processing, both particle size manipulation and the changes in physico-chemical properties of nutrients (e.g gelatinization of starch, denaturation of proteins) are options to shift the site of digestion of ß CAB International 2005 Quantitative Aspects of Ruminant Digestion and Metabolism, 2nd edition (eds J Dijkstra, J.M Forbes and J France) 627 628 A.F.B Van der Poel et al protein and starch from the rumen to the small intestine (Nocek and Tamminga, 1991) However, these options used in applied technology imply the manufacturers of rumen by-pass nutrients to use precision in controlling their processing methods The interest in manipulating the site of digestion through processing has ¨ increased during recent years Kaufman and Lupping (1982), Satter (1986), Broderick et al (1991), Nocek and Tamminga (1991), Schwab (1995) and Mills et al (1999) have published reviews on this topic Unfortunately, some of the methods may also render proteins or starch resistant against digestion in the small intestine (Broderick et al., 1991; Mills et al., 1999) To meet the protein requirements of high-yielding animals, the diet is usually supplemented with rumen undegradable protein from feedstuffs high in rumen undegradable protein, either by nature, or resulting from processing Santos et al (1998), reviewing publications in the period 1985 to 1997, concluded that increasing the amount of dietary undegradable protein did not consistently improve performance This implies that validation in the target animal of an increased protein value of diet ingredients by processing is important Similar reasoning applies for starch value of diet ingredients Thus, care must be taken when the in vivo verification of technological treatment is absent In this chapter, a brief description of relevant processing methods for ruminant feedstuffs is given and mechanisms and effects are discussed The main intention is to review existing knowledge on how the most relevant processing methods quantitatively affect protein and starch digestion, whilst effects on health in ruminants are briefly discussed The emphasis will be on nylon bag (in situ) studies, but in vivo and in vitro studies will be presented as well Feed Processing: Mechanisms and Methods Proteins are macromolecular polypeptides, consisting of covalently bound a-amino acid residues The sequence of these peptide-bound amino acids forms the primary structure of the protein The secondary structure of the polypeptide chain comprises helical coil, held together by non-covalent bonds, such as hydrogen bonds The tertiary structure is the folded and twisted positioning of the secondary structure, which is also stabilized by hydrogen bonds When two polypeptide ÀSH groups containing chains are close together, covalent disulphide bonds can occur, which cannot be easily broken down The way two or more polypeptides are merged together, often involving non-polypeptide groups, is the quarternary structure (Holum, 1982) Starch is a storage carbohydrate in many plants, and can comprise more than 70% of dry matter (DM) in cereals In most plants, a single starch granule is formed inside an amyloplast, whereas in some plants (e.g oats) several small granules are formed, which aggregate to a larger complex Starch contains two macromolecules of glucopyranose (glucose), viz the linear amylose and the branched amylopectin, which are organized in a semi-crystalline structure (Kotarski et al., 1992) Most of the starch is located in the endosperm Three Feed Processing: Effects on Nutrient Degradation and Digestibility 629 types of endosperm are distinguished: peripheral, corneous and floury endosperm Peripheral and corneous starch granules are surrounded by protein storage bodies, and embedded in an inaccessible matrix which consists mainly of protein and non-starch carbohydrates, whereas corneous starch has less cellular structure and a higher starch content Mechanisms Protein All methods that are applied to protect the protein have essentially a similar mechanism of rumen protection in that a stearic hindrance of enzymes in the rumen is established (Metcalf, 2001) The low pH in the abomasum causes the protein molecule to unwind, making the protease binding sites available again for digestion in the small intestine Heat treatment of protein results in structure stabilization and cross-linkages to carbohydrates, which protects them from ruminal hydrolysis or at least slows down their rate of degradation (Satter, 1986) The structure stabilization principally involves denaturation (Finley, 1989) In structural terms, denaturation is a disorganization of the overall molecular shape of a protein It can occur as an unfolding or uncoiling of a coiled or pleated structure, or as the separation of the protein into its subunits, which may then unfold or uncoil (Holum, 1982) Any temperature change in the environment of the protein which can influence the non-covalent interactions involved in the structure may lead to an alteration of the quarternary, tertiary and secondary structures Depending on the temperature, several processes may occur, ranging from only hydration and modification of the tertiary structure, to a complete alteration of the secondary structure and even the primary structure of the molecule (Finley, 1989) However, not only temperature plays a role during treatment, but also factors such as residence time and moisture level Various heat processing methods are available that differ in their mechanisms in view of their time–temperature relationship and also in other factors (e.g the use of shear) The occurrence of Maillard reactions is very common when heat processing is involved to modify proteins Lysine reacts with carbonyl compounds, usually originating from reducing sugars such as glucose, xylose and fructose (Cleale et al., 1987) Voragen et al (1995) have outlined the reactions and nutritional implications Other reactions may also occur, including the formation of isopeptide cross-links between lysine and asparagine or glutamine Additionally, methionine, cystine and tryptophan may be involved in the isopeptide cross-linking (Broderick et al., 1991) Metcalf (2001) has described various mechanisms for chemical treatments of dietary ingredients In the formaldehyde–protein interaction, the mixing of formaline (aqueous formaldehyde) and – if required – subsequent heat processing will form a rumen undegradable protein In this reaction, a precise level of formaline is attributed to a protein level and reactivity The reaction involves the bonding of the aldehyde group in formaldehyde to the amino group from 630 A.F.B Van der Poel et al amino acids in the peptide chain After a certain reaction time, a pH reversible methylene bridge is formed that is responsible for blocking the binding sites of bacterial peptidases In the tannin–protein interaction, tannins (polyphenol compounds) act by a chemical reaction with proteins that may be either reversible or irreversible in the abomasum Both reactions act by stearic hindrance in the rumen but the hydrolysis reaction only is susceptible to the low abomasal pH The irreversible condensation reaction will lead to an indigestible product (D’Mello, 1992) In the xylose–protein interaction, the mixing of xylose with protein prior to a heating process will block a number of enzyme-binding sites thereby increasing the level of undegradable protein During chelation (metal salt–protein interaction), the mixing of soluble metal salts and protein with additional steam processing will also result in rumen undegradable protein The underlying mechanism is the binding of metal salts to the protein, thereby blocking the binding of microbial enzymes, leading to the protection of the protein from rumen degradation Fat encapsulation of protein, using rumen inert fat (calcium soaps), involves the physical protection from digestion in the rumen of vegetable proteins In the abomasum, the proteins become available again since the low pH causes the release of the fatty acids from the soap Starch Several physical processes play a role during the heat processing of starch, such as swelling, gelatinization and retrogradation The magnitude of these processes depends on the particle size, but also largely on the temperature, the treatment time and the moisture level (Goelema et al., 1998) The exposure of starch to water combined with gradual heating results in swelling At low temperatures (below 60–808C), swelling is reversible after cooling and drying At higher temperatures, depending on the moisture level, gelatinization may take place (Lund, 1984) At this temperature, the granular structure is altered from semicrystalline to amorphous, which results in loss of its birefringence Gelatinization of individual starch granules occurs in a range of to 28C, but due to variation between granule fractions, it results in a 10 to 158C range for the total starch At low moisture contents (