Maillard protein digestibility

The effects of premelanoidins on nitrogen retention is explained by a reduction in protein digestibility due to (a) the inactivation of the proteolytic enzymes, (b) the formation of indigestible peptides (20,38), (c) the inhibitory effects on amino acid absorption (33) and (d) by a decrease in the efficiency of the protein synthesis. The Maillard products also have an inhibitory effect on the intestine saccharidases (39). 

Maillard reaction) or in the relative absence of sugars (lean meat protein). The reaction leads to a fall in protein digestibility and in the availability of most amino acids, in addition to that of lysine. Destruction of amino acids can also occur. 

The initial Schiff base is digestible but after the Amadori rearrangement, the products are not metabolically available. Since lysine is the amino acid most likely to be involved and is an essential amino acid, Maillard browning reduces the biological value of proteins. Interaction of lysine with lactose renders the adjacent peptide bond resistant to hydrolysis by trypsin, thereby reducing the digestibility of the protein. 

While the determination of total lysine is rather straightforward, the determination of free or available lysine is more problematic. In this situation, the term free is meant to imply that the e-amine of the lysine side chain has not reacted with various components of the sample matrix (most commonly carbohydrates via Maillard browning). This is important because reaction of the e-amine can render lysine nutritionally unavailable and the nutritive value for that protein is then diminished if lysine is the limiting amino acid (which is often the situation with soy proteins). While enzymatic digestion in the human gut may not release the modified lysine in a nutritionally available state, often these lysine adducts are labile to the standard acid hydrolysis in 6N HC1 at 110°C. This results in total lysine values that overestimate the amount of nutritionally available lysine. 

Availability of Starting Materials for Maillard Reaction in Vivo Not all starting materials present in the body are available for Maillard reaction. For example, trypotophan in blood, formed by digestion and absorption of tryptophan-rich proteins, is easily bound by plasma albumin, making it unavailable for further transformation. 

Bouvet JP, Pires R, Lunel-Fabiani F, Crescenzo-Chaigne B, Maillard P, Valla D, Opolon P, Pillot J Protein F. A novel F(ab)-binding factor, present in normal liver, and largely released in the digestive tract during hepatitis. J Immunol 1990 145 1176—1180. 

An undesirable aspect of heat and alkaline treatment is that crosslinking compounds such as lysinoalanine are formed and can lead to severely impaired digestibility and reduced bioavailability of the protein. Such changes cannot be completely separated from the isopeptide formations (6), the Maillard reactions (2) and the racemization reactions (8) discussed elsewhere in this volume. In this chapter the chemistry, the analysis, the toxicology and the occurrence of lysinoalanine will be reviewed. 

The loss of nutritive value when protein or protein-rich foods are heated or stored in the presence of carbohydrates has been studied by a large number of groups (8,10,12,18,29,30). A decrease in the digestibility of proteins and in the availability of amino acids and carbohydrates after the Maillard reaction is shown in Table II. It can be concluded from this that the biologic value of proteins has also decreased. For example, in Table in one can see significant decreases in amino acids of egg albu-t... 

At temperatures of around 100 °C, asparagine and glutamine side chains react with the side chain of bound lysine (Figure 2.50) with the ehmination of ammonia (the reaction is termed deamidation) and the peptide chains are connected by intermolecular and intramolecular transverse covalent bonds. These bonds, known as isopeptide bonds, consist of dipeptides -N-(P-aspartyl)-L-lysine and e-N-iy- glutamyl)-L-lysine, respectively. About 15% of lysyl residues can react in this way. The digestive proteases of some animals (e.g. chickens and rats) can spHt these bonds, but for humans the so-called bound lysine is not available. The result is the certain, but usually not very significant, reduction of the nutritional value of proteins. Reduction of the nutritional value can be important in cases where a diet is low in protein and the hmiting amino acid of the protein is lysine. The Maillard reaction often leads to an extensive loss of lysine. 

Maillard r cticm products in food are vay important not only as the fectors for flavor and color but also beneficial constituents for our health. The Maillard reacticm is the reaction between amino (e.g. amino acids, peptides and proteins) and carbonyl compounds (e.g. sugars, adds and aldehydes). Since the discovery of the Maillard reacticm, mudh res ch has been directed to reveal the v/hole sequence of this complicated reacticm 1,2). Some researchers have also been attempting to understand the biolc cal effects of these compounds including toxicity, digestibility and beneficial effects (1,3). It has been reported some Maillard reaction products have antimutagenic, antioxidant, antibiotic and anti-allergenic effects (2). 

Following the formation of the deoxyketosyl derivative, the reactions leading to the formation of the brown pigments or melanoidins (Fig. 3) are not well defined. These reactions, however, are responsible for numerous flavours and odours, possibly for toxicity, and for a further reduction in protein nutritive value via destruction of amino acids and reduced digestibility. There are thought to be three main pathways in the advanced Maillard reaction. 

During heat-processing of feeds, Maillard reaction products have been measured. These have been used as indicators of nutritive damage of food protein, which may be indicators of changes in intestinal digestibility of protein. There are two different approaches (1) measure the products derived from lysine (Lys) damage or (2) measure available Lys. 

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