Maillard browning reaction: a non-enzymatic browning Under some conditions, reducing sugars produce brown colors that are desirable and important in some foods. Other brown colors obtained upon heating or during long-term storage of foods containing reducing sugars are undesirable. Common browning of foods on heating or on storage is usually due to a chemical reaction between reducing sugars, mainly D-glucose, and a free amino acid or a free amino group of an amino acid that is part of a protein chain. Advantages of Maillard browning Maillard browning products, including soluble and insoluble polymers, are found where reducing sugars and amino acids, proteins, and/or other nitrogencontaining compounds are heated together, such as in soy sauce and bread crusts. Maillard reaction products are important contributors to the flavor of milk chocolate. The Maillard reaction is also important in the production of caramels, toffees, and fudges, during which reducing sugars also react with milk proteins. D-Glucose undergoes the browning reaction faster than does D-fructose. Application of heat is generally required for nonenzymatic browning. 1
Disadvantages of Maillard browning Reaction of reducing sugars with amino acids destroys the amino acid. This is of particular importance with L-Iysine, an essential amino acid whose -amino group can react when the amino acid is part of a protein molecule. A relationship has been found between formation of mutagenic compounds and cooking of protein-rich foods. Mutagenic heterocyclic amines have been isolated from broiled and fried meat and fish, and from beef extracts. Caramelization: general Heating of carbohydrates, in particular sucrose and reducing sugars, without nitrogen-containing compounds effects a complex group of reactions termed caramelization. Reaction is facilitated by small amounts of acids and certain salts. Mostly thermolysis causes dehydration of the sugar molecule with introduction of double bonds or formation of anhydro rings. Catalysts increase the reaction rate and are often used to direct the reaction to specific types of caramel colors, solubilities, and acidities. The non-enzymatic browning caramel pigments are large polymeric molecules with complex, variable, and unknown structures. It is these polymers that form the colloidal particles. Their rate of formation increases with increasing temperature and ph. Caramelization: production Brown caramel color made by heating a sucrose solution with ammonium bisulfite is used in cola soft drinks, other acidic beverages, baked goods, syrups, candies, pet foods, and dry seasonings. Its solutions are acidic (ph 2-4.5) and contain colloidal particles with negative charges. The acidic salt catalyzes cleavage of the glycosidic bond of sucrose; the ammonium ion participates in the Amadori rearrangement. Another caramel color, also made by heating sugar with ammonium salts, is reddish brown, imparts ph values of 4.2-4.8 to water, contains colloidal particles with positive charges, and is used in baked goods, syrups, and puddings. Caramel color made by heating sugar without an ammonium salt is also reddish brown, but contains colloidal particles with slightly negative charges and has a solution ph of 3-4. It is used in beer and other alcoholic beverages. Pyrolytic reactions of sugars * * ** *Flavor of bread **Flavor & sweetness enhancer 2
Oligosaccharides An oligosaccharide contains 2 to 20 (10) sugar units joined by glycosidic bonds. When a molecule contains more than 20 units, it is a polysaccharide. Disaccharides are glycosides in which the aglycon is a monosaccharide unit. A compound containing three monosaccharide units is a tri-saccharide. Oligosaccharides 2 Structure containing from 4 to 10 glycosyl units, whether linear or branched, are tetra-, penta-, hexa-, octa-, nona-, and decasaccharides, and so on. Because glycosidic bonds are part of acetal structures, they undergo acid-catalyzed hydrolysis, that is, cleavage in the presence of aqueous acid and heat. Only a few oligosaccharides occur in nature. Most are produced by hydrolysis of polysaccharides into smaller units. lactose lactase D-glucose + D-galactose 4-O- -D-glucopyranosyl-D-glucopyranose 4-O- -D-galactopyranosyl-D-glucopyranose 3
Lactose absorption Lactose is ingested in milk and other unfermented dairy products, such as ice cream. Fermented dairy products, such as most yogurt and cheese, contain less lactose because during fermentation some of the lactose is converted into lactic acid. Lactose stimulates intestinal adsorption and retention of calcium. Lactose is not digested until it reaches the small intestine, where the hydrolytic enzyme lactase is located. Lactase (a -galactosidase) is a membrane-bound enzyme located in the brush border epithelial cells of the small intestine. It catalyzes the hydrolysis of lactose into its constituent monosaccharides, D-glucose and D-galactose. Of the carbohydrates, only monosaccharides are absorbed from the intestines. Both D-glucose and D-galactose are rapidly absorbed and enter the blood stream. Lactose-intolerant If there is a deficiency of lactase, some lactose remains in the lumen of the small intestine. The presence of lactose tends to draw fluid into the lumen by osmosis. This fluid produces abdominal distention and cramps. From the small intestine, the lactose passes into the large intestine (colon) where it undergoes anaerobic bacterial fermentation to lactic acid (present as the lactate anion) and other short-chain acids. The increase in the concentration of molecules, that is, the increase in osmolality, results in still greater retention of fluid. In addition, the acidic products of fermentation lower the ph and irritate the lining of the colon, leading to an increased movement of the contents. Diarrhea is caused both by the retention of fluid and the increased movement of the intestinal contents. The gaseous products of fermentation cause bloating and cramping. The fate of lactose in the large intestine of persons with lactase deficiency -D-glucopyranosyl -D-fructofuranoside 4
Sucrose, raffinose, and stachyose Common & systematic names of oligosaccharides Homework: draw the molecular structures of these three molecules 5