HEAT-INDUCED CHANGES IN MILK 2. Some of the by-products of Maillard browning have strong flavours(e.g furfural, hydroxymethylfurfural) which alter the typical flavour of milk 3. 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 4. The polymerized products of Maillard browning can bind metals, especi 5. It has been suggested that some products of the Maillard reaction are toxic and or mutagenic but such effects are, at most, weak and possibly due to other consequences of browning, e.g. metal binding 6. The attachment of sugars to the protein increases its hydrophilicity; however, solubility may be reduced, probably due to cross linking of protein molecules The heat stability of milk is increased by the maillard reaction, probably via the production of carbonyls( section 9.7) The formation of brown pigments via the Maillard reaction, especially in model systems(e. g. glucose-glycine), usually follows zero-order kinetics but the loss of reactants has been found to follow first- or second-order kinetics in foods and model systems. Activation energies of 109, 116 and 139 kJ mol have been reported for the degradation of lysine, the forma tion of brown pigments and the production of hydroxymethylfurfural (HMF), respectively Browning can be monitored by measuring the intensity of brown colour, the formation of hydroxymethylfurfural(which may be measured spectro- photometrically, after reaction with thiobarbituric acid, or by hPlC, but which is not regarded as a very good indicator of Maillard browning), loss of available lysine(e.g by reaction with 2, 4-dinitrofluorobenzene)or by the formation of furosine. Furosine is formed on the acid hydrolysis of lactulosyl lysine(the principal Maillard product formed during the heating of milk) During acid hydrolysis, lactulosyl lysine is degraded to fructosylysine which is then converted to pyridosine, furosine and carboxymethyl lysine(Figure 9. 8). Furosine may be determined by ion-exchange chromatography, GC or HPLC, and is considered to be a very good indicator of Maillard browning and the severity of heat treatment of milk(erbersdobler and Dehn- Muller 1989 ). The effects of time and temperature on the formation of furosine are shown in Figure 9.9. The concentration of furosine is highly correlated with furosine in commercial UHT milks is shown in Figure 9 0 oncentration of the concentrations of HMF and carboxymethyl lysine. The Dicarbonyls, which are among the products of the Maillard reaction, can react with amines in the Strecker reaction, producing a variety of flavourful
HEAT-INDUCED CHANGES IN MILK 357 2. Some of the by-products of Maillard browning have strong flavours (e.g. furfural, hydroxymethylfurfural) which alter the typical flavour of milk. 3. 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. 4. The polymerized products of Maillard browning can bind metals, especially Fe. 5. It has been suggested that some products of the Maillard reaction are toxic and/or mutagenic but such effects are, at most, weak and possibly due to other consequences of browning, e.g. metal binding. 6. The attachment of sugars to the protein increases its hydrophilicity; however, solubility may be reduced, probably due to cross-linking of protein molecules. 7. The heat stability of milk is increased by the Maillard reaction, probably via the production of carbonyls (section 9.7). The formation of brown pigments via the Maillard reaction, especially in model systems (e.g. glucose-glycine), usually follows zero-order kinetics, but the loss of reactants has been found to follow first- or second-order kinetics in foods and model systems. Activation energies of 109, 116 and 139 kJ mo1-l have been reported for the degradation of lysine, the formation of brown pigments and the production of hydroxymethylfurfural (HMF), respectively. Browning can be monitored by measuring the intensity of brown colour, the formation of hydroxymethylfurfural (which may be measured spectrophotometrically, after reaction with thiobarbituric acid, or by HPLC, but which is not regarded as a very good indicator of Maillard browning), loss of available lysine (e.g. by reaction with 2,4-dinitrofluorobenzene) or by the formation of furosine. Furosine is formed on the acid hydrolysis of lactulosyl lysine (the principal Maillard product formed during the heating of milk). During acid hydrolysis, lactulosyl lysine is degraded to fructosylysine which is then converted to pyridosine, furosine and carboxymethyl lysine (Figure 9.8). Furosine may be determined by ion-exchange chromatography, GC or HPLC, and is considered to be a very good indicator of Maillard browning and the severity of heat treatment of milk (Erbersdobler and Dehn-Miiller, 1989). The effects of time and temperature on the formation of furosine are shown in Figure 9.9. The concentration of furosine is highly correlated with the concentrations of HMF and carboxymethyl lysine. The concentration of furosine in commercial UHT milks is shown in Figure 9.10. Dicarbonyls, which are among the products of the Maillard reaction, can react with amines in the Strecker reaction, producing a variety of flavourful
DAIRY CHEMISTRY AND BIOCHEMISTRY (Galactose)>Glucose-Lysine<R 1. Addition compound 2. N-substituted glycosylamine Pyridoxine 3a. schiff base 3b, Enol form (Galactose,> Eructosylysine< CH2)4 HC-OH HC-OH HIC-OH Figure 9.8 Initial steps of the maill iction with the formation of furosine(after hydrolysis with 7. 8 M HCD)as well as of N-E-ca chn -Muller, 1989)
358 .~.~..~...~..... / .~~...~........~~~~....~.. Oxidative cleavage rCOOH I DAIRY CHEMISTRY AND BIOCHEMISTRY t 1. Addition compound I 2. N-substituted glycosylamine 3a. Schiff base 3b. Enol form > Fructosylysh < (Galactose) (Glucose) H 0’ OCH3 II 0 Pvrldoslne J HC -OH HC -OH HZC -OH I I Ervthroluc &id OR RI C-CH-NH I 11 / W2)4 I NH I y+2 CGQH Carboxymethvl BROWNING Figure 9.8 Initial steps of the Maillard reaction with the formation of furosine (after hydrolysis with 7.8 M HCI) as well as of N-E-carboxymethyl lysine and erythronic acid (from Erbersdobler and Dehn-Muller, 1989)