Salvio Jiménez-Pérez

Free radical scavenging capacity of Maillard reaction products as related to colour and fluorescence

Abstract The free radical scavenging activity of Maillard reaction products (MRPs) produced by heating glucose or lactose with lysine, alanine or glycine was estimated directly by means of a 2,2-diphenyl-1-picrylhydrazyl radical (DPPH·) method. Model systems of sugar (lactose and glucose) and amino acid (alanine, glycine, and lysine) were heated without pH control at 100°C up to 24 h. Fluorescence (347 and 415 nm), browning (absorbance at 420 nm), colour parameters in the CIELab scale, and free radical scavenging activity were determined. Browning is not directly related to the free radical scavenging properties of MRPs formed at prolonged heating conditions. This study shows that fluorescence measurement of heated sugar/amino acid systems is more effective than browning to follow the formation of MRPs with free radical scavenging activities. This approach could be applied to optimise the formation of natural antioxidants during food processing where browning is not desirable for consumers.

Fluorescence associated with Maillard reaction in milk and milk-resembling systems

The progress of the Maillard reaction in milk and milk-resembling systems during heating was followed by monitoring free fluorescent intermediary compounds. Fluorescence intensity was examined over a wide temperature/time range (90–140 °C/0.5–30 min). Different kinetic behaviours were found in the presence or absence of amino groups in the system. Traces of fluorescence were detected when the lactose system without proteins was heated; the isomerization and degradation reactions of lactose could also generate fluorescent compounds. Fluorescence accumulation in the lactose system (without proteins) proceeded according to first-order reaction kinetics, whereas systems with lactose and casein, lactose and whey protein and milk fitted zero-order reaction kinetics. Apparent activation energies calculated were 61.3, 78.6, 59.6 and 83.9 kJ mol−1 for lactose system, lactose/casein system, lactose/whey protein system and milk, respectively. Moreover, levels of intermediary fluorescent compounds in Spanish commercial milk (pasteurized, UHT-treated and in-bottle sterilized milk) have been studied.

New methodologies for kinetic study of 5-(Hydroxymethyl)-furfural formation and reactive lysine blockage in heat-treated milk and model systems

New analytical techniques were used to study the kinetic behavior of 5-(hydroxmethyl) furfural (HMF) and blockage of available lysine in milk and model systems. Both determination of HMF by the chromatographic method and estimation of available lysine by the ortho-phthaldialdehyde (OPA) method at sterilization temperatures proved valid. Activation energies for the lysine loss reaction were 91.14, 112.41 and 66.67 kJ·mol-1 in the model systems and the milk respectively in fluorimetric determination with OPA. Activation energy for HMF determination ranged from 118.5 to 93.04 kJ·mol-1.

An enhanced liquid chromatographic method for 5-hydroxymethylfurfural determination in UHT milk

SummaryAn enhanced method of HPLC for the determination of 5-Hydroxymethyl-2-furfuraldehyde (HMF) in fluid milk was perfected. Its resolving capacity permitted the specific separation of this compound. The real concentration (μmol/l) of HMF was determined and not the HMF value, as in the colorimetric analysis, where an intermediary coloured compound (HMF-TBA) is utilised. The optimum pH condition (pH=4.0–4.2) for the mobile phase to prevent overlapping between the HMF peak and the peaks of the other compound (α and β) that coeluted with the HMF and also the optimum organic solvent content (3% or 7%) were established. This is a sensitive, quick and safe method for the determination of HMF in fluid milk.

Hydroxymethylfurfural determination in infant milk-based formulas by micellar electrokinetic capillary chromatography

Abstract A procedure for determining hydroxymethylfurfural (HMF) in milk-based products by micellar electrokinetic capillary chromatography (MECC) was settled on. The effect of the trichloroacetic acid on the migration time of HMF and presence of interference peaks were investigated. MECC procedure was applied on commercial liquid infant formulas and compared with the classical reversed-phase high performance liquid chromatography method, giving similar values of repeatability and recovery. HMF peak was well-resolved by using an uncoated fused-silica capillary (48.5 cm×50 μm i.d.) with 50 mM phosphate buffer (pH 7.5) containing 100 mM SDS as electrolyte, voltage at 20 kV (25°C). Sample injection was by pressure (50 mbar, hydrodynamic) for 2.5 s. HMF analysis by MECC was suitable for routine analysis since separation was completely achieved at 5 min.

The Maillard reaction during the ripening of Manchego cheese

The Maillard reaction was studied in semi-industrial Manchego cheese during ripening, through measurement of furosine and galactose. An accumulation of galactose was observed during the initial period of ripening, probably due to the inability of Streptococcus thermophilus to metabolise galactose. However, a considerable decrease in galactose content and increase in furosine amount were found after 15 days of ripening. No further formation of furosine was observed after 45 days. This fact could be attributable to the exhaustion of galactose due to its participation in Maillard reaction and/or its utilisation by the microorganisms Lactococcus and Lactobacillus. Furosine seems to be a useful indicator of the Maillard reaction during ripening of Manchego cheese.

Effect of Storage Temperature on Lactulose and 5-hydroxymethyl-furfural Formation in UHT Milk¹

The amounts of two heating-induced products were studied (lactulose and 5-hydroxymethyl-furfural) in UHT milk samples, directly and indirectly heat-treated, during a period of 90 d storage at five different temperatures. The results indicate an increase in 5-hydroxymethyl-furfural, as well as lactulose, taking place in the range of temperatures between 40-50°C.

Effect of storage temperature on galactose formation in UHT milk

Abstract Galactose has been studied from the standpoint of its suitability as a heat-induced parameter for processed milk. Galactose content in two directly and two indirectly UHT treated liquid milk samples were studied during storage until the end of its shelf-life (12 weeks) period at temperatures of 6, 20, 30, 40, and 50°C. No significant differences during storage were found with reference to the type of thermal process applied, nor with respect to the initial galactose content of the sample. Galactose formation was just directly related to storage time and temperature, being more significant at temperatures higher than 20°C. Galactose increased slightly during storage at ambient temperature (20°C) after the first weeks but kept constant under temperature of refrigeration until end of storage. Thermal history of the processed milk, mainly at a temperature above 20°C, could affect the usefulness of galactose as a heat-induced parameter.

Application of the clarifying reagent index as a heat-induced parameter in commercial milk

The absorbance at 340 nm of transparent modified milk solutions (designated as Clarifying Reagent index) from industrially processed samples as well as from milk-resembling systems was measured in order to test the validity of this index for determining the extent of previous heat treatment. A statistical study (n = 81) of the Clarifying Reagent index of different commercial milks was carried out. There was a significant difference (P < 0.001, Students t test) between three groups of samples: (1) raw and pasteurized milk, (2) UHT-treated milk, and (3) in-bottle sterilized milk. The Clarifying Reagent index showed significant relationships (P < 0.001, Students t test) with other heat-induced parameters such as hydroxymethylfurfural formation, loss of available lysine, and free fluorescent intermediary compounds formed by the Maillard reaction. Between 90 and 140°C, the Clarifying Reagent index of the clarified milk or milk-resembling systems followed zero-order kinetics as determined by nonlinear regression analysis.

Kinetics of the formation of galactose in milk during thermal processes

During the heating of milk a series of changes occur in its elements. It is proposed to use the galactose as an index for measuring the intensity of the thermal treatment applied to the milk. An enzymatic method was used to know the galactose content. This work is a kinetic study of its formation during heating and found that the process of formation followed a kinetic of zero order. The formation shows two clearly delimited phases, attributable to the two different methods of formation.