Carmen Romero

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.

Contribution of olive seed to the phenolic profile and related quality parameters of virgin olive oil

BACKGROUND Conflicting results have been reported about the effect of fruit de-stoning on the virgin olive oil (VOO) phenolic profile. The aim of the present study was to determine whether olive seed plays any role in the synthesis of this oil phenolic fraction. RESULTS Increases of around 25% of total phenolic compounds were observed in oils obtained from de-stoned olive fruits in three main Spanish cultivars. To investigate the involvement of olive seed in determining the phenolic profile of VOO, whole intact olive fruits were added with up to 400% olive stones. Excellent regression coefficients were found in general for the decrease of total phenolic compounds and, particularly, of o-diphenolics in the resulting oils. On the other hand, it was found that olive seed contains a high level of peroxidase (POX) activity (72.4 U g(-1) FW), accounting for more than 98% of total POX activity in the whole fruit. This activity is able to modify VOO phenolics in vitro, similar to the effect of adding stones during VOO extraction. CONCLUSION Olive seed plays an important role in determining VOO phenolic profile during the process to obtain an oil that seems to be associated with a high level of POX activity. Copyright

Oxygen concentration affects volatile compound biosynthesis during virgin olive oil production.

The effect of O 2 concentration on oil volatile compounds synthesized during the process to obtain virgin olive oil (VOO) was established. The study was carried out either on the whole process or within the main steps (milling and malaxation) of this process with two olive cultivars, Picual and Arbequina, at two ripening stages. Data show that O 2 control during milling has a negative impact on VOO volatile synthesis. This effect seems to depend on cultivar and on the ripening stage in cultivar Picual. Because most VOO volatiles are synthesized during olive fruit crushing at the milling step, O 2 control during malaxation seems to affect just slightly the volatile synthesis. The highest effect was observed when control of O 2 concentration was performed over the whole process. In this case, the content of volatile compounds of oils obtained from both cultivars and ripening stages showed quite similar trends.

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.

An improved method for determination of the activity of β-galactosidase in yoghurt by high-performance liquid chromatography

SummaryA new method has been developed for the HPLC determination of the activity of lactase from the microorganisms in yoghurt. The method is based upon the ability of β-galactosidase to hydrolyze lactose bonds in glucose and galactose. To determine such activity in yoghurt, 1 mL of yoghurt was diluted 1∶10 with 0.1 M phosphate buffer (pH 7) containing 2 % (w/v) lactose and 5 mM dithioerythritol as reducing agent; the mixture was the incubated and injected into the HPLC. Lactase activity remained high as long as the number of viable microorganisms did not fall below the minimum CFU·mL−1 (107). This method is more repeatable than conventional colorimetric determination, and may also be automated for routine analysis.