S. Garza

Kinetic models for colour changes in pear puree during heating at relatively high temperatures

Abstract The effect of thermal treatments on 11° Brix pear puree were studied at temperatures from 80°C to 98°C. Changes in colour measured with colorimetric parameters (reflectance spectra, colour difference, L *, a * and b * parameters), and the variation in 5-hydroxymethylfurfural (HMF) and sugars (hexoses and sucrose) content were used to evaluate non-enzymatic browning. A kinetic model based on a two-stage mechanism was applied to the changes in colour difference and a *. A first order kinetic model was applied to L * changes, while absorbance at 420 nm (A 420 ) for liquid fractions was described using a zero order kinetic model. Thermally treated samples became more reddish and suffered a slight loss of yellow hues. The effect of temperature on kinetic constants was described by an Arrhenius type equation. The presence of pulp in the samples implied that the activation energy was lower than in clarified juices with the same soluble solids content.

Non-enzymatic browning in peach puree during heating

Non-enzymatic browning in peach puree (11 Brix) due to extended thermal treatment was investigated. Absorbance at 420 nm, colorimetric parameters (CIE L, a, b and E), content in sugars (fructose, glucose and sucrose) and hydroxymethylfurfural were used to estimate the extent of nonenzymatic browning during heating at high temperatures (80, 85, 90, 95 and 98C) for 480 min. Zero and first order kinetics were applied to describe evolution of relative absorbance at 420 nm. Relative luminosity and sucrose content follow a first order kinetic. Colour diAerence was successfully adjusted to a model including two stages: the first one includes the colour formation and it follows a kinetic of order zero and the second includes the colour destruction and it follows a first order kinetic model. Both first and autocatalytical models can describe HMF formation. Parameter b was significantly reduced with heating time, especially at higher temperatures, on the contrary parameter a increases during heating. The Arrhenius model described well the temperature dependence of the reaction rate constant for all the parameters considered. # 1999 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.

Rheology of clarified cherry juices

The rheological behaviour of cherry juices with different soluble solids content (22–74 ° Brix) was studied at a wide range of temperatures (5–70 °C) using a concentric cylinder viscometer. The results indicated that these juices behave as Newtonian fluids. The effect of temperature on their viscosity can be described by means of an Arrhenius-type equation. Depending on soluble solids, the activation energies for viscous flow vary from 3.44 to 14.6 kcal · mol−1. The effect of soluble solids on viscosity can be described by an exponential equation. Experimental data were fitted to several models in order to describe the combined effect of temperature and soluble solids content. The equation below was found to be the best to describe the combined effect of temperature and soluble solids content on the dynamic viscosity.

Toxic effect of melanoidins from glucose–asparagine on trypsin activity

In this work the effect of the presence of the melanoidins from glucose-asparagine on the enzymatic activity of trypsin is studied. It was observed that an excess of N alpha-benzoyl-L-arginine ethyl ester (BAEE) has an inhibiting effect on this enzyme activity. The maximum reaction rate was obtained for a 0.06 mN substrate concentration. It is also observed that the presence of melanoidin inhibits the enzymatic activity of trypsin. This inhibition can be described as a linear mixed type where the inhibition constant alpha K(i) of the substrate-inhibitor complex is higher than the inhibition constant K(i) of the complex enzyme-inhibitor with a alpha value of 1.88.

Heat resistance of Saccharomyces cerevisiae strains isolated from spoiled peach puree

The heat resistance of three Saccharomyces cerevisiae strains isolated from spoiled peach puree, in McIlvaine buffer (pH 4 and 7) and peach puree (pH 3.9) was studied. The D60-values in buffer at pH 7 were 0.75, 1.32 and 0.14 min for the strains 173, 180 and 325, respectively. The pH of the buffer did not influence the heat resistance of the three strains studied. The thermal sensitivity for all strains assayed was higher when peach puree was used with D60-values of 0.53, 0.20 and 0.10 min for the strains 173, 180 and 325, respectively. The menstrua used had limited influence on the z-values, varying between 3 (strain 173, pH 4) and 4 (strain 325, peach puree).

Kinetics of Peach Clarified Juice Discoloration Process with an Adsorbent Resin

This paper deals with the effects of adsorbent resin upon kinetic process of peach juice adsorption de-coloration at different temperatures ranged from 10 °C to 50 °C. The adsorption equilibrium was quantified by means of adsorption isotherms in the range from 10 °C to 50 °C. Absorbance data at 420 nm were used to plot all the isotherms, which correlated reasonable well with the Freundlich and Langmuir isotherms. Also, the efficiency of the adsorption process was studied for different resin/juice ratios at different temperatures, from which it was observed that there was a notable improvement in efficiency as the resin content increased, while the increase in temperature was not so important in the process. The adsorption kinetics was also studied at 30 °C, for relationships of 1, 2, and 3 g resin/100g juice. A kinetic adsorption— desorption model in two simultaneous steps was proposed, where the adsorption step is considered as zero order and the desorption step as first order. This allows a global expression to be obtained that fits the experimental data appropriately to this kinetic type equation. The initial adsorption rate depended on the resin/juice relationship in such a way that the higher its value, the lower this relationship was. It was also shown that the equilibrium constant showed a similar tendency, its value being superior to the unit, which indicates that the retention stage over the resin prevails over the colored product desorption stage.