Albert Ibarz

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.

Extraction and characterization of pectin from stored peach pomace

Extraction of pectin from peach pomace stored for 10 months at 3°C and 95% RH was carried out at 60° and 80°C, in acidified solutions in the 1.20–2.53 pH range, for times varying between 10 and 80 min. The kinetics of extraction was studied and pectin yields from fresh and stored pomace under equivalent conditions of temperature, pH, and time were compared. The intrinsic viscosity and degree of esterification of selected pectin samples from stored pomace were analyzed and were also compared with the equivalent samples previously extracted from fresh pomace [Pagan, J., Ibarz, A., Llorca, M., & Coll, L. (1999). Quality of industrial pectin extracted from peach pomace at different pH and temperatures. Journal of the Science of Food and Agriculture, 79, 1038–1042]. Finally, a quality index for each sample was obtained to allow an analysis of the effect of storage on the quality of the extracted pectin.

Kinetic models of non-enzymatic browning in apple puree

The effects of thermal treatments on 11∞Brix apple puree were studied at temperatures from 80 to 98∞C. Colour changes (measured by reflectance spectroscopy, colour difference, L*, a* and b* and the evolution of 5-hydroxymethylfurfural (HMF) and sugars (hexoses and sucrose) were used to evaluate non-enzymatic browning. A kinetic model based on a two-stage mechanism was applied to the evolution of colour difference and a*. A first-order kinetic model was applied to L* evolution, while the evolution of absorbance at 420 nm (A420 ) of the liquid fraction 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 the kinetic constants was described by an Arrhenius-type equation. The presence of pulp in the samples led to activation energies for A 420 and sucrose which were lower than those found previously for clarified juices with the same soluble solids content. # 2000 Society of Chemical Industry

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.

Removal of dark compounds from clarified fruit juices by adsorption processes

Abstract To improve the colour of a browned clarified peach juice, an adsorption treatment using different kinds of activated carbon was investigated. The adsorption equilibrium was studied by adsorption isotherms in the range 10–50 °C for three types of activated carbon. Absorbance data at 420 nm were used to plot all the isotherms which are correlated reasonably well with the Langmuir and Freundlich isotherms. A kinetic study is also presented where changes with time of the following parameters were analyzed: absorbance at 420 nm, hydroxymethylfurfural (HMF) concentration, soluble solids content and pH of the juice for various amounts of the three activated carbons at different temperatures. Improvements of the juice colour due to the adsorption treatments were detected by a decrease in the absorbance at 420 nm and in the HMF content. Also, it was observed that the pH increased and the soluble solids decreased. The initial rate of the adsorption was very fast, but then it became quite slow and reached equilibrium in an asymptotic form. It was found then, that a contact time of 10–15 min is enough to obtain a suitable juice. The optimum processing temperature was found in the 30–50 °C range and the most effective carbon was the granular activated carbon with 2–4 mm particle size.

The rheology of semiliquid foods.

Publisher Summary This chapter introduces the basic food rheology concepts, such as shear rate, shear stress, viscosity, elasticity, and viscoelasticity, and describes the concept of the experimental methods for steady shear, extensional flow, and dynamic measurements. It discusses the principles of instrumentation, along with elaborating constitutive models based on steady shear flow, diluted solution molecular theories, and concentrate solutions. The chapter also discusses the applications of rheology in characterizing engineering properties of foods. Food rheology is concerned with the description of the mechanical properties of food materials under various deformation conditions. To understand and use rheological information, it is essential to have a mechanistic basis for the interpretation and correlation of the experimental data. Such interpretation of behavior in terms of theoretical mechanisms provides the guidelines needed to make sense of observations, to relate behavior to composition and structure, to predict and to modify properties, and to compare one experimental method with another.

Rheological Behavior of Tomato Juice: Steady-State Shear and Time-Dependent Modeling

IntroductionThe present work has evaluated the time-dependent and steady-state shear rheological properties of tomato juice.Materials and MethodsThree models were compared for describing the shear stress decay during shearing (Figoni–Shoemaker, Weltman, and Hahn–Ree–Eyring), and the parameters of each model were empirically related with the shear rate.ResultThe three evaluated models, as well as their modification as function of shear rate, described well the experimental data of tomato thixotropy. The Herschel–Bulkley and Falguera–Ibarz models have shown to be very adequate to describe the data from steady-state shear. The obtained data are potentially useful for future studies on food properties and process design.

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.

Kinetic and Multivariate Analysis of Polyphenol Oxidase Inactivation by High Pressure and Temperature Processing in Apple Juices made from Six Different Varieties

The effectiveness of high pressure processing on the inactivation of apple polyphenol oxidase has been widely investigated. However, from an industrial point of view, there is a need for assessing this effectiveness on each one of the different apple varieties that each company uses for their products. This piece of work fills in this gap, studying the effect of the variety on apple juice polyphenol oxidase inactivation. Six varieties were assayed (Braeburn, Fuji, Gala, Golden Delicious, Granny Smith, and Red Delicious), searching for this influence from two different approaches: a kinetic study, finding the best model for each one of them, and a multivariate part consisting of a principal component analysis of data.