Rheology of tomato and wheat dietary fibers in water and in suspensions of pimento purée

Highlights

• The flow of tomato and wheat dietary fiber aqueous suspensions was shear thinning.

• An equation to predict the apparent viscosity of fiber suspensions was given.

• The tomato and wheat dietary fibers stabilize the pimento purée.

• Sterilisation of purée with fibers decreases the apparent viscosity and influence the texture.

Abstract

The flow behaviour of the tomato and wheat dietary fibers in water and its effect on the rheology of the pimento purée were studied. Water retention, swelling, and fat adsorption capacity were determined for the characterization of the fibers. The flow behaviour of the fibers in water was measured with a concentric cylinder viscometer at 10–50 °C temperature range. It was shear thinning and fitted well to the power-law model for all fibers. The Arrhenius model described the effect of temperature on the apparent viscosity. The activation energy of flow range from 4.54 to 12.23 kJ/mol depending on the concentration. In all cases, the effect of concentration on apparent viscosity was more important than the effect of temperature. An exponential expression was given for the combined effect of temperature and concentration on the apparent viscosity. The rheological behaviour of suspensions of pimento purée with added dietary fibers was studied before and after sterilisation to prevent the separation for sedimentation. The experimental data were fitted to power law being the behaviour index similar to before sterilisation. The texture of pimento pure with fibers was determined at 20 °C. Sterilisation influenced the texture.

Introduction

The pimento purée prepared from dried pimentos is a complex suspension of cells and vascular bundles (Cepeda et al., 2000), whose viscosity is very concentration dependent (Cepeda and Gomez, 2002). As the serum has a low viscosity, sometimes the purée presents separation by sedimentation, which is a refuse factor for the consumer. An increase of the viscosity, by addition of dietary fibers could be the solution to the problem providing “body”, because of they contribute to the formation/stabilisation of emulsions, suspensions and foams (Diepenmaat-Wolters,1993). Moreover, the fibers also modify the texture adding the aforementioned beneficial health effects. So, dietary fiber exhibits laxation and blood cholesterol attenuation (Spiller, 1994) as well as blood glucose attenuation (Bijlani, 1985). It may also prevent cancer (Mccann et al., 2001), diabetes (Wang et al., 2002), heart disease (Fernandez, 2001) and obesity (Birketvedt et al., 2000). Furthermore, dietary fibers add to the foods excellent sensory qualities and might have technological functions such as gel formation, water binding and lipid binding.

In the open literature, the use of dietary fibers in the food industry has been described as thickeners in suspensions and also as texturing and bulking agents (Femenia et al., 1997). These effects are mainly ascribed to their hydration properties which are related to the chemical structure of the component polysaccharides and other factor such as porosity, particle size, ionic form, pH, temperature, ionic strength, types of ions in solution and stresses upon fibers (Elleuch et al., 2011, Thebaudin et al., 1997) and the way in which the food is processed. Generally, they are used in low concentration.

References were not found on the effect of adding dietetic fibers to pimento purée to modify its viscosity and texture. The tomato dietary fiber was chosen because it is an ingredient found along with the pepper in culinary preparations, also wheat fiber was chosen because of both are fully compatible with pimento puree and do not add flavour. Earlier works published on fiber suspensions indicate that the rheological behaviour of these materials nor only depends on fiber characteristics, but also on fiber orientation and matrix characteristics. The viscosity of fiber suspensions increase at low shear rates as concentration of fibers and/or the aspect ratio increase (Kamal and Mutel, 1989, Laun, 1984). At high shear rates, the viscosity of fiber suspensions approach the viscosity of the matrix, and became almost independent of fiber concentration and fiber characteristics (Czarnecki and White, 1980, Laun, 1984). Based on these antecedents, the behaviour of dietetic fibers in aqueous suspensions and as thickenings should be experimentally determined.

The functional properties of fibers have to be determined for its characterization. Consequently, hydration properties such as water retention capacity (WRC), swelling capacity (SW), and fat adsorption capacity (FAC) were determined. As the environmental conditions such as pH, ionic strength, nature of the ions can influence the hydration values of fibers containing polyelectrolytes determinations must be made at different pH and ionic strength (Fleury and Lahaye, 1991, Renard et al., 1994).

Stabilization of pimento purée in canning is necessary to avoid risk of microbiological by-products. Sterilization process generally involve high temperature which may alter the chemical composition and physicochemical, nutritional and functional properties of dietary fiber (Rodriguez et al., 2006). Because of the physicochemical properties of the fibers are modified slightly with sterilization this effect has to be determined experimentally.

The main objective of this work was to study of the effect of dietary fibers on the rheology of suspensions having pimento purée as model. This included: (1) the characterization of the fibers, (2) the study of the flow behaviour of aqueous suspensions of dietary fiber, and (3) the study of the effect of dietary fibers on the viscosity and texture of a suspension of pimento purée before and after sterilisation.