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Effect of structural characteristics on the rheology and nutritional quality of plant-tissue-based food suspensions. A study on carrot- and tomato-derived products

Boek - Dissertatie

Plant-tissue-based food suspensions, like soups, sauces and purées, consisting of a particle fraction and a continuous serum phase, are the final result of structure-enabling (e.g. blending, mixing, high-pressure homogenisation) and preservation unit operations (e.g. heating, high-pressure treatments) used during production. The structural organisation of these suspensions, governing their functional properties, is largely determined by the production process. In other words, there is a possibility to tailor the functional properties of plant-tissue-based food suspensions based on structure management by the use of controlled processing. Therefore the understanding and quantification of these process-structure-function relations has become a key research area and forms the basis for future improved and novel food process and product design. In the present study, specific attention was given to the rheology and the nutritional quality of plant-tissue-based suspensions and the relations between structural characteristics and these functional properties were investigated. First, the relation between structure and rheology of plant-tissue-based food suspensions was investigated. As the rheology of plant-tissue-based food suspensions is known to be influenced by the properties of both the particle and the serum phase, the effect of both phases on the resulting rheology was investigated separately. To start, the effect of particle characteristics on the rheology of carrot-derived and tomato-derived suspensions was investigated systematically. The reconstitution principle was used to obtain samples with particles with well-characterised structural properties, distinguishing this study from most rheological studies available in literature. A range of relatively monodisperse suspensions, containing either cell fragments, single cells (in the case of tomato) or cell clusters (in the case of carrot) with varying average particle size and pulp content (from 25 to 65 wt.%) were prepared. The influence of the serum properties on the rheological characteristics of the suspensions was excluded by reconstituting the particles in water rather than serum. All the investigated suspensions were non-Newtonian liquids exhibiting a yield stress. The undisrupted network structure of all suspensions could be classified as a weak gel with a rather low critical strain. Increase in yield stress and storage modulus with particle concentration was modelled using a power law model. The ratio of static yield stress to dynamic yield stress turned out to be larger for particles with a more irregular, less intact surface, showing the enhanced tendency of the latter particles to build up structure in rest conditions. Particle concentration, size and morphology (i.e. surface/shape/type) turned out to be key structural properties controlling the rheological parameters of plant-tissue-based food suspensions.Subsequently, the role of the serum phase in the rheology of plant-tissue based suspensions was investigated. To start, the influence of process-induced pectin changes on the kinematic viscosity of the serum phases of carrot- and tomato-derived suspensions was studied. Changes in pectin structure were monitored by determination of the degree of methoxylation, analysis of the molar mass distribution and immuno-dot blotting using anti-pectin antibodies. Characterisation of serum pectins revealed that a strong thermal treatment caused pectin thermosolubilisation and depolymerisation, especially in carrot sera, and high-pressure homogenisation provoked predominantly mechanical pectin solubilisation in carrot sera and molar mass changes in tomato sera. It was observed that the kinematic viscosity of carrot sera was mainly affected by the amount of solubilised pectin, whereas in tomato sera, changes in the kinematic viscosity could largely be explained in terms of differences in polysaccharide chain length. The degree of methoxylation of pectin, which has a broader distribution in tomato sera than in carrot sera, seemed to have a less pronounced role in determining the kinematic viscosity. In general, it could be concluded that serum viscosity is mainly determined by the amount and the molar mass of the solubilised pectin. Furthermore, the effect of the presence of a serum phase on the rheological properties of plant-tissue-based suspensions was investigated by comparing the rheology of reconstituted purées in water with those containing serum. For carrot as well as for tomato, replacing the serum phase by water led to a substantial decrease in network strength, especially in quiescent conditions. However, the influence of the serum viscosity on the rheology of plant-tissue-based suspensions was rather limited. In general, it could be concluded that the rheology of plant-tissue-based food suspensions is largely determined by the particle properties of the dispersed phase.To complete the investigation of the relation between the structural characteristics and the rheology of plant-tissue-based food suspensions, the effect of calcium ion (Ca2+) addition on the rheological properties of carrot-derived suspensions was studied. By separating the particle phase from the serum, and characterizing the rheology of both phases as a function of pectin degree of methoxylation, Ca2+ addition and pH, it was concluded that the particle phase rather than the serum phase was affected by Ca2+ addition. In carrot-derived suspensions containing intrinsic low-methoxylated pectin, Ca2+ addition caused a decrease in both network stiffness and strength, in particular at pH values above the pKa of galacturonic acid. Therefore, it was suggested that Ca2+ was screening the negatively charged pectin at the surface of the particles whereby the rheological characteristics of these suspensions, such as the yield stress and storage modulus, are reduced and the flow is facilitated. Ca2+ cross-link formation, if present at all, only had a limited effect on the rheological properties of carrot-derived suspensions. In a second part of this work, the relation between the structural characteristics and the carotenoid bioaccessibility of plant-tissue-based food suspensions was investigated. In particular, the effect of particle size on the relative all-E-ß-carotene and all-E-lycopene bioaccessibility in carrot- and tomato-derived suspensions was studied. Adding olive oil (2%) during digestion, especially as an oil-in-water emulsion, resulted in a substantial increase in carotenoid uptake in the micellar phase. Carotenoid bioaccessibility decreased with average particle size. Only particles smaller than an individual cell resulted in high carotenoid bioaccessibility values, pointing out the importance of the cell wall as main physical barrier for carotenoid release for the food matrix and subsequent micellarisation. These observations were valid for both carrot- and tomato-derived particles. Based on the relation obtained between particle size and carotenoid bioaccessibility, the relative all-E-ß-carotene and all-E-lycopene bioaccessibility in respectively carrot- and tomato-derived suspensions, were predicted. The calculated results indicated that carotenoid bioaccessibility in complex plant-tissue-based food suspensions is not only determined by the intactness of the cell wall, but is also affected by interactions between the structural compounds of the complex food matrix.
Jaar van publicatie:2013