Lucy Bialek

Effect of mechanical and thermal treatments on the microstructure and rheological properties of carrot, broccoli and tomato dispersions

BACKGROUND The food industry has shown an increased interest in the manufacture of healthier and more natural food products. By tailored processing fruit and vegetables can be used as structurants thus reducing artificial gums and stabilisers. The effect of different thermal and mechanical treatments, including high-pressure homogenisation, on the microstructural and rheological properties of carrot, broccoli and tomato dispersions was studied. As part of the rheological characterisation small oscillatory deformation as well as shear flow measurements were performed. RESULTS Carrot and broccoli showed a different behaviour from tomato under the conditions studied. Changing the order of thermal and mechanical treatment led to microstructures with different flow properties. The resulting microstructures differed in the manner of cell wall separation: either breaking across the cell walls or through the middle lamella. High-pressure homogenisation decreased the viscosity of carrot and broccoli dispersions, while it increased the viscosity of tomato. Cryo-scanning electron microscopy showed that the cell walls of carrot and broccoli remained as compact structures after homogenisation whereas tomato cell walls were considerably swollen. CONCLUSIONS Based on the type of vegetable, the different processes applied led to microstructures with different rheological properties. This study shows that particle size distribution, morphology and phase volume are important parameters to explain the complex relationship between rheology and microstructure for these types of systems.

Comprehensive metabolomics to evaluate the impact of industrial processing on the phytochemical composition of vegetable purees

The effects of conventional industrial processing steps on global phytochemical composition of broccoli, tomato and carrot purees were investigated by using a range of complementary targeted and untargeted metabolomics approaches including LC-PDA for vitamins, (1)H NMR for polar metabolites, accurate mass LC-QTOF MS for semi-polar metabolites, LC-MRM for oxylipins, and headspace GC-MS for volatile compounds. An initial exploratory experiment indicated that the order of blending and thermal treatments had the highest impact on the phytochemicals in the purees. This blending-heating order effect was investigated in more depth by performing alternate blending-heating sequences in triplicate on the same batches of broccoli, tomato and carrot. For each vegetable and particularly in broccoli, a large proportion of the metabolites detected in the purees was significantly influenced by the blending-heating order, amongst which were potential health-related phytochemicals and flavour compounds like vitamins C and E, carotenoids, flavonoids, glucosinolates and oxylipins. Our metabolomics data indicates that during processing the activity of a series of endogenous plant enzymes, such as lipoxygenases, peroxidases and glycosidases, including myrosinase in broccoli, is key to the final metabolite composition and related quality of the purees.

Effect of Enzyme Homogenization on the Physical Properties of Carrot Cell Wall Suspensions

Plant cell wall suspensions are widely present in daily food, such as soups, dressings, and sauces. Cell walls of edible plants are made up of an intricate biopolymer network of mainly cellulose microfibrils, pectins, and hemicelluloses. The current study aims to obtain a better insight in cell wall biopolymer interactions and their relation to physical functionalities of cell wall suspensions. In addition to standard thermal and mechanical treatments to effectively disrupt the cell wall, enzymes were added to hydrolyze specific classes of biopolymers. Incubation with pure cellulases, pectinases, or hemicellulases resulted in suspensions with different physical properties. The effect of the enzymes were measured on the rheology over an 8 h period, and the microstructure was investigated by cryo-scanning electron microscopy. Homogenization of the enzyme-treated system was further investigated. It was observed that there was an increase in the storage modulus after homogenizing the enzyme-treated samples. For example, after homogenizing the 8 h pectinase-treated sample, the storage modulus went from 30 to 70 Pa. This was due to an increase in the particle size and, hence, the volume fraction. The enzyme treatment weakened the cell wall; the additional homogenizing step caused the cell wall to break along the defects caused by the enzyme. This resulted in distinctly different cell wall microstructures based on the type and extent of the treatment.

Cell-Based Assay To Quantify the Antioxidant Effect of Food-Derived Carotenoids Enriched in Postprandial Human Chylomicrons

We developed a new method to evaluate the antioxidant effect of food products in a biological system. The antioxidant status of HepG2 cells was quantified after incubation with postprandial human chylomicrons after the intake of vegetable products. Three subjects consumed in a meal a vegetable soup containing 8.4 mg of β-carotene and 9 mg of lycopene. After 5 h, the subjects consumed a second meal without carotenoids. Blood samples were collected at basal time and every hour for 9 h. Chylomicrons were isolated from serum samples and used for both carotenoid quantification and HepG2 stimulation. Carotenoid in chylomicrons followed an inter-individual and bimodal carotenoid response. We demonstrated the antioxidant effect of postprandial chylomicrons in HepG2 at the time of maximum carotenoid concentration of chylomicrons with respect to basal time. This cell-based assay seems to be a useful method to evaluate the antioxidant effect of fruit and vegetable products in a biological system.

Effect of Enzymes on Serum and Particle Properties of Carrot Cell Suspensions

Rheological properties of cell wall suspensions strongly depend on particle size and particle-particle interactions. In the present study, an experimental method was developed to study the effect of particle elasticity and electrostatic interactions on the rheological properties of cell suspensions. Enzymes were used to selectively depolymerize the pectin (backbone) and proteins in suspensions. The enzymatic treatments affected the physical properties, thus a hypothesis for the structure-function relationship of these biopolymers was formulated. The enzymatic treatment directly affected particle properties, resulting in looser cell walls as visualized by cryo-SEM. The effect of the enzymatic treatment on the storage modulus was measured as a function of total solid content (below critical packing fraction). Furthermore, experiments were performed in the presence of varying concentrations of sodium chloride in order to change the Debye screening length. Such method assisted in decoupling the electrostatic effects from particle elasticity. In addition, particle properties were measured directly by applying a compressive strain on the particles and measuring the normal force. By fitting the normal stress relaxation with a Maxwell model, particle properties such as time scale of relaxation and elasticity were obtained. It is suggested that for carrot suspensions, pectins on the cell walls could contribute to the particle hardness. The pectins on carrot cell walls are responsible for electrostatic interactions between particles.