Maud Langton

Fine-stranded and particulate gels of β-lactoglobulin and whey protein at varying pH

Abstract The effects of pH on the gel network structure have been characterized by means of different microscopy techniques. β-Lactoglobulin forms white particulate gels in the region of pH 4–6, and transparent fine-stranded gels below and above this region. Microstructural analysis showed differences in the regularity and particle size of the particulate network structures. The fine-stranded gels formed at low pH were composed of stiff short strands, whereas the fine-stranded gels formed at high pH had longer, more flexible strands. A mixture of particulate and fine-stranded structures was found at the lower shift, at pH 4. At the higher shift pH 6 no mixtures of structures were found and either a particulate or a fine-stranded structure was formed. The microstructure of whey protein gels at different pH showed similarities with the microstructure of β-lactoglobulin gels at comparable pH. The results of the microstructure correlated with previously published data on fracture properties.

Microstructure and rheological behaviour of particulate β-lactoglobulin gels

Abstract The microstructure of the network as well as the strands of particulate β-lactoglobulin gels formed at pH 5.3 have been characterized by microscopy. The microstructural influence on the rheological properties both at small and large deformations has been measured. It was shown that the microstructure depends on the heating rate used. Gels formed at a fast heating rate (5–10°C/min) consisted of a homogeneous network with pore sizes of 20–30 μm. The strands were formed by evenly sized spherical particles linked like a flexible string of beads. At a slow heating rate (0.1–1°C/ min) the network had larger pores, ~100–150 μm. The network formed at 0.1°C/min was inhomogeneous, with regions of small and large pores. The particle size distribution was broader at a slow heating rate and the strands, formed by several particles fused together, were thicker. Tensile measurements of fracture properties showed that the gels formed at a fast heating rate had higher stress and strain at fracture due to the network structure. The size of the weakest element of the network was deduced from notch sensitivity measurements and correlated well with the pore size, i.e. the fracture starts at the largest pores. Viscoelastic measurements showed that the gels formed at a slow heating rate had a higher storage modulus, G′, which was explained by the microstructure of the strands. The thick strands of particles fused together were stiffer, thus causing a higher storage modulus than the flexible strands formed at a fast heating rate. The concentration dependence of G′ was measured, and a model assuming clustering of clusters was applied to the results. The model shows that the particulate gels are self-similar within the region of concentration measured, with a fractal dimension of ~2.5.

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

BACKGROUNDnThe 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.nnnRESULTSnCarrot 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.nnnCONCLUSIONSnBased 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.

Image analysis of particulate whey protein gels

Abstract Two different methods of image analysis have been used to characterize particulate gel networks quantitatively. The methods have been used to analyse the effect of different preparation conditions on the microstructure of whey protein gels. The microstructure has been characterized at different structural levels by light microscopy, transmission electron microscopy and scanning electron microscopy. The structural parameters have been quantified by digital image analysis and by using a group of experienced microscopists evaluating SEM-micrographs. A stereological approach was used to quantify pore size, particle size and amount of threads within the pores in volume weighted mean volumes. The mode of aggregation was determined by the expert microstructural panel. A 2-lev el fully factorial experimental design has been used, with heating rate (l–5°C/min), pH (4.6–5.4) and salt addition (0–0.1 mol/dm 3 ) as design parameters. The results showed that the heating rate and the pH had main effects on both the particle size and the pore size. The faster heating rate produced both pores and particles of smaller volume. The mean volume of the particles varied between 0.3 and 1.4μm 3 , which could be compared to diameters between 0.8 and 1.4 μm if a spherical shape is assumed. The size of pores and particles increased by an increase in pH. Pore volume was found to be affected by an interaction between heating rate and pH. The change in pH had a pronounced effect when the slower heating rate was used. If the voids were approximated with a spherical shape, the diameters varied between 10 and 40 μm, compared with the volumes between 1200 and 29 100 μm 3 . The interaction effects showed that the heating rate influenced the mode of aggregation at the higher pH 5.4, whereas the salt addition affected the mode of aggregation at the lower pH 4.6.

Inhomogeneous fine-stranded β-lactoglobulin gels

Abstract Inhomogeneities occur in the network of fine-stranded β-lactoglobulin gels. They have been characterized by electron microscopy and their influence on the rheological properties at small deformations measured. The inhomogeneities occurred as dense and loose regions in the network and their formation depended on the heating rate during gel formation (0.017–12°C/min) at pH 7.5 or on pH if close to where the network structure shifts from fine-stranded to particulate (pH 5.8–6.5). An inhomogeneous microstructure was formed at pH 7.5 with a slow heating rate, whereas the network was homogeneous after fast heating. These homogeneous gels had higher G′ than the inhomogeneous gels. In addition the character, as expressed by the frequency dependence, changed from strong homogeneous gels to weaker, more frequency-dependent inhomogeneous gels. The formation of the inhomogeneities was monitored at the slowest heating rate by measuring G ′ during gelation. G ′ showed a maximum around 70°C, indicating a separation into polymer-rich and polymer-poor regions in the network. Electron microscopy showed a transient, homogeneous network before the separation. Both types of inhomogeneous gels had broken frequency curves, i.e. G ′ f had two slopes, which were caused by different relaxation times in the dense and loose regions. Relaxation measurements of the inhomogeneous gels showed relaxation time spectra with two peaks. A model was used to confirm that a broken frequency curve may be caused by two peaks in the relaxation time spectrum.

Rheology and Microstructure of Carrot and Tomato Emulsions as a Result of High-Pressure Homogenization Conditions

High-pressure homogenization, as a way to further mechanically disrupt plant cells and cell walls compared to conventional blending, has been applied to thermally treated and comminuted carrot and tomato material in the presence of 5% olive oil. Mixes of both vegetables in a 1:1 ratio were also included. Both the effect of homogenization pressure and the effect of multiple process cycles were studied. The different microstructures generated were linked to different rheological properties analyzed by oscillatory and steady state measurements. The results showed that while carrot tissue requires a high shear input to be disrupted into cells and cell fragments, tomato cells were broken across the cell walls already at moderate shear input, and the nature of the tomato particles changed to amorphous aggregates, probably composed of cell contents and cell wall polymers. All the plant stabilized emulsions generated were stable against creaming under centrifugation. While for tomato a low-pressure multiple cycle and a high-pressure single-cycle process led to comparable microstructures and rheological properties, carrot showed different rheological properties after these treatments linked to differences in particle morphology. Mixes of carrot and tomato showed similar rheological properties after homogenizing in a single or in a split-stream process. Practical Application: Following consumers demand, the food industry has shown a growing interest in manufacturing products free of gums and stabilizers, which are often perceived as artificial. By tailored processing, fresh plant material could be used to structure food products in a more natural way while increasing their nutritional quality.

Dynamic analyses of sensory and microstructural properties of cream cheese

Flavour and texture in cream cheese depend on the microstructure. The objective of this work was to study the influence of fat content, salt content and homogenisation pressure on the microstructur ...

Confocal laser scanning microscopy and image analysis of kinetically trapped phase-separated gelatin/maltodextrin gels

Abstract The effect of phase separation on the gelatin/maltodextrin systems has been studied using confocal laser scanning microscopy and image analysis. Stereological image analysis has been used to analyse the effect of different cooling rates, holding times, holding temperatures and gelatin types on the microstructure at pHxa05.3. The quantified microstructural parameters were the volume-weighted mean volume, the interfacial area and the area fraction. A factorial experimental design was used, with cooling rate (0.2°C/min, 1°C/min, 10°C/min), holding time (0xa0min, 10xa0min, 20xa0min), holding temperature (20°C, 25°C, 30°C), and two different gelatin types (LH, PS) as design parameters. Gelatin lime hide (LH) has an isoelectric point of pHxa04.7, and gelatin pig skin (PS), has an isoelectric point of pHxa09.1. The composition was kept constant at 4% gelatin and 5% maltodextrin. The results showed that the phase-separated system was gelatin continuous. The size of the maltodextrin inclusions decreases with increasing cooling rate and was largest at the lowest cooling rate (0.2°C/min). Gelatin PS has larger maltodextrin inclusions and a smaller interfacial area than gelatin LH. The size of the maltodextrin inclusions varied in diameter between 3 and 10xa0μm for gelatin LH and between 3 and 18xa0μm for gelatin PS. The size of the maltodextrin inclusions increases with increasing holding time and was largest at 20xa0min. The interfacial area increases with increasing cooling rates and was largest at 10°C/min. A region was found where the phase separation and the gel formation competed with each other in connection with mobility. The residence time in that region and how fast the sample proceeds through it, are important for the morphology of the resulting microstructure.

Processing of tomato: impact on in vitro bioaccessibility of lycopene and textural properties

BACKGROUNDnHuman studies have demonstrated that processing of tomato can greatly increase lycopene bioavailability. However, the difference between processing methods is not widely investigated. In the current study different thermal treatments of tomato were evaluated with regard to their impact on in vitro bioaccessibility and retention of lycopene and beta-carotene as well as textural properties. Thermal treatments used were low (60 degrees C) and high (90 degrees C) temperature blanching followed by boiling.nnnRESULTSnLycopene was relatively stable during thermal treatment, whereas beta-carotene was significantly (P < 0.05) reduced by all heat treatments except for low temperature blanching. In vitro bioaccessibility of lycopene was significantly increased from 5.1 +/- 0.2 to 9.2 +/- 1.8 and 9.7 +/- 0.6 mg kg(-1) for low and high temperature blanching, respectively. An additional boiling step after blanching did not further improve lycopene bioaccessibility for any treatment, but significantly reduced the consistency of low temperature treated samples.nnnCONCLUSIONnChoice and order of processing treatments can have a large impact on both lycopene bioavailability and texture of tomato products. Further investigations are needed, but this study provides one of the first steps towards tomato products tailored to optimise nutritional benefits.

Microwave heat treatment of apple before air dehydration : Effects on physical properties and microstructure

Abstract Golden delicious apple cubes were heated with microwave energy of high intensity (20 W/g), as a pre-treatment before air-dehydration at 40°C, 60°C and 80°C. After the microwave treatment extending for 0.75 up to 5 min, the cubes were finish-dried with only forced air at 2 m/s. Dehydrated and rehydrated samples were analysed with a puncture test using a texture analyser. The microstructure of the samples was studied with confocal laser scanning microscopy (CLSM). Dried apple pieces were harder and more shrunk when pre-treated with microwaves, compared to only air dehydration. Despite the shrinkage and increased firmness, the rehydration capacity of microwave “blanched” apple cubes was higher than when they were only air-dehydrated. Image analysis of the rehydrated apples showed that large voids (up to 0.5 mm in diameter) had developed, and that these increased with air temperature. Higher magnification of the images revealed cell separation and disruption of cell walls, caused by the microwave heating.