Anne-Marie Hermansson

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.

Developments in the understanding of starch functionality

In this article, we describe how starch functionality can be explained in terms of structure. The behaviour of different types of starches is demonstrated by showing how the microstructure of potato and wheat starch is related to their rheological properties. The results illustrate the structural importance of amylose and amylopectin. The microstructure of a completely new type of genetically engineered potato amylopectin starch is presented for the first time.

Structure, mechanical and barrier properties of amylose and amylopectin films

The effect of film formation conditions on structure, mechanical properties and barrier properties of amylose and amylopectin films was studied. The films were prepared by solution-gel-casting of amylose and amylopectin from potato, with or without the addition of glycerol as plasticizer. Transmission electron micrographs showed that the network structure characteristic for the amylose gel was also found in the film state. The amylose films without glycerol plasticization exhibited a relatively high degree of B-type crystallinity, as revealed by wide-angle X-ray diffraction, whereas the unplasticized amylopectin films were amorphous. Although the addition of glycerol did not affect the crystallinity of the amylose films, glycerol-plasticized amylopectin formed B-type crystallinity, and the degree of crystallinity was dependent on the air humidity during film formation. The degree of crystallinity affected the mechanical properties of the amylopectin films, whereas the mechanical properties of the amylose films were influenced by the network microstructure. Oxygen and water vapour permeabilities were dependent neither on the degree of crystallinity in the films nor on the network structure.

Viscoelastic behaviour of β-lactoglobulin gel structures.

The viscoelastic properties of β-lactoglobulin gels, made by dissolution of β-lactoglobulin in distilled water, have been investigated by dynamic rheological measurements at pH 2.5–9.0. Gels formed at pH 4–6 were opaque and have been defined as aggregate or particle gels; at pH below or above this range gels were transparent and have been defined as fine-stranded gels. The aggregate gels were more strain sensitive during gelation and more frequency dependent than the fine-stranded gels. They also had a higher storage modulus than the fine-stranded gels at a constant β-lactoglobulin concentration of 12% (w/w). The critical concentration for gel formation was lower for the aggregate gels than for the fine-stranded gels, and as low as 1% in the pH range 4.5–5.5. This indicates a very open structure. The onset of gelation of 12% β-lactoglobulin gels at intermediate pH, when measured as G′ = G″ at 1 Hz or as the development of a measurable G′ takes place at temperatures far below the temperature of denaturation. This is not the case at higher or lower pH, where the onset of gelation is above the temperature of denaturation. The heating rate during gel formation was varied between 0.008 and 1°C/min. The gelation temperature of the fine-stranded gels was found to increase with an increasing heating rate.

Effects of potassium, sodium and calcium on the microstructure and rheological behaviour of kappa-carrageenan gels

Abstract The effects of cations and mixtures of cations on kappa-carrageenan gelation was studied by electron microscopy and viscoelastic measurements. On cooling only potassium-kappa-carrageenan formed a transient fine structure at 54-52°C. The structure became unstable at 51°C, whereafter long stiff superstrands with two to three strands in parallel formed a coarse network. The balance between the fine and the coarse gel strands was dependent on the potassium concentration. In contrast to the potassium form, sodium-kappa-carrageenan formed relatively weak gels and showed almost no dependence on the sodium ion concentration. The sodium form had a network structure with flexible superstrands of uniform thickness. The calcium-kappa-carrageenan formed relatively weak gels in a limited calcium concentration range, whereafter salting-out effects were observed. The calcium-induced gels had a very fine network of an entanglement type without any of the coarse superstrands seen in sodium or potassium forms. A transient gel structure was not found for sodium- or calcium-kappa-carrageenan. Strong synergistic effects were found between calcium and potassium. Depending on the ratio of potassium to calcium, it was possible to make 1% gels in the temperature range 20–40°C with storage moduli varying from 70 to 43 000 Pa. Apart from adding KCl to the pure calcium form and CaCl2 to the pure potassium form of kappa-carrageenan, intermediate ion forms were produced by ion exchange where the potassium to calcium ratio was varied between 0·2 and 0·8. Synergistic effects were also observed when sodium was added to potassium-kappa-carrageenan, but not when potassium was added to sodium-kappa-carrageenan. No synergistic effects were observed between calcium and sodium in any form.

Large deformation properties of β-lactoglobulin gel structures

Different gel structures formed by β-lactoglobulin dissolved in distilled water (12% w/w) at pH 3.0–7.5 have been investigated using tensile measurements at large deformations. Gels formed at pH 4–6 were opaque, whereas at pH values below or above this range they were transparent. The fracture properties showed large variations over the pH range studied. Gels formed at low pH were brittle with low strain and stress at fracture, as opposed to those formed at high pH, which were rubber-like with high strain and stress at fracture. Gels formed at intermediate pH (pH 4–6) had an intermediate, near-constant, strain at fracture. The fracture stress was, however, higher at pH 5.5–6.0 than at pH 4.0–5.2. The specific fracture energy resembled the stress at fracture, with a maximum of 6 J/m2 at pH 6.0. Gels formed at pH 4.5, 5.5, 6.5 and 7.5 were all notch-sensitive. The opaque gels were defined as aggregate gels and the transparent gels were defined as fine-stranded gels. The fracture properties clearly showed there are differences between the fine-stranded gels formed at high pH and those formed at low pH. The fracture stress demonstrated that there are structural differences within the pH range in which the aggregate gels are formed. The non-linearity of the stress-strain curve was the same for all fine-stranded gels, which had r-shaped stress-strain curves. The stress-strain curves of the aggregate gels were almost linear. The non-linearity did not influence the fracture properties. Youngs modulus showed two peaks, at pH 3.5 and 6.0, coinciding with the range where the structure changes between aggregated and fine-stranded. The stress at fracture also has a maximum at pH 6.0. The high elasticity and fracture stress may depend on strong, elastic areas in the network structure. Apart from the two peaks, Youngs modulus shows the same behaviour as the storage modulus, G′, measured at small deformations, but Youngs modulus is slightly larger than 3G′.

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.

Liquid Loss as Effected by Post mortem Ultrastructural Changes in Fish Muscle: Cod (Gadus morhua L and Salmon Salmo salar

This study was performed in order to assess the effect of early post mortem structural changes in the muscle upon the liquid-holding capacity of wild cod, net-pen-fed cod (fed cod) and farmed salmon. The liquid-holding capacity was measured by a low speed centrifugation test. Transmission electron microscopy was used to discover ultrastructural changes both in the connective tissue and in the myofibrils. Differential scanning calorimetric thermograms of the muscle proteins were recorded to elucidate whether fundamental differences did exist between the proteins of the raw material tested. Multivariate statistics were used to explicate the main tendencies of variations in the thermograms. The salmon muscle possessed much better liquid-holding properties than the cod muscle, and wild cod better than fed cod regardless of the storage time. Both fed cod and farmed salmon, underwent the most severe structural alterations, probably caused by the low muscle pH values. The higher liquid-holding capacity of the salmon muscle was related to species specific structural features and better stability of the muscle proteins. The myofibrils of the salmon muscle were denser and intra- and extracellular spaces were filled by fat and a granulated material. The differences in thermograms of muscle from wild and fed cod were largely explained by the variations in pH. The severe liquid loss of fed cod is due to a low pH induced denaturation and shrinkage of the myofibrils. Post mortem degradation of the endomysial layer and the sarcolemma may have further facilitated the release of liquid.

Improved water vapor barrier of whey protein films by addition of an acetylated monoglyceride

Abstract This study aimed to determine to what extent the water-vapor barrier of whey protein isolate (WPI) films could be improved by adding a lipid and make laminate and emulsion films. The laminate whey protein–lipid film decreased the water vapor permeability (WVP) 70 times compared with the WPI film. The WVP of the emulsion films was half the value of the WPI film and was not affected by changes in lipid concentration, whereas an increased homogenization led to a slight reduction in WVP. The mechanical properties showed that the lipid functioned as an apparent plasticizer by enhancing the fracture properties of the emulsion films. This effect increased with homogenization. The maximum strain at break was 117% compared with 50% for the less-homogenized emulsion films and 20% for the pure WPI films. Phase-separated emulsion films were produced with a concentration gradient of fat through the films, but pure bilayer films were not formed.

Rheological and microstructural evidence for transient states during gelation of kappa-carrageenan in the presence of potassium

Abstract Kappa-carrageenan gelation was studied by a combination of electron microscopy and dynamic viscoelastic measurements. There was an initial maximum in the storage modulus during cooling and gel formation of 1% kappa-carrageenan in the presence of potassium. The structure of the transient state was found to consist of a fine network structure, where the junction zones were believed to be double helices. The structure had the characteristics of a true gel but was unstable. When the temperature was lowered aggregation took place and the fine network structure was partly broken down. Ordered superstrands formed which aligned themselves in parallel or were densely packed together. The degree of aggregation depended on the potassium ion concentration. In the weakest gel formed in 0·01 m KCl, the superstrands did not form a network but were rather dispersed in the fine network structure. In 0·1 and 0·2 m KCl the superstrands formed a three-dimensional network, where aligned superstrands formed the junction zones and branching occurred when superstrands deviated from each other and aligned with new superstrands. In 0·1 m KCl the supermolecular network dominated but in 0·2 m KCl a mixed gel was formed of the fine network and the coarse supermolecular network. The mixed gel gave rise to the firmest gel.