Mats Stading

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.

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.

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.

Humidity-induced structural transitions in amylose and amylopectin films

The effect of humidity on the dynamic mechanical and barrier properties and on the microstructure of amylose and amylopectin films was studied. The films were solution-gel-cast, with and without gl ...

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.

Plasticization of zein: a thermomechanical, FTIR, and dielectric study.

Zein, the main seed storage protein of maize, has been widely studied as a possible source of material for the production of biodegradable plastic films. Plasticization of zein is critical to make functional films. While there have been a number of publications which report the behavior of systems with a wide variety of plasticizers, there have been few which attempt to examine the interactions of protein and plasticizer at the molecular level. In this paper, we report on the plasticizing effects of water, glycerol, and 2-mercaptoethanol, which were examined by a combination of spectroscopy (FTIR and dielectric) and thermomechanical methods. The results suggest that both water and glycerol are adsorbed onto the protein and form hydrogen bonds with the amide groups. The plasticizer then builds up in patches on the protein surface. 2-Mercaptoethanol only exhibited a weak plasticizing effect due probably to disulfide bond breaking.

Influence of skinning on lipid oxidation in different horizontal layers of herring (Clupea harengus) during frozen storage

This study was conducted to evaluate the effect of skinning and of compositional differences on the oxidative stability of various horizontal layers from herring (Clupea harengus) during frozen storage. Herring fillets, with and without skin, were stored at -18°C for 0, 3, 9, 16 and 28 weeks. After each storage period, the fillets were divided horizontally into three layers: ‘under skin’, ‘middle part’ and ‘inner part’. Each layer was then extracted for total lipids, in which peroxide value (PV), absorbance at 234 nm (A234) and 268 nm (A268) as well as lipid-soluble fluorescent oxidation products (FP) were measured. Prior to storage, the fat content, fatty acid pattern and α-tocopherol were also analysed. During storage of skinless fillets, the under skin layer increased most in PV, A234, A268 and FP (P<0·05), followed by the inner and middle parts. In fillets stored with skin, the high oxidation rate of the under skin layer lipids was suppressed, but this layer still gave rise to the highest responses. Firstly, these results point to the protective properties of the skin and, secondly, to the unfavourable composition of the under skin layer: a lot of dark muscle; the silver surface; the highest fat content and the lowest level of α-tocopherol. Concerning the fatty acid pattern in the three layers, the amount of C20: 5, C18:1 and C20:1 in the fat gradually decreased from the under skin layer towards the inner part of the fillet, whereas the opposite was true for C22: 6.

Rheological behaviour of mixed gels of κ-carrageenan-locust bean gum

Mixed gels of the K, Na and Ca-forms of ?-carrageenan and locust bean gum at a total polysaccharide concentration of 1% (w/w) were studied by dynamic viscoelastic measurements. Synergistic effects ...