Enhancement of the foaming properties of protein dried in the presence of trehalose.

Surface tension, foamability, and foam stability kinetics have been measured for the pure proteins bovine serum albumin (BSA) and beta-lactoglobulin, before and after aqueous solutions of the proteins had been subjected to different drying conditions, and also for whey protein concentrate (WPC). Pure proteins were air-dried, at 78 or 88 degrees C, in the presence and absence of sucrose or trehalose, at a mass ratio of 5:1 sugar/protein. WPC was spray-dried in the presence of various sugars: trehalose, sucrose, lactose, and lactitol. Spray-drying WPC without sugars resulted in a dramatic decrease in the foam stability, whereas drying in the presence of sugars gave better retention of the original foaming properties. Trehalose in particular resulted in almost complete retention of the foam stability observed for the nondried WPC. Pure beta-lactoglobulin showed similar behavior, but trehalose did not seem to afford the same protection to BSA.

Interfacial shear rheology of aged and heat-treated beta-lactoglobulin films: displacement by nonionic surfactant.

Interfacial shear rheology of adsorbed beta-lactoglobulin films (bulk protein concentration 10(-)(3) wt %) has been studied over the temperature range 20-90 degrees C using a two-dimensional Couette-type viscometer. Effects of the type of interface (air-water, triolein-water, and n-dodecane-water), the pH (2.0, 5.6, 6.0, 7.0, and 9.0), and the extent of the heat treatment have been assessed via measurements of changes in the apparent interfacial shear viscosity and elasticity before and after the addition of increasing amounts of nonionic surfactant Tween 20 (polyoxyethylene sorbitan monolaurate). The highest interfacial viscosities were obtained at the n-dodecane-water interface and the lowest at the air-water interface. Competitive displacement of protein from the interface by Tween 20 was easier at the air-water and n-dodecane-water interfaces as compared to the triolein-water interface. The surface shear viscosity was higher and the displacement by Tween 20 more difficult as the isoelectric point of the protein was approached, which is in agreement with the presence of a more strongly cross-linked protein network at the interface. The effect of heat treatment was dependent on the pH of the aqueous solution. No simple relationship between the surface rheological characteristics and the ease of displacement by Tween 20 could be inferred.

Dilatational rheology and foaming properties of sucrose monoesters in the presence of beta-lactoglobulin.

The foaming properties and the dilatational rheology of systems containing purified sucrose caprylate (SM800), caprate (SM1000), laurate (SM1200) and palmitate (SM1600) have been studied. Addition of beta-lactoglobulin (beta-lg) at a low concentration (0.050 wt.%) can aid the foam formation in the cases, where the surfactant concentration is insufficient to support foam formation. However, foams where both species co-existed exhibited poor stability. beta-lg was found to affect the dilatational properties of surfactant films even at low concentrations. It is thought that this could be related to the effect of the protein on the adsorption-desorption relaxation mechanism, or to the possible formation of a protein-surfactant complex in the bulk. The age of the protein film was also found to affect the kinetics of protein displacement by SM1000, as monitored by the change in the dilatational properties (Langmuir trough technique) and the relative reflectivity of the interface (Brewster angle microscopy) with time. An insoluble monolayer of sucrose stearate (Suc18) and beta-lg was also studied and it was found that the presence of small amounts of Suc18 in the protein film lead to a reduction of the interfacial elasticity. This is believed to be due to the disruption of the protein network. A possible mechanism could involve the obstruction of the hydrogen-bond intermolecular protein association by the strongly hydrated sucrose headgroup or the obstruction of the protein-protein hydrophobic interactions by the formation of an interfacial protein-surfactant complex.

Acid beverage floc: protein-saponin interactions and an unstable emulsion model.

The occurrence of acid beverage floc (ABF) in acidified carbonated beverages has long been attributed to the presence of saponins. We have examined this assertion and have found evidence to suggest that traces of protein may also be a key factor, along with lipid material present in the floc. Turbidity levels of beet sugar protein (0.001 wt.%) and saponin (0.001 wt.%) solutions were examined over time using spectrophotometry. At neutral pH, no change in turbidity was observed in any combination (individually or mixed). Furthermore, acidified (pH 2) saponin and protein solutions, considered separately, also exhibited no change. However, a mixture of equal concentrations at pH 2 showed an initial increase in turbidity up to 2 h after mixing, followed by a decrease over the ensuing 12 h. Interfacial tension measurements also indicated interactions between the protein and saponin at pH 2. Photon correlation spectroscopy (PCS-Malvern Zetasizer 4) was used to quantitatively examine the particle size distributions and aggregation of a model, highly dilute dispersion, of bromohexadecane in 20 wt.% sucrose solution, prepared with a jet homogenisor. Beet sugar saponin (0.001 wt.%) and protein (0.001 wt.%) were added to the dispersion, and their emulsion-stabilising effects examined via oil droplet size measurement over time. At neutral pH, the size of oil droplets in the dispersion was unaffected by the addition of saponin or protein. At pH 2, the presence of saponin again caused no effect on droplet size. However, in acid conditions, protein appeared to destabilise the dispersion. The results indicate that the key to controlling the ABF problem may be the ratio of saponin to protein in the product, which may or may not stabilise dispersed lipid, depending on their interactions.

A molecular and biochemical analysis of the structure of the cyanogenic beta-glucosidase (linamarase) from cassava (Manihot esculenta Cranz).

The cyanogenic beta-glucosidase (linamarase) of cassava is responsible for the first step in the sequential break-down of two related cyanoglucosides. Hydrolysis of these cyanoglucosides occurs following tissue damage and leads to the production of hydrocyanic acid. This mechanism is widely regarded as a defense mechanism against predation. A linamarase cDNA clone (pCAS5) was isolated from a cotyledon cDNA library using a white clover beta-glucosidase heterologous probe. The nucleotide and derived amino acid sequence is reported and five putative N-asparagine glycosylation sites are identified. Concanavalin A affinity chromatography and endoglycosidase H digestion demonstrate that linamarase from cassava is glycosylated, having high-mannose-type N-asparagine-linked oligosaccharides. Consistent with this structure and the extracellular location of the active enzyme is the identification of an N-terminal signal peptide on the deduced amino acid sequence of pCAS5.

Modulation of the degree and pattern of methyl-esterification of pectic homogalacturonan in plant cell walls. Implications for pectin methyl esterase action, matrix properties, and cell adhesion.

Homogalacturonan (HG) is a multifunctional pectic polysaccharide of the primary cell wall matrix of all land plants. HG is thought to be deposited in cell walls in a highly methyl-esterified form but can be subsequently de-esterified by wall-based pectin methyl esterases (PMEs) that have the capacity to remove methyl ester groups from HG. Plant PMEs typically occur in multigene families/isoforms, but the precise details of the functions of PMEs are far from clear. Most are thought to act in a processive or blockwise fashion resulting in domains of contiguous de-esterified galacturonic acid residues. Such de-esterified blocks of HG can be cross-linked by calcium resulting in gel formation and can contribute to intercellular adhesion. We demonstrate that, in addition to blockwise de-esterification, HG with a non-blockwise distribution of methyl esters is also an abundant feature of HG in primary plant cell walls. A partially methyl-esterified epitope of HG that is generated in greatest abundance by non-blockwise de-esterification is spatially regulated within the cell wall matrix and occurs at points of cell separation at intercellular spaces in parenchymatous tissues of pea and other angiosperms. Analysis of the properties of calcium-mediated gels formed from pectins containing HG domains with differing degrees and patterns of methyl-esterification indicated that HG with a non-blockwise pattern of methyl ester group distribution is likely to contribute distinct mechanical and porosity properties to the cell wall matrix. These findings have important implications for our understanding of both the action of pectin methyl esterases on matrix properties and mechanisms of intercellular adhesion and its loss in plants.