Perla Relkin

Reversibility of heat-induced conformational changes and surface exposed hydrophobic clusters of β-lactoglobulin: their role in heat-induced sol–gel state transition

The effects of heat-treatment on the conformational changes of beta-lactoglobulin were monitored by differential scanning calorimetry (DSC), binding properties to 1-anilino-8-naphthalenesulphonic acid (ANS) and to 5,5-dithio-bis (2-nitrobenzoic acid) (DTNB). The thermal transition of beta-lactoglobulin was 100% reversible on re-heating and its binding properties to the ANS fluorescent-dye and to the DTNB probe did not significantly change when the first heating was made to a temperature T < Tmax, that of the DSC maximal peak deviation of unheated solutions. When the solutions were heated to higher temperatures, the degree of reversibility of the thermal transition decreased, while the beta-lactoglobulin surface hydrophobicity increased. Furthermore, the time (tg) needed for the sol-gel state transitions was highly temperature-dependent for the solutions showing no significant reactivity with the DTNB probe, a high percentage of residual tertiary structure but a low surface hydrophobicity. For beta-lactoglobulin showing < 50% residual tertiary structure but high surface hydrophobicity, tg values were hardly temperature-dependent. The results are discussed in terms of the role of hydrophobic interactions in the aggregation process of denatured beta-lg molecules.

Influence of thermal history on the stability of gelatin gels

Gelatin gel properties have been studied using three techniques. Optical rotation measurements have shown that the lower the ageing temperature, the faster the helix content increases but the lower the helix stability. Rheological measurements show that a small increase in temperature leads to a melting of some junction zones followed by a build up of new ones. By ageing the gel at two successive temperatures one can show the existence of two populations of junction zones with different thermal stabilities. The same result is shown on melting thermograms obtained by differential scanning calorimetry. All these results are consistent with the hypothesis of the presence of junction zones of various lengths, the thermostability of which being proportional to their lengths.

Heat-induced gelation of porcine blood plasma proteins as affected by pH.

Porcine plasma is a by-product of the meat industry that can be used as a food ingredient. It is a protein mixture, hence its composition can be modified to meet specific functionality requirements. In the present paper, the gelation properties of plasma and its two major fractions (serum and albumin) have been studied at pH 4.5, 6.0 and 7.5. Polyacrylamide gel electrophoresis (SDS-PAGE) revealed that albumin was the constituent that remained soluble to a larger extent during heat-treatments, and that acidic coagulation occurred at pH 4.5, making weak interactions the predominating ones between protein aggregates. Differential scanning calorimetry (DSC) and rheological tests showed that both the thermal stability and the gelation point of protein solutions were lower as pH decreased. The textural properties and water-holding capacities of plasma and albumin gels were more pH-dependent than serum. Albumin gels were the weakest and those of plasma at pH 7.5, the strongest. It has been determined that interactions between protein fractions play a key role in the gelling properties due to synergistic effects. This knowledge should be useful in the engineering of a plasma derivative product designed for specific food requirements, by reformulating its natural composition and enhanced by controlling pH.

Study of the stability of egg white protein-based foams: effect of heating protein solution

The results reported in this study are concerned with the effects of pre-heating treatments of ovalbumin and lysozyme solutions on the formation, stability and texture morphology of their corresponding foams. It was shown that pre-heating solutions of these proteins to temperatures higher than that of their thermal denaturation led to an enhanced rate of surface pressure development and enhanced foaming properties relative to the unheated proteins. The effect of pre-heating was more clearly apparent for lysozyme, which has a higher rigidity than ovalbumin in the native conformational state. Furthermore, the evolution of liquid volume retained in the lysozyme- and ovalbumin-based foams and foam images during the destabilisation step indicated the slowest drainage and the finest, most homogeneous and most stable foam texture for foams formed from pre-heated protein solutions. n n© 2000 Society of Chemical Industry

Formation and structural heat-stability of β-lactoglobulin/surfactant complexes

Abstract The binding of two model surfactants, sodium dodecyl sulfate and dodecyltrimethylammonium bromide to β-lactoglobulin was studied at room temperature and the thermal stability of the resulting complexes was evaluated by differential scanning calorimetry (DSC) measurements. Binding isotherms indicated both ionic and hydrophobic interactions depending on both the charge of the protein and surfactant at different pHs and on the binding molar ratios of surfactant to the globular protein. Zeta potential measurements indicated charge neutralisation of the protein, under suitable conditions, which also lead to aggregation and precipitation of the proteins. Surface tension measurements indicated similarity between the two types of oppositely charged protein-surfactant complexes and a difference between them when protein and surfactants are similarly charged. DSC measurements revealed different behavior in protein conformation in the presence of the two surfactants. The results obtained at room temperature and upon heating are discussed in terms of the nature of the surfactant/protein interactions involved in the complex formation.

Effects of milk protein type and pre-heating on physical stability of whipped and frozen emulsions

Abstract Various milk protein mixtures were used to manufacture complex emulsions differing by their total protein concentration (1, 2.25 and 3.5%) and by their weight proportion of casein (from 0 to 75%) or whey proteins (WP) (containing from 10 to 80% β-lactoglobulin). Beside those changes in protein concentration and composition, impact of milk protein heat treatment, which was applied before emulsification, was also investigated. The resulting structuration effects on the corresponding emulsions were determined through changes in the particle size distribution and in the amount of adsorbed protein at the fat globule surface. Furthermore, fat destabilisation under a whipping and freezing steps was underlined as functions of the processing parameters (protein concentration and composition, and application or not of an additional heat treatment), and it was discussed in terms of the resulting amount of displaced protein from the oil–water interface.

Foam formation and stabilisation by pre-denatured ovalbumin

Abstract Various mild heat-treatments of ovalbumin solutions were applied to produce molecular species with different conformational states, and having different kinetics of adsorption to the air/water interface and different foaming properties. Molecular species with a higher degree of shear-induced deformation and a low degree of thermal conformational stability showed a slight enhancement of the rate of decrease of surface tension, 5 min after the creation of the fresh interface, and decreasing long-term values of surface tension. Solutions of ovalbumin molecular species exhibiting such initial structural patterns were shown to have enhanced foam capacity and stability against liquid drainage. Ovalbumin molecules with some degree of secondary and tertiary structural changes and increased viscosity, before adsorption at the air/water interface, were shown to be relevant to produce more or less hydrated foams with more or less stability against liquid drainage.

Impact of structural changes and aggregation on adsorption kinetics of ovalbumin at the water/air interface

Abstract Differential Scanning Calorimetry (DSC) was used to study the thermal stability of ovalbumin molecules at concentrations lying between 0.25% and 5% and at pHxa03 and pHxa07: critical temperatures such as temperature of the first increase in heat flow (Ti), peak temperature (Tp) and enthalpy change of heat reactions (ΔHapp) were determined. The results indicated that Tp did not vary significantly whatever the protein concentration, whereas the heat stability of protein conformational states was higher at pHxa07 than at pHxa03. These results were confirmed through the secondary structure changes upon heating, as monitored by circular dichroism measurements: at pHxa03, we observed the formation of molecular species probably associated by intermolecular β-sheets at T⩽Ti. Electrophoretic patterns of preheated solutions indicated the formation of aggregates at T

Non-covalent binding of benzaldehyde to β-lactoglobulin: characterisation by spectrofluorimetry and electrospray ionisation mass-spectrometry

β-Lactoglobulin (β-lg), which is found in the milk of several mammal species, is the most abundant protein in bovine whey (McKenzie, 1971). The interaction properties of β-lg with a large variety of small hydrophobic ligands have been extensively studied (Sawyer et al . 1998). β-Lg belongs to the super-family of hydrophobic molecule transporters called the lipocalins, which characteristically bind hydrophobic ligands inside a central calyx (Godovac-Zimmermann, 1988; Perez et al . 1989; Brownlow et al . 1997; Wu et al . 1997; Qin et al . 1998). At present, whey protein concentrates containing a high percentage of β-lg are commercially available in a very large quantity, but most of the processes used in milk technology involve heat-treatments that are known to affect the initial conformational state of β-lg. Such conformational changes have consequences for both the physicochemical and functional properties in food systems, including a decrease in the availability of lysine, due to Maillard reactions (Leonil et al . 1997) and a decrease in the affinity constant for binding to retinol (Laligant et al . 1991) and to flavour compounds (ONeil & Kinsella, 1988). In the field of protein-volatile compound interactions, we have recently reported a significant enhancement of foaming properties of β-lg solutions (50 m M -NaCl, pH 6) in the presence of aroma compounds such as isoamyl acetate (Marin & Relkin, 1999), benzaldehyde (BZA; Marin & Relkin, 2000) and vanillin (Relkin & Vermersh, 2000). The observed increase in foaming properties of β-lg was postulated to be due to formation of surface active complexes between β-lg and aroma compounds. In the present work we have investigated the interaction between a β-lg concentrate (prepared by ultra-diafiltration on an industrial scale) and BZA; particularly, evidence was sought for the presence of covalently bound monomers, dimers and lactolated monomers.

Native and hydrophobized human IGG: Enthalpies of heat-induced structural changes and adsorption onto silica

Differential scanning calorimetry (DSC) and isothermal calorimetric batch technique were used to monitor the heat-induced structural changes and adsorption properties of human immunoglobulin G (IgG), in native and hydrophobized states. The transition temperature (Tmax) and enthalpy of heat-induced conformational changes (ΔcalH) of IgG in solution as well as the enthalpy change accompanying the adsorption of IgG onto hydrophilic silica (ΔadsH), were shown to depend on the degree of the protein hydrophobicity (number of covalently attached alkyl chains). The adsorption enthalpy for all forms of IgG at all surface concentrations was found to be endothermic, that is the process is entropy driven. Factors affecting the IgG adsorption onto silica are discussed.