A novel pH dependent dimerization motif in beta lactoglobulin from pig (sus scrofa)

Biophysical Structural Chemistr

Cold gelation of globular proteins

Keywords : globular proteins, whey protein, ovalbumin, cold gelation, disulfide bonds, texture, gel hardnessProtein gelation in food products is important to obtain desirable sensory and textural properties. Cold gelation is a novel method to produce protein-based gels. It is a two step process in which first thermally induced protein aggregates are prepared by a heat treatment of a solution of native globular proteins at low ionic strength. After cooling, gelation of the dispersion of repulsive aggregates is induced in the second step by lowering the pH or by adding salt at ambient temperature. Cold-set gelation finds applications in food products with a delicate flavor and texture. In addition cold-set gels can be used as a thickening agent or in encapsulation of sensitive materials.The purpose of the research described in this thesis was to investigate the molecular mechanisms of the acid-induced cold gelation process. Therefore it was studied (i) how different aggregate properties determined the final properties of cold-set whey protein gels, (ii) how structural and rheological properties of the gels scaled with the protein concentration, and (iii) how the final gel properties depended on the use of different cysteine-containing globular proteins.The results demonstrated that reduction of the electrostatic repulsion initiated the formation of a randomly aggregated protein network by physical interactions. Surprisingly, additional covalent disulfide bonds were formed under the acid conditions. The disulfide bonds stabilized the initial network and increased the mechanical gel strength. The formation of disulfide bonds depended on the number and accessibility of thiol groups and disulfide bonds present in the various protein molecules. Therefore, the disulfide bonds are important control parameters that can be used to tune the texture of (cold-set) gels. In addition, the contour length of the linear-shaped aggregates prepared in the first step affected the mechanical gel strength of cold-set gels. For smaller aggregates percolation is preceded by the formation of clusters, yielding less effective contact points and therefore weaker gels compared to cold-set gels prepared from long fibrillar structures. Moreover, the length of the linear-shaped aggregates determined the appearance of the cold-set gels. Cold gelation is a relevant method for the application of globular proteins as an efficient structuring ingredient in food systems

Comparison of the aggregation behavior of soy and bovine whey protein hydrolysates

Abstract Soy-derived proteins (soy protein isolate, glycinin, and ß-conglycinin) and bovine whey-derived proteins (whey protein isolate, ¿-lactalbumin, ß-lactoglobulin) were hydrolyzed using subtilisin Carlsberg, chymotrypsin, trypsin, bromelain, and papain. The (in)solubility of the hydrolysates obtained was studied as a function of pH. At neutral pH, all soy-derived protein hydrolysates, particularly those from glycinin, obtained by hydrolysis with subtilisin Carlsberg, chymotrypsin, bromelain, and papain showed a stronger aggregation compared to the non-hydrolyzed ones. This increase in aggregation was not observed upon hydrolysis by trypsin. None of the whey-derived protein hydrolysates exhibited an increase in aggregation at neutral pH. The high abundance of theoretical cleavage sites in the hydrophobic regions of glycinin probably explains the stronger exposure of hydrophobic groups than for the other proteins, which is suggested to be the driving force in the aggregate formation. Keywords: Aggregation; Peptides; Soy protein; Whey protei

Cold-set globular protein gels: Interactions, structure and rheology as a function of protein concentration.

We identified the contribution of covalent and noncovalent interactions to the scaling behavior of the structural and rheological properties in a cold gelling protein system. The system we studied consisted of two types of whey protein aggregates, equal in size but different in the amount of accessible thiol groups at the surface of the aggregates. Analysis of the structural characteristics of acid-induced gels of both thiol-blocked and unmodified whey protein aggregates yielded a fractal dimension (2.3 ± 0.1), which is in line with other comparable protein networks. However, application of known fractal scaling equations to our rheological data yielded ambiguous results. It is suggested that acid-induced cold-gelation probably starts off as a fractal process, but is rapidly taken over by another mechanism at larger length scales (>100 nm). In addition, indications were found for disulfide cross-link-dependent structural rearrangements at smaller length scales

Physical and chemical interactions in cold gelation of food proteins

pH-Induced cold gelation of whey proteins is a two-step process. After protein aggregates have been prepared by heat treatment, gelation is established at ambient temperature by gradually lowering the pH. To demonstrate the importance of electrostatic interactions between aggregates during this latter process, -lactoglobulin aggregates with a decreased iso-electric point were prepared via succinylation of primary amino groups. The kinetics of pH-induced gelation was affected significantly, with the pH gelation curves shifting to lower pH after succinylation. With increasing modification, the pH of gelation decreased to about 2.5. In contrast, unmodified aggregates gel around pH 5. Increasing the iso-electric point of -lactoglobulin via methylation of carboxylic acid groups resulted in gelation at more alkaline pH values. Comparable results were obtained with whey protein isolate. At low pH disulfide cross-links between modified aggregates were not formed after gelation and the gels displayed both syneresis and spontaneous gel fracture, in this way resembling the morphology of previously characterized thiol-blocked whey protein isolate gels (Alting, et al., J. Agric. Food Chem. 2000, 48, 5001-5007). Our results clearly demonstrate the importance of the net electric charge of the aggregates during pH-induced gelation. In addition, the absence of disulfide bond formation between aggregates during low-pH gelation was demonstrated with the modified aggregate

Improved creaminess in stirred yoghurt through amylomaltase-treated starch domains

Amylomaltase-treated starch (ATS) has been described as an excellent fat-replacer in set yoghurt. Its functionality as a creaminess enhancer relates to the ability to form domains, which have melting and yielding characteristics in the mouth. This study was intended to understand the functionality of ATS in stirred yoghurt. Sensory evaluation of reduced fat stirred yoghurt samples showed that the creaminess perception of stirred yoghurt with 1.5% fat and 0.5% ATS was comparable with the reference full fat yoghurt containing 3% fat. This showed that ATS was even more effective as a creaminess enhancer in stirred yoghurt than in set yoghurt. This was related to the higher effectiveness of domain formation during storage after cooling in a dispersed system, as a result of stirring of the yoghurt gel, compared with a gelled set-yoghurt system

Fused deposition modelling of sodium caseinate dispersions

Only recently, researchers have started experimenting with 3D printing of foods. The aim of this study was to investigate 3D printed objects from sodium caseinate dispersions, exhibiting reversible gelation behaviour. Gelation and dispensing behaviour were explored and structures of different protein content and with oil droplets were prepared. Additions of pectin, sucrose and starch facilitated FDM of sodium caseinate dispersions with different layers and an enzymatic cross-linking procedure was enabled printing of caseinate dispersions at lower dry matter content. A modified Poiseuille equation for power law fluids was applied to describe dispensing behaviour of the caseinate dispersions and may be used to set dispenser pressure and line speeds. Finally, oil droplets are introduced in the dispersions with a premixing method and a dispenser with side-inlet. It is suggested that such specific spatial distribution of oil droplets provides more freedom in custom design of healthy foods, thus providing a niche for FDM of foods.</p

Number of thiol groups rather than the size of the aggregates determines the hardness of cold set whey protein gels

Variation of protein concentration during heating resulted in the formation of protein aggregates with clearly different structural and chemical characteristics. Heating conditions were chosen such that differences in the degree of aggregation were excluded. Acid induced gelation of dispersions of these aggregates resulted in gels with clearly different hardness. Although gel hardness seemed to correlate with the different structural aggregate features as reported before in literature, the differences in hardness could for the most part be cancelled by blocking of the thiol groups. Application of thiol-blocked protein aggregates enabled us to make a distinction between the effect of structural- and chemical-properties of the aggregates. Formation of larger disulfide cross-linked protein structures paralleled the increase in gel hardness and dominated the effect of structural characteristics on mechanical properties of cold-set gels. In addition, the effect of the presence of native non-aggregated protein on the final gel properties can be excluded, since in our gel-experiments most protein (>95%) participated in the formation of a protein network. Therefore, we can conclude that the hardness of cold set whey protein gels is determined by the number of thiol groups rather than by the size of the aggregates or other structural features

Improved creaminess in stirred yoghurt through amylomaltase-treated starch domains

Amylomaltase-treated starch (ATS) has been described as an excellent fat-replacer in set yoghurt. Its functionality as a creaminess enhancer relates to the ability to form domains, which have melting and yielding characteristics in the mouth. This study was intended to understand the functionality of ATS in stirred yoghurt. Sensory evaluation of reduced fat stirred yoghurt samples showed that the creaminess perception of stirred yoghurt with 1.5% fat and 0.5% ATS was comparable with the reference full fat yoghurt containing 3% fat. This showed that ATS was even more effective as a creaminess enhancer in stirred yoghurt than in set yoghurt. This was related to the higher effectiveness of domain formation during storage after cooling in a dispersed system, as a result of stirring of the yoghurt gel, compared with a gelled set-yoghurt system