Mixtures of gum arabic and whey protein (whey protein isolate, WP) form an electrostatic complex in a specific pH range. Three phase boundaries (pH(c), pHphi(1), pHphi(2)) have been determined using an original titration method, newly applied to complex coacervation. It consists of monitoring the turbidity and light scattering intensity under slow acidification in situ with glucono-delta-lactone. Furthermore, the particle size could also be measured in parallel by dynamic light scattering. When the pH is lowered, whey proteins and gum arabic first form soluble complexes. This boundary is designated as pH(c). When the interaction is stronger (at lower pH), phase separation takes place (at pHphi(1)). Finally, at pHphi(2) complexation was suppressed by the charge reduction of the gum arabic. The major constituent of the whey protein preparation used was beta-lactoglobulin (beta-lg), and it was shown that beta-lg was indeed the main complex-forming protein. Moreover, an increase of the ionic strength shifted the pH boundaries to lower pH values, which was summarized in a state diagram. The experimental pH(c) values were compared to a newly developed theory for polyelectrolyte adsorption on heterogeneous surfaces. Finally, the influence of the total biopolymer concentration (0-20% w/w) was represented in a phase diagram. For concentrations below 12%, the results are consistent with the theory on complex coacervation developed by Overbeek and Voorn. However, for concentrations above 12%, phase diagrams surprisingly revealed a "metastable" region delimited by a percolation line. Overall, a strong similarity is seen between the behavior of this system and a colloidal gas-liquid phase separation.

Feather keratins were extracted from chicken feathers with an aqueous solution of urea and 2-mercaptoethanol. The keratin solution obtained was dialyzed to remove the reagents. Upon dialysis, extensive protein aggregation occurred. To obtain stable solutions or dispersions in water, cysteine residues were modified prior to dialysis with iodoacetamide, iodoacetic acid, or bromosuccinic acid, thereby blocking free thiol groups and introducing hydrophilic groups. For the development of biodegradable materials with good mechanical properties from these biopolymers, disulfide bonds between the keratin molecules are needed. Therefore, cysteine residues were only partially modified by using different reagent/cysteine molar ratios. The reaction rate constants of iodoacetate with glutathione and 2-mercaptoethanol were successfully used to predict the degree of modification of keratin cysteine. It was shown that, for carboxymethylated keratin, fewer aggregates were formed for higher degrees of cysteine modification, while more protein was present as oligomers. Aggregates and oligomers were stabilized through intermolecular disulfide bonds.

No abstract

Free cysteine thiol groups of keratin extracted from chicken feathers were partially carboxymethylated with iodoacetic acid (25-76% cysteine modification). Stable dispersions were used for the preparation of films by solution casting. Glycerol was used as a plasticizer (0.05-0.47 g/g of keratin), and films were stored at a constant relative humidity (20, 30, 50, 70, or 90%). The degree of crystallinity in the films was higher when more cysteine residues were carboxymethylated. The films displayed an optimum in mechanical properties at approximately 50% cysteine carboxymethylation. The tensile strength at this optimum was 25 MPa, the E modulus, 350 MPa, and the elongation at break, 50%. Probably, this optimum was the result of both a decreasing amount of disulfide bonds and an increasing degree of crystallinity for higher degrees of cysteine modification. The influences of a higher amount of glycerol and of different storage conditions on the mechanical properties of films from keratin with a defined degree of cysteine modification were also investigated.

The development of new functional foods requires technologies for incorporating health-promoting ingredients into food without reducing their bioavailability or functionality. In many cases, microencapsulation can provide the necessary protection for these compounds, but in all cases bioavailability should be carefully studied. The present paper gives an overview of the application of various microencapsulation technologies to nutritionally-important compounds, i.e. vitamins, n-3 polyunsaturated fatty acids, Ca, Fe and antioxidants. It also gives a view on future technologies and trends in microencapsulation technology for nutritional applications.