Casein Gelation Research Articles

Temperature‐controlled gelation of casein concentrates enabled by the utilisation of acid whey permeate as a diafiltration medium in microfiltration

In this study, ultrafiltration permeate obtained from acid whey (AWP) was applied as a diafiltration (DF) medium during skim milk microfiltration. The thereby induced acidification of differently concentrated retentates was assessed regarding suitability to generate gels from micellar casein concentrates further downstream. While aggregation was avoided during cold DF, subsequent heating induced controlled spontaneous formation of gels with concentration‐ and pH‐dependent firmness ranging from 26 to 1420 Pa. By employing its typical compounds, AWP as a rather problematic dairy side stream could thus be utilised for the production of calcium‐rich acidic casein gels providing sensory characteristics of cultured dairy products.

Laccase and glucono-δ-lactone dual-induced gelation of casein and arabinoxylan: Microstructures, physicochemical properties, and pH-responsive release behavior

In this study, dual-induced casein and arabinoxylan composite gels (casein/AX gels) were successfully prepared by laccase and glucono-δ-lactone (GDL). The influence of AX amount on the microstructure and properties of casein/AX gels was also evaluated. Compared to that of the single casein gel, the rheological and textural properties of casein/AX gels were improved, which might be attributed to the co-existence of covalent and non-covalent interactions of AX and casein molecules, resulting in the interconnected network gel microstructure observed by scanning electron microscopy (SEM). Besides, with the increased amount of AX, casein/AX gels showed an increased water holding capacity and a decreased water mobility significantly (P < 0.05). Meanwhile, epigallocatechin gallate (EGCG) was loaded into casein/AX gels more efficiently and released in a pH-responsive pattern. Only 10–22% of the loaded EGCG was released from casein/AX gels in the simulated gastric fluid (SGF). In contrast, most EGCG was released in the simulated intestinal fluid (SIF), showing great potential to protect EGCG from degradation at low pH conditions.

Heat induced gelation of micellar casein with and without whey proteins in the presence of polyphosphate

The heat-induced gelation of aqueous micellar casein (MC) suspensions was studied as a function of the sodium polyphosphate concentration. The effect of replacing 30% of the casein with whey protein isolate (WP) was also investigated. Addition of less than 4 g L−1 polyphosphate inhibited gelation of intact micelles, but adding more than 6 g L−1 caused gelation of disintegrated micelles. At intermediate polyphosphate concentrations, no self-supporting gels were formed. The microstructure of gels formed at high polyphosphate concentrations was much more homogeneous than those formed at low polyphosphate concentrations. In the presence of WP, gelation was observed at all polyphosphate concentrations and systematically stiffer gels were formed. At C ≤ 6 g L−1, MC aggregated first followed by co-aggregation of WP, whereas at C ≥ 8 g L−1 WP gelled first followed by gelation of the caseins. Gels formed at higher polyphosphate concentrations were more heterogeneous when WP was present.

PROTEIN GELS: A MINI REVIEW

The use of gelatin as food additive for gelling properties was discussed in this review. Three products (i.e. gel dessert, yoghurt, and surimi) were selected as the main focus of this mini review since they characterized 3 different protein gelation system. The mechanism of gelatin gelation in water system was described by gel dessert, the gelation of casein in dairy product was presented by yoghurt, and the gelation of sarcoplasmic protein in meat system was exemplify by surimi. Overall, the mechanism of gelation mainly contributed by the networking of protein-protein which is strengthen by the intermolecular bond and intramolecular bond.

Hydrogen peroxide and ferulic acid-mediated oxidative cross-linking of casein catalyzed by horseradish peroxidase and the impacts on emulsifying property and microstructure of acidified gel

Horseradish peroxidase (HRP, EC 1.11.1.7) was used in this study to catalyze oxidative cross-linking of casein in the presence of hydrogen peroxide and cross-linker ferulic acid. Cross-linking of casein was demonstrated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and ultraviolet absorbance analysis. Oxidative cross-linked casein mediated by hydrogen peroxide and ferulic acid was prepared at casein concentration of 5% (w/ w), HRP addition of 3 mkat·g-1 proteins, ferulic acid addition of 6 mmol·l-1, 3% (w/w) H2O2 addition of 1 ml, reaction temperature 37°C and reaction time 3 h. Analysis results showed that the emulsifying activity index and emulsifying stability index of cross-linked casein prepared were enhanced compared to that of untreated casein totally. Microstructure of chemical acid-induced gel of cross-linked casein was observed under scanning electron microscopy and appeared to be more compact and uniform than that of casein. Hydrogen peroxide and ferulic acid-mediated oxidative cross-linking of casein catalyzed by horseradish peroxidase might have a beneficial to the emulsifying property or gelation of casein.

Cooling effects on a model rennet casein gel system: part I. Rheological characterization.

The gelation of a model rennet casein system was studied during cooling at different rates. During cooling, casein network structure development was proposed to evolve over a few steps at different length scales: molecules, particles, flocs, or network. Rennet casein flocs are fractal in nature, and fractal dimension and floc size are two variables affecting the rheology and microstructure of a rennet casein gel. Casein structure formation during cooling from 80 to 5 degrees C at four different rates (0.5, 0.1, 0.05, and 0.025 degrees C/min) was monitored by dynamic rheological tests, and a stronger gel developed at a slower cooling rate. During different cooling schedules, similar fractal dimensions were observed due to a lack of difference in the colloidal interactions. Differences among rheological data were possibly caused by variability in floc size, as observed in the second part of this paper. A larger number of smaller-sized flocs enabled gelation at a higher temperature and created a stronger network at a slower cooling rate. Controlling cooling schemes thus provides an approach for manipulating casein gelation and the microstructure for a system of fixed chemical compositions.

Kinetics of rennet casein gelation at different cooling rates.

A mathematical model was developed to quantitatively analyze the rheological data of rennet casein gelation at different cooling rates. Kinetic parameters were estimated and correlated with the microstructure development of the protein network. The kinetic model identified structure development upon cooling to be first order, and the network forming energies were estimated for four protein concentrations cooled at four rates. A lower energy for network formation was observed for a slower cooling rate and a higher protein concentration. This observation resulted from the availability of more flocs at a slower cooling rate and a higher casein concentration, simplifying floc cross-linking. By analyzing the kinetics during the aging process of casein gels, no difference in the reaction mechanism was observed. This study illustrated that structure formation resulted from the addition of flocs into the protein network: not all flocs were part of the network at a defined gel point. The incubation period following cooling integrated idle flocs into the network, thereby strengthening the gel. By understanding the gelation mechanism during cooling of rennet casein gels, the structure and thus quality of dairy products, such as processed cheese, may be better controlled.

Micellar Casein Gelation at High Sucrose Content

This article investigates the effect of sucrose addition on the formation of casein gels by acidification and/or renneting of pure micellar casein. Gelation kinetics and gel properties were followed by rheological methods, and microscopy and syneresis measurements were used to obtain a more complete characterization of the structures formed. Sucrose content has been identified as a key parameter for controlling the kinetics of aggregation and the strength of the final gels. Results have shown that the effect of sucrose on gelation can vary such that effects can be completely reversed depending on the gelation route used. During acid gelation, addition of up to 30% (wt/wt) sucrose causes gels to form more rapidly and at higher pH values, and to have higher viscoelastic moduli and a more homogeneous microstructure than those without sucrose. By contrast, gels formed by renneting in the presence of sucrose are weaker and have longer gelation times. It is proposed that sucrose reduces solvent quality and causes the collapse of the “hairy”κ-casein brush on the surface of the casein micelles. This may explain why sucrose increases the possibility of gel formation during acidification and reduces the degree of κ-casein hydrolysis during renneting.

Gelation of casein-whey mixtures: effects of heating whey proteins alone or in the presence of casein micelles.

The aim of the present work was to investigate the role of whey protein denaturation on the acid induced gelation of casein. This was studied by determining the effect of whey protein denaturation both in the presence and absence of casein micelles. The study showed that milk gelation kinetics and gel properties are greatly influenced by the heat treatment sequence. When the whey proteins are denatured separately and subsequently added to casein micelles, acid-induced gelation occurs more rapidly and leads to gels with a more particulated microstructure than gels made from co-heated systems. The gels resulting from heat-treatment of a mixture of pre-denatured whey protein with casein micelles are heterogeneous in nature due to particulates formed from casein micelles which are complexed with denatured whey proteins and also from separate whey protein aggregates. Whey proteins thus offer an opportunity not only to control casein gelation but also to control the level of syneresis, which can occur.

Cross-linking casein micelles by a microbial transglutaminase conditions for formation of transglutaminase-induced gels

The conditions for obtaining transglutaminase-induced casein gels have been studied by rheological measurements. The experiments have been performed with pure micellar casein. Different parameters were investigated including enzyme concentration, protein concentration, pH, ways of acidification, gelation temperature, etc. The conditions of gel formation are clearly based on a compromise between the conditions required for casein gelation and those controlling the intra- and inter-micellar cross-linking mechanisms of casein micelles. Such conditions involve control of enzyme activity (pre-incubation, pH and temperature dependence) as well as control of the casein micelle aggregation (pH and temperature dependence).

Rheological Properties of Thiolated and Succinylated Caseins

Gelation of casein may be improved by introduction of functional groups that are capable of forming disulfide bonds. Thiolated whole caseins were prepared with S-acetylmercaptosuccinic anhydride and succinylated whole caseins with succinic anhydride. Thiolated caseins formed gels at protein concentrations of 5% or more at ambient temperatures after treatment with a deacetylating agent, due to air oxidation of the sulfhydryl groups to form disulfide bonds. Gelation rate and gel strength increased with increasing concentration of casein. Gels formed at 10% protein concentration when only 2.6 lysines/mol were modified by the thiolating reagent. However, increased lysine modification was only slightly related to more rapid gelling rate or increased gel strength. Gels formed had mechanical spectra that resembled those of weak gels (gums) in terms of moduli behavior during frequency sweeps, although absolute values were greater. Succinylated caseins and native casein did not form gels under these experimental conditions. Keywords: Casein; gels; thiolated

Gelation of casein after modification by dialdehyde starch–Chemical and physical aspects

The gelation of casein after modification by dialdehyde starch (DAS) was studied by measuring its mechanical and swelling properties, the amount of sol fraction, its melting behaviour, amino acid analysis and polyacrylamide gel electrophoresis. Depending on the concentration of protein and DAS, fusible and infusible gels are formed. The transition from fusible into infusible gels takes place after increasing the amount of DAS, owing to an enhanced blockage of basic amino acid residues and crosslinking of polypeptide chains. The higher the concentration of protein, the lower is the concentration of DAS necessary for gelation. When the DAS concentration is high enough to form a pervading crosslinked network the crosslinking proceeds relatively fast and is nearly completed after one day. The correspondence of the time dependence of chemical andphysical parameters, the irreversible blocking of basic amino acids, especially lysine, the decrease of swelling and nitrogen extractability, the elastic component of creep compliance indicates that during a 7 day storage period the gels are completely crosslinked. Using a corrected Flory relation it was shown that an average of 3.5 crosslinks exist per one polypeptide chain. This number is similar to the number of blocked lysine groups of the same gels (3.6 - 4.5). The melting and creep behaviour of the gels, and especially the effect of sodium do-decyl sulphate on the creep compliance, point to the formation of a heterogeneous network which includes noncovalently connected mi-croassociations and covalently connected polypeptide chains as structural elements.

GELATION OF CASEIN AFTER MODIFICATION BY DIALDEHYDE STARCH‐CHEMICAL AND PHYSICAL ASPECTS2

The gelation of casein after modification by dialdehyde starch (DAS) was studied by measuring its mechanical and swelling properties, the amount of sol fraction, its melting behaviour, amino acid analysis and polyacrylamide gel electrophoresis. Depending on the concentration of protein and DAS, fusible and infusible gels are formed. The transition from fusible into infusible gels takes place after increasing the amount of DAS, owing to an enhanced blockage of basic amino acid residues and crosslinking of polypeptide chains. The higher the concentration of protein, the lower is the concentration of DAS necessary for gelation. When the DAS concentration is high enough to form a pervading crosslinked network the crosslinking proceeds relatively fast and is nearly completed after one day. The correspondence of the time dependence of chemical andphysical parameters, the irreversible blocking of basic amino acids, especially lysine, the decrease of swelling and nitrogen extractability, the elastic component of creep compliance indicates that during a 7 day storage period the gels are completely crosslinked. Using a corrected Flory relation it was shown that an average of 3.5 crosslinks exist per one polypeptide chain. This number is similar to the number of blocked lysine groups of the same gels (3.6 ‐ 4.5). The melting and creep behaviour of the gels, and especially the effect of sodium do‐decyl sulphate on the creep compliance, point to the formation of a heterogeneous network which includes noncovalently connected mi‐croassociations and covalently connected polypeptide chains as structural elements.

Gelation of casein and soybean globulins by transglutaminase.

Guinea pig liver transglutaminase is a Ca2+ dependent enzyme which catalyzes the formation of inter- and intramolecular ε-(γ-glutamyl)lysyl cross-links between protein molecules. We have found that solutions of several proteins (αs1-casein, and soybean 11S and 7S globulins) were gelatinized firmly by transglutaminase. The gel formation depended on the protein concentration. In the case of αs1-casein, a reaction mixture containing below 2% was incapable of gelation. However, above 3%, a firm gel was formed by transglutaminase. As to soybean 11S and 7S globulins, reaction mixtures containing below 5% did not form gels, while, above 8%, firm gels were formed. The protein solutions in the presence of EDTA, an inhibitor of transglutaminase, were not gelatinized on treatment with transglutaminase. Thus, transglutaminase and a higher concentration of a substrate protein are indispensable for firm gel formation. It is supposed that the protein gels are formed through covalent bonds with transglutaminase.

Chemical Modification of Proteins Part 6. Further Studies on Gelation of Casein after Modification by Dialdehyde Starch

AbstractThe physical and chemical changes in casein during the gelation owing to the chemical modification by dialdehyde starch (DAS) in 8–10% solution were studied using electron‐microscopy, measuring the mechanical properties (shear deformation), turbidimetric titration, and determination of amino groups.The gelation which proceeds during a storage time of 2 weeks is accomplished by the formation of a super‐molecular network structure. Depending on the conditions of modification (pH, percentage of DAS, protein concentration) a more or less dense network is formed. An increase of pH (in the range of pH 6,3–9,7) leads to an increase of gelation speed, which favours the formation of irregular network structures. The denser the network formed the smaller is the shear deformation measured by the device of Tolstoi. During the storage of the gels the plastic component of compliance decreases to a greater amount than the elastic one. Therefore the relative amount of elastic component increases.During the reaction with DAS the maximum of turbidity of the protein (solubility minimum) shifted to lower pH owing to a blockage of positively charged groups. On the contrary to the changes measured by physical methods, this shift is complete soon after a 1 day storage. Similarly the blockage of amino groups is finished in the most cases this time too.

Changes Related to Casein Precipitation in Frozen Concentrated Milk

Summary During storage of frozen concentrated skimmilk, lactose crystallization preceded protein precipitation. Cellulose acetate and free-boundary electrophoresis indicated that the precipitated protein was identical to normal casein. Casein precipitation was accompanied by losses of soluble calcium, phosphate, and citrate, but there was no change in the sodium or potassium content of centrifuged reconstituted milk. Precipitation occurred through interaction of the casein with colloidal tricalcium phosphate in the unfrozen portion. Cooling of concentrated skimmilk with agitation before freezing hastened lactose crystallization and subsequent casein precipitation, but quiescent cooling and freezing yielded a more stable product. The quiescently frozen samples were more stable when stored at +15F than at +5F, but samples cooled with agitation deteriorated more rapidly at the higher temperature. Glucose, fructose, sucrose, sorbitol, and citrate delayed lactose crystallization in frozen concentrated skimmilk and protected casein from precipitation, once crystallization had occurred. Carboxymethylcellulose and glycerol delayed lactose crystallization but did not protect casein. In fluid skimmilk systems containing 2 m KCl and 0.1 m (NH 4 ) 2 HPO 4 , added glucose, fructose, sucrose, lactose, and sorbitol delayed casein gelation and maintained ultrafiltrable calcium at higher concentrations than in controls containing no added carbohydrate. Results of this investigation suggest that sugars hinder the interaction of colloidal Ca 3 (PO 4 ) 2 with the casein micelle.