Serological studies on heat-induced interactions of α-lactalbumin and milk proteins

Paneer-An Indian soft cheese variant: a review

Paneer, a popular indigenous dairy product of India, is similar to an unripened variety of soft cheese which is used in the preparation of a variety of culinary dishes and snacks. It is obtained by heat and acid coagulation of milk, entrapping almost all the fat, casein complexed with denatured whey proteins and a portion of salts and lactose. Paneer is marble white in appearance, having firm, cohesive and spongy body with a close-knit texture and a sweetish-acidic-nutty flavour. Preparation of paneer using different types of milk and varied techniques results in wide variation in physico-chemical, microbiological and sensory quality of the product. Paneer blocks of required size are packaged in laminated plastic pouches, preferably vacuum packaged, heat sealed and stored under refrigeration. Paneer keeps well for about a day at ambient temperature and for about a week under refrigeration (7 °C). The spoilage of paneer is mainly due to bacterial action. Successful attempts have been made to enhance the shelf life of paneer. This review deals with the history, method of manufacture, factors affecting the quality, physico-chemical changes during manufacture, chemical composition and nutritional profile, packaging and shelf life of paneer.

Whey protein denaturation in heated milk and cheese whey

Quantitative polyacrylamide gel electrophoresis has been used to measure residual native whey proteins remaining after heat treatment of skim-milk and cheese whey in a kinetic study. The denaturation of α-lactalbumin (α-la) appeared to be first order, but was probably a second-order reaction displaying pseudo first-order kinetics. The denaturation of both β-lactoglobulin A and B (β-lgA and β-lgB) followed second-order kinetics while that of serum albumin was more complex, and could equally well be described as first or second order. Equations are given relating logk1(in s-1) to temperature for α-la denaturation in skim-milk between 70 and 95 °C and between 100 and 150 °C. Similarly, equations relating logk2(in lg-1s-1) to temperature are given for ²-lgA in skim-milk between 100 and 150 °C, and for ²-lgB between 95 and 150 °C. The relative heat stability of ²-lgA and ²-lgB was found to vary.

Below 95 °C ²-lgA appeared slightly more thermostable than ²-lgB in skim-milk, and the same was observed in cheese whey below 100 °C. Above these temperatures ²-lgB appeared more stable than ²-lgA.

Denaturation of ²-lgB was only slightly more rapid in skim-milk than in whey at temperatures below 95 °C, but was significantly slower at higher temperatures.

Heat stability of milk: influence of denaturable proteins and detergents on pH sensitivity

α-Lactalbumin and SDS in addition to β-lactoglobulin introduced pH sensitivity to the heat stability–pH curve of serum protein free casein micelles particularly by increasing stability in the pH range 6·4–6·7. Bovine serum albumin, ovalbumin and lysozyme caused marked destabilization of milk and casein micelle suspensions throughout the pH range 6·4–7·4. Tetramethyl ammonium bromide caused destabilization of milk at pH values > 7·0, but had no effect in the region of maximum stability while the non-ionic detergents Triton X-100 and Tween 80 had no effect on heat stability.

Ultrafiltration with a microfiltration membrane of acid skimmed and fat-enriched milk coagula: hydrodynamic, microscopic and rheological approaches

The effect of acidification method (microbiological with or without renneting, HCl addition) on mass transfer, fouling structure and the rheology of the retentate was studied in the ultrafiltration of skim milk coagula using a mineral microfiltration membrane. The increase in fouling with time appeared to determine permeate flow rates, which were higher in biological coagula, and the protein retention rates which were higher in chemical coagula. Fouling was investigated using scanning electron microscopy. The rheological study showed that at the same total solids, biological coagula were more viscous than chemical coagula. The initial coagula (total solids 97 g/kg) all displayed pseudoplastic behaviour at low shear velocities and Newtonian behaviour at high velocities. Ultrafiltration of fat-enriched milk coagulum to a dry weight corresponding to a soft cheese (total solids 334 g/kg; fat in total solids 60%) gave satisfactory permeate flow rates and protein retention rates. Performance was related to the composition of the product, the hydrodynamic parameters used and the resulting fouling. The rheological study showed that the initial coagulum behaved as a pseudoplastic body at low shear rate and for higher velocities as a Newtonian liquid. The concentrated retenate behaved as an ideal viscoplastic body (Bingham body).

A calorimetric study of the thermal denaturation of whey proteins in simulated milk ultrafiltrate

Differential scanning calorimetry (DSC) was used to study thermal transitions of the following whey proteins and enzymes in milk ultrafiltrate solution: β-lactoglobulin, α-lactalbumin, serum albumin, γ-globulin, apo- and Fe-lactoferrin, lysozyme, ribonuclease, α-chymotrypsin and xanthine oxidase. Denaturation enthalpies (ΔHD), denaturation temperatures (TD) and the half width of the denaturation peaks in DSC thermograms (ΔT½D) were determined and the degree of renaturation was estimated by rescanning previously denatured samples. A fair correlation between the results obtained by DSC and other more classical methods was found in general. However, for some proteins (α-lactalbumin, lysozyme, ribonuclease and xanthine oxidase), which have so far been considered relatively thermostable, calorimetry reveals conformational changes starting at temperatures as low as about 45 °C. In these cases thermostability observed after heat treatment of milk should be interpreted in terms of renaturation and not of high temperatures of denaturation.

Irreversible heat denaturation of bovine α-lactalbumin

The irreversible heat denaturation of α-lactalbumin (α-la) in 0·1 M-phosphate, pH 7·0, at 100 °C was studied using polyacrylamide-gel electrophoresis (PAGE). PAGE revealed two groups of bands, one moving faster than native α-la and one slower, in addition to some denatured protein which remained at the origin and some residual native α-la. The faster group had unchanged molecular weight, but an increase in charge, partly due to hydrolysis of glutamine and asparagine residues. The slower group was shown by two-dimensional sodium dodecyl sulphate-PAGE to be oligomers of denatured α-la; formation of the smaller oligomers preceded the larger ones. The oligomers reverted to monomers in the presence of dithiothreitol, showing that they were disulphide-linked aggregates of denatured α-la. Immuno-blots of the gels showed that both fast and slow groups of bands had irreversibly lost most of the antigenicity of the native protein.

Serological and fluorescence studies of the heat stability of bovine lactoferrin

The heat denaturation of Fe-saturated lactoferrin (If) and Fe-free lactoferrin (apo-lf) was studied using the methods of micro-complement fixation and fluorescence. It was established that the change in conformation of apo-lf, induced by iron binding, conferred a higher heat stability to the molecule: the changes were observed at temperatures above 40 °C for apo-lf and above 60 °C for If. The Fe-binding ability of the protein was partially independent of the degree of denaturation. Fluorescence analyses indicated that tryptophan residues were probably not directly involved in the metal binding. There was no evidence of antibodies interfering with the binding sites.

What establishes a protein as an allergen?

There is little known about the factors that determine the allergenicity of food proteins. Apparently, the ability of a food protein to induce an allergic response requires its presence in substantial amounts in the food supply, its durability during food processing, and its resistance to digestion in the gastrointestinal tract. In addition to the mode and degree of exposure, structural characteristics appear to play an important role for the capacity of a protein to modulate the immune response towards allergic reactions. Until now, however, there has been no indication for common structural characteristics of linear T cell or linear IgE (B cell) epitopes and the knowledge of structural characteristics of conformational IgE binding sites is very limited. Experimental data point only to certain surface areas of allergenic proteins which are important for IgE binding. Therefore, it is not possible to suggest any structural motif or conformational sequence pattern common to all allergenic proteins. Furthermore, glycosylation appears not to be a common critical determinant of allergenicity since food allergens comprise both glycoproteins and nonglycosylated proteins. Based on the few published three-dimensional structures of allergenic proteins including food proteins, one unifying feature of allergens appears to be their spherical shape. The three-dimensional structures of many more allergens have to be determined, however, to allow for a better understanding of the molecular basis of allergenicity. Most recently, new ideas have been introduced as to why certain biochemical or biologic functions such as enzymatic activities may predispose a protein to become an allergen. Proteolytically active allergens have been demonstrated to irritate the human mucosal surface, to enhance their own transmucosal uptake, and to augment IgE production. Therefore, the functional activity of some allergens may play a role among other factors in the process of sensitization and allergic responses.

Protein modification by thermal processing

This paper is a review concerning the way in which heat treatment can modify the allergenicity of food proteins. Any food protein may be allergenic if it can be absorbed intact, or as substantial fragments, through the gut mucosa and then evoke an immune (allergic) response. The intrinsic properties of the protein, the overall composition of the food, and the past processing history (especially thermal processing) all have an effect on the allergic potential. When a protein is denatured by heat, most of the original tertiary structure is lost, so that many of the sites recognized by antibodies on the native molecule are destroyed. There are many examples of allergenicity being reduced, but not eliminated, by heating. But heat-denatured proteins can also present new antigenic sites, uncovered by the unfolding process or created by new chemical reactions with other molecules present in the food (e.g., beta-lactoglobulin associating with alpha-lactalbumin in milk). We have found that heat-denatured beta-lactoglobulin has at least one new epitope, not found in the native state. Therefore, thermal processing can be part of a procedure for making hypoallergenic food, but will rarely be sufficient on its own. Increased understanding will help in evaluating novel proteins and processes.

Thermal denaturation of whey proteins and its effect in dairy technology

The irreversible denaturation of the most important whey protein fractions, namely beta-lactoglobulin A and B and alpha-lactalbumin were studied. The orders of the reactions, the rate constants and the activation energies were determined. The experiments were extended to include whey protein solutions of different concentrations and mixtures of whey proteins and caseins in different proportions. The kinetic data found by experiment make it possible to calculate in advance the precise degree of irreversible denaturation. It was found that the denaturation of beta-lactoglobulin was a good test parameter in technological studies and that there was a close correlation between the degree of denaturation and the results of important dairy processes.

Effects of temperature on food proteins and its implications on functional properties

This article surveys the knowledge in the area of protein structure and chemistry of denaturation prior to an indepth review of the effects of heat on soy, milk, and egg proteins. It also reviews the methods available to assess denaturation of proteins. Protein denaturation is an ambiguous phenomenon and the consequences of denaturation on the functional properties of proteins is further confounded by this ambiguity. For each of the three food proteins, the known chemistry of individual proteins is reviewed followed by observations made on changes induced by heat in each protein group. Food proteins are not pure entities and purification and physicochemical characterization of various components of the food proteins have not been thoroughly investigated. Further, food is a complex milieu of water, fat, carbohydrate, vitamin, minerals, etc. along with proteins, and processing affects not only each individual component in the food but also the nature and intensity of intercomponent interactions in a food.

Interference of lipid with determination of bovine serum albumin by electroimmunodiffusion

Electroimmunodiffusion of ultra-high-temperature processed milk demonstrated an apparent increase in bovine serum albumin at some processing temperatures. The cause of the phenomenon was traced to the lipid component of the milk. Caproic acid was used in model studies to evaluate lipid effects in unheated and heated systems. Addition of caproic acid to bovine serum albumin without heat treatment reduced the apparent concentration of bovine serum albumin. Heating at 78 degrees C resulted in an initial increase in the apparent bovine serum albumin concentration, followed by a decrease with further heating. These results indicate that the reaction of specific antibody with bovine serum albumin in milk is inhibited by lipid and that this inhibition is reversed by heat treatments below that causing marked conformational changes in the protein.

Heat-induced association of beta-lactoglobulin and casein micelles

Heat-induced association of beta-lactoglobulin (beta-lg) and casein micelles, mainly at 85--90 degrees C, was studied by means of preparative ultracentrifugation, using [3H]-labelled beta-lg. Qualitative aspects were studied by gel electrophoresis and electron microscopy. In this association, the formation of intermolecular S-S bonds between beta-lg and kappa-casein plays a role, but hydrophobic bonds are also involved. After heating mixtures containing 25 g/kg casein and 4 g/kg beta-lg at 90 degrees C for 20 min, the amount of beta-lg sedimented with the casein micelles by ultracentrifugation decreased by approximately 30% when the milk salt buffer system was reduced to 0.25 of its normal concentration; it decreased by about 20% when the pH was increased from 6.8 to 7.3 and increased by 15% when the pH was reduced from 6.8 to 5.8. When the beta-lg concentration was decreased from 4 to 2 g/kg, the amount of sedimented beta-lg decreased by 25% after heating at 90 degrees C for 6 min. After heating in milk salt buffer, the amount of sedimented beta-lg was 15% higher than after heating in salt solution which contained neither Ca nor citrate ions. Although alpha-lactalbumin (alpha-la) is involved in the heat-association of beta-lg and casein micelles, no influence of alpha-la on the amount of associated beta-lg was found. In addition to the heat-association product of beta-lg and whole casein micelles, an association product consisting mainly of beta-lg and kappa-casein was also formed. This product was observed by electron microscopy as noodle-like particles. The size of these particles depended on the beta-lg/kappa-casein ratio.