Effect of pH at heating on the acid-induced aggregation of casein micelles in reconstituted skim milk (original) (raw)
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Comparison of casein micelles in raw and reconstituted skim milk
Journal of Dairy Science, 2007
During the manufacture of skim milk powder, many important alterations to the casein micelles occur. This study investigates the nature and cause of these alterations and their reversibility upon reconstitution of the powders in water. Samples of skim milk and powder were taken at different stages of commercial production of low-, medium-, and high-heat powders. The nature and composition of the casein micelles were analyzed using a variety of analytical techniques including photon correlation spectroscopy, transmission electron microscopy, turbidity, and protein electrophoresis. It was found that during heat treatment, whey proteins are denatured and become attached to the casein micelles, resulting in larger micelles and more turbid milk. The extent of whey protein attachment to the micelles is directly related to the severity of the heat treatment. It also appeared that whey proteins denatured during heat treatment may continue to attach to casein micelles during water removal (evaporation and spraydrying). The process of water removal causes casein and Ca in the serum to become increasingly associated with the micelles. This results in much larger, denser micelles, increasing the turbidity while decreasing the viscosity of the milk. During reconstitution, the native equilibrium between colloidal Ca and serum Ca is slowly reestablished. The reequilibration of the caseins and detachment of the whey proteins occur even more slowly. The rate of reequilibration does not appear to be influenced by shear or temperature in the range of 4 to 40°C.
Effect of temperature and pH on the interactions of whey proteins with casein micelles in skim milk
Food Research International, 1996
Skim milk was heated at temperatures in the range 7559O"C, at pH values of 6.8, 6.2 and 5.8. The amounts of a-lactalbumin and p-lactoglobulin which interacted with the casein micelles during heat treatment were quantified by SDS-polyacrylamide gel electrophoresis of the micellar fractions isolated by ultracentrifugation. Both o-lactalbumin and ,&lactoglobulin appeared to interact similarly with casein micelles at temperatures up to 85°C. The amount of whey protein complexed with micelles increased with time, reaching plateau values that, at the highest temperatures, were comparable with the quantity present in the original skim milk. In general, faster reaction of the whey proteins with the micelles was found at lower pH and higher temperatures. The rates and extent of the reaction changed also when additional cr-lactalbumin and P-lactoglobulin isolates were added to milk before heating. The reaction between cl-lactatbumin and casein micelles depended to a relatively small extent upon environmental variations (pH and temperature), while /I-lactoglobulin interactions were more affected, so that a more complex behaviour may be attributed to the latter protein. Copyright 0 1996 Canadian Institute of Food Science and Technology
The mechanisms of the heat-induced interaction of whey proteins with casein micelles in milk
International Dairy Journal, 1999
The heat-induced interactions between whey proteins and casein micelles were investigated by de"ning the "nal product of the reaction when milk was heated at temperatures up to 903C. By looking at the changes of the interactions in skim milk and in resuspended casein micelles, to which di!erent amounts of whey protein had been added, information on the mechanisms that determine the heat-induced protein}protein interactions in milk was derived. The ratio of -lactalbumin and -lactoglobulin to -casein and the ratio of -lactalbumin to -lactoglobulin found in the micellar pellet were used as indices of these heat-induced reactions occurring in milk. The results suggested that at these low temperature (70}903C) with batch heating conditions, whey proteins form soluble complexes which act as intermediates in the heat-induced association of -lactalbumin and -lactoglobulin with the micelles. The presence of -lactoglobulin was necessary for any association of whey protein with casein micelles to occur; furthermore, the amount of -lactoglobulin found in the micellar pellet after heating seemed to be limited by a discrete number of binding sites available on the micelles.
International Dairy Journal, 2011
Heat treatment of milk at 85e95 C has long been reported to increase the pH of gelation and firmness of acid milk gels; hence its wide application in yoghurt manufacture. These changes have been attributed to the formation of heat-induced whey protein/k-casein complexes in the milk, to which heat-denatured whey protein ingredients may be substituted. However, variations in resulting gels show that a possible role of k-casein in determining the functional acid-gelation property of the complexes needs investigating. Model heat-induced whey protein/k-casein complexes were produced of k-casein content from 0 to 40% (w/w), but of similar size, secondary structure, surface hydrophobicity and thiol/disulphide distribution. These complexes were added to whey protein-free skim milk systems and the resulting acid-gelation behaviour of the milks was evaluated. The results showed a modification of the pH of gelation that was explained more by variation of the pI of complexes than by the k-casein content.
Journal of Dairy Research, 2006
The pH-dependent behaviour of soluble protein aggregates produced by the pre-heating of reconstituted skim milk at 90 8C for 10 min was studied, in order to understand the role of these aggregates in acid gelation of heated milk. The following milk samples were prepared: (1) control (unheated reconstituted milk, pH 6 . 5); (2) milk heat-treated at pH 6 . 5 (mHtd6 . 5) and (3) milk heat-treated at pH 7 . 2 (mHtd7 . 2). They were centrifuged and the supernatants (SPNT 1) pH-adjusted to yield a series of pH values ranging from 6 . 5 or 7 . 2 to 4 . 6 using HCl at 20 8C or GDL at 20 and 38 8C. pH-Adjusted SPNTs 1 were re-centrifuged. The resulting supernatants (SPNTs 2) were analysed by OD (at 600 and 280 nm) and SDS-PAGE in order to characterise proteins still soluble as a function of pH. Particle size in SPNTs 1 was analysed by Steric Exclusion Chromatography. The OD600 nm revealed that during acidification soluble casein in both control and heat-treated samples exhibits variations in its optical properties or size as previously shown with micellar casein. In heat-treated samples, soluble casein and heat-induced covalent soluble aggregates precipitate at the same pH value. A progressive acidification of the soluble phase did not separate them. Increasing the temperature of acidification from 20 to 38 8C resulted in an increase in the precipitation pH of the proteins. However choice of acidifier did not have a significant effect on OD profiles. The soluble covalent aggregates from mHtd7 . 2 were smaller, more numerous, and had a higher content of k-casein than mHtd6 . 5. Both types of aggregates began to precipitate at the same pH value but precipitation occurred over a narrower pH-range for soluble aggregates prepared from mHtd7 . 2. This may explain the higher gelation pH of mHtd7 . 2 compared with mHtd6 . 5.
Properties of casein micelles reformed from heated mixtures of milk and ethanol
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2003
Addition of an aqueous solution of ethanol ( /30% ethanol) to skim milk, followed by heating to /60 8C, resulted in the mixture becoming translucent; objectively, its Hunter L -value was reduced. The degree of translucence increased with increasing ethanol concentration and temperature; the effect was not fully reversible on cooling, as cooled mixtures had higher turbidity values than unheated mixtures. Following removal of ethanol by vacuum rotary evaporation and readjustment to the original milk solids concentration, the milk contained large protein aggregates (referred to as reformed micelles), the properties of which were studied. Milk containing reformed micelles had a type B heat coagulation time Á/pH profile and was less stable to ethanol than control milk. The mechanism responsible for these changes in the properties of milk appeared to be independent of calcium or other milk salts, but was probably linked to ethanol-induced denaturation of whey proteins. # address: a.kelly@ucc.ie (A.L. Kelly). Colloids and Surfaces A: Physicochem. Eng. Aspects 213 (2003) 265 Á/273 www.elsevier.com/locate/colsurfa 0927-7757/02/$ -see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 -7 7 5 7 ( 0 2 ) 0 0 5 1 9 -8
Journal of Agricultural and Food Chemistry, 2009
The effect of heat treatment of milk on the formation of acid gel was examined using confocal scanning laser microscopy and low-amplitude dynamic oscillation throughout acidification. Milk samples were reconstituted by mixing colloidal phase from unheated or preheated skim milk, labeled with rhodamine B isothiocyanate, with the aqueous phase from unheated or preheated milk, labeled with fluorescein isothiocyanate. Gels were made by acidification with glucono-δ-lactone. The presence of material from preheated milk, that is, either the colloidal or the aqueous phase or both, led to an increase in the gelation pH and in the final elastic modulus and to a more branched network with larger pores. During acidification, the heat-induced serum complexes and the casein micelles did not appear to form separated gels with time or in space. Moreover, the colocalization in the final network of serum heat-induced complexes and casein micelles is particularly well observed in the presence of an aqueous phase obtained from preheated milk. Finally, because the rheological and microstructural properties of acid gels containing either micelle-bound or serum heat-induced complexes were similar, it was suggested that the serum heat-induced complexes interacted with the casein micelles early in the course of acidification and that formation of the network did not differ significantly whether the heat-induced complexes were initially found in the aqueous phase of milk or bound to casein micelles.
Physico-chemical changes in casein micelles of buffalo and cow milks as a function of alkalinisation
Dairy Science and Technology, 2009
By modifying the forces (hydrophobic and electrostatic interactions, hydrogen bonding and presence of micellar calcium phosphate) responsible for the structure and the stability of casein micelles, alkalinisation induces a disruption of casein micelles in milk. The objective of this work was to compare the alkalinisation-induced physico-chemical changes of casein micelles of buffalo and cow milks with a special attention to the mineral fraction. The whiteness and viscosity were determined as global characteristics of milk. The aqueous and micellar phases of milks were ascertained for the distributions of the concentrations of nitrogen, casein molecules, calcium, inorganic phosphate and water as their supramolecular and molecular characteristics. These parameters were measured at six pH values between pH 6.7 and 10.8. Between pH 6.7 and 10.8, the whiteness decreased from 73.5 to 50.9 and from 71.3 to 50.9 units and the viscosity increased from 1.8 to 10.2 and from 1.5 to 4.8 mPa·s for buffalo and cow milks, respectively. Simultaneously, > 90% of nitrogen contents were in the supernatants of ultracentrifugation at pH 9.7 and 8.6 for buffalo and cow milks, respectively. Chromatographic analyses showed that caseins were totally solubilised at these pH values. Calcium and inorganic phosphate concentrations progressively increased in the supernatants of ultracentrifugation and decreased in the ultrafiltrates. At alkaline pH, the negative charge of caseins increased and the inorganic phosphate ion changed its ionisation state from HPO 4 2− to PO 4 3− form. This form has a greater affinity for calcium and can demineralise casein micelles. The consequences were modifications of protein-protein and protein-minerals interactions resulting in micellar disruption. The dissociations took place at pH 9.7 and 8.6 for buffalo and cow milks, respectively. These differences were due to higher concentrations of casein and minerals in buffalo than in cow milk, which were also our criteria of selection of the former as a