Proteins recovered from milks heated at alkaline pH values

Abstract

Approximately 95% of available nitrogen can be precipitated from milk on adjustment to pH 4.6 after heating at 90°C × 15 minutes at its natural pH and pH 7.5, while 89% can be precipitated after heating at pH 10.0 at 60°C × 3 minutes. Non‐recovered protein includes some serum albumin, β‐lactoglobulin, α‐lactalbumin and proteose peptones. Protein isolates precipitated from milk heated at pH >7.0 are more soluble in the pH range 6.0–7.0 than those precipitated from milk heated at its natural pH. Whey proteins complex onto the casein micelles after heating milk at its natural pH, while on heating at pH >7.0 whey proteins appear to interact with k‐casein in the serum phase. When N‐ethylmaleimide is present in milk during heating the percentage protein recovered on pH 4.6 precipitation is decreased, confirming that disulphide linkage is involved in complex formation. However, addition of β‐mercaptoethanol to recovered isolates did not result in dissociation of the casein/whey protein complex, suggesting that forces other than disulphide bonding are also involved in maintaining the complex.

Formation of heat-induced protein aggregates in milk as a means to recover the whey protein fraction in cheese manufacture, and potential of heat-treating milk at alkaline pH values in order to keep its rennet coagulation properties. A review

“…At present, the heat-treatment of milk at alkaline pH values has found little application in the restoration of the rennet coagulation properties of heated milk. In most cases, such a process was used to induce whey protein aggregates or protein-calcium complexes not otherwise present in the unheated milk, giving ingredients with modified viscosity, calcium content, or lactose content [18], to increase the yield of isolation of whey protein/casein complexes [25] or to increase the solubility of the isolates upon reconstitution [57]. In some other cases, alkaline heat-treatment has been used as a preliminary to the use of transglutaminase [145], sometimes in combination with rennet [28] to manufacture dairy ingredients.…”

“…The reactivity and accessibility of the free thiol of β-lactoglobulin was higher at pH 8.0 than at 7.0, as a result of the proximity of the pK value of the thiol (~8.2) [66]. As a result, the conversion of thiols into disulphide bonds increased with pH up to at least 6.9 [163] or up to pH 11 [113], as did the contribution of intermolecular disulphide bonds in the formation of heatinduced protein aggregates of β-lactoglobulin [67] or of κ-casein and whey protein in milk [57]. Conversely, the contribution of electrostatic interactions decreased as pH increased, as a result of the increased negative charge of the β-lactoglobulin (isoelectric pH ~ 5.4) [67].…”

“…In model solutions of β-lactoglobulin [66,68,163] or in skim milk [57], heated at pH values in the maximum range 3 to 10, heat-induced protein aggregates were formed at all pH values. The aggregation rate increased with pH values in the range 6.5 to 8.0, so that more aggregates were formed at pH 7.0 or 8.0 than at pH 6.5 [66,68].…”

“…Because they interact with κ-casein upon heating, the denatured whey proteins remain in the serum phase when the heattreatment is performed at pH > 6.9, whereas a high proportion of them can be sedimented with the casein micelles by ultracentrifugation when the heat-treatment is performed at lower pH values [57,117,[139][140][141]. Virtually no aggregates could be found in the serum phase of milk at pH < 6.7 [156].…”

A Comparison of Analytical Methods for Quantifying Denatured Whey Proteins and Their Correlation to Solubility

“…For example, that the protein which precipitated from milk heated above pH 7.5, which is where the rate of denaturation for β-lg was at a minimum, had better solubility properties than that from milk heated at its natural pH of 6.5, which is when the rate of denaturation of β-lg is at its highest (Grufferty and Mulvihill, 1987).…”