Changes in the acid gelation of skim milk as affected by heat-treatment and alkaline pH conditions

Abstract

In an attempt to improve the acid gelation properties of heated milk, or to reduce the heat load necessary to obtain significant acid gelation properties, skim milk adjusted at pH values ranging from 6.7 to 10.5 was heat-treated for 10 min at temperatures ranging from 25 to 95°C then neutralised to pH 6.7. Protein composition of the serum and micellar phases of milk and separation of the protein particles present in the serum phase of milk indicated that formation of micelle-bound heat-induced whey protein/κ-casein aggregates was prevented from pH 7.5 upward, to the benefit of serum aggregates. Aggregates were formed in milk at pH 9.5 and 10.5 even at mild heating temperature (65-75 °C), while temperatures of 85 or 95 °C were necessary at lower pH values. Size of the aggregates decreased as pH of heat-treatment increased. Quality of the results was however reduced because of the extensive dissociation of caseins on alkaline treatment and of side mechanisms like Maillard reaction at high pH and temperature. However, some relationships could be found between the occurrence of aggregated whey protein and κ-casein and higher elastic modulus of the acid gels after heating at low temperature/high pH or high temperature/low alkaline pH values of skim milk.

The effect of dietary crude protein and phosphorus on grass-fed dairy cow production, nutrient status, and milk heat stability

“…Gradient elution and peak detection were performed according to Mounsey and O'Kennedy (2009). The skim milk samples and standards were prepared according to Guyomarc'h et al (2007) and diluted volumetrically. All casein and whey standards were supplied by Sigma-Aldrich.…”

The effect of high and low levels of supplementation on milk production, nitrogen utilization efficiency, and milk protein fractions in late-lactation dairy cows

“…Gradient elution and peak detection were performed according to Mounsey and O'Kennedy (2009). The skim milk samples and standards were prepared according to Guyomarc'h et al (2007) and diluted volumetrically. All CN and whey standards were supplied by Sigma-Aldrich (Sigma-Aldrich, Vale Road, Arklow, Co. Wicklow, Ireland).…”

Changing the isoelectric point of the heat-induced whey protein complexes affects the acid gelation of skim milk

“…x, falls on approaching pH 4.0e4.5 (Anema & Klostermeyer, 1996;Guyomarc'h et al, 2007). Because the whey proteins also precipitate on acidification when denatured (de Wit, Hontelez-backx, & Adamse, 1988), the hypothesis soon arose that the coverage of the heated casein micelles by the micelleebound complexes could affect their electrostatic repulsion through elevating their apparent isoelectric point (pI) from w4.5 towards that of the major whey protein, b-lactoglobulin (pI w5.3) (Lucey, Teo, Munro, & Singh, 1997;Vasbinder, van Mil, Bot, & de Kruif, 2001).…”

“…Also, the measurement of the x potential of whey protein/k-casein complexes isolated from the serum phase of heated skim milk showed that their apparent pI was w4.5 (Guyomarc'h et al, 2007;Jean, Renan, Famelart, & Guyomarc'h, 2006), which no longer linked the pH of gelation of heated milk directly to the pI of the native b-lactoglobulin. Donato and Guyomarc'h (2009) further noted that the heat-induced whey protein/k-casein complexes also exhibit significant surface hydrophobicity and suggested that the increased hydrophobic attraction opposed electrostatic repulsion over a larger pH range, thus inducing destabilisation of the milk at higher pH values on acidification.…”