Effects of milk pH alteration on casein micelle size and gelation properties of milk

ABSTRACT

The effects of changes in pH above and below the natural pH of milk (ca 6.6) on the casein micelle size and the gelation properties of the pH adjusted and restored samples were investigated. The size of casein micelles increased at pH 7.0 and 7.5, then started to decrease above pH 8.5. It is postulated that at pH below 8.5 the casein micelles swell, while elevated pH causes their dissociation. Conversely, the size of casein micelles decreased on acidification to pH 5.5 and increased when the pH dropped below 5.5, due to the shrinkage of casein micelles at lower pH before their aggregation at pH below 5.5. In response to neutralising treated milk back to normal milk pH of 6.6, it was found that the casein micelle size of treated milk with a narrow range of pH change between 6.0 and 7.0 was reversible, while beyond this range the structure of casein micelles became irreversible. The restoration of casein micelle size was followed by the restoration of some parameters such as soluble calcium, ethanol stability, and milk whiteness. Acid-induced gelation did not change the elastic modulus, while rennet-induced gelation was affected by initial milk pH. In reference to the size of reversible range elastic modulus (G’) of acid or rennet gels made from restored milk, the sizes were similar to control milk after 6 h of gelation.

KEYWORDS:

• Casein micelle
• alkalination
• acidification
• gelation properties
• acid gel
• rennet gel

Acknowledgment

The authors are grateful for Dr John M. Schiller (Honorary Senior Fellow, School of Agriculture and Food Sciences, The University of Queensland), for his assistance in the proofreading of this manuscript.

Funding

The authors would like to thank Dairy Innovation Australia Ltd. (DIAL) for the financial support provided to undertake this research, DIKTI for the financial support granted to Hotnida Sinaga during her study.