Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/6/2015
Publication Date: 5/1/2015
Citation: Qi, P.X., Ren, D., Xiao, Y., Tomasula, P.M. 2015. Effect of homogenization and pasteurization on the structure and thermal stability of whey protein in milk. Journal of Dairy Science. 98(5):2884-2897. DOI: 10.3168/jds.2014-8920.
Interpretive Summary: The manner in which fluid milk is processed can affect the quality of the whey proteins it contains. Changes in the nutritional value, vitamin and mineral contents and functional properties such as foaming have been attributed to the heat treatment received, with the damage in milk quality and the whey proteins increasing with milk processing temperature. In the commercial plant, milk is first treated by homogenization which is designed to break down the fat molecules in milk to a small size so that they remain suspended rather than rise to the top and form a layer of cream and then followed by heating to preserve the milk. In this study, the effects of homogenization alone or in combination with heating at high temperature short time (HTST) pasteurization, heating at a minimum 72 deg C for 15 seconds, or ultra-high temperature (UHT) sterilization, heating at a minimum 135 deg C for 2 sec, on the properties of the whey proteins in milk were investigated. Changes in the molecules and structures of the proteins were assessed by using various advanced physical-chemical methods. Overall, the results showed that homogenization enhanced the damaging effects of UHT treatment on milk but did not affect HTST milk. The methodology demonstrated in this study can be used to gain insight on the behavior of milk proteins when processed and provides a new tool for assessing the impact of processing schemes on milk and dairy product nutrition and quality.
Technical Abstract: The effect of homogenization alone or in combination with high temperature, short time (HTST) pasteurization or UHT processing on the whey fraction of milk was investigated using highly sensitive spectroscopic techniques. In pilot plant trials, 1-L quantities of whole milk were homogenized in a two-stage homogenizer at 35 deg C (6.9 MPa/10.3 MPa) and, along with skim milk, were subjected to HTST pasteurization (72 deg C for 15s) or UHT processing (135 deg C for 2s). Other whole milk samples were processed using homogenization followed by either HTST pasteurization or UHT processing. The processed skim and whole milk samples were centrifuged further to remove fat and then acidified to pH 4.6 to isolate the corresponding whey fractions, and centrifuged again. The whey fractions were then purified using dialysis and investigated using the Circular Dichroism, Fourier Transform Infrared and Trp intrinsic fluorescence spectroscopic techniques. Results demonstrated that homogenization combined with UHT processing of milk caused not only changes in protein composition but also significant secondary structural loss, particularly in the amounts of antiparallel beta-sheet and alpha-helix, as well as diminished tertiary structural contact. In both cases of homogenization alone and followed by HTST treatments neither caused appreciable chemical changes, nor remarkable secondary structural reduction. But disruption was evident in the tertiary structural environment of the whey proteins due to homogenization of whole milk as shown by both the near-UV CD and Trp intrinsic fluorescence. In-depth thermal stability analyses revealed that even though processing of milk imposed little impairment to the secondary structural stability, the tertiary structural stability of whey protein was altered significantly. The following order was derived based on these studies: raw whole is greater than HTST, h, h-HTST is greater than s-HTST and s-UHT is greater than h-UHT. The methodology demonstrated in this study can be used to gain insight on the behavior of milk proteins when processed and provides a new tool for assessing the impact of processing schemes on milk and dairy product nutrition and quality.