Effect of high pressure processing on some physical properties of transglutaminase-treated skim milks

Abstract

Skim milk contains about 30-36 g L-1 proteins that are randomly distributed in the serum phase of the milk. Milk proteins include caseins and whey proteins and both play extremely important roles in the structural and nutritional aspects of milk. The main objective of this research was to investigate the changes in reconstituted skim milk with added transglutaminase (TGase) under high pressure processing (HPP) and adjusted pH conditions (5.3, 5.6, 5.9, 6.2, 6.5, 6.8, 7.1 and 7.4), in order to evaluate the ability of TGase to cross-link milk proteins and prevent the dissociation of the casein due to HPP. The results obtained from dynamic light scattering (DLS), diffusing-wave spectroscopy (DWS) and cryo-transmission electron microscopy (cryo-TEM) showed that the addition of TGase prevented casein micellar dissociation under HPP by cross-linking with the milk proteins in the range of 100-400 MPa at all pH conditions. Also, the high pressure induced changes in average particle size of milk proteins depend on the level of pressure applied, addition of TGase and pH. For samples without TGase, the average particle size decreased after 100 MPa or 150 MPa pressure treatment, depending on the pH value. The diameter sizes reduced from about 180 nm to 50% of their original sizes when the applied pressure increased to 400 MPa. On the other hand, the diameter sizes of milk with added TGase samples increased from about 190 nm to varies values depends on the pH and the pressure applied. This increase diameter size is attributed to the denaturation of the whey proteins which bridged the casein micelles through β-lactoglobulin-κ-casein interaction under high pressure. Finally, it can be said that the addition of TGase to milk prior to treatment by high pressure improved the structural stability of milk proteins against disruption. These findings can be used in the development of novel dairy products which might be subjected to high pressure treatment.