Influence of pH, NaCl and protein composition on the melting performance of casein gels made from concentrated skim milk
Special Symposium - Innovations in Food Technology (LMC Congress)
Meals - Convenience, Gastronomy & Quality (Food-1b)
Keywords: Concentrated milk, melting, cheese
During coagulation of concentrated milk for cheese production, water will not be separated from the protein gel and whey proteins are incorporated into the final cheese product. The incorporation of whey proteins increase the cheese yield but also decrease the flowability of the cheese upon melting. An increasing amount of cheese products are heated in food applications and then the flow behaviour upon melting becomes the crucial property. Depending on the food application, extensive (e.g. on pizza) or limited flow (e.g. in toasted sandwich) of the cheese product upon melting is desired. Thus, there is also potential in using concentrated milk for production of cheese products with limited or low level flowability during melting. In this study the production parameters (e.g. pH, NaCl content and protein composition) were varied to explore changes in the melting performance of casein gels made from concentrated skim milk. The results can be valuable for developing cheese products made from concentrated milk with designed melting performance.
Skim milk was concentrated 7 times using ultrafiltration or microfiltration. Different amounts of acid (i.e. glucono-δ-lactone) and NaCl were added to the skim milk concentrate. When the acidification was complete, rennet was added to induce coagulation and obtain casein gels. The casein gels had the same casein concentration (20.7 %, w/w) but different pH (5.2 to 6.2), NaCl concentrations (0 to 3.5 %, w/w) and whey protein concentrations (0.1 or 3.5 %, w/w). After storage (13°C, 2 to 30 days), the elastic properties and the flowability of casein gels were measured at 10 to 90°C with small amplitude oscillatory shear rheology and with uniaxial creep tests. Furthermore, melting performance of casein gels was measured on an oven (80 and 120 °C). The microstructure of gels before and after melting was revealed by confocal laser scanning microscopy.
At all pH, NaCl concentrations, times of storage or whey protein concentrations, the elasticity of casein gels decreased and flowability increased from 10 to 60°C. This was due to softening of the casein network and was most extensive in gels of pH 5.2 but also significant in gels with proteolytic breakdown of caseins after storage. Micrographs revealed that the microstructure in gels were the same before melting but less elastic gels at pH 5.2 caused more rearrangements and phase separation between protein and water during melting. At heating from 60 to 80°C, a small increase in elasticity of gels at all pH indicated that gel firming occurred and this resulted in decreased flowability. This was partly due to cross-linking of whey proteins above 60 °C but also the elasticity of gels containing only 0.1 % (w/w) whey protein increased from 60 to 80 °C. Thus, gel firming at cooking temperatures was also caused by changed interactions between caseins. After addition of NaCl the heat-induced gel firming of gels occurred at 70 °C and was less apparent.
When measuring the overall melting performance of gels in the oven (80 or 120°C) it was apparent that low elasticity and high flowability of gels (i.e. pH 5.2) before heat-induced gel firming occurred (i.e. at 60 to 90 °C) was crucial for obtaining an extensive flow of gels in the oven. Addition of NaCl could to some be used to increase the extent of flow of casein gels in the oven at 120 °C because heat-induced gel firming occurred at 70 °C instead of 60°C. Thus, addition of NaCl can together with a low pH and proteolytic breakdown of caseins during storage can be used to increase flowability of cheese products made from concentrated milk.
Presented Wednesday 19, 17:25 to 17:30, in session Meals - Convenience, Gastronomy & Quality (Food-1b).
