"The PCs were made from Cheddar cheese with an intact casein content of 89 g.100 g(-1) total casein and with calcium/casein ratios (mg.100 g(-1)) that were low (LCaCC, 19.6), medium (MCaCC, 24.2) or high (HCaCC, 29.8). The PCs were formulated from Cheddar cheese, butter oil, emulsifying salts, preservative and water; the blend was heated to 80 degrees C while continuously shearing at 2000 rpm for a total time of four minutes. The calcium/casein ratio of the resultant PCs, denoted LCaPC, MCaPC and HCaPC, were 17.86, 22.10 and 28.36 mg.100 g(-1), respectively. The PCs were analysed for composition, deformation properties on compression to 30% original sample dimensions, cooking properties (flow on heating at 280 degrees C for four minutes and 180 C for 7.5 minutes) and changes in viscoelasticity on heating from 25 to 80 degrees C using low-amplitude strain oscillation rheometry. All PCs had similar levels of moisture (similar to 47 g.100 g(-1)), protein (similar to 21 g.100 g(-1)), water-soluble nitrogen, WSN (similar to 74 g.100 g(-1) total N) and pH 4.6 soluble N (similar to 11 g.100 g(-1) total N). The fracture stress, fracture strain and firmness of HCaPC were significantly higher than those of MCaPC or LCaPC. The heat-induced flow of HCaPC and MCaPC was significantly lower than that of LCaPC. Heating of PCs from 25 to 88 degrees C resulted in a continual decrease in storage modulus (G') of HCaPC and MCaPC; however, G' for LCaPC decreased to 51 degrees C but thereafter increased sharply. The phase angle (delta) of all PCs increased to a maximum (delta(max)) at 45-54 degrees C and thereafter decreased, the decrease being most pronounced for LCaPC. Reducing the calcium content of the Cheddar cheese significantly increased delta(max) and reduced the value of G' at 25 degrees C, the time to reach delta(max) and the temperature at delta(max) of the resultant PC," wrote T.P. Guinee and colleagues.

The researchers concluded, "The results indicate that lower calcium levels in natural cheese give PCs that are softer and shorter (more brittle), that become more fluid and spreadable on heating, that acquire this fluidity in a shorter time and at a lower temperature, but they tend to be less stable on heating at high temperature (similar to 80-100 degrees C) for longer times (> 7 minutes)."

Guinee and colleagues published their study in Dairy Science & Technology ("The Effect of Calcium Content of Cheddar-style Cheese on the Biochemical and Rheological Properties of Processed Cheese. Dairy Science & Technology, 2009;89(3-4):317-333).

For more information, contact T.P. Guinee, Teagasc Moorepk, Moorepk Food Research Center, Fermoy, Cork, Ireland.

From the July 20, 2009, Prepared Foods E-dition