Changes in thermoviscoelastic and biochemical properties of Atroncau blancu and roxu Afuega'l Pitu cheese (PDO) during ripening

Thermoviscoelastic and biochemical properties of Afuega'l Pitu cheese blancu (B) and roxu (R), a Spanish acid-curd cheese made from cow's milk, were determined at 3, 15, 30 and 60 days of ripening. The contents of fat (>45 g/100 g total solids, TS) and NaCl (<3 g/100 g TS) were similar between varieties and did not change with ripening. Total solids, pH, nitrogen fractions and fat acidity increased with ripening time in B and R samples. Lactose content was higher in B than in R cheeses at 3 d, was degraded gradually throughout ripening but did not disappear totally in both cheeses at 60 d. Viscoelastic parameters were determined in the linear viscoelastic region (LVER) by stress sweeps (6.3 rad/s, 20 degrees C): the stress amplitude (sigma(max)) and complex modulus (G*) increased, while strain amplitude (gamma(max)) decreased up to 60 d. Mechanical spectra were also determined, in the LVER, at 20 degrees C, 50 degrees C and 75 degrees C, showing that all samples behaved as "true gels" (elastic modulus, G' > viscous modulus, G''), irrespective of temperature. The best gel properties (low values of loss factor, tan delta = G''/G', and minor frequency dependence, i.e., low n', n'' exponents), were observed at 20 degrees C for B30 and R30. The viscoelastic parameters (G(0)' and G(0)'') increased with ripening time, irrespective of the temperature, and for a fixed ripening time they were lower at higher temperature. At 75 degrees C, all samples exhibited notable frequency dependence (n'' >> n'), resulting in shear-induced gelation at low frequencies (high oscillation times) which was consistent with data in thermal profiles from 70 degrees C to 90 degrees C.