Large amplitude oscillatory shear stress (LAOStress) analysis for an acid-curd Spanish cheese: Afuega'l Pitu atroncau blancu and roxu (PDO)

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Large amplitude oscillatory shear stress (LAOStress) was used to study the nonlinear rheological properties of Afuega'l Pitu (PDO) cheese atroncau blancu and roxu, a Spanish acid-curd cheese made from cow's milk, from nine manufacturers. Composition, physicochemical and lipolytic parameters were determined. The moisture content varied between (23.60 ± 0.75)% and (45.84 ± 0.01)% and the percentage of salt-in-moisture was <(8.17 ± 0.86)%. Stress amplitude sweeps were conducted at 20 °C, 50 °C and 75 °C at frequencies of 0.628 rad/s, 6.28 rad/s and 31.4 rad/s, for each temperature. Experiments were analyzed using the MITlaos software modified to extract LAOStress quantitative material functions. The large number of experiments and the high dimensionality of the LAOS signals were carefully analyzed and reduced to understand the material response and extract the relevant properties. All cheeses showed a distinct signature that motivates us to introduce the concept of “pseudo-linear LAOS” where nonlinear rheology is evident in only some measures while others minimally change from the linear regime. We geometrically interpret these different LAOS measures as rotation and distortion of Lissajous curves; all cheeses showed pseudo-linear LAOS regimes where rotation is strong and distortion is minimal, i.e. first-harmonics change dramatically in a regime where higher-harmonics are small enough that Lissajous curves are nearly elliptical. The pseudo-linear behavior enabled low-dimensional data reduction and three metrics were discovered to be the most relevant: linear elastic modulus (G′), critical stress (σcrit) and strain (γcrit) amplitudes. All Afuega'l Pitu cheese samples softened as stress amplitude increased while maintaining a solid-like behavior (tanδ^1∼ 0.3), regardless of frequency and temperature. Moreover, the difference between blancu and roxu was statistically insignificant compared to difference between manufacturers. Linear G′ and nonlinear σcrit decreased with increasing temperature, while the nonlinear γcrit maintained a constant value at 50 °C and 75 °C. Although G′, σcrit and γcrit metrics establish comparison between cheeses, more metrics are still required to fully connect rheology to cheese texture and sensory profiles.