Microbiological, steady, and dynamic rheological characterization of boza samples: temperature sweep tests and applicability of the Cox–Merz rule

Boza is a beverage produced by lactic acid bacteria (LAB) and yeast, which affects rheological properties. In the present study, boza samples collected from a local market were characterized with reference to microbiological and rheological approaches. The boza samples were characterized with respect to their microbiological population, i.e. the numbers of LAB and yeast. The numbers of LAB and yeast were found to range from 2.88 × 10^6 to 2.06 × 10^10 CFU/mL and from 3.71 × 10^3 to 9.40 × 10^5 CFU/mL, respectively. Their steady and dynamic rheological properties were investigated within the temperature range of 0–50 °C. Apparent viscosity values (η) of the boza samples decreased with shear rate, indicating that the samples behaved as a pseudoplastic fluid. The Ostwald–de Waele model satisfactorily described (R^2 0.99) the flow behavior of the samples at 5 and 20 °C. Storage modulus (G′) values were higher than loss modulus (G″), indicating that boza samples had a solid-like structure. Temperature had a remarkable effect on the steady and dynamic properties of the samples; therefore, temperature sweep tests were also conducted to determine temperature dependency of the samples. The η50, G′, and G″ values decreased with increase in temperature. The Arrhenius equation could be satisfactorily used to describe changes in these parameters depending on temperature. A modified Cox–Merz rule with shift factors ranging from 1.0 to 2.2 was successfully applied to put forward a relationship between steady shear and dynamic shear viscosity data.