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IV. Resultados y discusión. Capítulo II 167
concentration of 0.4 g/100 mL using a phosphate buffer solution. The
Smoluchowsky mathematical model was used to convert the electrophoretic
mobility measurements into -potential values.
2.3.5 Colloidal stability
Serum retention capacity (SRC) of both non-fermented and fermented
milks was analysed by sample centrifugation (Medifriger-BL, JP-Selecta;
Spain). Conditions were 3,000 rpm/45 min/20 ºC and the amount of serum
separation was used to quantify sample stability.
2.3.6 Rheological behaviour
The rheological behaviour was characterised in a rotational rheometer
(HAAKE Rheostress 1, Thermo Electric Corporation; Germany) with a
sensor system of coaxial cylinders, type Z34DIN Ti. The shear stress ()
was measured as a function of shear rate ( ) from 0 to 512 s -1 , using 5
minutes to reach the maximum shear rate and another 5 to fall (up and down
curves). The Herschel-Bulkey model (Eq. 1) was fitted to the experimental
points of the up curve to determine the flow behaviour index (n),
consistency index (K) and yield stress ( y ) by using a non-linear procedure.
Apparent viscosities were calculated at 50 s -1 (Eq. 2), since shear rates
generated in mouth when food is being chewed and swallowed are between
10 and 100 s -1 (McClements, 2004).
y + K n (1)
n1
K (2)