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208 IV.Resultados y discusión. Capítulo II
diluted 1:1 (w/w) in deionised water and 30 L were removed and added to
3 mL of OPA reagent. The absorbance of the solutions was measured
spectrophotometrically at 340 nm in a quartz cuvette by using a UV-visible
spectrophotometer (Helios Zeta UV-vis, Thermo Scientific, USA) after 2
min incubation at room temperature. The starters’ proteolytic activity is
quantified as the difference in absorbance measured between fermented and
non-fermented hazelnut milks.
2.3.5 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 , taking 5
minutes to reach the maximum shear rate and another 5 to fall. The
Herschel-Bulkey model (Eq. 1) was fitted to the experimental points 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 the shear rates generated in mouth when
food is being chewed and swallowed are between 10 and 100 s -1
(McClements, 2004).
n
y
K
(1)
n1
K
(2)