Mango-papaya nectars were prepared by blending mango puree with
papaya puree (one of the cheaper fruits in the Egyptian market) in different
proportions (90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, 20:80 and 10:90).
The products were analysed for physicochemical, sensory characteristics and
rheological properties. Rheological measurements were made by Brookfield
Digital Viscometer Model DV-II+ with 18 rotational speeds at shear rate ranged
between 0.3 to 100 Sec-1 and at temperatures of 5-100 ºC. Four rheological
models (Bingham plastic, Power Law (Ostwald), IPC Paste and Casson) were
used to calculate the relationship between shear stress and shear rate. Mangopapaya
nectars behaved as a non-Newtonian fluids (pseudoplastic behavior) with
a yield stress, while the effect of temperature could be described by an Arrheniustype
equation. The effect of temperature on viscosity was very well correlated
with the Arrhenius equation (r > 0.999). Activation energy for viscous flow was
in the range of 2068.44 to 8534.32 J/mole depending on blending ratio. As the
ratio of papaya puree to mango puree increased, flow rate increased, while the
scores for sensory quality decreased. The nectar consisting of 80% mango puree
and 20% papaya puree with 17 ºBrix. and having a sensory score of 93.1 was
found to be the best. |
