Modelling the Rheological behaviour of tender coconut (Cocos nucifera L) water and its concentrates

The rheological behaviour of tender coconut (Cocos nucifera L) water was studied as a function of total soluble solid (TSS) content (5.3 to 52.9 °Brix) and its corresponding water activity (aw) (0.982 to 0.870) at a wide range of temperature (10 to 85°C) by controlled stress rheometer using concentric cylinders. The rheological parameter shear stress was measured up to the shear rate of 1000 s^-1. The investigation showed that tender coconut water and its concentrate behaved like a Newtonian fluid and the viscosity (η) was in the range 3.80 to 34.88 mPa s depending upon the concentration and temperature used. The temperature dependency on viscosity of tender coconut water was described by Arrhenius equation (r 0.96) and the activation energy (Ea) of viscous flow was in the range 5.268 to 20.798 kJ/mol depending upon the total soluble solid content. The effect of total soluble solid content on flow activation energy was described by exponential equation (r 0.92, rmse%=11.4, p 0.05) and that of water activity was described by power law equation (r 0.98, rmse %=5.54, p 0.01). The effect of total soluble solid content on viscosity of tender coconut water followed second order polynomial exponential equation (r 0.99, rmse % 3.98) at the temperature used. The effect of water activity on viscosity was described by both power law as well as exponential type relationship (r 0.99). The combined effect of temperature and total soluble solid content/water activity on viscosity was described by the equations η = 2.007x10^-2 exp (1710.5/T –0.0157C + 0.000728C²), (r 0.93, rmse%=3.95, p 0.05) η = 1.42 x 10^-2 (aw)^-11.217 exp (1705.96/T), (r 0.93, rmse%=3.95, p 0.05) η = 2891.7* exp (1704.20/T - 12.256aw), (r 0.93, rmse%=3.97, p 0.05) where η is viscosity in mPa s, T is temperature in °K and C is the total soluble solid content in °Brix, aw is the water activity (-).