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 oBrix) and its corresponding water activity (aw) (0.982 to 0.870) at a wide range of temperature (10 to 85oC) 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 equationsη = 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 oK and C is the total soluble solid content in oBrix, aw is the water activity (-).