Contribution of colloidal forces to the viscosity and stability of cloudy apple juice

Abstract

Cloudy apple juice (CAJ) was considered to be a dilute colloidal dispersion of electrically charged, hydrophilic particles in an electrolyte solution (serum). Experimental data of relative (CAJ/serum) viscosity as a function of particle volume fraction, η r( ϕ), was modeled as the sum of a ‘hard-sphere’ contribution ( η r hs ) plus a ‘colloidal forces’ contribution ( η r cf ) . Theoretical values of η r hs ( ϕ) were obtained with Einstein's equation for dilute suspensions of non-interacting, rigid spheres. Semi-empirical values of ( η r cf ) were found to be proportional to ϕ 1.22, lower than the theoretical ϕ 2. The difference was attributed to the effect of the energy barrier or activation energy between pairs of particles ( U Max). The value of U Max at each ϕ was obtained from the maximum of total interaction potential curves as function of inter-particle distance, U( x). In its turn, U( x) was modeled with the extended DLVO theory as the balance between attractive Van der Waals, repulsive electrostatic, and repulsive hydration energies. The term U Max was found to be a function of ϕ and the hydration pressure constant ( P 0), which was unknown for CAJs particles. This function was introduced in an empirical model proposed in this work, η r cf = α ( U Max / k B T ) ϕ , and correlated with semi-empirical values, giving α=0.483 and P 0=2.45×10 6 N m −2. According to this result, hydration forces (even reduced by hydrophobic interactions between pectin molecules) played the main role in the stability of CAJ particles.