Unsteady 2D natural convection driven sterilization of a non-Newtonian liquid inside a square container caused by external natural convection of a non-Newtonian fluid has been studied numerically. Carboxy-methyl-cellulose was chosen as the internal non-Newtonian liquid food and external heating fluid, both treated in terms of a Power Law model. Conjugate convective fluid and heat transport, described in terms of non linear coupled continuity, momentum, and energy equations, were solved by using the finite volume method with the SIMPLE algorithm. Time evolution for velocity and temperature distributions for inner liquid food and outer fluid flow are described. The effect of assuming pseudoplastic (n = 0.6), Newtonian (n = 1.0) and dilatant (n = 1.4) fluid models on fluid dynamics, natural convection and sterilization is analyzed. (C) 2013 Elsevier Ltd. All rights reserved.