The effect of free convection on apparent vitamin degradation kinetics

Experimentally determined vitamin degradation kinetic parameters are vital input for process optimization, e.g., of nutrient rich beverages such as milk and fruit juices. The standard method of obtaining these parameters is to subject a test-tube of the nutrient (in its food matrix) to a water bath at elevated temperatures and measure remaining concentration as a function of time. It is well known that accurate kinetic fitting requires measuring the temperature inside the test tube as a function of time (due to lag it is not identical to the water-bath temperature), and this is included in contemporary studies. However, the importance of axial temperature gradients for valid kinetic parameter estimations has not been discussed to the same extent. This contribution combines experiments and physical modelling to show how an axial temperature gradient evolves in a test tube (R = 13 mm, Z = 75 mm), due to free convection currents (T∞ – T0 = 57–71 °C). Consequently, the test tube temperature is not uniform in such an experiment. Thus, the positioning of the thermocouple in a test-tube-in-water bath experiment is essential for ensuring a valid estimation of the kinetic parameters. Simulations suggest that positioning it half-way between top and bottom is a suitable best practice.