Model selection for oxygen consumption by azalea cuttings and Echinacea plantlets in a closed system

In the Agro-Food Industry, equilibrium modified atmosphere packages have proven their usefulness in prolonging the shelf life of minimally processed fruits and vegetables while minimizing quality loss. However, the ornamental industry can also benefit from the advantages of modified atmosphere packaging. For an efficient selection of the right packaging materials and dimensions, it is of utmost importance to know the respiration rate of the plants at the desired storage conditions. Based on oxygen concentration measurements as a function of the time in hermetically closed bottles containing azalea cuttings or Echinacea plantlets, several model equations were fitted and compared. Bottles were simultaneously stored at five different temperatures (2, 5, 8, 10 and 14°C for azalea and 15, 20, 23, 27 and 31°C for Echinacea). The initial headspace composition consisted of 10% oxygen + 90% nitrogen. The most optimal model for each species was selected based on 1) the significance of the model coefficients, 2) (over)fitting by predicted residual error sum of squares and Akaike’s Information Criterion, and 3) distribution of the standardized residuals as a function of the fitted values. In a second stage, the effect of the temperature on the model coefficients was estimated with an Arrhenius equation and a single nonlinear mixed-effects model was developed per plant species. For both plant species, a third-degree polynomial with temperature-dependent coefficients performed best according to the corresponding AIC-values. Taking into account a random effect for the intercept and the linear coefficient further improved the models. In the case of Echinacea, the model could be optimized further by adding a random effect for the quadratic coefficient.