Mixed models for repeated multicategorical response: modelling the time effect of physical treatments on strawberry sepal quality

Abstract

Generalised linear mixed models for multicategorical responses (GLMM) were applied to study the effect of a UV-C and light pulse treatment on the visual quality of strawberry sepals. GLMM works well to analyse repeated quality measures over time on the same subject. The concept of random intercepts and slopes allowed a description of the biological variability inherently present in batches of fruit. The linear mixed models were adapted for multicategorical response according to the threshold concept described in the literature. It was found that UV-C treatment slows down the quality decay of the sepals for doses up to 0.1 J/cm2, but when higher doses were applied the treatment became destructive and the sepals dehydrated and discoloured brown. Since the fungal growth on the strawberry fruit flesh is inhibited significantly starting from a dose of 0.05 J/cm2, an optimal dose of 0.1 J/cm2 is recommended to improve the quality retention of the strawberry including the visual aspect of the sepals. The pulsed light dose appeared not to influence the sepal quality.