Using mixed-effects models to estimate the effect of harvest date and its interactions with post-harvest storage regime on apple fruit firmness

The effects of harvest date and storage on apple (Malus pumila Mill.) fruit firmness have been studied using numerous cultivars. Many factors, such as cultivar, year, orchard location, and storage condition appear to confound the effects of harvest date, making it difficult to predict how firmness would change over time. Firmness in 'Minneiska' fruit, a cross between 'Honeycrisp' and `Minnewashta', was measured over several harvests at multiple locations in three States over 2 years, with harvests occurring over 3 - 4 weeks in each year. Linear mixed-effects models were used to isolate the effects of year, orchard, storage duration, and storage conditions on harvest date effects on firmness. The models indicated that harvest date had little effect on fruit firmness at harvest, regardless of orchard location, and the variance due to year was small. Storage had a large effect on firmness, but the differences between air storage and controlled atmosphere storage were not significant. Simulations were performed to determine the effect of sample size on fixed effect estimates, especially harvest date. The results of these simulations suggested that increasing the sample number beyond ten fruit per treatment had little impact on the estimates.