Objective method for measuring firmness of iceberg lettuce

Traditional hand compression firmness scores of iceberg lettuce (Lactuca sa tiva L.) heads were compared with force-deformation data collected from par allel-plate compression tests conducted with a universal testing machine. S ample deformation was measured over a load range of 30 to 40 N. A quadratic response surface best described the relationship between hand firmness sco res (1 to 5 scale) and three measurements of sample deformation (mm). Sampl e deformation was as precise as hand compression in measuring lettuce firmn ess, and it provided improved reproducibility by eliminating much of the hu man error. Although adequate for most firm heads, the predictive ability of the statistical model was weak for soft heads (when the hand firmness scor e was <2), and for heads with inconsistencies in firmness because of uneven leaf distribution. The minimum sample size required to determine accuratel y the mean firmness score (+/-0.5 units) of a population of harvested lettu ce was approximate to 20 heads. This may be a disadvantage, since sampling one head requires approximate to 1.5 minutes. Overall, the instrument-based method measures lettuce firmness as precisely as the hand compression meth od, and provides a standardized, objective measurement for postharvest rese archers when exchanging or reporting firmness data.