The effect of Ca2+ and cellular structure on apple firmness and acoustic emission

Abstract

This study presents the influence of calcium lactate treatment (0–6 %) and cellular composition on two mechanical attributes: firmness and total acoustic emission (AE) events registered in the puncture test of apple tissue. The experiment was performed on five apple cultivars stored for nine months in a normal or controlled atmosphere. The microstructure was characterized quantitatively on control samples by confocal scanning laser microscope followed by image analysis. The mean area and perimeter of detected objects, which were either cells or spaces, and estimated cell wall fraction were used for the characterization of the tissue microstructure. Treatment with increased Ca2+ concentration caused a significant increase in firmness and total AE events. The increase was more pronounced in the case of total AE events which better reflected an increase in brittleness of apple tissue than firmness parameter. The effect was particularly positive in the case of very soft apples (stored in a normal atmosphere) due to greater extent of de-esterification of homogalacturonan. Analysis of the results, together with a review of the literature, suggests that calcium importantly changes the cracking mode of tissue, from intercellular debonding to cell wall rupturing. Firmness and the total AE events significantly relate negatively to the object’s size and positively to estimated cell wall fraction. A rough accordance of the number of split cells open across cell walls by puncture probe and the total AE events registered in the test, which was saturated in high Ca2+ concentrations, was noticed.