Analysis of the dynamic mechanical properties of apple tissue and relationships with the intracellular water status, gas distribution, histological properties and chemical composition

• A study of determinants of dynamic mechanical behavior of apple tissue was performed.
• Subcellular water status and distribution were linked to mechanical properties.
• Apparent microporosity was linked to mechanical properties.
• Different cell wall components impact viscous and elastic behavior independently.

Mechanical properties of fruit depend on many parameters, including microporosity, cell characteristics and cell wall composition/structure. Recent developments in quantitative MRI provide the possibility of studying fruit through local measurements of multi-exponential T2 reflecting water status and distribution at the subcellular level, and apparent microporosity. In the present study, dynamic mechanical analysis provided Young’s modulus and the damping factor, reflecting elastic and viscous properties of apple tissue, respectively. These measurements were compared to quantitative MRI measurements, biochemical analysis (water, free sugar and alcohol-insoluble solid contents, cell wall composition) and histological measurements, all performed on apple parenchyma tissue. The aim was to investigate the respective contributions of microporosity, intracellular water status and distribution, and chemical composition to mechanical properties characterizing fruit texture quality.Correlations between the measurements were studied for six apple cultivars with storage times varying from one to six months. A general behavior pattern independent of cultivar and storage time provided greater understanding of the parameters involved in mechanical properties. Cell wall arabinose composition and water status associated with the vacuole were correlated with Young’s modulus, the xylose and hemicellulose compositions were correlated with the damping factor and the relative amount of water attributed to the cytoplasm and the apparent microporosity of the samples were correlated with both the damping factor and Young’s modulus. Modification of the water pool attributed to the extracellular water/cell wall was shown to be involved in softening of the tissue. Comparison of the different measurements showed that variations in relaxation times were not explained by the water content, indicating that other phenomena such as membrane permeability, vacuole size and/or solute composition were predominant in the relaxation mechanism. A correlation between water pools attributed to the vacuole and cytoplasm evidenced water exchange occurring between the two compartments.