Chemical and biochemical changes of pumpkin (Cucumis moschata, Duch.) in relation to osmotic stress

MARINA DE ESCALADA PLA; MARISA DELBON; ANA M. ROJAS; LIA GERSCHENSON

JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE

Wiley Interscience

Lugar: Chichester; Año: 2005 vol. 85 p. 1852 - 1860

0022-5142

Raw tissue of a pumpkin (Cucumis moschata Duch) popularly known in Argentina as ?calabacita criolla? was rheologically characterized by large and small deformations after a stepwise adjustment of its turgor pressure to determine the relative contributions of turgor pressure, cell wall (CW) and middle lamella (ML) to the mechanical behavior. Microscopic observations (light microscopy, LM, and transmission electron microscopy, TEM) were performed in order to explain the textural behavior observed. In general, firmness at failure, residual relaxation force and dynamic moduli increased with turgor pressure. For all the hypotonic solutions (PEG concentrations lower than 250 mol m-3) a gain of tissue-weight was observed, trend that indicated a null occurrence of cell bursting as confirmed by observation through LM and TEM of undamaged cell membranes with swelled cytoplasms. Anyhow, no significative change in relative volume was detected for tissue equilibrated in the 0-130 mol m-3 PEG concentration range fact that might be related to the scarce elasticity of CW. Peaks of failure were observed during compression for tissue equilibrated at all osmotic conditions and their presence was associated with CW-integrity. Plasmolysed tissue did not show a residual force at infinite time of relaxation. Incipient plasmolysis was better detected by textural studies than by volume change and it was confirmed by LM. Significant correlations were obtained between residual relaxation force and either storage moduli or phase shift angle , revealing that relaxation out of linear viscoelasticity range and dynamic assays provide analogous information concerning tissue behavior.