Cucumber-softening rates

Effect of Monovalent Ions on Cucumber Softening Rates... 

Once it had been demonstrated that NaCl enhanced the rate of cucumber tissue softening, the next question was whether and in what way calcium ions would inhibit the rate of softening. To answer this question a combination of 1.5 M NaCl and 0 to 80 mM calcium ions were added first to blanched cucumber slices and then to cucumber mesocarp tissue (47) (Fig. 5). In both experiments there was an excellent fit of a hyperbolic curve to the softening rates as a function of calcium added, since the hyperbolic model accounted for over 99% of the experimental variation. For the cucumber slices (Fig. 5a), half of the observed inhibition of softening rate occurred at 6.3 mM calcium. For the mesocarp pieces (Fig. 5b), half maximal inhibition occurred at 1.5 itiM calcium ion. These results indicated that even in the presence of high NaCl concentrations low calcium ion concentrations could saturate some binding site that resulted in inhibition of texture loss. 

Fig. 4. Effect of NaCl concentration on the rate of cucumber tissue softening in acidic conditions, a) Softening rates of cucumber slices, b) Softening rates and pectin methylation changes in cucumber mesocarp tissue pieces. From reference 43.

Fig. 6. Relationship between the natural concentration of calcium ions in cucumbers and softening rates of cucumber tissue in 0.6% acetic acid and 1.5 M NaCl. Fran reference 47.

Fig. 8. Comparison of the inhibitory effect of 10 mM lanthanide ions on cucumber tissue softening rates relative to the inhibitory effect of calcium ion. Fran reference 47.

Now let us consider recent results from this laboratory concerning the effects of metal ions on the rates of texture changes in acidified cucumber tissue texture. To investigate the effect of NaCl on cucumber texture in the absence of enzymatic degradation, the firmness changes in blanched tissue under acid conditions, pH 3.2-3.8, were measured (43). The procedure was to blanch cucumber slices or mesocarp tissue pieces in boiling water for 3 min to inactivate pectinesterase (19). After cooling, the tissue samples were covered with a brine which contained acetic acid and S02, to equilibrate at 0.6% and 200 ppm, respectively, and the appropriate amount of NaCl. The tissue equilibrated with the brine in the cold and then samples were incubated at 44C so they would soften reasonably rapidly. Firmness of the tissue was evaluated at the... 

Table II. Effect of 0.25 M alkali metal ions on the rate of softening of blanched cucumber mesocarp tissue held at 44°C...

---