Unravelling the mechanism that causes translucency in cut cucumber and tomato fruits.

MSc-thesis abstract (submitted 20 October 2014):
Translucency is a physiological disorder that affects the quality of many fresh cut products, among them are cucumber (Cucumis sativus L.) and tomato (Solanum lycopersicum L.) fruits.
It is suggested that flesh translucency is caused when the intercellular spaces are filled with liquid. It has been proposed for cucumber that the liquid that fills the intercellular space is from flesh cells that have lost their membrane integrity due to chilling injury. Whereas in tomato it is proposed that the locular gel is involved in translucency development.
So far the processes involved in translucency of cut fruits are not clearly understood. In order to increase the understanding of translucency, four experiments were done using cucumber and tomato fruit slices. Each experiment was repeated twice, one using 5mm and another using 2mm cucumber slices originated from different batches. Furthermore the data were analyzed independently.

The first experiment aimed to develop a scale for measuring translucency of cucumber slices; so cucumbers were cut and immersed (sunk) in deionized water. Later they were placed inside a vacuum chamber at different pressures in order to induce five different degrees of translucency clearly differentiated by eye. After that RGB images were taken using an image processing system. The parameter associated with the visual translucency evaluation was the sum of the R, G and B values using a black background (sRGBb). A fully translucent and non-translucent pericarp was associated with low and high sRGBb values respectively. The relationship found between sRGBb and the visual evaluation of translucency was exponential.

The second experiment aimed to evaluate the effect of acidic solutions (pH 4 and 6) on translucency of cucumber slices. The slices were immersed in acidic solutions obtained by combining solutions of either sodium citrate dihydrate with DL malic acid or NaCl with HCl in different ratios. Translucency was significantly affected by pH solutions and the mechanism by which these differences were caused seemed to be associated to membrane damage, based on the results of ion leakage. In the case of the acidic solutions of both compositions, the solutions of pH 6 caused a higher translucency and ion leakage than the solutions of pH 4.

The third experiment aimed to evaluate the effect of chilling temperatures and two concentrations of malic acid (33mM and 66mM) or sodium citrate (0.1M and 0.2M) on translucency of cucumber slices. The cucumbers were cut in half and each part of the cucumbers was stored in a fridge for three days at 4°C or 12°C. Later the cucumber parts were cut into slices and immersed in the solutions. Chilling temperatures did not have a clear effect on translucency in slices of both thickness. Increasing the concentration of malic acid from 33mM to 66mM did not have a significant effect on translucency. In the case of sodium citrate increasing the concentration from 0.1M to 0.2M had a significant effect. The translucency caused by the malic acid and sodium citrate treatments agreed with the results of ion leakage.

The fourth experiment aimed to evaluate the effect that tomato locular gel could have on translucency in cucumber slices. The treatments consisted in storing together a cucumber slice between two tomato slices of the same developmental stage without and with physical contact. Tomato fruits of green, breaker and red developmental stages were used. In the treatments without physical contact, the tomato slices of different developmental stages did not induce translucency. Translucency was developed in cucumber slices stored with contact between two tomato slices. The effect of tomato developmental stage was observed only in cucumber slices of 5mm thickness, where red tomato slices in contact with a cucumber slice caused the highest translucency in the last one. For these treatments the development of translucency agreed with the results of ion leakage.