Preliminary study on the influence of turgor pressure on body reflectance of red laser light as a ripeness indicator for apples

A preliminary experiment was carried out to obtain information on the possible use of body reflectance of red laser light as a ripeness indicator for Jonagold apples (Malus domestica Borkh.). The influence of turgor changes on reflectance was especially investigated. Two groups of 20 Jonagold apples were stored at 20 degrees C and a relative humidity of 65 or 95% for a period of 16 days. The difference in storage conditions was designed to induce a difference in turgor pressure between the two groups. Every second day, different non-destructive techniques were used to monitor the change in fruit characteristics during storage. Two red light laser diode systems having different output powers (3 or 12.5 mW) and a characteristic wavelength in the chlorophyll absorption band, were used to measure optical reflectance. A computer vision system was used to calculate the number of pixels having an intensity between set threshold values. Fruit firmness was measured using the non-destructive acoustic response technique and a destructive puncture test. The colour of the apple skin was measured with a colorimeter. Decrease in apple stiffness was dependent on the mass loss, but independent of the rate of mass loss. There was no significant difference in colour change between the two groups, which indicates that they ripened at the same rate. The pixel number of the scattered and reflected laser image could be used as a ripeness indicator. Both the high and low power laser were suitable to perform the measurements. There was no significant influence of relative humidity during storage on the pixel count. The results suggest that the body reflectance was not directly dependent on mass loss and decrease in turgor pressure. It depended, however, on other factors changing with time, such as colour of the skin and loss of chlorophyll. The preliminary results presented in this paper should be validated by a full study on a larger number of apples under varying initial and experimental conditions. (C) 1999 Elsevier Science B.V. All rights reserved.