A comparison of mesocarp cell dyeing between ‘Golden Del. Reinders' apples with different flesh texture

Abstract：【Objective】‘Golden Del. Reinders' is a mutation of‘Golden Delicious' and has the advantages of high yield, large fruit, and very low susceptibility to rusty. However, the texture of the fruit can be influenced by many factors and is prone to change during storage, which affects the appearance quality and limits the consumers' acceptance. The fruit texture can be divided into hard, soft and extremely soft types with sensory evaluation and hardness test methods. But the evaluation results may be unsuitable because of differences in apple varieties and in evaluation criterions. In order to further distinguish the different textures of‘Golden Del. Reinders', we detected the apple fruit texture by hand-slices, staining with various dyes, sensory evaluation and hardness test, and distinguished the texture of‘Golden Del. Reinders' from the perspective of pulp cell activity and provided a new method to evaluate the distinctive texture.【Methods】The observation of the morphology of cells and tissues under an optical microscope is the most simple and direct method. The apple tissue and morphology of cells under three different treatments, including natural water loss in the normal environment of storage, heat treatment in the boiling water bath, and dry treatment that accelerated the loss of water, were observed. Three dyes including red ink, Evans blue and FDA were used, and the dyeing flake slices (FDA staining was observed using the excitation light of 488 nm and emission light 530 nm under a fluorescence microscope) could show different textures of apple. By observing the differences of the colors resulted from the staining with 5% red ink, 0.5% Evans blue diluted by 1 280 times, and 0.01% FDA dyeing, we speculated the correlation between the fruit texture and the cell activity. Image-Pro Plus was used to quantitatively analyze the color values of the mesocarp (cells) after staining, which reflects the activity of pulp cells and textures more concisely and clearer. At the same time, the accuracy of the test could be ensured with parallel experiments, and the average value of the test was selected for statistical analysis to improve the test accuracy.【Results】We divided the fruit of‘Golden Del. Reinders' stored at 4 ℃ for two months into three groups based on fruit texture determined by sensory evaluation, which were the hard, the soft and the extremely soft groups. The highest scores were found in the hard group, fruit of which were delicate, chewy, crispy and full of juice. The soft group was moderate, with pulp easily chewed, poor taste and little juice. Because of the diversity of fruit texture in‘Golden Del. Reinders', the textures could not be accurately distinguished simply by the hardness value. With the softening of the apple fruit, single layered cell arrangement was observable; cell deformation became more obvious; and cell layers decreased. The color value of the fruit in the hard group differed significantly from the other groups, indicating that it was higher in the active pulp cells than in the deactivated cells. The pulp cell activity in all groups decreased gradually. After FDA staining, the fluorescence intensity in the pulp of the hard group was high, and the pulp of fruit under boiling water bath treatment and drying treatment showed no fluorescence. The result indicated that FDA could be utilized to detect the activity of apple pulp cells. As the texture of the fruit changed from hard to soft and to extremely soft, the pulp cells became separated with increased intercellular space. The fluorescence became partially concentrated at the edge of the cell and the intracellular fluorescence turned weaker, indicating that the activity of the pulp cells was decreasing. These phenomena were consistent with the results of red ink and Evans blue staining.【Conclusion】In summary, the FDA was the best way to distinguish different textures of‘Golden Del. Reinders' among the three dyes tested and the hard pulp cells had high activity. During softening of the pulp, cell activity reflected by FDA fluorescence value decreased.