Fundamental Studies on Mechanical Citrus Fruit Harvesting (I)

Biophysical Properties and Qualities of Citrus Unshiu Related to Detachment

Abstract

For the improvement of fruit removal equipment, engineers need a better understanding of the biological and physical properties of citrus fruits to establish their harvester design parameters. The purpose of this research is to obtain quantitative and correlative information about Citrus Unshiu (Satsuma Orange) on detach ment force, torsional moment for shearing stem, coefficient of friction at fruit-rind, the effect of growth regulator on hastening fruit abscission and the qualties of fruits when an external force is exerted upon them.
1) The fruit detachment force by the tensile testing instrument was 3-8kg, average 5.35kg, and the correlation between detachment force and fruit weight (fruit size) was recognized. But the mature Citrus Unshiu's rind was so tender and apt to float that a portion of peel was torn with stem. This was observed more than 50% of total fruits in some variety during detachment. Therefore, it is not suitable for harvesting fruits to detach by only tensile force. The leaf detachment force was 1-2.3kg, average 1.58kg, but leaf removal during vibratory harvesting was nothing to be anxious about, because leaf mass was too small in comparision with fruit mass.
2) In order to accelerate chemical control of fruit abscission, fruits was soaked in Ethrel (2-chloroethyl-phosphonic acid) solusion (48×10^-4mg/l). The effect of Ethrel on detachment increased in proportion to the days elapsed since treatment date, and detachment force became under 2kg 8 days later. This fact may enable us to predict fruit detachment without tearing rind, because the minimum tensile strength of fruit-rind is 2-3kg.
3) Torsional moment used to shear the stem of Citrus Unshiu was 0.5-4.9kg-cm, average 1.1kg-cm, and it correlated with fruit size or stem diameter. If fruits were detached by only torsional operation, about 300 degree of twist angle was required to shear the stem.
4) Statical coefficient of friction between fruit rind of Citrus Unshiu and Acryle sheet, Lauan board was above 0.8. It may be considered to use the frictional resistance of fruit-rind as the binding force to support fruit during detachment operation.
5) When fruits were subjected to drop repeatedly to ground just after sampling, their acid was remarkably decreased and their sugar was increased in proportion to days elapsed since dropping treatment. Therefore, Brix-acid ratio was rapidly increased but the quality of fruits was fast deteriorated, too. These fruits will not be adapted for storage.