Changes in the mechanical properties of white seedless grapes during different maturity stages

Grapes، both in ripe and unripe forms، are used to produce different food products. This fruit is much sensitive to mechanical damage because of their soft texture and thin skin. This study investigates the effect of the period of maturation of white seedless grapes (Vitis vinifera L.) on changes of some mechanical properties of grape and its skin and flesh. Changes in the value of toughness، ultimate strength، breaking force and energy، stiffness and modulus of elasticity were determined in grapes harvested from a local vineyard during ripening stage. Simultaneously with increasing ºBrix and decreasing titrable acidity during ripening، the value of true density of the fruits increased from 1. 073±0. 004 to 1. 106±0. 007 g/cm3. Both the value of ultimate strength (σpu) and toughness (Upu) of grape flesh، jointly increased in maturation stage with a positive correlation (R2=0. 8626، P=0. 0025 at a confidence level of 0. 99)، while the value of modulus of elasticity decreased in the first weeks during ripening then increased in the last weeks of the period. All parameters obtained from tensile test of the skin، including ultimate strength، modulus of elasticity and toughness decreased in the maturation stage. In this period، maximum and minimum of the fruit’s skin ultimate strength were calculated by 3. 176 and 2. 011 MPa، respectively. Also modulus of elasticity of the skin decreased from 9. 515 to 6. 127 MPa and toughness changed from 0. 876 to 0. 413 mJ/mm3 for the duration of ripening. Resistance to rupture (Rupture force) of whole berries was measured using compression test ranged from 7. 80 to 9. 18 N. The results of penetration test on the whole fruit showed that breaking force، breaking energy and stiffness decrease throughout ripening.