TY - JOUR

T1 - Mechanical properties of apple skin are determined by epidermis and hypodermis

AU - Khanal, Bishnu P.

AU - Knoche, Moritz

PY - 2014/3/1

Y1 - 2014/3/1

N2 - Mechanical failure of the fruit skin is an early event in the etiology of the disorders russeting and skin spots in a number of apple cultivars including 'Elstar' (Malus ×domestica Borkh.). The objective was to quantify the mechanical properties of excised epidermal segments (ES) of fruit skin and of enzymatically isolated cuticular membranes (CM) using uniaxial tensile tests. ES thickness ranged from 0.25 to 1.8 mm because thin ES samples of more uniform thickness are difficult to prepare. Sample values for stiffness (S), maximum force (Fmax) and strain at Fmax (εmax) were recorded. Measured values were adjusted by regression to refer to a hypothetical standard ES of 0.5 mm thickness. Generally, S and Fmax values were positively related to ES thickness during the preharvest period from 51 to 141 days after full bloom (DAFB) and during the postharvest period from 141 to 259 DAFB in cold storage (1.78 °C, 92% relative humidity). The εmax recorded were independent of ES thickness. The S of a standardized ES decreased slightly from 51 to 90 DAFB, then increased up to 161 DAFB, and then declined. There were essentially no differences in S recorded for isolated CM and ES. The Fmax and εmax were highest in young fruit at 51 DAFB but decreased steadily toward harvest and continued to decrease in cold storage after harvest but at a lower rate. The Fmax and εmax were markedly lower for CM samples than for ES ones. Monitoring the increased incidence of CM microcracking during a tensile test performed on an ES revealed that CM failure preceded ES failure. The decrease in the Fmax for ES during fruit development was accounted for in part by a decrease in the mass of cell wall per unit surface area. Our results show that the epidermal and hypodermal cell layers represent the structural backbone of an apple skin during preand postharvest development. Furthermore, CM microcracking has limited relevance to the overall mechanical properties of the skin.

AB - Mechanical failure of the fruit skin is an early event in the etiology of the disorders russeting and skin spots in a number of apple cultivars including 'Elstar' (Malus ×domestica Borkh.). The objective was to quantify the mechanical properties of excised epidermal segments (ES) of fruit skin and of enzymatically isolated cuticular membranes (CM) using uniaxial tensile tests. ES thickness ranged from 0.25 to 1.8 mm because thin ES samples of more uniform thickness are difficult to prepare. Sample values for stiffness (S), maximum force (Fmax) and strain at Fmax (εmax) were recorded. Measured values were adjusted by regression to refer to a hypothetical standard ES of 0.5 mm thickness. Generally, S and Fmax values were positively related to ES thickness during the preharvest period from 51 to 141 days after full bloom (DAFB) and during the postharvest period from 141 to 259 DAFB in cold storage (1.78 °C, 92% relative humidity). The εmax recorded were independent of ES thickness. The S of a standardized ES decreased slightly from 51 to 90 DAFB, then increased up to 161 DAFB, and then declined. There were essentially no differences in S recorded for isolated CM and ES. The Fmax and εmax were highest in young fruit at 51 DAFB but decreased steadily toward harvest and continued to decrease in cold storage after harvest but at a lower rate. The Fmax and εmax were markedly lower for CM samples than for ES ones. Monitoring the increased incidence of CM microcracking during a tensile test performed on an ES revealed that CM failure preceded ES failure. The decrease in the Fmax for ES during fruit development was accounted for in part by a decrease in the mass of cell wall per unit surface area. Our results show that the epidermal and hypodermal cell layers represent the structural backbone of an apple skin during preand postharvest development. Furthermore, CM microcracking has limited relevance to the overall mechanical properties of the skin.

KW - Cracking

KW - Cuticle

KW - Extensibility

KW - Fracture

KW - Malus ×domestica

KW - Stiffness

KW - Strain

KW - Stress

UR - http://www.scopus.com/inward/record.url?scp=84896335234&partnerID=8YFLogxK

U2 - 10.21273/jashs.139.2.139

DO - 10.21273/jashs.139.2.139

M3 - Article

AN - SCOPUS:84896335234

VL - 139

SP - 139

EP - 147

JO - Journal of the American Society for Horticultural Science

JF - Journal of the American Society for Horticultural Science

SN - 0003-1062

IS - 2

ER -