SIMULTANEOUS INSTRUMENTAL MEASUREMENT OF FIRMNESS AND JUICINESS OF APPLE TISSUE DISCS

• Published in: Journal of texture studies Vol. 34; no. 3; pp. 271 - 285
• Main Authors: HARKER, F. ROGER, WHITE, ANNE, FREETH, BRUCE, GUNSON, F. ANNE, TRIGGS, CHRIS M.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.08.2003; Blackwell
• Subjects: Biological and medical sciences; Food industries; Fruit and vegetable industries; Fundamental and applied biological sciences. Psychology; Apple; Fruit; Mechanical properties; Firmness; Measurement method; Fruit juice; Maturity; Cultivar
• ISSN: 0022-4901; 1745-4603
• DOI: 10.1111/j.1745-4603.2003.tb01061.x

Discs of apple tissue were compressed to 75% of their original height. Throughout compression, the force‐distance curve was collected and the electrical impedance of the discs was measured at two frequencies of alternating current—1 kHz and 1 MHz. Electrical impedance was separated into its resistive and reactive components, and at these particular frequencies changes in resistance predominated. Measurements at 1 kHz indicate the resistance of extracellular regions of the discs (ruptured cells as well as those regions external to the plasma membrane), while measurements at 1 MHz indicate the resistance of the entire disc (combined intracellular and extracellular regions). Juice was released from the discs as a result of damage to cells and the extrusion of cellular fluid into intercellular air spaces. This resulted in a decline in electrical resistance at 1 kHz, but little change to the resistance at 1 MHz. Changes in juice release as determined by electrical measurement were related to the mechanical properties of the discs. Generally, the release of juice occurred after the inflection point on the force‐distance curves, but much earlier than mechanical failure (indicated by maximum force). The extent of tissue damage was determined from the relative decrease in resistance at 1 kHz, and was found to vary among apple cultivars and in response to fruit ripening.