Dynamic Force and Deformation

Impact. The impact response of a product is directly related to its mechanical properties, mass, and shape. Impact is the rapid collision of two objects, whether one or both objects are in motion. Impact testing techniques include drop, falling mass (or impact probe), and impact ram. A number of impact parameters have been proposed to measure horticultural product firmness, including peak force and time in contact with the impacting object. Impact responses have been studied for apples, blueberries, cherries, kiwifruit, peaches, pears, potatoes, and tomatoes (4446). One potential problem with impact tests on fruits and vegetables is that some bruising may occur.

Vibration. Sonic (or acoustic) vibrations encompass the audible frequencies from 20 Hz to about 15 kHz and ultrasonic vibrations are above that range. When an object is excited by periodic forces, such as at sonic frequencies, or by certain types of impact it vibrates. Resonance (maximum vibration) occurs at certain frequencies that depend on the mechanical properties, size, shape, and density of the product. The firmer the flesh, the higher the resonant frequency for products of the same size and shape. The traditional watermelon ripeness test is based on the acoustic principle, where one thumps the melon and listens to the pitch (frequency) of the resonance. Resonant frequency, wave propagation velocity, attenuation, and reflection are the important parameters for evaluating texture of horticultural commodities. The sonic vibration method is nondestructive and is suitable for rapid firmness measurement. Sonic measurement generally represents the mechanical properties of the entire product, unlike puncture or compression tests that sample localized tissues. Sonic measurements are excellent for following changes in individuals over time in research applications and are suitable for determining average firmness of grower lots of fruit. Sonics may not be suitable for sorting operations as they have not always proved capable of predicting firmness of individual fruit as determined with a penetrometer (4753). Some of the fruit tested have been apples, avocados, bananas, grapes, kiwifruit, head lettuce, mangoes, melons, peaches, pears, pineapples, and tomatoes.

Ultrasonic measurement has generally not been successful for quality measurement on fruits and vegetables despite its success in medicine and animal studies. The structure and air spaces in fruits and vegetables make it difficult to transmit sufficient ultrasonic energy through them to obtain useful measurements (54). Wave propagation velocity, attenuation, and reflection are the important ultrasonic parameters for quality evaluation of horticultural commodities. Skin texture of oranges and cracks in tomatoes could be evaluated by reflectance and backscat-ter, respectively (54). Hollow heart in potatoes was detectable (55), but bruises in apples could not be readily detected (56). Ultrasonic measurements correlated well with firmness or ripeness of melon and avocado (57). However, a more powerful ultrasonic source is required to penetrate most fruits and vegetables.

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