Attempts to correlate sensory measurements with more objective instrumental measurements go back to the beginning of the century. Researchers have always wanted to relate what a person perceives to the physical variables which can be measured by so-called objective means (Moskowitz, 1983). Szczesniak and Ilker (1988) and Casutt et al. (1994) correlated sensory perception of juiciness with experimental parameters. They found that juiciness was a multifactorial attribute positively correlated to the amount of juice present in the food. Sensory perceived juiciness was negatively correlated with mealiness. Finney (1971) used sensory and instrumental tests to evaluate Red Delicious apples. He observed that sensory firmness correlated better with Magness-Taylor firmness than with sonic firmness measurements. Diehl and Hamann (1979) concluded from instrumental and sensory studies on Red Delicious apples that the sensory crispiness was directly related to the modulus values from uniaxial compression tests and torsion tests, while graininess, a measure of mealiness, was inversely related to the modulus values and the shear stress at failure in torsion. In general, graininess was a texture parameter which gave negative correlations with the other sensory parameters used to describe the apples. Harker et al. (1997) found a curvilinear relationship between sensory perceived hardness and tensile strength, puncture strength and shear strength for Royal Gala apples. Paoletti et al. (1993) tried to discriminate among apple cultivars and different levels of quality for their mechanical and texture characteristics by instrumental and sensory methods. They found high correlation values between most of the instrumentally and sensory measured parameters.
The potential of several instrumental techniques in the study of mealiness in apples will be discussed further. First, microscopic images of mealy tissue will be compared to those of non-mealy tissue. Subsequently, the confined compression test will be described as a destructive measurement technique to measure mealiness. Finally, the potential of several non-destructive techniques to measure or detect mealiness will be assessed: ultrasonic wave propagation, nuclear magnetic resonance relaxometry and imaging, NIR reflectance spectroscopy, aroma analysis, acoustic impulse response technique and electrical impedance.
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