Nuclear magnetic resonance relaxometry and imaging

Nuclear magnetic resonance (NMR) relaxometry and imaging have been evaluated as a technique to measure mealiness. Barreiro and colleagues used magnetic resonance techniques to assess mealiness in apples (Barreiro et al., 1999) and peaches (Barreiro et al., 1998b). Magnetic resonance techniques rely on the magnetic properties that some atomic nuclei have. When placed in a magnetic field, the natural magnetic dipoles of the nuclei reorient themselves along the magnetic field. After excitation they return to their equilibrium position. The rate at which this happens can be expressed by two relaxation times (T1 and T2) and is a function of the texture of the material (Smith and Lange, 1998). Barreiro et al. (1999) found that the variability of the T2 values inside an apple was larger than that between apples. However, a difference between the average T2 value of fresh apples and that of apples stored in mealiness-enhancing conditions was noticed; minimum T2 values were shown to be significantly lower for mealy apples than for fresh apples, indicating that a more desegregated structure and a lower juiciness content lead to lower T2 values. Furthermore, they also found that the T2 maps of mealy apples showed a regional variation of contrast which was not shown for non-mealy apples. Not all results found for apples were similar to those found for peaches (Barreiro et al., 1999; Barreiro et al., 2000). This might indicate that the development of mealiness in both fruits is caused by different underlying physiological changes. The magnetic resonance images of mealy apples also showed a regional variation of contrast which was not the case for non-mealy apples (Barreiro et a/., 2000). This variationofcontrastwassimilartotheNMR images of apples with internal breakdownalthoughthecontrast wassmaller.

9.8 Near-infrared reflectancespectroscopy

Near infrared (NIR) spectroscopy has been evaluated in the past as a nondestructive measurement technique for measuringJonagoldapple qualityattrib-utes, like soluble solids, pH and firmness (Lammertyn et al., 1998). Experiments with NIR reflectance spectroscopy indicated that mealiness could be measured in a non-destructive way (Nicolai et al., 1999). A specialfixedsample-presentation module, developed for an existing spectrophotometer (Fig. 9.3) allows rapid acquisition of diffuse reflectance spectra by simply putting the unpeeled apple or another fruit on top of an optical window, supported by self-centring mechanics. However, as reported in Nicolai' et al. (1999), the calibration models contain many principal components and need to be improved further for mealiness prediction.

9.9 Aroma, sugar and acid analysis

Because cells of mealy tissue do not break during mastication, the flavour compounds are not liberated. This may explain the fact that the aroma of mealy apples

Fig. 9.3 Near infrared (NIR) spectrophotometer with sample presentation accessory.

was perceived by the sensory panels to be at a lower intensity compared to fresh apples. However, gas chromatographic measurements of the headspace when investigating the aroma compounds of Cox and Jonagold apples revealed that the concentration of aroma compounds in the static headspace even increased in mealy apples while the aroma determined by sensory panel clearly decreased. Sugars and organic acids were measured by means of high performance liquid chromatography (HPLC), for both non-mealy and mealy apples. A linear relationship between malic acid content and sensory mealiness was observed (Nicolai et al., 1999).

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