Instrumental and Ultrasonic Techniques in Quality Evaluation of Fresh Fruit and Vegetables

Non-destructive ultrasonic pitch and catch ultrasound measurement of sound velocity was used to assess ripeness in ‘Envy’ apples during storage and to detect brown heart in swede. Ultrasonic group velocity was measured (path length over the transit time) through intact apples along the axial and radial directions of the mature and more mature fruit every two weeks for eight weeks at 4C and 20C. The velocity measurement was also conducted on the defective and non-defective Brown Heart (BH) swedes in an axial direction. Compression, puncture, and sugar level tests were also carried on the two maturity fruit groups, together with a puncture test on the vegetables. The differences between the ultrasonic velocity measured in the axial and radial directions in apples was significantly correlated with the firmness (as assessed by the compression and puncture tests) of the fruit and this is possibly due to increased homogeneity of the fruit during senescence.
The correlations between ripeness and ultrasonic velocity in apples, and BH and ultrasonic velocity in swede were supported by the hypothesis of changes of volume fraction of air-water in the parenchyma. The parenchyma of the ripening apple was suggested to have undergone changes of cell compositions of the starch-sugar conversion, cell walls disassembly, and middle lamella disintegration during storage. These changes caused the accumulation of air-water mixtures in the cells, indicating the ripening process in apples. The PCA clearly discriminated the ripening apples based on the weeks of storage (weeks 2 to 8), the maturity levels (mature and more mature fruit), and the orientations of ultrasonic velocity measurements (the axial and radial directions). Meanwhile, the defective BH was suggested to cause the increasing ‘water-core’ of the internal volume in swede parenchyma. This finding was supported by the dissimilar TPA curves between the BH and the healthy swedes. The ultrasonic technique offers an alternative online, fast, economical, non-destructive assessment of firmness for the apples at different ripening stages, storage durations, and storage temperatures. It may assess the fruit ripeness along the postharvest chain and can evaluate the presence and levels of BH of an individual swede. Therefore, this technique signifies cost savings and high standard quality in fruit and vegetables.