Analysis of changes in fruit tissue after the pulsed electric field treatment using optical coherence tomography

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Abstract
The pulsed electric field (PEF) is one of the non-thermal methods used in the food industry for prolonging food preservation or obtaining better quality of end products. The structure of fruit and vegetable tissues subjected to PEF treatment changes under the influence of short-term high voltage electrical impulses. In this process, the hydrophilic spaces in the cell membranes occur. The authors present the results of the assessment of structural changes in fruit subjected to PEF, using the textural analysis of sub-peel layers. The images were obtained by optical coherence tomography (OCT) at an infrared wavelength of 1300 nm. The OCT cross-sections revealed the zone of strong infrared light reflection from internal structures indicating the loss of parenchymatic tissue integrity. The intensity of these changes depended on fruit type and used parameters of PEF. It was shown that the increasing intensity of the electric field affecting the tissue structure of raspberry or grape fruit generally increases the entropy, standard deviation and the mean of their OCT images. Changes in these feature values are usually not proportional to the field strength (0, 3.3, 5 kV/cm) and depend on the depth below the fruit surface. The raspberry fruit is more sensitive to PEF because at the strength of 5 kV/cm the corresponding features of grape fruit behave similarly with 10 times more field pulses. The OCT method can be used to assess noninvasively the suitability of fruit for further stages of processing, e.g. in PEF assisted pressing of fruit juice.

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