Physiological and biochemical aspects of vegetable processing. A case study on carrots

University dissertation from Food Engineering Deptartment, Lund University

Abstract: The aim of the present study was to enhance our understanding of vegetables as living organisms, interacting dynamically with the environment, and to explore the possible influence of metabolic changes on subsequent processing operations. Growing conditions, harvesting and handling in the packing house and storage conditions are key events leading to a defined “physiological status” of the vegetable that is going to be consumed as a fresh product or a processed product. Different events during the lifetime of vegetables are reviewed and analysed from the point of view of the physiological transformations taking place in each of them. Aspects of plant stress physiology were of particular interest. The physiological response to low-temperature stress, known as cold acclimation, was studied using carrots. The presence and accumulation of an antifreeze protein (AFP) in the carrot cell wall was used as a biological marker for the study of the induction of cold acclimation in carrot taproots. It was demonstrated that AFP gene expression was rapidly upregulated in response to low-temperature exposure. Furthermore, the induction of this gene was clearly affected by the temperature at which the taproots had been grown. The results also indicated that the development of cold acclimation in stored carrots, shown by the accumulation of AFP, was consistent with a high storage potential of the harvested taproots. The influence of cold acclimation on the mechanical strength of the harvested roots was studied. Tissue rigidity increased after 12 weeks of storage, at which time a higher accumulation of AFP was found. It was suggested that oxidative cross-linking between extensin proteins in the cell wall may be part of the mechanism behind the storage-induced firmness of carrots. The potential benefits of cold acclimation for the enhancement of quality upon freezing in the food industry are reviewed. The damaging action of a preceding blanching operation is discussed and the alternative of using mild blanching to minimize tissue damage was investigated. Isothermal calorimetry was used as a technique to quantify cell damage due to mild blanching of carrot slices. The results suggest that the mild blanching treatment, intended to inactivate the enzymes responsible for the development of off-flavours, may be accompanied by the inactivation of cell metabolism and hence, cell damage to about 70% of the cells in the carrot slices. Radiation in the far-infrared region was tested as a technological alternative to minimize cell damage. Radiation in the far-infrared region damaged cells to a depth of only 0.5 mm in the carrot slices, preserving cell integrity and most of the texture characteristics of the raw tissue. Basic scientific studies on the changes in vegetable cellular systems before harvest and after harvest and processing are necessary to provide the knowledge needed by food engineers on vegetables as biological materials. The use of this knowledge is of critical importance as a tool for process optimization in the food industry. The present study represents a contribution to this knowledge.

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