ABSTRACT
References ................................................................................................................ 75
In spoilage of fresh-cut fruits and vegetables, two major patterns can be defined, which are different but influence each other. A distinction needs to be made between physiological spoilage (due to enzymatic and metabolic activity of the living plant tissue) and microbiological spoilage (due to proliferation of microorganisms). Processing fresh fruits and vegetables removes the natural protection of the epidermis and destroys the internal compartmentalization that separates enzymes from substrates. Consequently, plant tissues suffer physical damages that make them much more perishable than when the original product is intact (Artés et al. 2007). Moreover, processing results in a stress response by the produce characterized by an increased respiration rate (wound respiration) and ethylene production, leading to faster metabolic rates (Rosen and Kader 1989; Howard et al. 1994; Watada et al. 1996; Saltveit 1999; Kang and Saltveit 2002; Knee and Miller 2002; Laurila and Ahvenainen 2002; Surjadinata and Cisneros-Zevallos 2003; Artés et al. 2007). In addition to these changes in metabolic rates, damage of the plant tissue leads to exposure to air, desiccation, and the bringing together of enzymes with substrates, all leading to quality degradation (Klein 1987; King and Bolin 1989; Roura et al. 2000; Knee and Miller 2002). Next to physiological processes, the release of nutrients on the cut surfaces allows the growth of microorganisms. Under natural conditions, the outer layer of the plant tissue consists of a hydrophobic surface providing a natural barrier for microorganisms (Lund 1992). Due to damage of the surface, nutrients are released from the plant tissue, which can be used by microorganisms as shown by Mercier and Lindow (2000) after inoculating Pseudomonas fluorescens on different leaves of vegetable plants. Densities of microorganisms were directly correlated with the amount of sugars present on the surface of leaves, and these sugars were the limiting factor with regard to colonization. Related to the presence of damaged areas, Babic et al. (1996) found that microbiological populations were situated on cut surfaces of spinach after 12 days of storage at 10°C. Brocklehurst and Lund (1981) found after inoculation of celery, that soft rot could not be caused on unwounded tissue, possibly due to limited proliferation. Damaged spots on plant tissue thus provide a better substrate for microbiological growth by providing nutrients (King et al. 1991; Zagory 1999). Some of the microorganisms produce pectinolytic enzymes degrading texture and as such provide more nutrients for microbiological activity (e.g., during soft rotting, spoilage of leafy vegetables such as fresh-cut lettuce). Commodities that are susceptible to a high degree of nutrient release will result in intense microbiological proliferation (e.g., during spoilage of intensively cut fruits and vegetables such as cucumber cubes, zucchini slices, and melon parts). Moreover, high microbial loads can induce increasing respiration rates of the produce as shown by Saftner et al. (2006) in the case of fresh-cut melon. Yeasts and molds, which are less sensitive to a low pH, will develop on fruits with lower pH (e.g., strawberries, raspberries, and nectarines), while on the more pH-neutral vegetables, bacterial growth can be detected.
