Mode Of Action

Heat treatments can interact directly or indirectly with pathogens and/or fresh harvested produce via several responses. The efficacy of heat on pathogens is usually measured by reduced viability of the heated propagules [14,40]. Heat effects may be lethal or sublethal, and pathogen kill is not always proportional to the temperature-time product of the treatment [85,86]. Heat treatments may cause changes in nuclei and cell walls, denature proteins, destroy mitochondria and outer membranes, disrupt vacuolar membranes, and form gaps in the spore cytoplasm, which lead to reducing inoculum level [14].

Applying HWRB to melon and citrus fruits resulted in a 3 to 4 log reduction of the total microbial colony forming units (CFU) of the epiphytic microorganism population, compared to untreated control fruit [27,30,34]. Scanning electron microscopy (SEM) showed that HWRB removed fungal spores from the fruit surface, and partially or entirely sealed natural openings in the epidermis (Figure 20.2) [25,27,30,33]. As a result of heat treatments that reduce fungal viability, the effective inoculum concentration that causes decay development is reduced, thus reducing rot development [86]. In addition, sealing epidermal cracks with heat treatment could reduce sites of fungal penetration into the fruit, thus reducing decay incidence [13]. Schirra and D'Hallewin [62] and Ben-Yehoshua [64] reported that hot water dips of grapefruit and mandarins redistributed the epicuticular wax layers, which sealed or partially sealed cracks, thus improving physical barriers to pathogen invasion.

Hot water immersion or rinse was found to inhibit ripening processes as measured by relatively low respiration rate and ethylene evolution, and slow color development, compared with nonheated control fruit. In addition, heat treatment prevented postharvest geotropic curvature of vegetables [13,87,88]. Fruits that ripen slowly have less susceptibility to fungal attack during storage [47]. Ben-Yehoshua [64] reported that heat treatment induced resistance of grapefruits to decay caused by P. digitatum by delaying the breakdown and disappearance of preformed antifungal compounds. The heated citrus fruit had higher concentrations of the phytoalexin scoparone, which in turn was correlated with antifungal activity in the fruit extract [64]. However, a 10-minute hot water dip caused a faster decrease of antifungal compounds and the earlier appearance of rot symptoms in treated avocado, compared to nonheated fruit [89].

Hot water treatments caused a delay in spore germination and fungal growth in citrus fruit [37]. This was explained by a building up or improvement

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