450 Produce Degradation: Reaction Pathways and their Prevention
suggested that infiltration by this pathogen may be partly linked to capillary action,
which pulled the inoculum into the apple core over an extended period of time (18 h).
Therefore, if the wash water for fresh produce is contaminated with pathogenic
bacteria, the internal tissues of these products will most likely become contaminated
following ingress of water.
The infiltration of bacteria with water taken up by warm fresh produce immersed
in cold wash water can be explained by the general gas law. As the temperature of
fruits and vegetables is lowered the gases trapped in their tissues contract and create
a reduced internal pressure. This reduction in pressure forces the combined atmo-
spheric and hydrostatic pressure on the produce to equilibrate with the internal
pressure and thus permits infiltration of water (Bartz, 1982). Zhuang et al. (1995)
demonstrated that significantly higher populations of Salmonella Montevideo infil-
trated the core tissue of tomatoes (25°C) immersed in a cell suspension of the
organism at 10°C compared with tomatoes immersed at 25 or 37°C. Relatively warm
(26 to 40°C) tomatoes submerged for 10 min or longer in cool (20 to 22°C)
suspensions of Serratia marcescens, Erwinia carotovora, Pseudomonas aeruginosa,
or Pseudomonas marginalis were infiltrated by these bacteria (Bartz and Showalter,
1981). Infiltration of bacteria was attributed to a negative temperature differential
between the water and the tomatoes. The creation of a positive temperature differ-
ential (water temperature higher than that of the immersed product) can decrease
the extent of bacterial infiltration in fruits (Bartz, 1981, 1991; Zhang et al., 1995;
Burnett et al., 2000).
14.5.1.2 Hydrostatic Pressure
Fruits and vegetables deposited in dump tanks can endure various levels of hydro-
static pressure depending how deeply they are submerged in the wash water. Bartz
(1982) demonstrated that certain levels of hydrostatic pressure increased infiltration
of E. carotovora in tomatoes. Tomatoes were immersed in water or in E. carotovora
cell suspensions in a 19-L pressure cooker at different pressure levels of air entering
the sealed vessel to simulate various depths in water. Tomatoes immersed in the
aqueous cell suspension with no hydrostatic pressure did not gain weight or develop
decay (soft rot). Tomatoes subjected to a hydrostatic pressure of 24 inches for 2 min
exhibited no detectable increase in weight but had a 20% incidence in decay,
indicating infiltration of E. carotovora. Based on this result even a slight infiltration
of water into fruits could lead to bacterial internalization. A hydrostatic pressure of
24 inches for 10 min resulted in an average weight gain of 0.1 g/fruit and a 40%
incidence of decay. In contrast, an increase in hydrostatic pressure to 48 inches for only
1 sec resulted in a weight gain of less than 0.1 g/fruit but there was a 70% incidence
of decay. It is likely that internalization of bacteria is initiated almost instantaneously
when hydrostatic pressure is applied to the surfaces of fruit (Bartz, 1999). The impact
of hydrostatic pressure on bacterial infiltration in fruits is relevant to situations in which
packing houses may overfill dump tanks to maintain efficient operation of the packing
line. In overfilling dump tanks some of the fruits will be submerged under many layers
of fruit and remain in the water for extended periods of time. Bartz (1999) suggested
that such situations are likely to lead to infiltration of wash water.