Bacterial Infiltration and Internalization in Fruits and Vegetables 451
14.5.1.3 Surfactants
The addition of surfactants to enhance the cleaning efficacy of wash water can affect
the extent of bacterial infiltration into fruits and vegetables. The influence of added
surfactants in wash water on the infiltration of tomatoes with E. carotovora was
studied (Bartz, 1982). Surfactants such as Triton X-100 and Tergitol-NPX were
added to water or to cell suspensions of E. carotovora to give concentrations ranging
from 0.001 to 1.0% weight/volume. Percentage weight increases in tomatoes were
0.23, 0.35, 0.38, 1.02, and 0.68 for surfactant concentrations of 0.0 (control), 0.001,
0.01, 0.1, and 1.0%, respectively. Weight increases in tomatoes were highly corre-
lated to surfactant concentration. Correlations were 0.96 and 0.99, respectively, for
two separate tests. The incidence of decay in tomatoes increased with an increase
in water uptake. Surprisingly, less infiltration resulted from using 1.0% surfactant
compared to a concentration of 0.1%. Tomatoes treated with water that contained
1.0% surfactant appeared to exude liquid from the stem scars following removal of
the fruits from the wash water (Bartz, 1982). This loss of liquid might have contrib-
uted to the relatively lower percentage weight increase in tomatoes treated with the
highest concentration of surfactant (1%) used in the study. Apart from infiltration
and subsequent decay of tomatoes by E. carotovora, tomatoes submerged in tap
water with added surfactant developed decay from naturally occurring spoilage
organisms on the fruit surface (Bartz, 1982).
The extent of bacterial infiltration into tomatoes is enhanced in the presence of
surfactant and hydrostatic pressure. Tomatoes immersed at a depth of 12 inches in
cell suspensions of E. carotovora without added surfactant were infiltrated by the
organism if exposure time was 10 min or more. Weight gain in the fruits submerged
for 0, 2, or 5 min was not significantly different. However, when tomatoes were
immersed at a depth of 27 inches in cell suspensions with added surfactant, infiltra-
tion of water and E. carotovora occurred within 2 min and increased dramatically
by 5 min (Bartz, 1982).
14.5.1.4 Type of Cooling System
The type of system used to cool fruits and vegetables may affect the extent of
bacterial infiltration into these products. Merker et al. (1999) suggested that the
potential for infiltration of bacteria (from contaminated water) into intact citrus fruits
may be increased when the fruits are placed in a dump tank or hydrocooler. Vigneault
et al. (2000) demonstrated that a laboratory-scale shower hydrocooler reduced the
temperature of tomatoes from 35 to 15°C within 13.3 min, whereas a flume cooler
gave the same temperature reduction in 10.5 min. In both cooling systems the
tomatoes gained weight due to infiltration of water. However, tomatoes cooled in
the shower hydrocooler exhibited larger increases in weight than those cooled in the
flume. Interestingly, significantly larger weight increases occurred in tomatoes with
an upward orientation of the stem scars under the shower. The authors suggested
that this increase in weight was due water continuously flooding the pores of the
stem scar. Increased uptake of water by tomatoes in a shower cooler could lead to
increased infiltration of bacteria from contaminated cooling water into the fruit.