Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

434 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
Table 20.6
Examples of SPR analysis of contaminants commonly found in fruit and vegetables
Analyte Limit of detection Reference
Salmonella enteritidis, Listeria monocytogenes 10 6 cells/mL Koubová et al. (2001)
Salmonella groups B, D, and E 1.7 × 10 3 cfu/mL Bokken et al. (2003)
Staphylococcal enterotoxin B 0.5 ng/mL Homola (2003)
Staphylococcal enterotoxin B 1.0 ng/mL Nedelkov et al. (2000)
Salmonella typhimurium 10 2 –10 9 cfu/mL Oh et al. (2004)
Atrazine 0.5 mg/mL Minunni and Mascini (1993)
Atrazine 1–100 mg/mL Nakamura et al. (2003)
2,4-D, 2,4,5-T 0.1 mg/mL Schlecht et al. (2002)
contamination of food is a frequent occurrence. Leonard et al. (2005) developed a rapid
SPR-based detection method for the detection of L. monocytogenes. A polyclonal antibody
was generated to detect Internalin B (InlB), a protein on the surface of L. monocytogenes
that participates with Internalin A (InlA) in the invasion of mammalian cells. An inhibition
assay was then used to monitor the presence of cells in solution. This involved the incubation
of the specific polyclonal antibody with varying concentrations of cells. They were then
injected over a CM5 sensor chip with immobilized anti-InlB antibodies. A decrease in the
antibody binding of InlB when increasing concentrations of L. monocytogenes cells were
present in the original sample was observed, and a detection limit level of less than 10 6
cells/mL of sample was reported. Hearty et al. (2006) generated a monoclonal antibody that
specifically interacted with the InlA surface protein and showed that this antibody could
be used to detect Listeria InlA using a Biacore-based assay. Tully et al. (2005) demonstrated
the use of quantum dot-labeled antibodies specific for InA for the immunostaining
of L. monocytogenes cells, and these have major potential for use with fluorescence-based
sensor formats. Table 20.6 shows further examples of the use of SPR instruments to detect
and measure the presence of bacteria and pesticides commonly found in fruit and
vegetables.
Mycotoxins are toxic compounds that are synthesized as secondary metabolites by
fungal strains. The fungus Fusarium moniliforme produces the carcinogenic mycotoxin,
Fumonisin B 1. This is one of a number of major fungal species that can infect corn
(http://fumonisin.noneto.com/). Mullett et al. (1998) developed an SPR method to detect
this mycotoxin by immobilizing varying concentrations of Fumonisin B 1 -specific antibodies
onto a gold layer and then passing a standard solution of Fumonisin B 1 over the surface prior
to measuring the change in SPR. Their assay had a detection limit of 50 ng/mL. This research
group states that the homemade SPR device used in their research could be implemented as
an early-stage method in the screening of large numbers of potentially contaminated food
samples. Positive samples could then be analyzed further by more elaborate methods of
analysis, such as liquid chromatography/electrospray ionization mass spectrometry (Hartl
and Humpf, 1999).
The monitoring of the presence of aflatoxins, which are naturally occurring mycotoxins
produced by several strains of Aspergillus spp., in fruit, vegetable, and food produce
is of great significance. Produce that is prone to contamination includes nuts (almonds,
walnuts), cereals (rice, wheat, maize), and oilseeds (soybean and peanuts). Aflatoxin monitoring
is also important as consumption of infected produce can manifest in carcinoma of
the liver (Bhatnagar and Ehrlich, 2002; Bennett and Klich, 2003). While approximately 16