Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

432 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
whereby a small amount of enzyme (lactate oxidase) was injected into an internal delivery
flow system and was held in direct spatial contact with an amperometric transducer by a
semipermeable membrane. Measurements were determined by the enzymatic conversion of
L-lactate to pyruvateand H 2 O 2 . This method could detect concentrations of L-lactate in the
range of 0.10–2.51 mM. All assay measurements were compared to an established spectrophotometric
assay. However, the authors state that the biosensor method has a distinct
advantage as the measurement can be performed in less than 3 min compared to 30–35 min
for the spectrophotometric method.
Voss and Galensa (2000) used an electrochemical detection protocol to monitor the
presence and concentration of amino acids in fruit juice. Amino acids were separated on a
lithium cation–exchange column, and an amperometric biosensor was used to monitor the
production of H 2 O 2 . This protocol could detect concentrations ranging from 0.1 mg/mL
to 5 mg/mL (D-Pro and D-Met to L-Ala, respectively). D-Alanine could be detected in
concentrations of 0.5 mg/mL. In this case, the presence of D-alanine may be indicative of
bacterial contamination.
Wheat is a universal component of food produce, and the monitoring of pesticide
residues in this product is of critical importance. Del Carlo et al. (2005) quantified the
amount of phosphothionate insecticide (pirimiphos-methyl) present in durum wheat using
an electrochemical biosensor. They used an AChE-inhibition assay and obtained a calibration
curve between 25 and 1,000 ng/mL with a detection limit of 38 ng/mL. When real
samples of durum wheat were analyzed, the limit of detection increased to 65–133 ng/mL
because of the sample matrix. In EU, regulations the pirimiphos-methyl MRL is 5 mg/kg.
A comprehensive review paper has been published that examines biosensors that use
enzyme inhibition to measure the presence of pollutants and toxic compounds in food
samples (Amine et al., 2006). Other types of biosensors have also been used for postharvest
analysis. Pogačnik and Franko (2003) developed a photothermal biosensor to measure
organophosphate and carbamate compounds in salads, lettuce, and onions. This approach
used thermal lens spectrometry, a technique that depends on the adsorption of optical
radiation in the sample, generating heat. This introduces a change in the RI, and through
this analysis, concentrations can be determined. Paraoxonwas detected in all of the samples
tested by this protocol. Another optical biosensor was developed to detect and quantify
carbamate residues in vegetables. It was observed that changes in the concentration of
carbamate could be monitored using chlorophenol red (Xavier et al., 2000).
Keay and McNeil (1998) developed a protocol that utilized a competitive ELISA that
exploits a disposable screen-printed horseradish peroxidase (HRP)-modified electrode as
the detector element in tandem with a single-test immunomembrane to measure traces of
atrazine. A monoclonal antibody for atrazine was immobilized onto a membrane that was
subsequently placed onto the electrode surface, and the assay used a glucose oxidase label
attached to the atrazine molecule. The quantification of atrazine in solution was permitted
by the competition between labeled and unlabeled atrazine for binding to the immobilized
antibody.
Another novel approach for the detection of atrazine involved the incorporation of
liposomes (small lamellar vesicles) into a competitive assay format. One of the advantages
over other formats, including disposable immunostrip assays, was the fast turnaround time
for analysis (7 min compared to 20 min for the immunostrip assays) (Bäumner and Schmid,
1998). Concentrations of 0.1 μg/L of atrazine could be detected with this system. In another