Electrochemical biosensors
Electrochemical sensors can be classified according to their transduction
principleas amperometric, potentiometricor conductometricsensors (Meadows,
1996).Theamperometricsensorfor blood glucoseconcentrationbasedon
glucose oxidaseis commercially widely used,but systemsfor foodanalytessuch
as ethanol (Yaoet al., 2000),ascorbic acid (Akyilmazand DincÀkaya,1999),free
fattyacids(Schmidtet al., 1996)and differentcarbohydrates(Bilitewskiet al.,
1993)havebeendeveloped. For this kindof transducer the currentproduced by
an electroactive speciesis measuredand correlated to the concentrationof the
analyte (Heldman, 2003).
It is also possible to use an amperometric bi-enzymesystem as described as
an dairyindustry applicationby Schelleret al.(1990): a combinationof -
galactosidase and glucose oxidase has been used to determinate lactose
concentrationby the reduction of oxygenin reaction(19.2)or the oxidation
of H 2 O 2 ; measurementswereperformedusing a Pt-Ag/AgCl oxygenelectrode.lactoseáH 2 Oˇˇˇˇˇˇˇˇˇ!ˇgalactosidase
D-galactoseá - D-glucose (19.1)- D-glucoseáO 2 ˇˇˇˇˇˇˇˇˇ!
glucose oxidase
D-glucano--lactoneáH 2 O 2 (19.2)In potentiometrya membrane is measuring a potentialresulting from the
differencein concentrationof H+or other positiveionsacross the membrane.
Examplesare described by Verma and Singh(2003)for qualitycontrolin milk,
and by Ercoleet al.(2003)forEscherichiacolidetectionin vegetable. Con-
ductometricsensors use conductive polymers, whichconvertthe biochemical
interactioninto an electrical signal.Recently, Mubammad-Tahirand Alocilja
(2003)usedthis techniquefor the detectionofE. coliandSalmonella.
In contrast to chemical or physicalsensors enzymeassays (forelectro-
chemical biosensorsand/or in general) are in general unusable for continuous
measurements, i.e. enzymaticefficiency is not constant overtime(O'Connell
and Guilbault, 2001b). To enableconstantmeasurements(in-line/on-line)the
flow injection analysis providesa solution by constantlysending samples with
recoveryphases betweeneachsample. This recoveryis usedto clean the sensor
fromsample(residues)and to return the responsebackto baseline.Thermalbiosensors
Thermalbiosensors were first developed in the early 1970s and used for
continuousmeasurementsand enzyme-reactor control,but theyare rarelyused
for foodcontrol. An overview of possibilitiesis given by Ramanathanet al.
(1999), including the monitoring of acetaldehyde, ethanol, glucose and
penicillinV in industrialfermentationsystems.The principleis characterised
by immobilisedenzymes, whichevolveheatduringtheircatalysedreaction,
whichis in proportion to the amountof substrate in the sample(Harbornet al.,
1997).The advantage of this technique is the independence of opticalproperties,
whichenables to measure a broadrangeof bioanalyteswithone instrument.
296 Handbookof hygiene controlin the foodindustry