The overall objective was to develop planar, disposable, direct reading sensors for sugars, ascorbate, malate and citrate in cut fresh citrus fruit. The particular objectives were to (i) integrate promising enzyme chemistries with amperometric sensor/meter combinations, (ii) to evaluate systems in simulated and actual fruit sample matrices, (iii) to achieve operational simplicity by achievement of high linearity, low interference biosensors.
A hand-held meter for universal application to amperometric enzyme biosensors. Enzyme electrodes for glucose, ascorbate, malate and citrate, which except for citrate, can operate in undiluted, unstirred fruit samples. These robust sensors are suitable for extralaboratory testing by unskilled personnel in the form of disposable dip-sticks. They are at different stages of further development and scale-up production and are currently being targeted at various commercial agro-food sectors.
The sugar and total acid content of fruits provides an objective, quantitative measure of the ripeness and acceptability for sale. At each stage of the fruit handling process from harvesting through to storage distribution and point of sale, the testing of these parameters would provide a valuable means of standardisation and quality assurance. Other than cumbersome laboratory analysis, there has hitherto been no direct, biochemical measurement of sugars/acids in fruits. Enzyme based biosensors in principle, offer the capability for reagentless, selective measurement, but because of sample interfering problems this potential has yet to be realised.
To achieve the desired operational biosensor requirements, a range of novel polymeric membranes were developed to control sugar/acid access to an immobilised oxidase or dehydrogenase enzyme layer mounted on noble metal electrodes. For ascorbate, simple enzymeless, direct reading devices were produced; for citrate more complex multi-enzyme cascade was needed. For glucose, for example, linearity was extended to 2M, with minimal interference and a maintained activity at pH 2.4. By using spin-coating, membranes of controlled thickness were reproducible and rapidly-deposited in situ onto screen-printed strip electrodes. Sensor development was aided by studies of basic enzymology and by computer simulation of sensor responses; progress with signal amplification and processing resulted in a simple hand-held meter.
Funding SchemeCSC - Cost-sharing contracts
PE11 2AL Spalding