Malate Biosensors for the Monitoring of Malolactic Fermentation: Different Approaches

Enzyme-based amperometric biosensors for malic acid - A review

Malic acid is a key flavour component of many fruits and vegetables. There is significant interest in technologies for monitoring its concentration, particularly in winemaking. In this review we systematically and comprehensively chart progress in the development of enzyme-based amperometric biosensors for malic acid. We summarise the components and analytical parameters of malic acid sensors that have been reported over the past four decades, discussing their merits and pitfalls in terms of accuracy, sensitivity, linear range, response time and stability. We discuss how advances in electrode materials, electron mediators and the use of coupled enzymes have improved sensitivity and minimised interference, but also uncover a trade-off between sensitivity and linear range. A particular focus of our review is the three types of malate oxidoreductase enzyme that have been used in malic acid biosensors. We describe their different properties and conclude that identifying and/or engineering superior alternatives will be a key future direction for improving the commercial utility of malic acid biosensors.

Rapid Determination of Ochratoxin A in Grape and Its Commodities Based on a Label-Free Impedimetric Aptasensor Constructed by Layer-by-Layer Self-Assembly

A simple and sensitive label-free impedimetric aptasensor for rapid determination of ochratoxin A (OTA) has been developed, which was based on the combination between thiolated aptamer and gold nanoparticles by layer-by-layer self-assembly. Because of the interaction between aptamer and OTA, the relative normalized electron-transfer resistance (ΔR_ct) values obtained by electrochemical impedance spectroscopy (EIS) was proportional to the concentration of OTA and showed a good linear relationship from 0.1 to 10.0 ng/mL, with a lower detection limit (0.030 ng/mL) than one-step thiolated DNA aptasensor. The established method was successfully applied to detect and analyze OTA in table wine and grape juice, and the recovery was 90.56%–104.21% when PVP effective removed of phenolic substances. The label-free impedimetric aptasensor was used for rapid detection and quantitation of OTA in the inoculated grapes with the Aspergillus Nigri (H1), and the production of OTA (62.4 μg/kg, 20 μg/kg) far exceeded the maximum levels of 2 μg/kg after inoculation for three days. The developed method exhibited a good specificity, high sensitivity, time-efficient, and it could be applied to detect the OTA concentration in grape and its commodities.

Comparison of biosensors based on gold and nanocomposite electrodes for monitoring of malic acid in wine

Amperometric biosensors based on a gold planar electrode and on two types of nanocomposite electrodes consisting of multi-walled carbon nanotubes for the determination of L-malic acid designed for wine-makers were developed. The biosensors designed for wine-makers were constructed by immobilization of L-malate dehydrogenase and diaphorase within chitosan layers on the surface of the electrodes. The coenzyme NAD+ and the electrochemical mediator ferricyanide were present in the measuring solution. The current resulting from re-oxidation of produced ferrocyanide was measured at a working potential of +300 mV against an Ag/AgCl reference electrode. The biosensor based on a gold electrode showed linearity over the range 10–520 µM with a detection limit of 5.41 µM. Calibration curves for biosensors utilizing nanocomposites were obtained both with the linear range of 10 to 610 µM. The detection limits were 1.57 and 1.77 µM, respectively. The biosensors showed satisfactory operational stability (no loss of sensitivity after 30 consecutive measurements) and storage stability (90% of the initial sensitivity after one year of storage at room temperature). The results obtained from measurements of wine samples were in a good correlation with the standard HPLC method. Satisfactory biosensor sensitivity, specificity and stability allowed their successful commercialization.

Biosensors as analytical tools in food fermentation industry

The food industries need rapid and affordable methods to assure the quality ofproducts and process control. Biosensors, combining a biological recognition element and a sensitive transducer, are versatile analytical tools that offer advantages as classical analytical methods due to their inherent specificity, selectivity and simplicity. This paper reviews the recent trends in the development and applications of biosensors used in food fermentation industry, focusing on amperometric enzymatic and microbial sensors.

Surface plasmon resonance biosensor for the detection of ochratoxin A in cereals and beverages

Ochratoxins are a group of mycotoxins produced as secondary metabolites by fungi which contaminate a large variety of food and feed commodities. Due to their teratogenic and carcinogenic properties, ochratoxins present a serious hazard to human and animal health. There is an increasing need to establish a simple sensitive method to detect these toxins. Here we report a rapid and highly sensitive surface plasmon resonance (SPR) assay of ochratoxin A (OTA) using Au nanoparticles for signal enhancement on a mixed self-assembled monolayer (mSAM) surface. A competitive immunoassay format was used for the development of the OTA immunoassay, which is based on the immobilization of target OTA through its ovalbumin (OVA) conjugate with a polyethylene glycol (PEG) linker. The new OTA conjugate (OTA-PEG-OVA) showed remarkably enhanced performance characteristics compared with those based on the immobilization of a commercial bovine serum albumin BSA-OTA conjugate without a PEG linker. Although OTA concentrations as low as 1.5 ng mL(-1) could be directly detected on this surface, the limit of detection (LOD) can be dramatically improved to 0.042 ng mL(-1) for OTA by applying large gold nanoparticles (40 nm) for signal enhancement. Various chemical conditions to minimize the influence of the food matrix on assay performance were also investigated. Grain samples were simply extracted with 50% methanol and liquid samples treated with poly(vinylpyrrolidone) (PVP) (3 or 5%), without any sample clean-up or pre-concentration step prior to analysis. The LODs for OTA in oats and corn were 0.3 and 0.5 ng g(-1), respectively, while in wine and other beverages, LODs ranged from 0.058 to 0.4 ng mL(-1). No cross-reactivity was observed with three other common mycotoxins. In addition, the mSAM/OTA-PEG-OVA surface exhibited high stability with over 600 binding/regeneration cycles. This approach with simple sample preparation provides a powerful tool for the rapid and sensitive quantitative determination of OTA in food matrices.

Synergistic effect of mediator-carbon nanotube composites for dehydrogenases and peroxidases based biosensors

Very sensitive, low cost and reliable NADH and H(2)O(2) sensors were realised and used for development of enzyme based biosensors. The active surface of the electrodes was modified with a nanocomposite obtained by modification of SWNT with a proper mediator: Meldola Blue (for NADH) and Prussian Blue (for H(2)O(2)). Low applied potential of -50 mV vs. Ag/AgCl reference electrode proved the synergistic effect of nanocomposite materials towards NADH and H(2)O(2) detection. Biosensors for malic acid and alkylphenols have been developed, using mediator-functionalized-SWNT-based electrodes and two different classes of enzymes: NAD(+)-dependent dehydrogenases and peroxidases. Immobilization of the enzymes was realised using a series of different procedures - adsorption, Nafion membrane, sol-gel and glutaraldehyde, in order to find the best configuration for a good operational stability. A higher sensitivity comparing with other reported biosensors of about 12.41 mA/M.cm(2) was obtained for l-malic acid biosensor with enzyme immobilised in Nafion membrane. Phenol, 4-t-octylphenol and 4-n-nonylphenol were used as standard compounds for HRP based biosensor. Fast biosensor response and comparable detection limit with HPLC methods were achieved.