title: Fluorescent sensing layer for the determination of L-malic acid in wine.
abstract: An enzymatic method for determining L-malic acid in wine based on an L-malate sensing layer with nicotinamide adenine dinucleotide (NAD+), L-malate dehydrogenase (L-MDH) and diaphorase (DI), immobilized by sol-gel technology, was constructed and evaluated. The sol-gel glass was prepared with tetramethoxysilane (TMOS), water and HCl. L-MDH catalyzes the reaction between L-malate and NAD+, producing NADH, whose fluorescence (lambdaexc=340 nm, lambdaem=430 nm) could be directly related to the amount of L-malate. NADH is converted to NAD+ by applying hexacyanoferrate(III) as oxidant in the presence of DI. Some parameters affecting sol-gel encapsulation and the pH of the enzymatic reaction were studied. The sensing layer has a dynamic range of 0.1-1.0 g/L of L-malate and a long-term storage stability of 25 days. It exhibits acceptable reproducibility [sr(%) approximately 10] and allows six regenerations. The content of L-malic acid was determined for different types of wine, and polyvinylpolypyrrolidone (PVPP) was used as a bleaching agent with red wine. The results obtained for the wine samples using the sensing layer are comparable to those obtained from a reference method based on UV-vis molecular absorption spectrometry, if the matrix effect is corrected for.
authors.affiliation: Laboratory of Analytical Chemistry, Department of Chemistry, University of La Rioja, Madre de Dios 51, 26006, Logroño, Spain.
authors.forename: Félix
authors.initials: F
authors.lastname: Gallarta
authors.forename: Francisco Javier
authors.initials: FJ
authors.lastname: Sáinz
authors.forename: Cecilia
authors.initials: C
authors.lastname: Sáenz
mesh_terms: D005998:Glycine; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D008293:Malates / Q000032:analysis*; D044367:Phase Transition; D015203:Reproducibility of Results; D012996:Solutions; D013050:Spectrometry, Fluorescence / Q000379:methods*; D013997:Time Factors; D014920:Wine / Q000032:analysis*
chemical_list: D008293:Malates; D012996:Solutions; C030298:malic acid; D005998:Glycine