Meagan D. Mercurio

Measurement of condensed tannins and dry matter in red grape homogenates using near infrared spectroscopy and partial least squares.

Samples (n = 620) of homogenized red grape berries were analyzed using a visible and near-infrared (NIR) spectrophotometer (400-2500 nm) in reflectance. The spectra and the analytical data were used to develop partial least-squares calibrations to predict dry matter (DM) content and condensed tannins (CT) concentrations. The coefficient of determination in cross-validation and the standard error of cross-validation were 0.92 and 0.83% w/w for DM and 0.86 and 0.46 mg/g epicatechin equivalents for CT, respectively. The standard error in prediction was 1.34% w/w for DM and 0.89 mg/g epicatechin equivalents for CT, respectively. By implementing a NIR spectroscopy method to measure DM and CT in red grape homogenates, we have developed an approach that is suited to large-scale compositional analysis in commercial wine production facilities, as it enables the analysis of large numbers of samples needed to stream batches of fruit. From an economical point of view, the calibration models could be achieved with relatively small data sets. Thus, NIR offers a suitable and efficient tool for the simultaneous measurement of DM and CT in addition to other important parameters in red grape homogenates such as total anthocyanins, total soluble solids, and pH, with minimal sample preparation and low cost.

Tannin quantification in red grapes and wine: comparison of polysaccharide- and protein-based tannin precipitation techniques and their ability to model wine astringency.

Quantification of red grape tannin and red wine tannin using the methyl cellulose precipitable (MCP) tannin assay and the Adams-Harbertson (A-H) tannin assay were investigated. The study allowed for direct comparison between the repeatability of the assays and for the assessment of other practical considerations such as time efficiency, ease of practice, and throughput, and assessed the relationships between tannin quantification by both analytical techniques. A strong correlation between the two analytical techniques was observed when quantifying grape tannin (r(2) = 0.96), and a good correlation was observed for wine tannins (r(2) = 0.80). However, significant differences in the reported tannin values for the analytical techniques were observed (approximately 3-fold). To explore potential reasons for the difference, investigations were undertaken to determine how several variables influenced the final tannin quantification for both assays. These variables included differences in the amount of tannin precipitated (monitored by HPLC), assay matrix variables, and the monomers used to report the final values. The relationship between tannin quantification and wine astringency was assessed for the MCP and A-H tannin assays, and both showed strong correlations with perceived wine astringency (r(2) = 0.83 and r(2) = 0.90, respectively). The work described here gives guidance to those wanting to understand how the values between the two assays relate; however, a conclusive explanation for the differences in values between the MCP and A-H tannin assays remains unclear, and further work in this area is required.

Volatile and Color Composition of Young and Model-Aged Shiraz Wines As Affected by Diammonium Phosphate Supplementation Before Alcoholic Fermentation

A Shiraz must with low yeast assimilable nitrogen (YAN) was supplemented with two concentrations of diammonium phosphate (DAP) and then fermented with maceration on grape skins. The nonvolatile, volatile, and color composition of the final wines were investigated. Ethanol and residual sugars were not affected by DAP supplementation, while glycerol, SO 2, and residual YAN increased and acetic acid decreased. DAP-supplemented treatments gave rise to higher concentrations of acetates, fatty acids, and fatty acid ethyl esters but lower concentrations of branched-chain fatty acids and their ethyl esters. No major difference between treatments was observed for higher alcohols, monoterpenes, norisoprenoids, and low-molecular-weight sulfur compounds. DAP-supplemented fermentations resulted in wines with higher concentrations of malvidin-3-glucoside, higher color intensity, and altered color tonality. Model aging studies indicated that higher concentrations of esters are still present in wines from the DAP-treated fermentations after aging. DAP supplementation also resulted in increased concentrations of dimethyl sulfide after model aging. It can be concluded that DAP treatment of a low YAN must fermented by maceration on skins can significantly affect wine color, aroma, and flavor.

Rapid Isolation of Red Wine Polymeric Polyphenols by Solid-Phase Extraction

A rapid technique for the isolation of polymeric polyphenols from red wine has been developed and validated. A copolymer reversed-phase SPE cartridge was utilized in conjunction with predominantly organic eluents to provide three phenolic fractions from red wine without the need for sample pretreatment. The first fraction contained the bulk of the monomeric and oligomeric phenolic material, while the second and third fractions contained the polymeric polyphenolic compounds, as determined by HPLC analysis. The two polymeric polyphenolic fractions differed in their solubility and extent of pigmentation, and the differences appeared to be related to wine age. This method contrasted with other available fractionation techniques because the interfering, nonpolymeric material can be removed in a single wash fraction, while the polymeric material is separated into two distinct fractions based on their diverse physicochemical properties. It is anticipated that the rapid access to discrete polymeric fractions afforded by this method will be of benefit in furthering the understanding of red wine polymeric polyphenols.

Rapid measurement of methyl cellulose precipitable tannins using ultraviolet spectroscopy with chemometrics: application to red wine and inter-laboratory calibration transfer.

Information relating to tannin concentration in grapes and wine is not currently available simply and rapidly enough to inform decision-making by grape growers, winemakers, and wine researchers. Spectroscopy and chemometrics have been implemented for the analysis of critical grape and wine parameters and offer a possible solution for rapid tannin analysis. We report here the development and validation of an ultraviolet (UV) spectral calibration for the prediction of tannin concentration in red wines. Such spectral calibrations reduce the time and resource requirements involved in measuring tannins. A diverse calibration set (n = 204) was prepared with samples of Australian wines of five varieties (Cabernet Sauvignon, Shiraz, Merlot, Pinot Noir, and Durif), from regions spanning the wine grape growing areas of Australia, with varying climate and soils, and with vintages ranging from 1991 to 2007. The relationship between tannin measured by the methyl cellulose precipitation (MCP) reference method at 280 nm and tannin predicted with a multiple linear regression (MLR) calibration, using ultraviolet (UV) absorbance at 250, 270, 280, 290, and 315 nm, was strong (r 2 val=0.92; SECV=0.20 g/L). An independent validation set (n = 85) was predicted using the MLR algorithm developed with the calibration set and gave confidence in the ability to predict new samples, independent of the samples used to prepare the calibration (r 2 val = 0.94; SEP = 0.18 g/L). The MLR algorithm could also predict tannin in fermenting wines (r 2 val=0.76; SEP=0.18 g/L), but worked best from the second day of ferment on. This study also explored instrument-to-instrument transfer of a spectral calibration for MCP tannin. After slope and bias adjustments of the calibration, efficient calibration transfer to other laboratories was clearly demonstrated, with all instruments in the study effectively giving identical results on a transfer set.