Berry Color as an Indicator of Quality

smGrapesFindings show that severely water-stressed vines yield juice with more color, while machine-pruning produces more color than hand-pruning.

This year's Unified Wine and Grape Symposium featured an interesting session about the possibility of determining winegrape quality by measuring the color of the grapes. Some Australian wineries are using color as one of six parameters they evaluate when determining grower payment. Given that premise, the session speakers discussed whether there is indeed a corre lation between berry color and red wine quality.

At this session, CSU Fresno's Robert Wample presented some interesting research that he and others have been conducting at Golden State Vineyards. The research has been focused upon aspects of irrigation and canopy management. Wample also discussed some very interesting technologies used to collect data for this project.

The project involves two different pruning regimes, specifically machine-pruning and hand-pruning, and three irrigation regimes, called "low" (-1.1MPa), "moderate" and "severe" vine water-stress. These were combined to give six sets of five replicates. It should be noted that at -1.5MPa, the so-called "severe" vine water-stress wasn't actually that severe in the context of the typical Central Valley summer.

One of the goals of the experiment was to control the early season canopy development while providing the vine with enough water to ripen fruit. Wample noted that the actual treatments, in and of themselves, were not as important as the attempt to correlate vineyard practices with the resultant winegrape and wine quality. As this research continues, it is hoped that it will enable researchers to provide winemakers and viticulturalists with better recommendations.

Since some of the data collected as part of this project included grape berry color as well as analysis of the resulting juice and wine, Wample was able to discuss how the treatments affected the winegrape color.

Wample noted that they managed the irrigation regimes using three tools. First they calculated the crop evapotranspiration and applied the appropriate amount of water for each irrigation scheme. Second, they monitored vine water-stress with a pressure chamber. Finally, they directly monitored the soil moisture. The data was recorded throughout the growing season.

The berries per cluster, average berry weight and yield per acre were recorded at harvest. Additionally, the resultant fruit was sampled, using a detailed cluster sampling technique, and analyzed using both "wet chemistry" and Fourier Transformed Infrared Spectroscopy (FTIR). FTIR is the methodology used by the FOSS GrapeSCAN/WineSCAN.

FTIR is an analytical tool for characterizing and identifying organic molecules. Using the IR spectrum, chemical bonds and the molecular structure of organic compounds can be identified. FTIR spectroscopy measures the vibrations of the molecular chemical bonds in the infrared spectrum. These oscillations interact with an infrared beam directed onto the chemicals. As the IR light hits the material, it exchanges energy between the beam and chemicals when the frequencies are synchronous or "in resonance." With FTIR, the intensity of the infrared beam is measured before and after it interacts with the sample as a function of the light's frequency. The resulting infrared spectrum is the ratio of intensities plotted against frequency. From this plot, one can determine the identities, surrounding environments and concentrations of the chemical bonds in the sample.

Additionally, Wample's team used a Near Infra-Red Spectrometer (NIRS) to measure Brix, pH and titratable acidity (TA) on individual berries on the vine. This is an exceedingly interesting development because NIRS is both very fast, just seconds per sample, and non-destructive. Furthermore, the probe for the NIRS can be swapped, allowing a transflectance probe to be fitted. This allows the NIRS to be used in the winery as well as in the vineyard.

Wample's group found that the more "severe" vine water-stress treatments yielded juice with more color and a higher anthocyanin content. Additionally, the juice from the machine-pruned treatments seemed to have more color than the juice from the corresponding hand-pruned treatments. Wample stressed, however, that this is only for color released into the juice and not final wine color. More work needs to be done before this next step can be taken.

Preliminary sensory analysis seems to indicate that the grapes from the intermediate irrigation treatment for both the machine- and hand-pruned replicates have preferable sensory characteristics to the other irrigation treatments.

The wines from this experiment were just finishing malolactic fermentation during Unified, so it was not possible for Wample to present any conclusive sensory analysis of the final wines. However, the color of the wines was consistent with the juice insofar as the color of the individual irrigation treatments increased vine water-stress. Unlike the unfermented juice samples, however, the wine from the hand-pruned treatments appeared to have more color than that from the machine-pruned treatments.