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Quantification of Berry Ripening, Flavor Development and Harvest Time by Means of the Phytogram
Agricultural Electronics Corporation
Tucson, Arizona 85703-1291
email@example.com 520 624 7656
A major aspect of viticulture is quantification of berry ripening, flavor development and harvest time. These three aspects are interconnected. Berry ripening is visually observed in terms of color change, softening of the berry and brix level. Flavor development is discerned by direct tasting of the berry in the field and by destructive chemical testing for acidity and concentrations of significant constituents. Harvest time is a decision based on appraisal of the "ripeness" and "flavor."
The discussion which follows is based on petiole and berry monitoring of over two hundred block-seasons. Data from these measurements was matched with brix levels, acidity and grower and wine maker appraisals of the berries and wine produced from the berries. This has led to numerical scales of "ripeness" and "flavor."
In the PHYTOGRAM permanent electrochemical sensors are implanted in extra cellular region of petiole and berry tissue of wine grapes. These sensors measure the "water content" and proton concentration change in the tissue in the vicinity of the sensor. Water content in this discussion is the magnitude of the water that wets the sensor surface inside the petiole or berry. Proton concentration is the number of protons per unit volume present in the water on the surface of the sensor.
The output of the sensors indicate petioles and berries tissue exhibit diurnal cycles of water content and proton concentration The magnitude, phase and minimum value of these cycles change during the season as the petiole and berries respond to weather, water and fertilizer application. It is possible to quantify the ripening process in terms of changes in the magnitude, phase and minimum values of the diurnal cycles.
During most of the season, the diurnal cycles of the petiole are the more useful indicator of vine status. The reason for this is the intrinsic attempt of the vine to protect the berries from any adverse influences such as water stress. This protective mechanism results in wide changes in petiole water content concomitant with minor, insignificant changes in berry water content. In the latter part of the season, the berry diurnal cycles are the main indicator of ripeness.
The major characteristic of the water content diurnal cycle of the berry which relates to ripeness is the minimum value of the cycle. The minimum value has a maximum value of 40 nfd/mm in the period before veraison. This minimum value declines steadily in the period following veraison depending on the irrigation schedule.
The berry minimum water content value near harvest varies with variety. But empirical results indicate the following ranges are optimum at harvest (in nfd/mm):
Cabernet sauvignon 12-15
Pinot noir 10-14
Pinot grigio 10-12
In general, the lower the value, the better the wine. Dimpling of the berry will occur at values of about 10 or lower.
While these are water content values at harvest, there is still the important question of flavor development. Empirical results indicate enhanced flavor comes from holding these values for a sustained period prior to harvest. Holding minimum berry water content to this optimum range for a sustained period becomes the operating objective of late season irrigation scheduling. At the present time the recommended holding period is two to three weeks.
An index of metabolic activity, MA4, was developed in 2006 which gives further insight into berry ripeness and flavor. MA4 measures the phase and magnitude of the water content diurnal cycle relative to the sun cycle. The daily values of this indicator are positive if the vine is not in a water deficit condition. But when the petiole and berry enter a significant water deficit, the daily values of MA4 become negative. For example, the extreme low vapor pressure deficit values of September 22-23 in 2006 shifted the index from positive to neutral or negative in many blocks in northern and central coast California. Harvest time should occur when the MA4 index value is neutral or negative.
There are indications that the gradual shift of the MA4 index is a result of gradual dysfunction of transport of sugars into the berry.
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