Enology Notes

Enology Notes #53, July 15, 2002

To: Regional Wine Producers

From: Bruce Zoecklein

Subject: Grape Sampling, Asynchronous Berry Development, 2002 Vintage

Grape Sampling.
Regardless of what maturity gauges are used, an important and universal concern is accurate vineyard sampling. Nothing is more heterogeneous than grapes from the same vineyard at a given moment, even from the same variety. How do you know if your sampling is representative of what is actually in the vineyard?

Each season I hear complaints regarding the difference between a grower’s reported Brix values and what the winery noted when they processed the fruit. The difference can be as much as 2 °Brix, always with the winery’s value lower than that reported by the grower. This variance between the grower’s analysis and the true value has a significant and negative impact on wine quality.

We have two basic choices in grape sampling: cluster sampling or berry sampling. With cluster sampling we have a further choice of gathering clusters from throughout the vineyard or using one or more targeted vines. Some vines always develop more quickly than others.

Therefore, it is risky to determine the harvest date based on a single vine. From the vast number of studies conducted, we know that the variance between the sample analysis and the true vineyard analysis, regardless of the analyte, is smallest if we take a minimum number of berries from a large number of vines.

The information below provides information on the number of clusters and berries needed to obtain specific levels of accuracy. Guidelines regarding sampling procedures are provided immediately afterwards. It should be noted that there is a general tendency to select the most mature berries. If you examine a cluster prior to berry sampling you are most likely to pick berries which are more mature. Therefore, berry sampling should involve locating the fruit zone, and then sampling without examining the clusters or berries.

Berry Sampling
+/- 1.0 °Brix 2x100 berries
+/- 0.5 °Brix 5x100 berries
Cluster Sampling
+/- 1.0 °Brix 10 clusters

Berry Sampling
Avoid edge rows and the first two vines in a row Collect samples from both sides of the vine For each row, estimate the proportion of shaded bunches and sample accordingly Collect berries from top, middle and bottom of the cluster Randomize the side of the cluster sampled Maximum sample area should be less than 2 ha

The three factors which have a major role in maturation dynamics are heat, light and soil moisture. Therefore, variation of these within a vineyard block can result in significant sample variation. About 90% of the variation in berry sampling is believed to come from variation in the position of the cluster on the vine and the degree of sun exposure. The vineyard must be sampled based on the degree of fruit exposure.

Asynchronous Berry Development
The problem of representative sampling is compounded by asynchronous berry, cluster or vine development. A crop with asynchronous berries, for example, has a mixture of development states, resulting in a proportion of berries with optimal qualities diluted by berries which are inferior.

We tend to assume that two vineyards or vineyard blocks with the same or similar Brix values will give similar wines. This can be far from the truth, even before differences arise from processing. If a winery reports a Brix of 22°, the juice might be composed of a narrow distribution of a few berries at 20° and a few at 24° Brix with the majority nearer to 22°. However, there may be a much wider distribution with berries below 18° and greater than 24°.

Because Brix is a distribution average, juices with similar Brix values can produce wines which are quite different due to variations in aroma/flavor and phenol content. The importance of asynchronous development is often overlooked and is a major factor limiting the quality potential of some wines.

The effect of asynchronous berry development on the aroma/flavor profile can be profound. The result is that a proportion of berries with optimal qualities being are diluted by those berries which are inferior.

While berries are all harvested at once, their optimum aroma matures at different times. Vines that produce the best quality wines are those with less variability. The lower the vineyard uniformity, the greater the negative influence on wine quality and the greater the sample volume needed to help get an accurate assessment of maturity.

2002 Vintage
As a result of the late frost damage we experienced in May, the potential of un-uniform ripening is quite large.

The possible extent of the problem can be determined by the careful noting of color change uniformity at veraison.

An additional way of monitoring vineyard uniformity is to carefully evaluate maturity samples. For example, noting the variation in the degrees Brix from each of five, 100 berry samples will help to provide an understanding of the extent of uneven development.

Uneven cluster development requires either fruit culling and/or multiple harvest dates.

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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293
Email: bzoeckle@vt.edu