In western Oregon the titratable acidity of grapes at
harvest may in some seasons be higher than desirable for
making quality wine, due to the retention of malic acid.
The purposes of this study were 1) to investigate the
effects of a vineyard cultural practice, cluster exposure
at veraison by basal leaf removal, as a means of reducing
the malate content and titratable acidity of grapes, and 2)
to develop a rapid, simple, and inexpensive test procedure
by which smaller wineries and vineyards could evaluate the
effects of their own field experiments on the malate and
tartrate content of their grapes.
1. At veraison, clusters of Chardonnay grapes were a)
exposed to the sun by removal of all leaves opposite or
below the clusters, b) treated as in a) but shaded with
shadecloth, c) exposed to the sun by tying back leaves
opposite or below the clusters, or d) left untreated as a
control. Clusters of White Riesling were exposed to the
sun by similar leaf removal a) 10 days before veraison, b)
10 days after veraison, or c) untreated. Clusters of Pinot Noir were exposed a) at veraison, b) 2 weeks after
veraison, or c) untreated. Exposed clusters received 3 to
3.5 times more light than shaded clusters and up to 32%
more heat, with temperature differences between exposed and
shaded treatments being most pronounced during cool, sunny
weather. None of the treatments had any effect on juice or
berry malate, tartrate, or potassium content; however,
exposed clusters of Pinot Noir had a lower pH (.03) and
higher titratable acidity (.06%) than the control at
harvest. Cluster exposure of Chardonnay increased
sunburning of grapes, and cluster exposure of Pinot Noir at
veraison caused a 1% reduction in juice soluble solids
concentration at harvest. The detrimental effects of
cluster exposure by basal leaf removal at veraison, as well
as the lack of any major effect on the acid content of the
berries, suggest that the practice has no value for acid
reduction during a warm, dry maturation season in western
Oregon.
2. A rapid, simple procedure for the estimation of the
malate and tartrate content of grape juice is described.
The procedure, which requires only a pH meter for
instrumentation, does not directly measure malate and
tartrate but instead measures their buffering effect.
Samples are titrated between pH 2.70-3.00 and pH 4.50-4.80
and the titrant volumes required are compared to two sets
of empirically derived standard curves. The malate and tartrate composition of the sample may be determined by a graphical or algebraic method. The use of the estimation
method, its advantages, and its limitations are illustrated
with different viticultural trials. The estimation error
(estimated value - measured value) was influenced by many
factors including maturity, season, vineyard location, and
cultivar. Standard deviations of the estimation error for
malate and tartrate in mature grapes were equal to 9% and
15%, respectively, of the mean malate and tartrate
concentrations in pooled Pinot Noir and Chardonnay samples
from different vineyards and years. The estimation error
is probably due to interference from other buffers present
in juice. Although not as accurate as existing analytical
methods, the estimation method appears potentially useful
for determining relative effects of treatments in vineyard
trials where analytical equipment is unavailable or for
monitoring malate decline during maturation of grapes. / Graduation date: 1987
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/27084 |
| Date | 01 May 1987 |
| Creators | Norton, Kerry M. |
| Contributors | Heatherbell, David A. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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