By Linda Bisson
Department of Enology & Viticulture, UC Davis
Grape maturity can be defined as the physiological age of the berry
on the vine. The berry functions to attract animals for dispersal
of grape seeds. Dispersal is always critical, but even more so if
the vine is under limiting growth conditions. This is the main reason
that a certain amount of vine stress is beneficial for development
of grape flavorants. Berry ripening is therefore tightly coordinated
with seed development.
Viticulturists have identified three stages of berry development following
flowering: green berry; arrest of green berry development and pause
before the onset of ripening; and veraison or ripening (Figure 1).
|From an enological perspective,
veraison should be subdivided into different sub-stages based upon
berry metabolism and the continued transport of substances to the
vine (Figure 1). During veraison, water, sugars, and nitrogen compounds
are transported to the berry via the phloem. Sucrose is hydrolyzed
to glucose and fructose in the berry. Berry flavor and aroma compounds
are synthesized within the berry. It is not clear whether the synthesis
of these compounds is controlled by hormonal signals from the rest
of the vine or occurs in the berry independently of other vine influences.
Arrest of phloem transport and onset of dehydration are both expected
to dramatically influence berry metabolic activities and composition.
At some point, the synthesis of desirable enological characteristics
ceases in the grape berry. This is the optimum time at which to harvest
the fruit, prior to deterioration of berry characters. It is not clear
what factors direct loss of berry flavorants and when this degradation
It is therefore important to define the optimal grape maturity for
wine production and to develop clear chemical or biochemical traits
that can be used to define the peak of ripeness. This article will
survey our current understanding of maturity assessment. Clearly,
further work is needed to define the parameters best associated with
optimal ripeness of grapes for wine production.
The definition of optimal maturity will vary depending upon the style
of wine being made; the working definition of quality; varietal; rootstock;
site; interaction of varietal, rootstock and site; seasonal specific
factors; viticultural practices; and downstream processing events
If a clear descriptive analysis of the quality target exists, then
the time of harvest can be optimized to meet those goals. Several
grape and cluster characteristics have been used to assess ripeness
(Figure 2). There are, of course, other non-compositional factors
that influence the decision to harvest, including labor availability;
seasonal changes such as rainfall; heat waves; tank space limitations;
and other factors beyond the winemakers control. But these will
not be considered here.
Sugar is a component often used to assess ripeness. Sugar content
increases during ripening and is therefore a function of berry age.
Sugar is also relatively easy to assess, adding to its value as an
index of ripeness.
Sugar levels appear to be fairly uniform across the population of
berries, meaning that the coefficient of variance is low (less than
10%), and thus the value at the press pan can be predicted with reasonable
accuracy if appropriate vineyard sampling protocols are followed.
Variance is much greater if the fruit is not uniform across the clusters
(as found with Zinfandel) or if the variation in cluster microenvironment
is not correctly accounted for in the vineyard sampling protocol.
However, several studies have shown no relationship between sugar
levels and accumulation of grape berry flavorants. Thus, while sugar
can indicate maturity level, it is not clear whether it is the best
index of optimal maturity.
Figure 3 shows a typical profile of changes in berry sugar composition
during ripening. There is an initial rapid phase of sugar accumulation,
but at some point during berry development and aging, the vine ceases
transport of sugar to the fruit. Further increases in sugar concentration
are due to dehydration.
|Sometimes sugar accumulation
will cease due to unfavorable environmental conditions such
as very high or low vineyard temperatures but resume once the
conditions have changed. It is important to be able to distinguish
a transient effect from a permanent cessation of transport of materials
via the phloem.
Once phloem transport has ended, any further increases in sugar level
will be due to loss of water, not continued synthesis and translocation
Assessing changes in berry weight, and noting the point at which average
berry weight starts to decrease significantly while sugar content
increases, can indicate the onset of dehydration. However, this can
be quite difficult to monitor where fruit maturity is not uniform
across the clusters, or where great differences in berry weight already
exist across the population. That is, the variation in normal
berry weight would obscure early detection of dehydration onset.
Assessments of acidity are also used to define the optimal time of
harvest. This can be evaluated as either pH or titratable acidity
or both. Changes in pH are complex and not necessarily a function
of berry age.
However, a historical index of ripeness suggests that optimal sugar
/ acidity balance is achieved if the product of the Brix value times
the square of the pH is in the range of 220 to 260. For example, a
22† Brix juice at pH 3.2 would yield a value of 225.3. Late harvest
fruit at a higher pH (24† Brix at pH 3.6, for example) would yield
a value (311) outside of this range.
Another scale relates titratable acidity and sugar level. In this
case, the Brix value divided by the TA (g/100 ml tartaric acid equivalents)
should yield a number around 3032 for table wine production.
For a juice at 22† Brix with a TA value of 0.8, the number obtained
would be 27.5. For a 24† Brix juice or must, the TA value could not
drop much below 0.8 to meet this standard. Other authors suggest that
this value can be higher (3738) for late harvest fruit.[5,6]
However, the sugar:acidity ratio is quite variable across different
varieties and growing conditions, and these kinds of universal rules
of thumb may be of little general predictive value for wine quality
especially if indiscriminately applied. Further, it is not
clear whether the optimal sugar (ethanol):acidity balance always coincides
with optimal maturity of grape flavorants.
From a survey of the literature, only the level of norisoprenoid flavorants
appears to be correlated with sugar. The norisopreniods are 13
carbon terpenoid molecules thought to be derived from the degradation
of carotenoids. In general, the 13 carbon norisoprenoid characters
(grassy, tobacco, smoky, kerosene, tea, honey) are far more stable
than the fruity components (red fruit notes).
Changes in acidity level, as they reflect berry metabolic activities,
may be useful to assess. It is well known that malate is consumed
as an energy source in the berry during veraison, so malate levels
decrease relative to tartrate (Figure 4).[1,3,5,6] Tartrate levels
generally remain constant during veraison, but may rise slightly during
grape dehydration. Malate levels decrease as the acid is consumed
by the fruit, and seem to plateau at a low level, roughly 2 to 3 g/L.[5,6]
|The grape may catabolize sugar
if malate levels decline too much, but this varies dramatically by
varietal. Malate levels may be quite high in some red varietals post
alcoholic fermentation. The synthesis of many grape flavor and aroma
compounds requires energy, but the factors leading to cessation of
synthesis of berry flavorants have not been well defined.
Attempts to use the ratio of malate to tartrate as an index of ripeness
also suffer from a lack of correlation of this ratio with grape flavorant
development. Further, the ratio itself is quite variable across different
varieties and growing conditions and seasons, and is of little true
One factor that has been reported in the literature as a useful indicator
of berry status is arginine levels. In theory, a decline in arginine
content signals a deterioration of the fruit. However, arginine levels
are so variable and subject to varietal and seasonal modification
that this parameter is not always reliable.
The glutathione content of grapes also increases at the onset of veraison
and during ripening, but it is not clear whether the level of this
compound is correlated with flavor development. A chemical marker
of the onset of fruit (specifically flavorant) deterioration would
be ideal from an enological perspective, however.
Several studies have evaluated the potential use of various berry
metabolites associated with varietal character. Increases in total
phenolic content have been associated with maturity. In a principal
component analysis of various ripening indices, phenolic content emerged
as a key defining factor of grape maturity.
Anthocyanin levels have also been associated with maturity, but
dramatic effects of environmental and cultural conditions on anthocyanin
pigment accumulation have been reported. Malvidin-3-glucoside
appeared to be unresponsive to growing conditions, with levels increasing
as a function of maturity only. An optical fiber probe capable
of providing a rapid assessment of both total phenolic and anthocyanin
content is being developed that might prove useful in the assessment
of optimal maturity.
With all these factors making correlaton to maturity flavors and other
compounds that are easily measurable difficult, it is generally accepted
that optimal maturity can be assessed only by monitoring levels of
grape flavorants themselves. However, all of these components are
dramatically affected by growing conditions and viticultural practices.
For example, bunch shading decreases the content of norisoprenoid
glycoside conjugates (as does vine shading), but the effect of cluster
microclimate exerts more of an influence than the vine environment.
Light exposure increases the levels of 2-methoxy-3-isopropyl and 2-methoxy-3-isobutyl
pyrazines in unripe grapes, but it also catalyzes photodecomposition
of these compounds in mature grapes.
Nitrogen and water availability also exert a strong impact on grape
flavorant composition and timing of ripening.[16,17]
Free and bound terpene levels have also been used to assess berry
flavorant development and potential, but this relies on assessment
of a single family of components. Many types of flavorants are present
in the form of glycosidic precursors. Analysis of the total precursor
level by assessment of the glycoside glucose (GG) content of the grapes
may yield a more complete picture of the flavorant potential.
Pruning practices do not appear to affect free terpene content, but
they dramatically impact levels of the glycoside terpene precursors.
In one study, total GG precursors were higher in the skins of minimally-pruned
vines, but this did not correlate with overall wine quality. Thus,
any index of ripeness needs to be modified depending upon site-specific
factors and cultural practices.
Total protein content has been evaluated as a function of ripening,
but found to be too variable. Specific proteins or their enzymatic
activities have also been investigated, and some appear to be promising
indices of berry physiological status. But this work is in its infancy.[8,20]
Molecular tools are being developed at UC Davis and elsewhere that
are defining critical protein changes in the berry during ripening.
This work should yield a collection of proteins or enzymatic activities
that are correlated with grape maturity.
All of the factors noted above (sugar levels, acidity, pH, specific
flavorants, metabolite levels, and proteins) change in the berry with
time of ripening and are relatively easy to monitor. However, it is
not clear whether they are truly related to each other or to any other
types of fruit quality assessment.
Tasting the fruit is obviously important in assessing its flavor and
aroma components. A typical progression of character qualities described
for red grapes is presented in Figure 5. This temporal pattern is
consistent with my own experience as to the order in which different
types of characters appear in the fruit, but not with the order in
which they disappear.
|In Cabernet, for example,
sometimes at first appearance of jam there is no herbaceousness, while
at other times it is still strongly present. This suggests that rates
of deterioration of specific flavorants are not coupled to rates of
appearance of others.
It can be as important to taste for the absence of negative characters
as it is to focus only on the appearance and nature of positive components.
The loss of vegetation and unripe characters can be observed from
tasting of the fruit or skins.
Although berry taste is often regarded as the most accurate assessment
of flavorant status, this can, in practice, be quite difficult to
perform in an unbiased manner. The phenomena of flavors masking other
flavors and the fact that many aroma compounds are present as glycosyl-glucose
(GG) precursors released during fermentation and aging make tasting
only a rough approximation of the flavor potential of the wine.
Also, humans can differ markedly in their thresholds for detection
of many compounds, so one can never be certain whether grapes will
taste the same to different individuals. Rather than assess the presence
of fruit complexity, it may be more advantageous to taste for the
absence of characters associated with unripe fruit.
Grape maturation can also be evaluated by assessing physical properties
of the berry such as firmness and deformability. Berry softening
is due to changes in composition of cell walls of the fruit, particularly
due to pectin and xyloglucan depolymerization [22,23] which accompanies
arrest of xylem flow to the fruit.
Some winemakers taste seeds in order to assess grape maturity. However,
seed bitterness may be overpowering, especially to super tasters.
Many individuals may not be able to accurately discriminate levels
of seed bitterness. Physical characteristics of the seeds color,
texture, and brittleness may be more important indicators of
seed and therefore berry maturity.
It is common to look at seeds, waiting for them to turn from bright
green to tan-brown and begin to dry or become woodier-looking and
feeling as an assessment of maturity. This has some enological value
also, since more mature seeds in a maceration tank will yield less
bitter and harsh tannins if they happen to be broken on pumpover,
or during pressing.
Cluster stems can also be evaluated to assess berry ripeness. Stems
undergo a change from green unripe to brown or ripe stems to overripe
or brittle stems. These changes are varietal-specific. In some varieties,
the stems never ripen beyond the green stage.
Figure 6 presents the characters associated with different degrees
of stem ripeness. It is believed that stem ripeness parallels berry
maturity, but this has not been rigorously demonstrated. Stems can
be tasted, as is the case with seeds. Depending on processing conditions,
the presence of unripe stems may lead to extraction of undesirable
Tasting of berries can be fatiguing as well as subject to taster bias.
Clearly the ideal method to assess optimal maturity would be numerical,
and dependent upon the level of key or signature compounds of the
varietal / style. Toward that end, assessments of phenolic compounds,
anthocyanin content, and terpene (free and bound) levels have been
proposed. However, it is not clear how any of these individual
characters correlates with overall grape quality.
Most berry flavorants are likely synthesized independently of each
other in the berry, and high levels of one are not necessarily correlated
with high levels of another. Synthesis of most flavorant molecules
varies dramatically with the season and vineyard practices.
There are several techniques that allow for global compositional profiling
of the fruit. Techniques such as solid phase microextraction (SPME)
have great potential for quantitative assessment of a variety of chemical
aroma compounds in wine, particularly if coupled to enzymatic
treatments to release the aroma potential of wine from precursor compounds.
This technique is more direct and easier to perform than many other
analyses. It does not require extensive sample processing or extraction
or modification of components to be analyzed.
If the optimal aroma composition of fruit at harvest can be defined,
SPME can be a useful tool for routine assessment of optimal maturity.
It is equally imperative to have an index of the cessation of flavor
and aroma development in the fruit. Grape flavorants display different
rates of loss in the fruit while on the vine. Loss of the red fruit
characters in Grenache can be quite dramatic following rainfall late
in berry ripening, while other characters are more stable. An understanding
of factors leading to rapid loss of optimal maturity is consequently
critical as well.
In addition to effects of on-the-vine berry aging on flavor and aroma
compounds, it is clear that the microbial flora of the fruit also
changes during maturation. Our experience with Grenache indicates
an increase in problems associated with bad lactics (off-flavors,
arrest of yeast fermentation) at higher maturity, as assessed by sugar
Since the grape microbial flora has a strong impact on wine composition,
it is important to develop rapid and reliable tools for the assessment
of berry flora. This may be partly due to higher pH values at higher
levels of sugar maturity, so undesirable bacteria are encouraged.
The criteria for optimal maturity are multi-faceted. Several important
classes of compounds change during ripening and maturation of the
fruit on the vine. These characters do not change in a highly coordinated
fashion, and instead suggest a series of independently regulated
pathways of synthesis. Each pathway is impacted by seasonal factors
and vineyard practices, and the effect varies by varietal.
Simultaneous analysis of all pertinent quality factors may be prohibitive
both time-wise and economically. Vineyard or site-specific indices
of optimal ripeness may need to be developed.
It is also important to correlate grape composition with finished
wine composition. Many flavor and aroma components are present in
a precursor or undetectable form. These compounds can be hydrolyzed,
becoming detectable during fermentation and aging.
It is critical to achieve an understanding of the relationship between
grape flavorants and wine quality. New analytical techniques are
being developed that should provide great assistance in future assessment
of optimal maturity.
However, it is unlikely that any single index of maturity will be
discovered that can be indiscriminately applied in all growing conditions
and to all varietals. Historical experience with specific vineyards
and growing regions will continue to be a critical factor in determining
the optimal maturity of the fruit.
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