Machine harvesting versus handpicking:
impacts on tropical and green characters in Sauvignon Blanc wines
Paul Kilmartin
Introduction
CONTINUED IMPROVEMENTS IN
mechanical harvesters mean that an
acceptable level of wine quality is readily
achieved with most grape harvests.
Indeed the speed with which the grapes
can be mechanically harvested means
that the crop can be brought in at its
optimum, in addition to cost savings to
the labour involved in handpicking. At
the same time, the effects of oxidation
with machine-harvested grapes have
been linked to a loss of grape quality
(Arfelli et al., 2010), and the ability to
sort out unwanted grapes and non-grape
material is seen as a further issue in
favour of manual harvesting (Handbook
of Enology, 2006). This can lead to the
view that the best quality wine can only
be obtained by the handpicking of grapes,
if the added cost can be justified. With
many wine styles, this could well be true,
but in the case of Sauvignon Blanc wines
with intense tropical/fruity and green
characters, an exception to this rule has
been found.
Sauvignon Blanc wines with intense
tropical and passionfruit characters
typically have high levels of the varietal
thiols 3-mercaptohexanol (3MH) and
3-mercaptohexyl
acetate
(3MHA)
(Tominaga et al. 2000). These varietal
thiols were indeed found to be important
in surveys of commercial Sauvignon
Blanc wines undertaken at the University
of Auckland (Benkwitz et al., 2012a). A
feature of many of the more intensely
flavoured wines is a combination of high
tropical/passionfruit characters with
a strong green edge, where additional
wine aroma compounds will make
an impact. However, when we turned
our attention to the sub-regions of the
Marlborough grapegrowing area, looking
to define different styles of Sauvignon
Blanc wines, some problems arose. For
her PhD study in 2008, Sara Jouanneau,
supervised by Dr Laura Nicolau,
obtained grape bunches from across
the Marlborough region, and sent these
back to Auckland for consistent smallscale winemaking. This was needed to
avoid the impact of variable winemaking
practices, and to examine what was
specific to the grape source. Instead of
3MH concentrations averaging in the
thousands of ng/L, as obtained in past
surveys of commercial wines (Benkwitz
et al., 2012a; Mateo-Vivaracho et al. 2010),
the 3MH levels ranged from 122 to 1235
ng/L, and remained untypically low in
further Sauvignon Blanc wines made
from handpicked grapes.
To overcome this problem in 2009
and 2010, Jouanneau moved to juices
obtained from the commercial harvesting
and pressing operation, following from
mechanical harvesting of the grapes.
Higher varietal thiol levels were found
in the wines, and Jouanneau completed
a wide survey of the Marlborough subregions for several classes of aroma
compounds. The results of this study
have been accepted for publication
(Jouanneau et al. 2012), and among the
findings are an indication that 3MH
levels across Marlborough can be higher
in some years than others. Also wines
from the southern Awatere valley, wellknown for greener aroma characters, did
not show higher concentrations of the
methoxypyrazines usually associated
with these aromas. We will return to
this issue later in the article, but note
here that these grapes need to travel the
greatest distance by truck, from vineyard
to winery, compared to grapes coming
from the other Marlborough sub-regions.
Harvesting trial
At the same time Laura Nicolau remained
convinced from Jouanneau’s first trial
year that there was something important
for Sauvignon Blanc wine aroma in the
handling of the grape samples, and that
this should be investigated in future
projects. And so we set up a harvesting
trial in 2010 to look at wine composition
from grapes taken at different points in
the harvesting process. Five sets of grape
and juice samples from five Marlborough
vineyards were included in the study.
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82 Grapegrower & Winemaker 4000
3MH
"grapefruit/ passion fruit"
per.thresh. 60 ng/L
Wine A
Wine B
Wine C
Wine D
Wine E
4000
Wine 3MH (ng/L)
5000
3000
2000
3000
2000
1000
1000
0
0
0.0
0.2
0.4
0.6
0.8
1.0
Juice A 420
Hand-picked Harvester Winery Hopper Free Run Pressed to 1 Bar
Figure 3. Left: 3MH concentrations in the five sets of Sauvignon Blanc wines at five harvesting
points; right: wine 3MH versus juice 420nm absorbance across the 25 samples.
8
IBMP
"green pepper"
per.thresh. 2 ng/L
20
Wine A
Wine B
Wine C
Wine D
Wine E
6
4
2
0
15
linalool ( μ g/L)
The wines made from juices that had
been mechanically harvested produced
higher 3MH concentrations in some
cases, but a number of exceptions were
noted (Figure 3). All of the pressed
to 1 bar juices produced wines with
3MH concentrations less than 750ng/L,
and confirmed results from a previous
pressing trial where lower 3MH values
were seen with wines made from heavily
pressed juices (Patel et al. 2010). Most of
the wines made from handpicked grapes
were low in 3MH content, with the
exception of one sample that produced
an unusually high value (wine D in
Figure 3). Further low 3MH wines were
also observed, particularly when the
juice samples were found to be subject to
some oxidation and browning, given by
a higher 420nm absorbance (Figure 3). At
the same time it needs to be recognised
that some juices do not have the potential
to produce high thiol wines, regardless
of how the grapes are processed. Similar
trends were seen with the acetate ester
3MHA, of particular importance to young
Sauvignon Blanc wines and wines stored
under cooler conditions (Makhotkina et
al. 2012).
The state of juice oxidation has been
examined in a further harvesting trial
undertaken in 2011, in which different
sulfite additions were made to Sauvignon
Blanc juices at harvest, with the finding
Figure 2. Left: Grapes and juice being transferred into a truck for transport to the winery, and right:
tipping into the winery receival hopper.
3-mercaptohexanol (ng/L)
Wine aroma results
Varietal thiols
Figure 1. Left: Thomas Allen collecting grapes and juice from a mechanical harvester in April 2010;
and right: pressing the 15kg grape lots at a water pressure of 2.5 to 3 bar for 20 minutes.
3-isobutyl-2-methoxypyrazine (ng/L)
Firstly 15kg of handpicked grapes were
taken immediately prior to mechanical
harvesting of the fruit, and a second 15
kg harvester sample was taken as the
grapes and juice came off the mechanical
harvester (Figure 1).
A third 15kg sample was taken as
the grapes were tipped from the truck
into the receival winery hopper (Figure
2), and were also pressed off using an
80L basket press (Figure 1). Two more
samples were taken from the commercial
pressing operation, the first a free run
juice and a final pressed to 1 bar sample.
The juices were then cold settled for 36
hours, and were inoculated with Lalvin
yeast strain, EC1118 Saccharomyces
cerevisiae, at 0.2g/L, in triplicate 750mL
green wine bottles. A rubber bung
and 100µL plastic pipet tip filled with
glass wool was inserted for CO2 release
during fermentation. The subsequent
wines were analysed for varietal thiols,
as reported earlier (Allen et al., 2011),
and for a wide range of compounds
that make an important contribution to
the aroma of Sauvignon Blanc wines,
including methoxypyrazines, terpenes,
C6-alcohols, higher alcohols and esters
(Benkwitz et al., 2012b).
Wine A
Wine B
Wine C
Wine D
Wine E
linalool
"fruity/ citrus/ floral/ lavender"
per.thresh. 25 μg/L
10
5
Hand-picked Harvester Winery Hopper Free Run Pressed to 1 Bar
0
Hand-picked
Harvester Winery Hopper Free Run Pressed to 1 Bar
Figure 4. Left: 2-methoxy-isobutylpyrazine (MIBP) and right: linalool concentrations in the five sets
of Sauvignon Blanc wines at five harvesting points.
that much lower 3MH concentrations
were obtained in mechanically harvested
juices with either nil or a small (c. 30 mg/
kg) additions of SO2. In the present trial,
the variable air exposure during truck
transport prior to sampling at the winery
hopper and variable sulfite gradients
throughout the truck load could have
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influenced the 3MH formation capacity
of the individual juice samples. On the
other hand, the concentration of a further
important varietal thiol, 4-mercapto4-methylpentan-2-one (4MMP), was
not greatly affected by the choice of
harvesting method and sampling point
(Allen et al., 2011).
October 2012 – Issue 585
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Methoxypyrazines and terpenes
It has been previously shown that the main methoxypyrazine
found in Sauvignon Blanc wines, 2-methoxy-isobutylpyarzine
(MIBP), is highly water soluble and is readily extracted from
grapes to must (Roujou de Boubee et al., 2002). In the present
trial the concentration of MIBP was very similar regardless of
harvesting point and the method of grape harvesting (Figure 4).
Even heavy pressing to 1 bar did not raise the methoxypyrazine
concentrations. Likewise similar concentrations were obtained
across the harvesting points for the terpenes, of which only
linalool was present at concentrations close to the perception
threshold of 25µg/L (Figure 4).
24/8/12
11:40 AM
Page 1
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Fatty acids, higher alcohols and esters
A lack of consistent differences from harvesting method and
sampling point was observed with further aroma compounds,
including fatty acids like octanoic acid, and many of the higher
alcohols, ethyl esters and acetate esters present in the wines.
Results for isoamyl alcohol and isoamyl acetate are presented
in Figure 5, two compounds that were found at concentrations
well above their respective perception thresholds.
3e+5
Isoamyl alcohol (µg/L)
3e+5
isoamyl alcohol
"whiskey / malt / burnt"
per.thresh. 30,000 μg/L
Wine A
Wine B
Wine C
Wine D
Wine E
2e+5
2e+5
1e+5
5e+4
0
Isoamyl acetate (μg/L)
10000
Hand-picked Harvester Winery Hopper Free Run Pressed to 1 Bar
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Wine A
Wine B
Wine C
Wine D
Wine E
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Figure 5. Above: isoamyl alcohol, and below: isoamyl acetate concentrations
in the five sets of Sauvignon Blanc wines at five harvesting points.
C6-alcohols and associated acetate esters
The situation was very different with the C6-alcohols,
responsible for green and grassy aromas in many wines. Even
more so than with 3MH, there was a marked increase in the
concentration of the C6-alcohols for wines made from grapes
that had been through the mechanical harvester, compared
to very low values for wines made from handpicked grapes.
In the case of hexanol, values below the perception threshold
October 2012 – Issue 585
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Grapegrower & Winemaker
83
of 1100µg/L were seen in the wines made from handpicked
grapes, while the perception threshold was exceeded with all
of the remaining wines (Figure 6). Even higher levels were seen
in wines made from the heavily pressed juices, and this trend
went counter to the 3MH case, where lower values were seen
with the “pressed to 1 bar” wines (Figure 3). The values for
hexanol obtained here can be compared to averages in the range
of 2500 to 3000µg/L for commercial New Zealand Sauvignon
Blanc wines made predominantly from machine harvested fruit
(Benkwitz et al. 2012a). For this study, the sensory panel gave
the descriptor 'bourbon' to hexanol and 'apple lolly/candy' to
hexyl acetate.
Very similar trends were seen with a further C6-alcohol,
cis-3-hexenol (at times called ‘leaf alcohol’). It has been noted
in the past that higher levels of cis-3-hexenol can appear in
musts that include grape leaves (Joslin et al., 1978). The acetate
esters corresponding to these two C6-alcohols, namely hexyl
acetate and cis-3-hexenol acetate, mirrored their C6-alcohol
counterparts in showing increases in concentration in moving
from handpicked to machine-harvested samples, and to higher
levels with the heavier pressed juices (Figure 6). These acetate
esters thus followed a different trend to the other acetate esters,
such as isoamyl acetate (Figure 5) analysed in the present study.
In considering which compounds contribute to the greener
characters in Sauvignon Blanc wines, more attention should
be given to the C6-alcohols in wines made from machine
harvested fruit. The contribution of the C6-alcohols, noted
for grassy aromas, may match or even exceed that due to
the methoxypyrazines, described as capsicum. Further
contributions to greener characters can also come from certain
hexanol (μg/L)
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hexanol
"Flower/ green/ cut grass"
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Wine A
Wine B
Wine C
Wine D
Wine E
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Figure 6. Above: hexanol, and below: hexyl acetate concentrations in the
five sets of Sauvignon blanc wines at five harvesting points.
October 2012 – Issue 585
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85
winemaking
varietal thiols (stalky/box tree), and from dimethyl sulfide
(canned asparagus) in older wines. During longer periods of
truck transport, particularly if sulfite levels become depleted,
juice oxidation may lead to the formation of greater quantities
of C6-aldehydes in the juices and ultimately C6-alcohols in the
wines. This may apply to some wines made in Marlborough
from grapes sourced in the Awatere Valley, where longer
transport times to the winery are typically required.
Conclusions
The method of harvesting, machine harvesting versus
handpicking, and the sampling point in the commercial pressing
operation, had varying effects on the levels of aroma compounds
in Sauvignon Blanc wines. Some compounds were present at
similar concentrations regardless of harvesting method and
pressing conditions, such as the methoxypyrazines, 4MMP,
fatty acids, terpenes, higher alcohols and their acetate esters.
The compounds that were lower in concentration in the wines
made from the hand-picked grapes were the C6-alcohols and
their acetate esters and in most cases the varietal thiols 3MH and
3MHA. Higher values of the C6-alcohols were found in wines
made from the heavily pressed juices. To obtain a Sauvignon
Blanc wine with intense tropical and green characters, machine
harvesting of the grapes would be favoured, so long as adequate
protection of the juice from oxidation is ensured through the
use of adequate sulfite applications, to maintain higher varietal
thiol levels. While this study has focused on Sauvignon Blanc,
many of the trends observed above can be expected in other
wine varieties.
Acknowledgements
The winemaking and aroma analyses reported above were
undertaken by Thomas Allen, Sara Jouanneau, Leandro Dias
Araujo and Mandy Herbst-Johnstone at the University of
Auckland. We also thank Villa Maria Estate and Maryvale
Vineyard for Sauvignon Blanc grape and juice samples, along
with Claire Grose for assistance with fermentation of the
experimental wines at the Marlborough Wine Research Centre
in Blenheim.
Paul Kilmartin, Wine Science Programme, School of Chemical Sciences, The
University of Auckland, New Zealand
References
Allen, T., Herbst-Johnstone, M., Girault, M., Butler, P., Logan, G., Jouanneau, S.,
Nicolau, L., and Kilmartin, P.A. 2011. Influence of grape harvesting steps on varietal
thiol aromas in Sauvignon Blanc wines. J. Agric. Food Chem. 59:10641-10650.
Arfelli, G., Sartini, E., Bordini, F., Caprara, C., and Pezzi, F. 2010. Mechanical
harvesting optimization and postharvest treatments to improve wine quality. J. Int.
Sci. Vigne Vin. 44:101-115.
Benkwitz, F., Tominaga, T., Kilmartin, P.A., Lund, C., Wohlers, M., and Nicolau.
L. 2012a. Identifying the chemical composition related to the distinct flavour
characteristics of New Zealand Sauvignon Blanc wines. Am. J. Enol. Vitic. 63:62-72.
Benkwitz F., Nicolau. L., Lund. C., Beresford. M., Wohlers. M., and Kilmartin. P.A.,
2012b. Evaluation of key odorants in Sauvignon Blanc wines using three different
methodologies, J. Agric. Food Chem. 60:6293-6302.
Handbook of Enology: The Microbiology of Wine and Vinifications. 2006. p. 302304.
Joslyn, W.S. and Ough, C.S. 1978. Cause and fate of certain C6 compounds
formed enzymatically in macerated grape leaves during harvest and wine
fermentation. Am. J. Enol. Vitic. 29:11-17.
Jouanneau, S., Weaver, R.J., Nicolau, L., Herbst-Johnstone, M., Benkwitz, F., and
Kilmartin, P.A. 2012. Sub-regional survey of aroma compounds in Marlborough
Sauvignon Blanc wines, Aust. J. Grape Wine Res. 18:329-343.
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86 Grapegrower & Winemaker Makhotkina, O., Pineau, B., and Kilmartin, P.A. 2012. Effect of storage temperature
on the chemical composition and sensory profile of Sauvignon Blanc wines, Aust.
J. Grape Wine Res. 18:91-99.
Mateo-Vivaracho, L., Zapata, J., Cacho, J., and Ferreira, V. 2010. Analysis,
occurrence, and potential sensory significance of five polyfunctional mercaptans in
white wines. J. Agric. Food Chem. 58:10184-10194.
Patel, P., Herbst-Johnstone, M., Lee, S.A., Gardner, R.C., Weaver, R., Nicolau, L.,
and Kilmartin, P.A. 2010. Influence of juice pressing conditions on polyphenols,
antioxidants and varietal aroma of Sauvignon Blanc microferments, J. Agric. Food
Chem. 58:7280-7288.
Roujou de Boubee, D., Cumsille, A. M., Pons, M., and Dubordieu, D. 2002. Location
of 2-methoxy-3-isobutylpyrazine in Cabernet Sauvignon grape bunches and its
extractability during vinification. Am. J. Enol. Vitic. 53:1-5.
Tominaga, T., Baltenweck-Guyot, R., Peyrot des Gachons, C., and Dubourdieu, D.
2000. Contribution of volatile thiols to the aromas of white wines made from several
Vitis vinifera grape varieties. Am. J. Enol. Vitic. 51:178-181.
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October 2012 – Issue 585