Ref: C0456
Comparison of two grapes shrivels techniques:
mechanical ventilation and dehumidification
Piernicola Masella, Lorenzo Guerrini, Fabio Baldi, Paolo Spugnoli, and Alessandro Parenti.
GESAAF – Università degli Studi di Firenze, 50144 Firenze, Piazzale delle Cascine 16.
Correspondence: [email protected]
Abstract
The raisin wines are produced with grapes that have been dried to concentrate their juice.
The dehydration could be natural, when the grapes are picked up and stocked for a certain
time, or induced, controlling the environment of the room where the berries are placed after
the harvest. The water losses from grapes could be accelerated with ventilation, when an air
flux is forced on berries, or with dehumidification, the removal of moisture from the room. The
manner adopted to accelerate the grapes shrivel process could affect some berries chemical
parameters, and consequently the produced wine. Hence, a comparison between these
techniques has been done. Trials were carried out using grapes, cultivar Corvina. For one
hundred days four parameters were monitored: berries weight, sugar content, total acidity,
and malic acid concentration. Dehumidification halves the grapes weight in 100 days,
improving the water losses of about 20% than the ventilation. As a consequence, the sugar
concentration raise up faster in the dehumidification thesis. In fact, at the end of the tests,
ventilated berries shows 322 g/kg of sugars, while dehidratated ones 338 g/kg. The two
treatments give different trends of total acidity and malic acid concentration. In fact,
ventilation does not affect the total acidity, while dehumidification increases it from 6.5 g/kg to
8.8 g/kg. The malic acid concentration is not affected by dehumidification, while its
concentration decrease with ventilation from 2.9 g/kg to 1.5 g/kg. Hence, the shrivel
technique affects the four considered parameters, giving different berries in terms of weight,
sugar, acidity and malic acid content.
Keywords: Straw wines, Moisture Control, Ventilation
Proceedings International Conference of Agricultural Engineering, Zurich, 06-10.07.2014 – www.eurageng.eu
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1
Introduction
Off-vine grape dehydration is a technique used to produce several sweet wine such as
Vinsanto from Toscana, Trentino or Veneto, Recioto di Soave, Passito di Gambellara or della
Valpolicella, Torcolato, Cinque Terre Sciacchetrà, Vins de Paille of Jura, Strohwein of Austria,
and Ribeiro Toasted (Figueriredo-Gonzalez et al., 2013). During the dehydration grapes
undergo to many changes in chemical profile, due to water losses and to changes in berries
metabolism. As a result, berries becoming more dark; sugar concentration rises, and the
acidic profile changes (De Santis et al., 2012). The dehydration process could be carried out
naturally, or in chambers under controlled conditions. In chambers parameters such
temperature, humidity, and air flow rate on grapes are controlled to guarantee a faster berries
dehydration, ensuring a high final sugar concentration. Particularly, higher temperature, lower
humidity, and faster flow rate accelerate the process. Especially higher ventilation can
improve water losses from berries owing to the removal of the high humidity boundary layer.
Therefore, technologies using high ventilation and relatively high temperature has been
proposed for production of quality straw wine (Amati et al., 1983). Moreover, direct removal
of water around the grapes by dehumidification can accelerate the raising process. Hence,
the climatic chamber approach allows producers to reduce the berry drying process time,
and to separate the process from climatic conditions of each year (Serratosa et al., 2008).
Furthermore, risk related to slow drying rate (i.e. insect attack, or fungi producing toxin
attack) are reduced (Pangavhane et al., 1999).
2
Materials and methods
Trials were carried out for 100 days on grapes (cv Corvina) in two dedicated rooms, one
equipped with a ventilation system, and one with ventilation and dehumification system.
Grapes were stored for test in pallett plateaux. During the tests the evolution of grapes
quality was monitored with the analysis of four parameters: berries weight, berries sugar
content, malic acid concentration, and total titratable acidity. The weight losses are assessed
randomly sampling about 1 kg of grapes, and measuring the weight every 20 days. The other
analises were performed randomly sampling 1 kg of berries. These analises were repeated
every 20 days too.
2.1
Ventilation system (V)
In ventilation system air flows on grapes to facilitate the dehydration. In this system fans
forced air circulation from outside into the room, removing the humidity, and avoiding the
development of stagnant air microclimate near the grapes. The fans work during all day,
while the outside windows are automatically opened when the outside relative humidity is
less than the inside relative humidity. When this condition became false, and the outdoor
humidity became higher the windows were automatically closed.
2.2
Dehumidification system (D)
Dehumidification system is the evolution of the previously described ventilation system. The
system uses the outside air as main raising agent, with the previously described regulation
criterion of difference between inside and outside relative humidity. Furthermore, when is not
possible to open windows for high external humidity the dehumidifier is activated until a preset internal humidity value is reached.
3
Results and Discussions
The considered drying period was 100 days, and the berry weight losses are shown in figure
1. In the first 20 days berries lose about 10 % of their initial weight, and there are no
difference between V and D. Afterward, at 40th day V further removes a 8% of the total
weight, while D further removes 21%. Total weight losses after 40 days were 20% for V, and
31% for D. At the end of the drying period the final losses were 31% for V (this amount of
losses has been reached by D after 40 days), and 49% for D. As a consequence, the grape
sugar concentration rises from 190 g kg-1 to 322 g kg-1 in the system with ventilation, and up
to 338 g kg-1 in the system with dehumidification. Hence, despite D removes about the 18%
of water more than V, the final sugar concentration of berries from the two systems seems to
be quite similar (difference is 16 g kg -1). Sugar concentration linearly increases until day 60
or day 80 for D, and V respectively, then the increment becomes slower. In fact, during the
shrivel process the increment rate of sugar content depends on the balance between the
water evaporation from berries (incrementing effect) and sugar consumption due to
biochemical reaction (decrementing effect). The occurrence of a metabolic change from
aerobic to anaerobic may be hypothesized in our test conditions. In fact, during the
dehydration process abscissic acid and proline are accumulated, and the activity of
lipoxygenase (LOX) and alcohol dehydrogenase (ADH) enzymes increase (Costantini et al.,
2006). The activation of LOX depresses the cells grapes membrane permeability to oxygen,
shifting the metabolism from aerobic to anaerobic, consequently activating the ADH. Under
these conditions other enzymes able to degrade sugars or malic acid to pyruvic acid can be
activated (Marquez et al., 2012). Hence, the pyruvic acid, along with acetaldehyde and
ethanol, could be considered precursors of pyranoanthocyanins and methylmethyne-bonded
anthocyanin-flavanol condensation adducts. According to Bellincontro et al. 2004, the
metabolic shift from aerobic to anaerobic took place at a berries weight loss of about 1015%, corresponding to about 20 days for D, and about 40 days for V. The initial total acidity
of grapes is quite high (6.5 g kg-1), probably because of the meteorological trend during
summer. However, it remain quite constant in V (final total acidity 6.7 g kg -1), while it increase
in D until a final value of 8.8 g kg -1. The increment in titratable acidity has been well
documented for raising grapes, consistently with what was observed in D. By contrast, in V
this increment is probably masked by the lose in malic acid, consistently with the decrease in
malate concentration from 2.9 g kg -1 to 1.5 g kg-1 (figure 2), while malic acid concentration is
quite constant in D (final concentration is 2.4 g kg -1). It is known that malic acid is rapidly
consumed in the early stages of slow grape dehydration, whereas rapid dehydration can
mask the malate loss (Bellincontro et al., 2004). The latter behavior occurs at different extent
in our tests.
4
Conclusions
A faster dehydration rate is provided by the dehumidification system, which was able to halve
the grapes weigh in 100 days, while the ventilation cause a reduction of about the 30%. The
berries final sugar content is 322 g kg -1 in ventilation, and 338 g kg-1 in dehumidification.
Hence, the grater weight loss in the latter system do not concurs with an effective berries
enrichment in term of sugar. Total acidity remains quite constant with ventilation, and
increase from 6.5 g kg-1 to 8.8 g kg-1 with dehumidification. On the other hand, the malic acid
concentration remain constant in dehumidification, and decrease from 2.9 g kg -1 to 1.5 g kg-1
with ventilation. Hence, the dehumidification could be used to reduce the raising time, and
consequently the risk related to molds and insects. On the other hand, it could modify the
acidic profile of the must.
5
References
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l'appassimento delle uve. Vigne Vini, 10, 27-35.
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Food and Agriculture, 84(13), 1791–1800.
Costantini, V., Bellincontro, A., De Santis, D., Botondi, R., & Mencarelli, F. (2006). Metabolic
change of Malvasia grapes for wine production during postharvest drying. Journal of
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De Sanctis, F., Silvestrini, M. G., Luneia, R., Botondi, R., Bellincontro, A., & Mencarelli, F.
(2012). Postharvest dehydration of wine white grapes to increase genistein, daidzein and the
main carotenoids. Food Chemistry, 135(3), 1619–25.
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Pangavhane, D. R., Sawhney, R. L., & Sarsavadia, P. N. (1999). Effect of various dipping
pretreatment on drying kinetics of Thompson seedless grapes. Journal of Food Engineering,
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Serratosa, M. P., Lopez-Toledano, A., Medina, M., & Merida, J. (2008). Drying of Pedro
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120
weight losses (%)
100
80
60
V
D
40
20
0
0
20
40
60
80
100
120
days
Figure 1: weight losses during grape dehydration. V is weight losses due to ventilation, and D to
dehydration
10
9
concentration (g/kg)
8
7
6
TA V
TA D
MA V
MA D
5
4
3
2
1
0
0
20
40
60
80
100
120
days
Figure 2: trends in titratable acidity (TA), and malic acid concentration (MA) during raisin. V is for
ventilation, D is for dehydration.