Agriculture Reference
In-Depth Information
Harvesting and post-harvest management
Temperature control
The temperature of freshly picked grapes should be as low
as possible to limit biochemical alteration processes
(Rankine 1997). Grapes are often picked at night with
mechanical harvesters and transported in refrigerated
trucks when long distance trips are necessary. Solid carbon
dioxide (CO
2
) or dry ice is sometimes used to cool
harvested grapes (see below).
Juice and jelly production
Juice is an important product in some countries (see
section
Product statistics
). For white grapes, the juice is
extracted from the crushed fruits using a basket press. The
juice is filtered through cloth and bottled. It may be
preserved either by adding SO
2
, sodium benzoate or by
pasteurisation. Although the flavour of the juice is better
from cold-pressed fruits, hot pressing can increase yields
by as much as 20%. Coloured grapes need to be heated for
10-15 min at 60-63°C in order to extract the coloured
anthocyanins treatment (Patil
et al
. 1995).
Browning is one of the most important quality changes
in grape juice, especially from white grapes (Yokotsuka
et al
. 1988). Intensity of browning depends on the activity
of polyphenol oxidase and the type and concentration of
certain phenolic compounds in the juice (Sapis
et al
.
1983a). Soluble proteins can cause haze and sediments in
the final product and levels are exacerbated by damage to
the fruit caused by mechanical harvesting and long distant
transport which delays pressing (Pocock
et al
. 1998).
Sediments in the juices can be removed by clarification
with kaoline or bentonite treatment, freezing or enzyme
treatment (Patil
et al
. 1995).
Some grape juice is clarified for use in blended juices. The
prevention of 'wine stone' or the crystallisation of tartrate is
a problem for long-term storage of the juice. Exchanging
sodium for potassium in the juice by ion exchange will
change the relatively insoluble potassium hydrogen tartrate
for the more soluble sodium salt (Arthey & Ashurst 2001).
In the United States, about 30% of juice produced is
used to make jelly. The most important cultivar in the
United States for juice and jelly production is Concord
(Patil
et al
. 1995). Grape juice is occasionally concentrated
following de-acidification and removal of water usually by
evaporation under a partial vacuum. Concentrated grape
juice can have a sugar precipitate if it is concentrated above
about 55° Brix but this will quickly re-dissolve on dilution
(Arthey & Ashurst).
Oxidation control
Some antioxidants (ascorbic acid, sulphur dioxide) are
sometimes introduced into the transport bin just after
harvest, but SO
2
addition is not recommended before
de-stemming as it favours the extraction of compounds
with a grassy taste (Ribéreau-Gayon
et al
. 1998). Limiting
oxygen access to the grapes after mechanical harvest can
also help to minimise unwanted oxidative changes (Flanzy
1998) and inert gases (CO
2
, N
2
) have been tested during
transport.
Before pressing grapes used for white wines, CO
2
is
sometimes added to protect the grapes from oxidation and
cool them down (for example using solid CO
2
). On-line
addition of CO
2
gas in the must as an antioxidant is
sometimes set-up between the hopper and the storage tank,
or the pre-fermenting vat.
Sorting
Sorting is a crucial step to get the best of hand-harvested
grapes, and may limit the quantity of rotten bunches to be
sent to a given tank, thus increasing the must quality and
limiting the amount of sulphur dioxide required. Ideally
sorting is carried out in the vineyard or it can be done on
arrival in the winery. For mechanically harvested grapes,
systems separating stems from juice are recommended to
avoid a grassy taste (Ribéreau-Gayon 1998).
Drying
Natural drying on shelves or on the ground, for example as
carried out in Jerez, Spain, is another post-harvest step
aiming to concentrate the sugars and obtain a higher
alcohol degree. The yield in juice is usually very low, as
little as 300 L / ton of grapes (Ribéreau-Gayon
et al
. 1998).
The vineyard has to be in a suitable climatic area (at least
dry autumn afternoons to avoid strong rot developments).
Alternatively, artificial drying using forced warm air may
be used. The association of partial dehydration and changes
in protein patterns has been studied recently (Di Carli
et al
.
2010).
POST-HARVEST TECHNOLOGY FOR
DRIED GRAPES
Introduction
The main dried products of grapes are raisins, sultanas and
currants. Raisins are the second most important product of
the grape vine after wine (Shanmugavelue 1989).
Cultivars
The main grape types used for commercial drying are all
V. vinifera
cultivars (Jackson and Looney 1999). Currants
