Agriculture Reference
In-Depth Information
are universally produced from the small dark seedless
Zante type grape (known as Black Corinth in California)
although other grapes (e.g. the Australian Carina are used;
Arthey & Ashurst 2001). In Australia both Thompson's
seedless and Sultana grapes are used to make sultanas. In
California, Thompson's seedless are used to make raisins.
In both California and Europe, Sultanas are used to make
sultanas. Muscat raisins are known as such in Europe and
America, while in Australia, they are known simply as rai-
sins. These raisins are sweeter than other types. The Muscat
grapes have large berries with seeds which may or may not
be removed after drying (Arthey & Ashurst 2001).
hydroxide (NaOH), citric acid or a mixture such as an
alkaline, oil-in water emulsion. There may be benefits to
combining alkali dipping with a microwave pre-treatment to
reduce total drying time (Kostaropoulos & Saravacos 1995).
It appears that the dip changes the structure of the waxy
bloom making it more permeable to water (Rojchev &
Botiyanski 1998). It also seems that it makes the grape
more transparent to infrared rays, allowing a better radiant
heat uptake. It can speed up the drying process by several
weeks (from 4-5 weeks to 8-14 days). Pre-treatments such
as NaOH and citric acid have been shown to cause a sub-
stantial reduction in cell wall pectins (Femenia
et al
. 1998).
Muscat grapes are usually alkaline treated due to their
larger size. Those that are not treated are carefully handled
so as not to damage the bloom. The resulting dried fruits
are used for high-class outlets such as health food stores
(Arthey & Ashurst 2001). To optimise quality, various
combinations of pre-treatments may need to be evaluated
for particular cultivars and under local conditions
(Gowda 2000).
Harvest maturity indices
A high-quality dried product depends on the harvest quality.
This is determined by the berry size, the uniformity and bril-
liance of the berry colour, the texture of the skin and pulp,
the moisture content, chemical composition and presence of
decay and foreign matter. There is evidence that the timing
of harvest can be critical to final quality, not just in terms of
berry sugar content but that it affects other parameters such
as product colour (Uhlig & Clingeleffer 1998).
Improving product quality
A light colour for dried grape products is considered highly
desirable. The extent of browning in the dried product is
determined amongst other things by the activity of poly-
phenol oxidase (PPO) particularly in the skin of the berry.
Cultivars with a naturally low level of PPO dry to a lighter
colour than others (Rathien & Robinson 1992). Low (<21°
Brix) or very high sugar levels (>23° Brix) can increase
browning in the dried fruit (Uhlig & Clingeleffer 1998).
Berries exposed to the sun before harvest tend to produce
darker dried product than shaded fruits (Uhlig 1998).
In some countries, sultanas and occasionally raisin
grapes are treated with sulphur dioxide to bleach the fruit
and give a more golden colour. The fruits are placed in
purpose-built fumigation chambers (houses). Sulphur is
then burned in a draught channel under the chamber, the
gas enters the chamber and treats the fruit. Residues up to
2000 mg/kg are permitted. The fruits are then dried by one
of the methods described above.
Some products may be given a light coating of mineral
oil to improve handling and prevent stickiness and clump-
ing when packaged.
Harvesting
Grapes for drying are usually hand-picked. Generally,
machine harvesting causes too much berry damage but
the canes can be pruned mechanically with the bunches
still attached and hung to dry on the vine (Jackson &
Looney 1999).
Drying technology
A review of raisin production is given by Arthey and
Ashurst (2001), Patil
et al
. (1995) and Waskar (1993). For
efficient drying, grapes should have a high sugar content of
20-24° Brix. The grapes may be dried naturally (common
in California, Iran and the USSR) or they may be pre-treated
to speed up the drying process. A solar drier that is substan-
tially more efficient than natural drying for grapes has been
described by Fuller
et al
. (1990). The moisture reduces
from about 70% to about 15%. Differences in the thickness
and toughness of the skin between varieties influence the
rate of water loss in raisin making (Winkler
et al
. 1974).
The raisins are then winnowed mechanically to remove the
capstem, leaves and stem pieces. After washing and
grading, the raisins are filled into packs ranging in size
from a few grams to bulk packs of about 12.5 kg for other
food manufacturers to use.
For some products it is normal to dip the grapes before
drying in a solution of potassium carbonate (2.5-4.5%)
containing a 'dipping' oil. Other dips include sodium
Problems of dried grapes
An important quality problem of dried grapes is the migra-
tion and crystallisation of sugars on the outside known as
sugaring. Skin characteristics influence the degree of
sugaring of natural raisins during storage. The delicate skin
of Monukka raisins renders them susceptible to sugaring,
