Agriculture Reference
In-Depth Information
A number of storage methods were tested by CIAT/NRI
(Diop & Calverley 1998, Westby 2000), Conical heaps or
clamps with 300-500 kg roots were constructed with a
layer of straw on the base and another layer of straw cov-
ering the roots, with an additional layer of soil. Openings
were left at the bottom of the heap to provide ventilation.
Although this storage method could successfully store roots
for more than four weeks, it was found to be difficult to
adapt to changing seasons, in terms of ventilation and the
risks of water damage. There was also had a high labour
requirement.
Another method tested was to use wooden crates with
alternate layers of sawdust and cassava roots, starting and
finishing with a layer of sawdust. Wood shavings and, peat
and other packing material could also be used as long as the
material was moist but not wet. Physiological deterioration
occurred if the material was too dry so that wound-healing
was impaired, while microbial decay accelerated when it
was too wet. With this method 75% of the roots remained
acceptable after four weeks in store, provided the roots
were packed immediately on the day of harvest. With
a delay of only one day only 50% of the roots were rated as
acceptable. The costs of this method make it suitable for
export but not domestic trade.
Probably the most practical method of storing cassava
roots for domestic (usually urban) markets is the use of
airtight plastic bags. When also treated with a fungicide
such as thiabendazole, the roots can be kept for two to
three weeks. This method was successfully introduced
to Ghana (Crentsil
et al
. 1995) where it was shown that
household bleach (0.95% active chlorine) was almost as
effective as thiabendazole. In practice most people used
no fungicide, but sprinkled the roots with water, and
often used woven rice or cocoa sacks. Even with these
adaptations wound-healing can occur and storage of 7-10
days is possible
More expensive methods of storage such as freezing and
waxing have been used for export markets primarily for
export markets to Europe and America. Low temperature
storage is also feasible taking into account that as a tropical
product cassava is likely to be susceptible to chilling injury.
of Cassava in Africa (COSCA) supported by the Rockefeller
Foundation has studied the range of products in Africa,
covering Cote D'Ivoire, Ghana, Nigeria, Democratic
Republic of Congo, Uganda and Tanzania (NRI 1992).
Only a brief summary will be given here.
Products may be separated into dry or moist. Dry
products, including chips,
gari
,
farinha
,
cassava bread
are
generally less time-consuming to produce and the most
practical processed form, with a longer storage shelf life.
Moist products, such as
attieke
,
chikwangue
,
batons
can
take a long time to prepare, and their shelf life is relatively
short (Diop & Calverley 1998).
For many products fermentation is an important part of
the process. This essentially refers to all moist products
and many of the dry products. Fermented products can be
divided into three types depending on type of fermentation
(Westby & Twiddy 1992):
1. Products that are fermented in grated form include
gari
,
where roasted granules are fermented in sacks,
attieke
from Cote d'Ivoire, where steamed granules are
fermented, and fermented pastes such as
agbelina
from
Ghana and
placali
from Cote d'Ivoire. These products
undergo a lactic acid fermentation associated with a
decrease in pH.
2. Processing using underwater fermentation is practised
across Africa e.g.
akpu
,
fufu
,
chikwangue
. In this case at
the start of fermentation a wide mix of flora is found,
but this is later dominated by lactic acid fermentation
and yeasts.
3. The third method involves fermenting whole roots in a
heap as in the case of
udaga
produced in Tanzania. The
main fermenting organisms in this case are
Rhizopus,
Mucor, Penicillium, Fusarium
.
In many cases when products are dried, where the drying
process is slow, for example when sun-drying is used out of
the dry season, fermentation occurs, even though it is not
intended. This is usually not a problem, but can lead to
contamination with mycotoxins.
Processing is important for removal of cyanogens.
Breakdown of cellular structure is key to this process. As
described previously, the cyanogenic glucosides present in
fresh cassava roots are linamarin (93%) and lotaustralin
(7%) (Nartey 1978). These are located in the cell vacuole.
During processing if the cells are broken the cyanogenic
glucosides can be hydrolysed to the corresponding ketone
(cyanohydrin) and glucose by the endogenous enzyme,
linamarase, which is located in the cell wall (de Bruijn
1973, Nartey 1978). Cyanohydrins will then break down
Cassava processing
The processing of cassava into more storable forms offers
an opportunity to overcome the perishability of the fresh
root. Processing is also important for the removal of cyano-
gens to produce a safe product. In the case of cassava, a
wide variety of products are produced; especially in Africa
and South America. The full range of products has been
reviewed elsewhere; for example the Collaborative Study
