Agriculture Reference
In-Depth Information
Table 18.8
Main Rotting Pathogens of Fresh Cassava Roots.
Pathogen
Occurrence
Symptoms
Control
Botryodiplodia
theobromae
Columbia, Costa
Rica, Brazil,
Nigeria, India
Discolouration of internal
tissue, grey mould on
cut surfaces.
Roots may be infected prior to harvest via
growth cracks or due to harvest damage.
Can be controlled by post-harvest fungicide
or low temperature storage.
Fusarium solani
Roots may be infested prior to or during
harvest.
Can be controlled by post-harvest fungicide.
Other rotting pathogens that are significant post-harvest:
Aspergillus
, bacterial soft rot (primary rot usually due to
Erwinia
spp.),
Mucor
,
Phytophthora
,
Rhizopus
,
Sclerotium
,
Sphaerostilbe
, and
Trichoderma
.
Columbia, Costa Rica
Nigeria, India
Puerto Rico
Dry rot, discoloured flesh.
White surface mould
in humid conditions.
Source: Collated from Snowdon (1991).
non-enzymatically at a rate dependent upon pH and
temperature (Cooke 1978) with their stability increasing at
acidic pH values.
Several studies have been undertaken to examine the
rate of cyanogenic glucoside breakdown at different stages
of processing. Grating, sun-drying and roasting all
contribute to cellular breakdown, as does the fermentation
process itself. (Mlingi & Bainbridge 1994, Jones
et al
.
1994, Bainbridge
et al
. 1998). Dissemination of informa-
tion on the relative efficiency of different processes is very
important, especially in regions where new cassava varie-
ties with higher cyanogenic potential have been introduced.
This has happened, for example, where varieties with
increased resistance to African Cassava mosaic virus have
been released (Bockett 1997, Bainbridge
et al
. 1997).
Table 18.8. As storage of the fresh root is relatively short-
term, insect pests are only really significant for the dried
products. Losses of dried cassava products of up to 75% have
been reported in Tanzania as a result of infestation by Larger
Grain Borer (
Prostephanus truncates
) (Hodges
et al
. 1985).
STORAGE HANDLING AND PROCESSING OF
SWEET POTATO
Sweet potato is a commodity which is handled and stored
in both temperate and tropical countries and in situations
covering a wide range in the availability of resources.
Where facilities are available for temperature controlled
storage sweet potatoes can be stored for more than 6
months, however, in the absence of temperature control
storage life is generally restricted to a few weeks.
Cassava processing for international trade
In many countries cassava is dried for export. The principal
markets for cassava products are in Europe - the European
Union being the most important for dried roots - and for
cassava starch the United States, the United Kingdom and
Japan. Although complete statistics of world trade in
cassava products are not available, thus making it difficult
to estimate the total quantity entering international trade,
the import statistics of the EEC and the United States show
a substantial increase in recent years, particularly for dried
cassava roots. Cassava starch is used as a raw material for
a wide range of food products and industrial goods,
including paper, cardboard, textile plywood, glue and
alcohol (Tonukari 2004).
Handling and storage in developed countries
In the United States, storage temperatures of 12.5°C to
15°C and a relative humidity of 85% or higher are
recommended (UC Davis 2008). Under these conditions
storage is possible for 6-10 months. Sprouting may occur
after 6 months, but it is not common practice to use sprout
suppressants. Chilling injury may occur at lower storage
temperatures. Curing is standard practice for which roots
are placed at 25-32°C with relative humidity greater than
90% for up to one week. Minimising root damage during
harvesting and subsequent handling is considered very
important for successful storage. Thus roots are not washed
before storage. Irrigation may be stopped 2-3 weeks before
harvest to allow the tops to start drying, and this may lead
to some toughening of the root periderm as seen in potatoes.
Cassava Post-harvest Pests and Diseases
As indicated above where fresh roots rot it is often an
indication of the onset of post-harvest physiological
deterioration. The most common rots are summarised in
Handling and storage in developing countries
In developing countries where roads may be rough,
packaging to cushion damage and temperature controlled
