Agriculture Reference
In-Depth Information
storage are too expensive, sweet potato roots rarely keep
for more than 2-3 weeks (Rees
et al
. 2001, Tomlins
et al
.
2002). In this case a wide range in keeping quality of
different varieties has been observed, and one approach to
extending shelf life would be the introduction of cultivars
with better keeping qualities, an approach that would cause
no additional cost to farmers and traders (Rees
et al
. 2003).
In-ground storage is practiced, but not as widely as for
cassava. The main problem is that at the onset of the dry
season infestation by the sweet potato weevil (
Cylas
spp)
becomes significant and damages the crop (see below).
Traditional storage technologies for sweet potato roots
have been reported in tropical countries such as Bangladesh
(Jenkins 1982), India (Prasad
et al
. 1981, Ray & Ravi 2005),
Tanzania (Tomlins
et al
. 2007) and Kenya (Karuri & Ojijo
1994, Karuri & Hagenimana 1995). The success of these
storage technologies, however, has been variable (Ray &
Ravi 2005). A review of storage methods (Ray & Ravi 2005)
has highlighted the need for further assessment of the factors
affecting storage because an understanding of these factors
remains incomplete. In Tanzania, a study located at
a research station investigated stores that varied by store type
(pit or heap), cultivar and ventilation by measuring O
2
and
CO
2
levels, relative humidity, temperature, root condition
and weight loss (Van Oirschot
et al
. 2007). The findings
indicated that the main factors that improved storability of
fresh sweet potato under tropical conditions were the use of
good-quality roots free of damage and disease, not lining the
stores with grass, and avoiding temperature build-up in the
stores. The type of store (pit or heap), cultivar and ventila-
tion were all found to have minimal effect on root keeping
qualities. The study concluded that fresh roots could be
stored for up to 12 weeks and that, by this time, stored roots
may taste sweeter than freshly harvested ones (Tomlins
et al
.
2007). The design of two stores that were tested in Tanzania
are shown in Figure 18.5.
(a)
Heap store
SHADE
Sweetpotatoes
Roof
Drainage
channel
Drainage
channel
Soil
(b)
Pit store
SHADE
Roof
Drainage
channel
Drainage
channel
Soil
Sweetpotatoes
Figure 18.5
Construction of heap and pit stores in
the Lake Zone of Tanzania.
snack products (Owori & Agona 2003). Substitution of
wheat flour, either with fresh, grated roots or sweet potato
flour, is gaining a foothold in the snack product market in
Kenya and Uganda and bread containing biofortified sweet
potato in Mozambique (Tomlins
et al
. 2009). Promotion of
commercial processing of primary products would increase
the utilization of sweet potato flour as an ingredient in
snack product processing. There is great variation in the
processing characteristics of sweet potato cultivars but
generally dry matter is an important characteristic.
While many new sweet potato products have been
developed in Africa, the priority is still predominantly in the
improvement of the quality of the flour after drying and stor-
age (van Hal 2000). Recent initiatives have focused on the
Processing of sweet potato
In countries where sweet potato is a staple, one strategy to
ensure the availability of sufficient food, especially during
the lean seasons and where sweet potato cultivation is lim-
ited to only one season in a year, is to process the roots into
dried chips and store them in this form (Owori & Agona
2003). The dried chips are either reconstituted by boiling
or are ground into flour which may or may not be mixed
with millet/sorghum flour for making porridge. Sweet
potato processing for human consumption in many
countries is not yet commercialized. Studies in some coun-
tries have, however, investigated the feasibility of sweet
potato as a partial substitute for imported wheat flour in
