Agriculture Reference
In-Depth Information
of yams at harvest affects the post-harvest behaviour of the
tubers, so that the optimum storage methods may differ. It
appears that less mature tubers may store better with less
ventilation (higher humidity) (Rees & Bancroft 2003).
yam flour. As the major cities in southwest Nigeia sell
amala in the public restaurants, amala is becoming widely
consumed. Thus, the demand for yam flour is on the rise.
Studies in Benin and Nigeria have indicated that there is
a significant problem of aflatoxin contamination due to
poor drying and storage (Adeleke 2009). According to
surveys carried out in Benin by Bassa (2000), 98% of the
analyzed dried sticks had an aflatoxin content higher than
the European limit of 4 ppb and 6% of dried sticks beyond
the OMS standard of 20 ppb. These results were supported
by those obtained subsequently by Fagbohoun (2001). The
use of preservatives to inhibit mould, possibly using
traditional herbs, during the processing of amala should be
investigated as a means of control.
Processing
By far the greater part of the yam crop in West Africa is
consumed fresh. Fresh tubers are prepared for eating as
boiled or pounded yam, mashed, fried or baked. The prepa-
ration of pounded yam involves peeling, boiling and
pounding the yam tissue until a sticky, elastic dough is pro-
duced. This is called pounded yam or yam fufu. Some
small-scale processing of yam occurs notably in South
West Nigeria and in Benin, the processing is mainly to uti-
lise damaged tubers at harvest.
Given the problems of storing fresh tubers, drying of
tubers, especially those that have been damaged, soon after
harvest and converting them into dried slices or milling
into flour for
amala
or
fufu
is an effective strategy for
extending availability.
Yam flour is generally regarded as an inferior substitute
for freshly pounded yam because it is often made from
damaged tubers. Nevertheless, consumption of products
such as amala made from yam flour is becoming more
popular in urban centres.
The most common method of drying yam is to cut it into
sticks and sun-dry it. Drying of sticks of yam for example
is practiced in central and northern areas of Benin
(Dossou, R.A.
Pers. Comm
.). Tubers are sliced to a
thickness of about 10 mm, more or less, depending on the
dryness of the weather. The slices are then parboiled and
allowed to cool in the cooking water. The parboiled slices
are peeled and dried in the sun to reduce the moisture
content. The dried slices are then ground to flour in a
wooden mortar and repeatedly sieved to produce a uniform
texture. Small, hand-operated or engine-driven corn mills
or flourmills are used in some areas.
The blanching/parboiling stage reduces discolouration
(oxidation of phenolics) during drying, and can also reduce
subsequent damage by insects and fungal pathogens.
Treatment with sodium bisulphate may also be used to pre-
vent phenolic oxidation. Parboiling can also change textural
properties. Higher temperatures make the resulting dried
sticks softer and gives a lower temperature of gelatinization.
The yam flour is rehydrated and reconstituted into
fufu
or
amala
and eaten with a soup containing fish, meat and/
or vegetables.
Industrial processing is increasing, especially in Nigeria
(Akaroda M.
Pers. Comm
.). The most important industrially
processed yam product found in the market is the pounded
Post-harvest pests and diseases of yam
It has been reported that insect pest attack in storage can
result in as much as 20% losses (Okoedo-Okojie &
Onemolcase 2009). However, of the range of factors that
precipitate losses of marketable tubers, rots are the most
immediately apparent causing both primary and secondary
damage. An extensive list of fungal and some bacterial
organisms have been isolated from decaying yams
(e.g. Coursey 1967; GTZ 1995; Nwankiti
et al
. 1988;
Ogundana 1982; Thompson
et al
. 1977). The main
spoilage microrganisms responsible for rotting are listed
in Table 18.10.
Aspergillus, Lasiodiplodia, Fusarium.
Table 18.10
List of Micro-organisms Isolated
from Yam Tuber Rots.
Aspergillus
sp.
Aspergillus niger
van Tiegh.
Aspergillus flavus
Link ex Fr.
Chanephora cucurbitarum
(Berk and Fac.)Thax
Erwinia carotovora
.
Fusarium sp
.
Fusarium culmorum (W.G. Smith) Sacc.
Fusarium solani
(Mart.) Sacc.
Fusarium oxysporium
Schlecht.
Lasiodiplodia theobromae
Pat.
Mucor
sp.
Pencillium sp
.
Pencillium brevi-compactum
Dierckx
Pencillium oxalicum
Currie and Thom.
Rhizopus
sp.
Rhizopus stolonifer
(Ehrenb) Lind
Trichoderma
sp.
Source: Rees and Bancroft (2003).
