Agriculture Reference
In-Depth Information
Table 18.4
Typical Composition of Sweet Potato
Roots.
more efficiently than cassava, and thereby reduce both water
loss and pathogen entry through wounds.
Levels in storage root
(units as indicated)
The process of wound-healing in sweet potato and
its role in determining storability
The healing response of surface tissues in root crops such as
sweet potato is very important for the protection of damaged
roots against water loss and pathogen invasion, and is key to
ensuring good keeping quality. It has been demonstrated
that a wide range in shelf life of sweet potato cultivars is
associated with variability in the efficiency of wound-healing
(van Oirschot
et al
. 2006); Although sweet potato cultivars
are generally all able to heal wounds efficiently when
placed at high humidity, they differ significantly in their
ability to heal wounds when kept at low humidity (Van
Oirschot
et al
. 2002, 2006). Wound-healing of roots and
tubers, notably potatoes, has been studied extensively under
controlled high humidity conditions (Walter & Schadel
1983, Burton 1989, St Amand & Randle 1991). However,
tissue response at low humidity, which is generally more
relevant for plant tissues, both under natural conditions and
notably marketed commodities wherever the commodity is
handled in the absence of temperature/humidity control,
has been largely overlooked. Given its importance for sweet
potato shelf life this has recently become a focus for
investigation in sweet potato.
Descriptions of the process of wound-healing in sweet
potato date from the 1920s when Weimer and Harter (1921)
described how moisture and temperature affect the wound
periderm formation and the efficiency of the wound cork in
preventing infection. Artschwager and Starret (1931)
distinguished three stages of healing: (1) desiccation of sev-
eral cell layers of parenchyma, (2) thickening of cell walls
(lignification) in underlying cell layers and (3) formation of
the wound periderm. The desiccation of cell layers in which
the cells on the surface dry out and die is the first response
after wounding. At low humidity, poor wound-healing is
associated with a thick desiccated layer and slow or
incomplete lignification (Van Oirschot
et al
. 2006).
Continuity of the lignified layer is vital for effective
wound-healing, presumably to act as an effective barrier to
water loss and pathogen invasion (Van Oirschot
et al
. 2006).
A method for assessing efficiency of wound healing based
on assessing the continuity of lignified layers by phloroglu-
cinol staining (lignification score) was developed by Van
Oirschot
et al
. (2002, 2006), and has been used as a tool for
screening sweet potato germplasm.
A relationship of sweet potato root dry matter content
both with shelf life (Rees
et al
. 2003) and with wound-healing
efficiency, has been reported previously (Van Oirschot
et al
.
Component
Dry matter (%)
14.7-39.9
Total sugar (% DM)
5.6-46.8
Starch (%DM)
12.3-70.8
Protein (% DM)
2.7-10.4
Protein (% FW)
0.5-2.4
Fat (% FW)
1.8-6.4
Carotene (mg/100g)
0-13.3
Ascorbic acid (μg/g fresh tissue)
68-284
Crude fibre (g dry fibre/g fresh
tissue)
0.04-0.35
Source: Adapted from Collins (1989).
potato presently consumed in sub-Saharan Africa is white
or yellow fleshed, orange fleshed varieties biofortified with
pro-vitamin A are becoming more widespread. More than
three million children under the age of five in the region
suffer from vitamin A related blindness, and vitamin A defi-
ciency is one of the leading casues of early childhood death
and a major risk factor for pregnant women. Adding 100 g
of orange fleshed sweet potato to the daily diet can prevent
vitamin A deficiency in children, dramatically reduce
maternal mortality and lower the risk of mother-to-child
transmission of HIV/AIDS (van Jaarsveld
et al
. 2005).
Ex -ante impact assessment study indicated that introduc-
ing the new high-beta carotene varieties that meet local
preferences would benefit an estimated 50 million children
under the age of six who are currently at risk in addition to
significant benefits for childbearing women (Kapinga
et al
.
2005, Low
et al
. 2001, 2007).
The root is also an important source of ascorbic acid
(Vitamin C) with some cultivars containing more than
30 mg/100 g (Visser & Burrows 1983).
Storability of sweet potato
Sweet potato roots do not exhibit the post-harvest physio-
logical deterioration described above for cassava, and even
without temperature control can, with care, be kept for
weeks rather than days. Thus in East Africa sweet potatoes
have a shelf life of one to two weeks during marketing (Rees
et al
. 2001). In storage trials carried out under simulated
marketing conditions in Tanzania, it was found that rates of
root weight loss and rotting both varied considerably among
cultivars. This has been attributed to variation in wound-
healing efficiency. Sweet potatoes are able to wound-heal
