Agriculture Reference
In-Depth Information
Table 18.3
Composition of Cassava Roots.
where the distance between production and consumption
increases (e.g. Westby 2002). The whole process has been
the subject of considerable investigation as an improved
understanding is perceived as key to extending cassava shelf
life and hence its potential to improve food security.
% in peel
(8-15% of
the root)
% in whole roots
Dry matter
35
30
Post-harvest physiological deterioration in Cassava
Post-harvest physiological deterioration (PPD) in cassava
has been identified as an enzymatically driven response
that occurs throughout the root as a result of wounding and
oxidative stress incurred at harvest. The first symptoms
involve a discolouration of the vascular tissues that spreads
from the wound sites, and then into the storage parenchyma
(Plate 18.1). The pattern of fluorescence observed when
tissues are exposed to UV light indicates the production of
phenolic compounds including scopoletin and tylose. The
blue/black vascular streaking has been attributed to the
peroxidase-mediated oxidation of scopoletin (Wheatley &
Schwabe 1985). Reilly
et al
. (2003) support this hypothe-
sis by demonstrating the presence of both reaction
components and peroxidase activity. Coloured occlusions
and tyloses appear to block xylem vessels. Other symptoms
related to PPD include increases in respiration and
mobilisation of starch to sugars by the activity of acid
invertase, changes in lipid composition and increased
synthesis of phenols, diterpenes and catechins as well as
increased synthesis of ethylene.
PPD appears to be a continuous cascade of wound
response spreading through the root. It is interesting that
when roots remain attached to the plant they are capable of
normal wound repair (Mwenje
et al
. 1998), it is only after
harvest that wound repair and down modulation appear to
be inadequate, leading to this cascade response. This
response is not found in the other main root crops for which
the storage roots/tubers are reproductive.
Thus, PPD is not considered to be degenerative, but an
active response of the cassava root involving gene activation,
protein synthesis and secondary metabolite accumulation.
Inhibition of protein synthesis by chemical inhibitors or
heat treatment slows the process (Uritani
et al
. 1984).
CDNA-AFLP analysis indicates that genes expressed during
PPD include defensive compounds such as antimicrobials,
antioxidants and, cell wall components. Up-regulation of
genes related to defence and wound healing is observed
such as phenyl alanine ammonia lyase (PAL), b-glucanase
and hydroxyproline-rich glycoproteins and of proteases,
protease inhibitors and other genes implicated in senescence
or programmed cell death responses in other plant systems
% of dry matter (DM)
Carbohydrate
(mainly starch)
89
75
Lipid 1 2
Protein 2.5 4
Fibre 4.5 12
Ash 3 5
Calcium 0.125 —
Phosphorus 0.1 —
Thiamine 0.125 mg/100g DM —
Iron 2.2 mg/100g DM —
Ascorbate 0.39 —
Source: Adapted from Balagopalan
et al
. (1988) and
Silvestre (1989).
and phloem (11-20% of total root weight). The innermost
tissues, constituting about 85% of the total root weight are
the parenchyma, made up of starch containing cells and the
xylem vessels (Alves 2002).
Cassava root composition
Table 18.3 summarises the typical composition of a cas-
sava root. Despite its high starch content cassava is low in
protein and key micronutrients (zinc, iron and vitamins).
When disrupted the root tissues may produce toxic hydro-
gen cyanide which can have very important implications
for human health (see below)
Cassava root deterioration
Of the main starchy root crops cassava has the shortest shelf
life. Two types of post-harvest deterioration are recognised in
cassava; primary physiological deterioration which involves
internal discolouration and the initial cause of loss of
acceptability, and secondary deterioration due to microbial
spoilage (Booth & Coursey 1974). Physiological deterioration
is thought to be a physiological response to tissue damage
during harvesting. In most cases it is seen as a blue-black
discolouration of the vascular tissue which is generally called
vascular streaking. These initial symptoms are followed by a
more general discolouration of the tissue. Where cassava is
harvested and consumed locally this is not a great problem,
but it is a serious constraint for the development of marketing,
