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African Journal of Biotechnology Vol. 5 (5), pp. 384-395, 1 March 2006
Available online at http://www.academicjournals.org/AJB
ISSN 1684–5315 © 2006 Academic Journals
Review
Sorghum grain as human food in Africa: relevance of
content of starch and amylase activities
Mamoudou H. Dicko1,2,3*, Harry Gruppen2, Alfred S. Traoré1, Alphons G. J. Voragen and
Willem J. H. van Berkel3
1Laboratoire de Biochimie, UFR-SVT, CRSBAN, Université de Ouagadougou, 03 BP. 7021, Ouagadougou 03, Burkina
Faso,
2Laboratory of Food Chemistry, Department of Agrotechnology and Food Sciences, Wageningen University, PO Box
8129, 6700 EV Wageningen, The Netherlands.
3Laboratory of Biochemistry, Department of Agrotechnology and Food Sciences, Wageningen University, PO Box 8128,
6700 ET Wageningen, The Netherlands.
Accepted 17 November, 2005
Sorghum is a staple food grain in many semi-arid and tropic areas of the world, notably in Sub-Saharan
Africa because of its good adaptation to hard environments and its good yield of production. Among
important biochemical components for sorghum processing are levels of starch (amylose and
amylopectin) and starch depolymerizing enzymes. Current research focus on identifying varieties
meeting specific agricultural and food requirements from the great biodiversity of sorghums to insure
food security. Results show that some sorghums are rich sources of micronutrients (minerals and
vitamins) and macronutrients (carbohydrates, proteins and fat). Sorghum has a resistant starch, which
makes it interesting for obese and diabetic people. In addition, sorghum may be an alternative food for
people who are allergic to gluten. Malts of some sorghum varieties display α
αα
α-amylase and ß-amylase
activities comparable to those of barley, making them useful for various agro-industrial foods. The
feature of sorghum as a food in developing as well as in developed countries is discussed. A particular
emphasis is made on the impact of starch and starch degrading enzymes in the use of sorghum for
some African foods, e.g. “tô”, thin porridges for infants, granulated foods “couscous”, local beer
“dolo”, as well agro-industrial foods such as lager beer and bread.
Key words: sorghum, α-amylase, β-amylase, starch, infant porridge, beer, couscous, dolo, tô, bread.
INTRODUCTION
Sorghum [Sorghum bicolor (L.) Moench], a tropical plant
belonging to the family of Poaceae, is one of the most
important crops in Africa, Asia and Latin America (Figure
1; Anglani, 1998). More than 35% of sorghum is grown
directly for human consumption. The rest is used
primarily for animal feed, alcohol production and
industrial products (FAO, 1995; Awika and Rooney,
2004a). The current annual production of 60 million tons
is increasing due to the introduction of improved varieties
and breeding conditions. Several improved sorghum
varieties adapted to semi-arid and tropic environments
*Corresponding authors E-mail: : mdicko@univ-ouaga.bf, Tel:
+226 70272643, Fax: +226 50307242.
are released every year by sorghum breeders. Selection
of varieties meeting specific local food and industrial
requirements from this great biodiversity is of high
importance for food security. In developing countries in
general and particularly in West Africa, demand for
sorghum is increasing. This is due to not only the growing
population, but also to the countries policy to enhance its
processing and industrial utilization (Akintayo and Sedgo,
2001). More than 7000 sorghum varieties have been
identified (Kangama and Rumei, 2005); therefore there is
a need of their further characterization to the molecular
level with respect to food quality. The acquisition of good
quality grain is fundamental to produce acceptable food
products from sorghum. Sorghum while playing a crucial
role in food security in Africa, it is also source of income
of house-hold (Anglani, 1998). In West Africa, ungermina-
Dicko et al. 385
Figure 1. Sorghum bicolor (L.) Moench.
ted sorghum grains are generally used for the preparation
of “tô”, porridge, and couscous. Malted sorghum is used
in the process of local beer “dolo” (reddish, cloudy or
opaque), infant porridge and non-fermented beverages.
Starch is the main reserve polysaccharide of the plant
kingdom, and the principal source of carbohydrate from
agricultural origin, being the end product of
photosynthesis. Sorghum grains like all cereals grains
are comprised primarily of starch. -Amylases are
endoenzymes that randomly split -(1→4)-linkages in
starch. β-Amylases on the other hand, are exogluco-
sidases releasing maltose units from starch. The purpose
of this review was to overview the current knowledge on
the relevance of starch content and the activities of
amylases in sorghum food processing in West Africa.
BOTANICAL DESCRIPTION, PRODUCTION AND
UTILIZATION OF SORGHUM
Description and production
S. bicolor is the fifth most important cereal crop after
wheat, rice, maize, and barley in terms of production
(FAO, 2005). Total world annual sorghum production is
about 60 million tons from cultivated area of 46 millions
ha. Most important producers are the United States,
Nigeria, Sudan, Mexico, China, India, Ethiopia, Argentina,
Burkina Faso, Brazil, and Australia (Figure 2). Burkina
Faso is the world leader of sorghum production and
consumption per inhabitant (FAO, 2005). Sorghum is a
plant belonging to the tribe of Andropogoneae and the
family of Poaceae. In 1753, Linnaeus described three
species of cultivated sorghum: Holcus sorghum, Holcus
saccaratus and Holcus tricolor. In 1794, Moench
distinguished the genus Sorghum from the genus Holcus,
and in 1805 Person suggested the name Sorghum
vulgare for Holcus sorghum (L.). In 1961, Clayton
proposed the name Sorghum bicolor (L.) Moench as the
correct name for cultivated sorghum and this is currently
the accepted one (Doggett, 1988).
Like most angiosperm (flowering plant) lineages,
sorghum is thought to be ∼200 million years old
(Paterson et al., 2003). Sorghum, maize, rice and wheat
diverged from a common ancestor only 50-70 million
years ago (Paterson et al., 2003). The main races of
cultivated sorghum are: bicolor, vulgare, caudatum, kafir,
guinea, and durra (Deu et al., 1994; BSTID-NRC, 1996).
Common names of sorghum vary from continent to
country levels. The most encountered names are:
kafferkoren, soedangras, suikergierst, or suiker-sorghum
(The Netherlands), kaoliang (China), mtatam, shallu or
feterita (East Africa), durra (Egypt), chicken corn,
sorghum or guinea corn (United Kingdom), jola, jowar,
jawa, cholam, bisinga, durra or shallu (India), kaffir corn
(South Africa), milo, sorgo, sudangrass or sorghum
(USA), milo (Middle East Africa) and great millet, guinea
corn, feterita, sorghum or sorgho (West Africa). Sorghum
is C4 crop, whom certain varieties also posses “stay
green” genes that enable them to perform photosynthesis
permanently. It is particularly adapted to drought prone
areas: hot, semi-arid tropical environments with 400-600
mm rainfall-areas that are too dry for other cereals.
Sorghum is also found in temperate regions and at
altitudes of up to 2300 meters in the tropics. It is well
suited to heavy soils commonly found in the tropics,
where tolerance to water logging is often required.
Sorghum is a vigorous grass that varies between 0–6
m in height. It has deep and spread roots with a solid
stem. Leaves are long (0.3-1.4 m) and wide (1-13 cm),
386 Afr. J. Biotechnol.
Figure 2. Annual sorghum production and cultivated area throughout the world. Data are from FAO (2005).
Figure 3. Structure of sorghum grain (Sautier and O’Deye, 1989). Reproduced
with the permission of Harmattan Editions, Paris.
with flat or wavy margins. The flower is a panicle, usually
erect, but sometimes recurved to form a goose neck
(Figure 3). Grain or caryopse is usually covered by
glumes. Glumes are the maternal plant tissues in the
panicle that holds the developing caryopses after
pollination. The caryopse is rounded and bluntly pointed,
from 4–8 mm in diameter and varying in size, shape and
color with variety (Figure 3). Caryopse color is an
important trait that affects grain quality in sorghum.
Sorghum caryopse is composed of three main parts:
seed coat (testa or pericarp), germ (embryo) and
endosperm (storage tissue). In some sorghum genotypes
the testa is highly pigmented. The presence of pigment
and the color is a genetic character controlled by the R
and Y genes (Waniska, 2000). The thickness of the testa
layer is not uniform and is governed by the Z gene. In
some genotypes there is a partial testa, while in others it
is not apparent or is absent.
Distribution
It is believed that sorghum originated in Africa, more
precisely in Ethiopia, between 5000 and 7000 years ago
(ICRISAT, 2005). From there, it was distributed along the
trade and shipping routes around the African continent,
and through the Middle East to India at least 3000 years
0
2
4
6
8
10
Argentina Australia Brazil Burkina
Faso
China Ethiopia India Mexico Nigeria Sudan USA others
Country
Production and cultivated area
Production (million tons)
Cultivated area (million ha)
Yield of production (t/ha)
ago. It then journeyed along the Silk Route into China.
Sorghum was first taken to North America in the 1700-
1800's through the slave trade from West Africa. It was
re-introduced in Africa in the late 19th century for
commercial cultivation and spread to South America and
Australia. Sorghum is now widely found in the dry areas
of Africa, Asia (India and China), the Americas and
Australia.
Sorghum is genetically diverse. The world sorghum
germplasms are deposited at the International Crop
Research Institute for the Semi-Arid Tropics (ICRISAT,
Patancheru) in India. ICRISAT holds about 36,000
germplasm accessions of this crop. The varieties are
distinguished on the basis of morphological traits,
differences in isoenzyme patterns and DNA
polymorphism (Chantereau and Nicou, 1991; Ollitraut et
al., 1989a, 1989b; Zongo, 1991; Tao et al., 1993; Vierling
et al., 1994, Deu et al., 1994). The sorghum genome is
currently sequenced (Paterson et al., 2003;
http://fungen.org/Sorghum.htm). Sorghum has 2n=20
chromosomes and is estimated to contain 750 Mb being
twice the genome of rice and six times the genome of
Arabidopsis (Passardi et al., 2004). Thus, a rough
estimate of the total number of genes in sorghum based
upon the currently 107 652 known expressed sequence
tag (EST) data would be between 35 000 and 40 000
(http://fungen.org/Sorghum.htm).
Worldwide utilization
Sorghum is grown in the United States, Australia, and
other developed nations essentially for animal feed.
However, in Africa and Asia the grain is used both for
human nutrition and animal feed. It is estimated that more
than 300 millions people from developing countries
essentially rely on sorghum as source of energy (Godwin
and Gray, 2000). The main foods prepared with sorghum
are: tortillas (Latino America), thin porridge, e.g. “bouillie”
(Africa and Asia), stiff porridge, e.g. tô (West Africa),
couscous (Africa), injera (Ethiopia), nasha and kisra
(Sudan), traditional beers, e.g. dolo, tchapallo, pito,
burukutu, etc. (Africa), ogi (Nigeria), baked products
(USA, Japan, Africa), etc. Tortillas are a kind of chips
prepared from sorghum alone or by mixing sorghum with
maize and cassava (Anglani, 1998). Nasha is a traditional
weaning food (infant porridge) prepared by fermentation
of sorghum flour (Graham et al., 1986). Ogi is an
example of traditional fermented sorghum food used as
weaning food, which has been upgraded to a semi-
industrial scale (Achi, 2005). Injera is a local fermented
pancake-like bread prepared in Ethiopia from sorghum
(Yetneberk et al., 2004). Kisra is traditional bread
prepared from fermented dough of sorghum (Mahgoub et
al., 1999).
Tô is prepared by cooking slurry of sorghum flour. Thin
porridges (usually used as weaning food) are also prepa-
Dicko et al. 387
red in the same manner with less amount of flour to
obtain a fluid end-product. Often sorghum porridges are
characterized by thick pastes that may form rather stiff
gels depending on variety used. Porridges prepared with
malted sorghums have several order of magnitudes lower
viscosities than those of non-malted sorghums (Malleshi
and Desikachar, 1988; Dicko et al. 2006). These
porridges are particularly useful for the formulation of
weaning foods for infants because of their high energy
density (Traoré et al, 2004). Furthermore the use of
exogenous sources of α-amylase from higher West
African plants is useful in reducing the viscosity of cereal
porridges, including sorghum ones (Dicko et al., 2005).
Couscous is a steamed and granulated traditional
African food originating from North Africa. The traditional
method of preparing couscous is a steam-cook process
in a special pot called “couscoussière”. Couscous is
prepared by mixing flour with water to obtain
agglomerated flour-water mixtures. The agglomerates are
then put on top of the “couscoussière”. The stew cooks in
the bottom pot while the granules are steamed on top.
Sorghum varieties differ in their couscous-making ability.
White sorghum varieties from tan plants yield the best
couscous product (Galiba et al., 1988). Couscous quality
criteria include size uniformity, color, stickiness, and
mouth-feel (Aboubacar and Hamaker, 1999).
Dolo is a reddish, cloudy or opaque local beer
prepared essentially from red sorghum malt (Hilhorst,
1986). The primary quality criterion of selection of
sorghum varieties for beer is their potential to produce
malt with high -amylase and β-amylase activities
(Verbruggen, 1996; Taylor and Dewar, 2001). The
sorghum malting process starts by immersing the grain in
water to activate hydrolytic enzymes. Traditional
germination involves seedling growth in warm water-
saturated air for 3 to 5 days (Hilhorst, 1986). The
germinated grain is then dried to moisture content of 10-
12%. The malt obtained is used to prepare dolo. Briefly,
dolo preparation starts by mixing sorghum malt flour with
water (1:10, w/v). The mixture is decanted and the
supernatant (containing hydrolytic enzymes) is separated
from the precipitate (containing starch). Water is added to
the precipitate and the mixture is boiled to gelatinize
starch, but the supernatant is not boiled. It is interesting
to note that people, who originally prepare dolo
(“dolotières”), empirically know that the supernatant
contains “some things”, e.g. enzymes that are
thermolabile so they should not be boiled. After cooling,
the precipitate is filtered to separate soluble components
(starch, sugars, proteins, etc.) and the spent (used as
animal feed). The mashing step (incubation of hydrolytic
enzymes with their substrates) consists of combining the
previous supernatant and the filtrate at 50-60ºC for 12-16
h, to obtain the wort. The wort is cooked, and then re-
cooled overnight to room temperature (35-40ºC). The
cooled wort is a sweet non-fermented beverage highly
appreciated by children. It is traditionally called “soft
388 Afr. J. Biotechnol.
dolo”. The fermentation (1-2 days) is initiated by addition
of dried yeasts to the wort. The final product, dolo, is
separated from yeasts by filtration. Characteristics of dolo
are: alcohol content (2-4%, v/v), pH 4-5, stability at room
temperature (12-16h), red color, and opaque
appearance. In West African countries, the governments
have encouraged the research on the preparation of
lager beer from sorghum. However, except in Nigeria,
until now it is not successful because of the lack of real
financial support.
In most West African countries, sorghum alone
accounts for 50% of the total cereal crop land area.
Therefore, true food security will be hard to achieve in
those countries without a significant improvement of the
production, use, and marketing of this major staple
cereal. The yield is 1000-3000 kg/ha, while in the other
countries (Argentina, China and USA) it is 3000-4000
kg/ha (Figure 2) (FAO, 2005). The low production in West
Africa is essentially due to biotic (insects, fungal
diseases, weeds, etc.) and abiotic stresses (drought,
logging, photoperiod, soil quality, etc.). Most of the
cultivated varieties in this area have white, brown, yellow
or red caryopses.
Sorghum alone is not considered as a bread making
cereal because of the lack of gluten, but addition of 20-
50% sorghum flour to wheat flour produces excellent
bread (Anglani, 1998; Carson et al., 2000; Hugo et al.,
2000, 2003). Among interesting features of sorghum
utilization is biscuits and other cooked products (Olatunji
et al., 1989). In the USA and Japan, sorghum utilization
as human food is increasing because of its use in snacks
and cookies (Rooney and Waniska, 2004). Sorghum has
been intentionally introduced in China for food needs and
it is becoming one of the most important crops in this
country (Kangama and Rumei, 2005). The future promise
of sorghum in the developed world is for wheat
substitution for people allergic to gluten (Fenster, 2003).
In addition, pasta products, such as spaghetti and
macaroni made from semolina or wheat could be made
with mixtures of composite flour consisting of 30-50%
sorghum in wheat (Hugo et al., 2000, 2003). Pre-cooked
sorghum flours mixed with vitamins and exogenous
sources of proteins (peanuts or soybeans) are
commercially available in many African countries for the
preparation of instant soft porridge for infants. Sorghum
can be puffed, popped, shredded and flaked to produce
ready-to-eat breakfast cereals.
Sorghum starch is successfully applied for the
production of bio-ethanol (Suresh et al., 1999; Aggarwal
et al., 2001). In Nigeria and South Africa, sorghum is
industrially used for the production of lager beer (Taylor
and Dewar, 2001). More information on sorghum
utilization for human nutrition can be found elsewhere
(FAO, 1995; Anglani, 1998; Taylor and Dewar, 2001;
Awika and Rooney, 2004a, Rooney and Waniska, 2004).
SORGHUM GRAIN COMPOSITION AND NUTRITIVE
VALUE
Starch is the main component of sorghum grain, followed
by proteins, non-starch polysaccharides (NSP) and fat
(Table 1). The average energetic value of whole sorghum
grain flour is 356 kcal/100g (BSTID-NRC, 1996).
Sorghum has a macromolecular composition similar to
that of maize and wheat (BSTID-NRC, 1996). However,
sorghum contains resistant starch, which impairs its
digestibility, notably for infants (FAO, 1995). This
resistance is desired in other applications to fight human
obesity and to feed diabetic people. Foods prepared from
high tannin sorghums varieties have a longer passage in
the stomach (Awika and Rooney, 2004a). Edible products
incorporating slowly digestible starch are known to exhibit
a low glycemic index and increase satiety (Shin et al.,
2004).
Sorghum contains non-starch polysaccharides (NSP),
mainly located in the pericarp and endosperm cell walls,
with proportions in the kernel ranging from 2 to 7%
depending on variety (Knudsen and Munck, 1985;
Verbruggen et al., 1993). The NSP in sorghum grain are
essentially constituted of arabinoxylans and other β-
glucans representing 55% and 40% of the total NSP
(Verbruggen et al., 1993; Hatfield et al., 1999). Verbru-
ggen and co-workers (1993, 1998) found arabinoxylans
from sorghum to be glucuronoarabinoxylans containing
ferulic acid and p-coumaric acid. Arabinoxylans, being
one of the major NSP present in sorghum cell walls, play
an important role in the processing of sorghum for baking
and brewing (Rouau, 1993; Verbruggen et al., 1998). The
other β-glucans comprise cellulose (1,4-β-D-glucans),
curdlan-type glucans (1,3-β-D-glucans), and lichenan-
type glucans (1,3; 1,4-β-D-glucans) (Knudsen and
Munck, 1985; Verbruggen et al., 1993, 1996, 1998).
These β-glucans are predominantly water-unextractable,
and form viscous and sticky solutions. In brewing,
together with arabinoxylans, they are associated with
processing problems like poor wort and beer filtration
rates and the occurrence of haze (Aisien and Muts, 1987;
Dufour et al., 1992). Sorghum also contains non-
carbohydrate cell-wall polymers such as lignins with
proportions constituting up to 20% of the total cell wall
materials (Hatfied et al., 1999).
The protein content in whole sorghum grain is in the
range of 7 to 15% (FAO, 1995; Beta et al., 1995). Using
the solubility-based classification (Jambunatan et al.,
1975), sorghum proteins have been divided into
albumins, globulins, kafirins (aqueous alcohol-soluble
prolamins), cross-linked kafirins and glutelins. The
kafirins comprise about 50-70% of the proteins (Hamaker
et al., 1995; Oria et al., 1995; Duodu et al., 2003). α-
Kafirins (23 and 25 kDa) make up about 80% of the total
kafirins and are considered the principal storage proteins
of sorghum, whereas β-kafirins (16, 18, and 20 kDa), and
γ-kafirin (28 kDa) comprise about 5% and 15% of total
Dicko et al. 389
Table 1. Proximate composition of sorghum graina.
Macro-components
(g/ 100g f. m.)
Essential amino-
acids
(mg/100g,
d.m.)
Vitamins
(mg/100g d. m.)
Minerals
(mg/ 100g d. m.)
Carbohydrates
65
-
80
Leu
832
-
Vit.
-
A
21 RE**
Ca
21
Starch
60
-
75
Ile
215
-
Thiamin
0.35
Cl
57
Amylose
12
-
22
Met/Cys*
190
-
Riboflavin
0.14
Cu
1.8
Amylopectin
45
-
55
Lys
126
-
Niacin
2.8
I
0.029
Non starch
2
-
7
Phe/Tyr*
567
-
Pyridoxine
0.5
Fe
5.7
Low M
w
carbohydrates
2
-
4
Thr
189
-
Biotin
0.007
Mg
140
Proteins
7
-
15
Trp
63
-
187
Pantothenat
1.0
P
368
-
Kafirins
4
-
8
Val
313
-
Vitamin C
<0.001
K
220
ß
-
Kafirins
0.2
-
0.5
Arg*
500
-
Na
19
γ
-
Kafirins
0.7
-
1.6
His*
200
-
Zn
2.5
Other proteins
2
-
5
Fat
1.5
-
6
Ash
1
-
4
Moisture
8
-
12
aSources: Verbruggen et al. (1993, 1996); FAO (1995), Hamaker et al., (1995), BSTID-NRC (1996), Glew et al. (1997), Duodu et al.
(2003), Dicko et al. (2006). *Not strictly essential amino-acids,**RE = retinol equivalent; f.m. = fresh matter, d. m. = dry matter; NSP =
non starch polysaccharides.
Table 2. Comparison of starch components and amylase activitiesa in ungerminated (g-) and germinated (g+)
sorghum varieties.
Variety code Total starch
(%) Amylose (%) Amylopectin (%) α
αα
α-Amylase
(
U mg
-1
)
β
ββ
β-Amylase
(
U mg
-1
)
g- g+ g- g+ g- g+ g- g+ g- g+
V1
66.1
58.1
21.5
14.8
44.6
43.3
1.4
12.8
3.5
5.4
V2
59.5
55.2
11.5
9.1
48.0
46.1
3.1
6.8
0.9
1.9
V3
59.5
56.2
10.2
9.2
49.3
47.0
0.6
1.7
1.2
2.8
V4
66.0
63.2
17.1
17.0
48.9
46.2
1.1
1.4
0.6
1.1
V5
66.5
60.2
16.1
10.0
50.4
50.2
3.4
7.9
1.3
1.7
V6
66.2
58.1
16.8
12.0
49.4
46.1
3.9
7.2
1.7
1.1
V7
68.5
58.1
16.7
12.8
51.8
45.3
4.3
5.6
2.2
4.6
V8
61.6
59.5
10.9
10.4
50.7
49.1
1.6
1.9
1.0
2.9
V9
65.2
61.6
12.3
9.2
52.9
52.4
0.2
1.9
1.
7
3.9
V10
63.7
58.1
11.6
10.7
52.1
47.4
0.3
4.9
1.8
3.8
V11
61.6
59.5
12.6
10.7
48.9
48.8
0.6
0.9
1.9
8.7
V12
62.3
59.2
14.8
13.3
47.5
45.9
0.4
2.9
1.5
6.1
V13
62.5
56.7
11.0
7.3
51.5
49.4
2.5
3.9
1.1
4.0
V14
68.5
61.6
14.2
9.7
54.3
51.9
0.8
3.2
0.9
2
.7
V15
62.3
60.8
14.3
12.9
48.0
48.0
5.3
6.0
2.1
2.8
V16
61.6
59.5
11.5
9.5
50.1
50.0
1.6
1.7
1.6
2.1
V17
60.2
59.3
12.6
12.2
47.6
47.1
6.9
8.4
1.7
2.6
V18
64.2
59.2
15.5
11.4
48.7
47.8
1.4
1.6
1.1
3.1
V19
61.6
57.5
10.9
10.1
50.7
47.4
11.3
11.5
2.0
4
.7
V20
64.1
58.8
13.2
9.7
50.9
49.1
6.6
11.0
2.6
4.3
V21
62.5
61.6
10.8
10.1
51.7
51.5
1.2
8.7
1.3
8.7
V22
62.6
61.5
13.2
12.2
49.4
49.3
5.4
7.8
2.8
7.3
V23
64.5
59.2
14.3
12.9
50.2
46.3
1.1
1.7
1.6
3.6
V24
67.6
60.2
17.2
13.8
50.4
46.4
4.3
7.5
2.5
3.
2
V25
61.6
59.1
11.6
11.4
50.0
47.7
4.0
16.4
2.7
1.7
V26
65.2
60.2
14.8
11.1
50.4
49.1
3.5
5.5
2.5
3.0
V27
66.1
60.2
15.6
10.9
50.5
49.3
1.4
1.8
1.0
0.9
V28
62.0
61.6
11.2
10.2
50.8
51.4
10.2
16.3
0.9
2.1
V29
65.1
59.8
15.7
12.0
49.4
47.8
4.8
6.4
2.0
2.0
V30
58.6
58.3
11.4
11.1
47.2
47.2
5.3
10.3
5.1
3.7
V31
57.2
55.2
11.5
10.0
45.7
45.2
4.9
13.8
2.0
5.4
390 Afr. J. Biotechnol.
Table 2. con td. Comparison of starch components and amylase activitiesa in ungerminated (g-) and germinated
(g+) sorghum varieties.
V32
63.5
61.0
11.9
10.2
51.6
50.8
1.4
12.2
2.2
5.7
V33
65.5
61.6
12.5
9.9
53.0
51.7
2.2
13.5
1.2
3.4
V34
59.5
58.5
11.0
10.1
48.5
48.4
4.1
5.9
2.1
2.3
V35
65.0
62.0
14.8
12.1
50.2
49.9
3.4
4.9
3.1
1.9
V36
65.7
60.1
15.1
14.2
50.6
45.9
0.7
10.1
1.5
2.4
V37
66.2
63.8
14.7
12.6
51.5
51.1
1.3
2.0
13.0
9.0
V38
61.6
60.8
12.8
12.4
48.8
48.4
2.7
3.3
6.7
1.6
V39
63.5
60.1
13.5
13.3
50.0
46.8
1.8
14.6
1.6
2.0
V40
62.6
61.6
11.2
11.1
51.4
50.5
0.8
1
4.1
3.5
2.9
V41
63.5
60.2
14.2
11.4
49.3
48.8
3.7
13.7
3.4
4.8
V42
64.2
61.7
12.7
10.6
51.5
51.1
5.1
14.3
1.8
2.2
V43
64.4
59.5
15.3
11.1
49.1
48.4
1.7
12.1
1.2
2.4
V44
62.5
57.4
15.0
10.3
47.5
47.2
3.1
13.2
1.1
1.1
V45
59.5
58.7
13.5
13.0
46.0
45.7
0
.8
16.1
2.9
3.4
V46
58.1
57.2
10.9
10.8
47.2
46.4
1.4
13.0
1.2
1.1
V47
61.6
58.2
12.7
11.7
48.9
46.5
2.3
10.1
12.5
13.9
V48
59.5
58.8
12.3
11.5
47.2
47.3
4.0
10.6
7.4
4.3
V49
59.5
58.1
11.9
11.4
47.6
46.8
0.8
12.4
1.0
2.1
V50
60.5
58.8
11.4
11.4
49.1
47.5
3.5
13.3
1.5
1.5
Mean
63.0
59.5
13.4
11.3
49.6
48.2
3.0
8.1
2.5
3.6
R
ange
variation
57
-
69
55
-
64
10
-
17
9
-
16
45
-
54
43
-
52
0.4
-
11
0.9
-
16
0.6
-
13
0.9
-
14
Standard error
3.8
0.8
3
0.4
0.2
aSpecific activities (units/mg of protein). Data are from Dicko et al. (2006).
kafirins, respectively. The nutritional quality of sorghum
proteins is poor because these kafirins are protease
resistant (Badi et al., 1990; Oria et al., 1995; Anglani,
1998). However, a wide variability according to variety
has been observed with respect to the levels of proteins
in sorghum (Reddy and Eswara, 2002). The protein
digestibility of sorghum may decrease upon cooking
(Axtell et al., 1981; Taylor and Taylor, 2002), but pre-
fermentation may increase the digestibility (Taylor and
Taylor, 2002). The low digestibility is due to protein-
protein, protein-carbohydrate, protein-(poly) phenol and
carbohydrate-(poly) phenol interactions (Knudsen et al.,
1988; Axtell, 1981, Hamaker et al., 1987; Cherney, et
1992, Taylor and Taylor, 2002).
The fat in sorghum grain (mainly present in the germ)
is rich in polyunsaturated fatty acids (Glew et al., 1997).
The fatty acid composition of sorghum fat (linoleic acid
49%, oleic 31%, palmitic 14%, linolenic 2.7%, stearic
2.1%, etc.) is similar in content to that of corn fat, but it is
more unsaturated (Knudsen et al., 1988; Adeyeye and
Ajewole, 1992; FAO, 1995).
Sorghum is a good source of vitamins, notably the B
vitamins (thiamin, riboflavin, pyridoxine, etc.), and the
liposoluble vitamins A, D, E and K.
Sorghum is reported to be a good source of more than
20 minerals (BSTID-NRC, 1996). Sorghum is also rich in
phosphorus, potassium, iron and zinc (Glew et al., 1997;
Anglani, 1998). Zinc (an important metal for pregnant
women) deficiency is more common in corn and wheat
than in sorghum (Hopkins et al., 1998).
Effect of germination on sorghum composition
The physiological maturity of sorghum grain generally
occurs 50 days after anthesis, and marks the end of
nutrient delivery and the beginning of senescence, and
caryopse desiccation (Waniska, 2000). The mature grain
is then harvested and stored. In a dormant stage, it is
characterized by dehydration and a dramatic decrease of
metabolic activity. Germination is induced by rehydration
of the seed, which increases both respiration and
metabolic activity thus allowing the mobilization of
primary and secondary metabolites (Limami et al., 2002).
Therefore, the biochemical composition between
ungerminated and germinated kernels is different.
Germination induces the synthesis of hydrolytic
enzymes, e.g. starch degrading enzymes, and proteases.
The reduction of phytic acid, some flavonoids and
proanthocyanidins has been observed during germination
(FAO, 1995; Traoré et al., 2004). The breakdown of
protease resistant prolamins (Mazhar and
Chandrashekar, 1993) and the increase of the availability
of minerals (iron, zinc, etc.) and essential amino acids
(principally Lys, Tyr and Met) upon germination has also
been reported (FAO, 1995; Anglani, 1998). Germination
of sorghum is important for the preparation of weaning
foods with low paste viscosity and high energy density
(Malleshi and Desikachar, 1988). While germination
usually has positive aspects, it is important to note that it
increases the content of nitrilosides (cyanogenetic β-
glycosides, e.g. dhurrin) of the grain (Ahmed et al., 1996;
Traoré et al., 2004). These compounds release cyanide
(prussic acid) which may be removed either by heating
the flour or removing shoots, roots and the germs, but
removing the latter reduced the content in -amylase
(Uvere et al., 2000; Traoré et al., 2004). Upon
germination, the initially low content of vitamin C is
strongly increased (Taur, et al., 1984).
STARCH AND STARCH DEGRADING ENZYMES IN
SORGHUM GRAIN
Starch
Starch is the primary source of stored energy in cereal
grains. Starch is deposited as granules in the endosperm
cells, being the main constituent of the endosperm.
Sorghum starch granules have diameters ranging from 5
to 25 µm (average 15 µm). Sorghum starch has a specific
particularity because of its high gelatinization temperature
(70-75ºC), which decreases its industrial application
(Dufour et al., 1992; Taylor, 1992). Native starch granules
are essentially insoluble in cold water. The term
“gelatinization” is used to describe the swelling and
hydratation of granular starches (Zobel, 1984). Starch
gelatinization is the disruption of molecular orders within
the starch granule manifested in irreversible changes in
properties such as granular swelling, native crystalline
melting, loss of birefringence, and starch solubilization.
These changes render all or part of the material in
granules soluble and consequently enable to contribute
to food properties such as texture, viscosity, and moisture
retention (Whistler and BeMiller, 1997). The point of initial
gelatinization and the range over which it occurs is
governed by the starch structure. Sorghum starch is
classified as type-B, e.g. a moderate swelling starch
compared to type-A starches (potato, tapioca, waxy
sorghum, etc.), which are high swelling starches (Beta
and Corke, 2001). The retrogradation involves reasso-
ciation of the molecules and occurs when the starch is
cooled, and this is dependent on the ratio of amylose and
amylopectin. Enzymatic sorghum starch hydrolysis or
chemical treatment can improve its technological
properties (Zhang et al., 1999).
Regardless of the botanical source, starch is
structurally composed of two high molecular weight
homopolysaccharides known as amylose and
amylopectin. Amylose content in mature sorghum grain is
varietal dependent. While waxy sorghums do not contain
amylose (level < 1%), the content of amylose in normal
sorghums is ranging from 10 to 17% (w/w, fresh weight
basis) (Table 2), constituting approximately 20-30% of
starch. There is no significant difference between red and
white sorghum grains in their starch contents (Dicko et
al., 2006). The screening of starch content in 50 sorghum
varieties before and after germination showed that there
is an inter-varietal difference of content in these compou-
Dicko et al. 391
nds (Dicko et al. 2006).
Amylose is composed of essentially homogenous
linear units of α-(14)-D-glucopyranose, which can form
helicoidal structures in solution (Manners, 1974; Jarvis
and Walker, 1993). The interior of the helix is
hydrophobic, allowing amylose to form a complex with
free fatty acids, iodine, etc. (Fennema, 1985). There is a
significant inter-varietal difference of content of amylose
among sorghum varieties (Beta and Corke, 2001; Dicko
et al., 2006). Although, some varieties contained
relatively little amylose, waxy sorghum were not found
among cultivated sorghums in West Africa (Dicko et al.,
2006). This is probably because cultivated varieties were
primarily selected and bred for tô, for which high amylose
content is required (Bello et al., 1999; Trouche et al.,
2000).
Amylopectin is constituted of short chains of α-(14)-
D-glucopyranose (majority 10-20 units in sorghum starch)
branched to α-(16)-D-glucopyranoses to form a highly
ramified structure (Blennow et al., 2001). The content of
amylopectin is varietal dependent. The content of
amylopectin in sorghum is ranging from 45 to 54% (w/w,
fresh weight basis) (Table 2). Contrary to amylose the
levels of amylopectin in sorghum varieties are not
significantly different (Dicko et al., 2006).
There are relationships between the levels of starch
component and sorghum utilization for several foods.
Content of starch and starch components such as
amylose and amylopectin may give directions for the
selection of sorghum varieties for specific foods (Dicko et
al., 2006). For instance, in “tô” preparation, the formation
of a thick paste linked to high amylose content is
necessary. On the other hand, sorghum varieties with low
viscosity are desired in the formulation of weaning foods
with high energy density (WHO, 1998). In that case low
amylose content and high -amylase activities are
determinant. Since amylose has a higher gelatinization
temperature than amylopectin (Whistler et al., 1984),
sorghum with low amylose content could be targeted for
industrial brewing. Amylose is more susceptible to
retrogradation than amylopectin and waxy sorghum is
less viscous than normal sorghum (FAO, 1995). Low
amylose-containing sorghum varieties are also preferred
for extrusion-cooking because they give better functional
characteristics of the extrudates, such as enzyme-
susceptibility and solubility (Gomez et al., 1988).
Sorghum varieties with low amylose content may be
recommended for infant porridges preparation.
-Amylase and β
ββ
β-amylase
Starch degradation in plants is accomplished by
α-amylase [α-1,4-D-glucan 4-glucanohydrolase, EC
3.1.1.1], β-amylase [α-1,4-D-glucan maltohydrolase, EC
3.1.1.2], amyloglucosidase [α-1,4-glucan-glucohydrolase,
EC 3.1.1.3] and starch phosphorylase [1,4-α-D- glucan
392 Afr. J. Biotechnol.
Table 3. Apparent determinant (bio)chemical markers for sorghum food processing.
Food Grain
color Germination Starch
content Amylose
content α
αα
α-amylase and
β
ββ
β-amylase
References
Tô white no high high low 1, 2,3, 4
Dolo red yes high high high 2, 3, 5
Couscous white no medium low medium 2, 3, 5
Infant porridge white yes high low high 3, 5, 6, 7, 8
Industrial beer white yes high low high 9, 10, 11, 12, 13
Bread white no high high high 1, 11
1Gomez et al., 1988; 2Trouche et al., 2000; 3Dicko et al. 2006, 4Bello et al., 1990; 5Dicko et al. 2006; 6Malleshi and Desikachar, 1988;
7Traoré et al., 2004; 8Thaoge et al., 2003; 9Dufour et al., 1992; 10Taylor and Robbins, 1993; 11FAO, 1995; 12Nwanguma and Eze, 1995;
13Uriyo and Eigel, 2000.
: phosphate α-D-glucosyltransferase, EC 2.4.1.1] action
on -(14)-linkages (Manners, 1974). Amylases are
hydrolytic enzymes, which depolymerize starch according
to a classic acid-base mechanism. -Amylases are
endo-enzymes that randomly split α-(1→4)-linkages in
starch with retention of anomeric configuration of glucose
residues. β-Amylase is an exoglucosidase acting from the
non-reducing end, releasing β-maltose units from starch,
hence the name β-amylase (Kaplan and Guy, 2004). The
β-maltoses released undergo mutarotation into -maltose
(and Akerman, 1973; Dicko et al., 2000). Both -amylase
and β-amylase cannot split the -(1→6)-linkages in
amylopectin. Therefore, the degradation of starch by
these enzymes is incomplete. In addition, plant amylases
scarcely hydrolyze raw starch: their action is lower than
5% hydrolysis (Dicko et al., 1999).
Sorghum amylases were first detected in 1928 and
partially purified since that time by solvent fractionation
(Patwardhan and Norris, 1928). The starch-liquefying or
dextrinizing power is referred to -amylase activity, while
the starch-saccharifying or saccharolytic power is
referred to as β-amylase activity. Sorghum -amylases
and β-amylases occur as glycosylated (2-3 glycoforms),
anionic (pI 4-5) isoenzymes of different molecular weights
(Mundy, 1982; Okon and Uwaifo, 1984). Two -amylase
isoenzymes with molecular masses of 41.5 and 42.7 kDa
(Mundy, 1982) and three β-amylases with molecular
masses of 20, 40 and 60 kDa were purified from sorghum
grain (Okon and Uwaifo, 1984). Red sorghum grains
have generally higher amylase activities than white ones
(Dicko et al., 2006).
One of the constraints of utilizing sorghum varieties in
industrial brewing is the low activity of starch degrading
enzymes. For instance, in Nigeria, sorghum has become
the predominant cereal for industrial scale malting and
brewing of beer, following legislation banning the
importation of barley and wheat (Hug et al., 1991). The
major disadvantage encountered using sorghum in
brewery is its low content or absence of β-amylase
(Taylor and Robbins, 1993; Swanston et al. 1993;
Verbruggen, 1996). Comparison of the effect of
germination on α-amylase and β-amylase activities in
sorghum varieties is rare, because most studies focused
only on germinated grains (Dufour et al., 1992; Beta et
al., 1999). Results about the effect of germination on
amylase activities are sometimes contradictory. Contrary
to previous results stating that α-amylase and β-amylase
activities were not detected in ungerminated sorghum
varieties (Ahmed et al., 1996), it is shown that these
enzymes are present in all sorghum varieties (Dicko et
al., 2006). The results of β-amylase screening in sorghum
may be biased because of the used chemical assays, of
which some are less specific (ferricaynide, 3,5-dinitro-
salicylic acid assays, etc.) for β-amylase than the
standard assay described by McCleary BV and Codd
(1989). While β-amylase activity did not show an overall
increase after germination, α-amylase activity increased
up to 20 fold in some varieties (Dicko et al., 2006). The
main reason for the difference of the effect of germination
between -amylase and β-amylase may be due to the
fact that β-amylase is unlikely de novo synthesized during
germination (Ziegler, 1999). Using a statistically signi-
ficant number of samples, it is shown that although -
amylase and β-amylase have a common substrate, e.g.
starch, their activities are not correlated in sorghum grain
both before and after germination (Beta et al., 1999,
Dicko et al., 2006). The clear polymorphism of β-amylase
and α-amylase activities in sorghum varieties may give
direction for the selection of sorghum varieties containing
these enzymes for specific food utilization.
It is interesting to note that amylase activities could be
correlated with food processing of sorghum. For instance,
low α-amylase activity of tô varieties is beneficial to
obtain a relatively stick porridge (Dicko et al., 2006). For
“dolo” and industrial beer preparations, high α-amylase
and β-amylase activities are desired (Dufour et al., 1992;
Taylor and Robbins, 1993). The high amylase activities
probably explain the preference for red sorghums for the
preparation of “dolo”. In industrial brewing, a specific
interest exists in high β-amylase-containing sorghum
varieties (Dufour et al., 1992; Taylor and Robbins, 1993;
Verbruggen et al., 1993, 1996). Interestingly some malted
sorghum varieties contain β-amylase activities
comparable to that of barley malt (Beta et al., 1995, Dicko
et al., 2006). These varieties can be suggested for
industrial brewing. A constraint in the utilization of
sorghum for industrial brewing is the high starch
gelatinization temperature (Okafor and Aniche, 1987).
For couscous preparation varieties displaying low -
amylase activity are required to avoid starch
dextrinization during the process.
In most West African countries bakers do not use
composite sorghum/wheat flour. However, acceptable
bread can be produced with 30-50% sorghum
substitution for wheat (Anglani, 1998; Carson et al.,
2000). In analogy with wheat dough, the use of sorghum
flour possessing suitable -amylase activity could
increase bread quality (Hilhorst et al., 1999). For bread
making, it is important to have sorghum lines with low
amylose contents (Lee et al., 2001; Martin et al., 2004).
These criteria may give directions for selecting sorghum
varieties for bread making.
The (bio)chemical characteristics screened, i.e. starch
and starch degrading enzymes are suggested to serve as
determinants for sorghum utilization for various foods.
The criteria of recommendation of varieties having
important (bio)chemical constituents as quality-grade
markers for the preparation for foods are shown in Table
3. It is important to stress that the suitability of sorghum
varieties for food and beverages is also dependent on the
chemical and physical properties of kernels and process
conditions. The recommendations made in Table 3 may
also depend on technological levels and consumer
acceptance for confirmations.
CONCLUSION
Sorghum is a staple food in Africa that has a high
environment tolerance and a great (bio) chemical
diversity. The content of starch as well as amylase is
highly polymorph among sorghum varieties. These
findings show that it is possible to use the content of
starch and starch degrading enzymes to give directions
for selecting the most suitable sorghum varieties for
specific food processing.
ACKNOWLEDGMENTS
The organization for the prohibition of chemical weapons
(OPCW) via the International Foundation for Science
(Stockholm, Sweden), is acknowledged for supporting the
research carried out by Dr. M. H. DICKO.
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