International Journal of Food Science and Technology 1999, 34, 365–370
The quality assessment of gari produced by using
microwave energy
Ibok Oduro1 & Brian Clarke2*
1 Present address: Department of Biochemistry (Food Science Section), University of Science and Technology, Kumasi,
Ghana
2 School of Agriculture, Food and Environment, Silsoe College, Cranfield University, Bedfordshire, England, MK45
4DT, UK
(Received 22 October 1996; Accepted in revised form 26 March 1999)
Summary
A novel technique using microwave energy for roasting and drying or garification of fermented cassava mash is investigated. The quality of gari produced using this method
compared favourably with the standards quoted in the literature and with those of commercial gari purchased from a London market, in terms of colour, swelling capacity,
moisture, pH and total acidity.
Keywords
Cassava, cooking, drying, gelatinization.
Introduction
Gari is the term used in West Africa for the partially gelatinized, fermented form of cassava
(Manihot esculenta Crantz) (Ajibola et al., 1987a;
Asiedu, 1989). It is also known as farinha de mandioca in South America, although variations exist
in the length of time of fermentation. Cassava is a
highly perishable, starchy root crop which starts
to deteriorate within two or three days after harvest if not processed. Up to 300 kg/person/year of
cassava is consumed, with an estimate of 30M
tonnes/year consumed in Nigeria alone. Gari is a
preservable, popular form of processed cassava
which accounts for about 70% of the cassava
eaten in Nigeria for example. In 1996 it was estimated by FAO that 70M tonnes were grown in
Africa, (FAO, 1996) and it continues to be a
major source of energy for millions of people.
One of the drawbacks however, is that it consumes large amounts of fuel in its preparation,
especially in the traditional technique where it is
roasted and dried in a flat pan over an open
*Correspondent: Fax: +44 1525 863277;
e-mail: [email protected]
© 1999 Blackwell Science Ltd
wood fire. Population drift from rural to urban
areas has increased the demand for ready packaged gari and there is a great need for better ways
of preparing this food at both domestic and factory scale.
The normal production stages (Lancaster et al.,
1982; Bruinsma et al., 1985) consist of washing,
peeling and grating the roots, and then fermenting the mash anaerobically for two to five days,
pressing to remove the water which exudes during
the fermentation stage, to give a moisture content
of about 50%. The mash is then usually sieved
before it is finally roasted and dried.
Domestically, small open pans are used for frying, but in an attempt to increase production,
larger rectangular pans of up to 2.5 3 1.5m are
being introduced. For example, these are promoted by the Rural Agro-Industrial Development
Scheme (RAIDS) of Nigeria and manufactured
by several small companies in Ibadan. Similarly
they are made by Agrico (Accra North, Ghana).
In the automated factory systems, gently sloping,
semi-circular troughs up to one metre diameter
are often used, with rotating angled scrapers to
move the gari along the trough (Fabrico Eng. &
Production Co. Ltd., Issele-Uku, Nigeria). Flat
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Quality of gari produced by microwave cooking I. Oduro & B. Clarke
circular plate roasters are available up to 3m in
diameter from Maquinas D’Andrea (Sao Paulo,
Brazil), which also use mechanical scrapers to
move the gari in a spiral fashion. Rotary drum
roasters with internal baffle lifters have also been
used in the past, which slope gently in the same
way as the semi-circular trough. The important
features are that a minimum temperature of
about 80 °C is reached for the onset of gelatinization of starch, and then maintained over a
residence time which permits a high percentage of
the starch to be converted. The only other
requirement is that some mixing or movement is
carried out to prevent re-agglomeration of the
granules or sticking of the gari to the base.
Energy sources for gari production are quite
varied and although wood is the principal fuel in
rural districts, factories are increasingly using gas,
oil and coal in West Africa. Wood may, of
course, be considered as free, but it is a much
depleted resource, and although it may be renewable in some areas, there is generally a large
shortfall in timber availability in Africa and people are having to travel further and further in
order to find sufficient wood for domestic or
commercial usage. Electrical energy is fairly reliable in many urban townships in Africa, and oil
is produced in Nigeria making the use of oilpowered electricity an attractive option for that
country. Microwave energy was considered as a
possible means of treatment due to its efficiency
in heat transfer, (Oduro 1995) and the first stage
was to see if the product could be prepared satisfactorily.
The use of microwave ovens in food processing
has been used in both domestic batch systems and
continuous flow commercial systems for many
years. Decareau (1985) reviewed many of these
applications and the expanded use of microwave
ovens since 1965 and listed the many advantages
of these systems such as energy saving, rapid
heating potential, shorter processing times and
waste elimination. Cook-in-container (including
plastic containers), defrosting and drying are
other advantages of the system.
The assessment of product quality, for either
export or local use, is usually based on certain
physical and chemical features. The quality standard for gari varies between countries as individual markets differ in their preferences. Variations
in flavour depend mainly on the length of fermentation time, but variety of cassava and
method of preparation are also important. It is
necessary therefore to make sure that any new
techniques of preparation are acceptable as well
as being technically possible.
Gari quality parameters have been reported by
Sanni (1991); Chuzel et al. (1988); Codex
Alimentarius Commission 1986 and Ingram
(1975), but because most of the gari is processed
by small scale producers, the application of these
standards is not systematic. On the other hand
because microwave techniques remove many of
the risks associated with open frying pans, the
chance of both small or large factories meeting
the gari standard specification using microwave
techniques could be much higher. Furthermore,
microwave technology lends itself to better control over processing conditions such as time and
temperature, so that a higher quality may be
guaranteed. The authors further observed that
microwave techniques require little handling and
provide a high degree of protection to the product during processing, thus resulting in a higher
standard of hygiene compared to that of other
techniques. Domestic scale microwave ovens are
available in West Africa and commercial scale
microwave equipment could be imported if it
were found to satisfy all the processing requirements. Other problems at commercial scale such
as maintenance, reliability of electrical supply,
training of manpower etc., would have to be
addressed at some stage. For the present, however, a study was deemed necessary to see if gari
could be produced satisfactorily by microwave
technology, and to determine any technical limitations or advantages of this approach. Furthermore, the objective was to assess the quality of
gari so produced in terms of swelling capacity,
colour, moisture content, pH and total titratable
acidity.
Materials and methods
Equipment
A domestic microwave oven with a turntable
(Toshiba model ER-7825 E/EW) was used.
The input power was 1250W and the typical
output power for water based products was
650W. The oven was operated from the ordinary
International Journal of Food Science and Technology 1999, 34, 365–370
© 1999 Blackwell Science Ltd
Quality of gari produced by microwave cooking I. Oduro & B. Clarke
single phase mains supply and operated at a frequency of 2450MHz.
Procedures
Cassava mash (500g) with a wet particle size of
between 3 and 5 mm was prepared by hand peeling and grating in a food processor. It was then
allowed to ferment spontaneously in an incubator
at 30 °C for four days. A cotton bag of the wet
mash was then subjected to a heavy weight (10
kg) for several hours to expel the excess water
down to a water content of about 50% (wet
basis). The mash had a pH value of 4 when it was
placed in the microwave oven. At 45 s intervals,
the oven was switched off and the mash removed
and stirred briskly by hand for one minute in
order to prevent the formation of lumps. The
mash was then returned to the oven for the next
phase of the cooking cycle. After the predetermined cooking time intervals: 9, 12, 15, 18 and 21
minutes, the sample was removed from the oven
and the moisture content was determined by the
oven method. The gari samples were then dried
down to about 10% moisture (wet basis) in an
ordinary electrical laboratory oven at 60 °C in
readiness for testing for swelling capacity, colour,
pH and total acidity.
room temperature, sealed with a rubber bung and
then mixed by inverting. After two minutes the
inversion was repeated and the sample left for a
further three minutes, making a total of five minutes. Swelling capacity was calculated as the volume of gari in water after five minutes, divided by
the initial volume of gari. Each measurement was
replicated three times.
Moisture content
Triplicate samples of the microwaved product,
weighing 10 g, were desiccated in a drying oven
at 130 °C for three hours, then cooled, weighed,
and the moisture content calculated on a wet
basis percent.
pH and total titratable acidity
Samples weighing 10g were homogenized in 50 ml
of distilled water for eight minutes using a magnetic stirrer. The pH was measured using a
Jenway pH meter (Jenway Ltd, Dunmow, Essex,
UK). The homogenized samples were filtered, 25
ml of the filtrate was titrated with 0.1M NaOH
using phenolphthalein as an indicator and the
percentage total acidity expressed as lactic acid
equivalent. The measurements were replicated
three times.
Analyses of the gari produced by microwave
Colour
Swelling capacity was determined by the method
devised by Bainbridge et al. (1996), similar to that
used by Ajibola et al. (1987b). A 10 ml volume of
gari was placed into a 50 ml measuring cylinder
and topped up to 50 ml with distilled water at
Colour was measured using a Minolta
Chromameter (CR-200) colour difference meter
(Minolta Co. Ltd, Osaka, Japan). The values read
were L* and b* to indicate lightness and intensity
of the yellowness of gari respectively, but a* val-
Table 1 Physico-chemical characteristics (colour, pH, total titratable acidity and moisture content) of micro-waved gari
samples. The standard deviations of triplicate measurements are given in parenthesis
Time
[min]
L*
0
9
12
15
18
21
Commercial
92.3
82.5
84.4
85.1
87.2
86.4
81.6
b*
(0.01)
(0.98)
(0.35)
(1.09)
(0.32)
(0.60)
(0.46)
© 1999 Blackwell Science Ltd
10.0
19.1
18.3
17.6
21.6
23.7
22.7
pH
(0.02)
(0.28)
(0.30)
(0.96)
(0.49)
(0.73)
(0.86)
3.98
3.99
4.00
4.01
4.01
4.05
(0.11)
(0.10)
(0.11)
(0.00)
(0.00)
(0.60)
Total titratable
acidity [g of lactic
acid/100g of gari]
Moisture
content
(%wb)
Swelling
capacity
1.1
1.2
1.1
1.1
1.1
0.6
35.8 (2.7)
30.7 (2.1)
24.4 (2.0)
16.5 (1.7)
9.0 (1.6)
2.7
3.1
3.1
3.3
3.3
(0.01)
(0.03)
(0.05)
(0.03)
(0.11)
(0.00
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Quality of gari produced by microwave cooking I. Oduro & B. Clarke
ues were not recorded as they measure levels of
green or redness which is not relevant in this case.
High L* values indicate lighter colour and higher
b* values indicate a more yellow colour. The
measurements were replicated three times.
Results and discussion
Quality analysis
Table 1 gives details of all the quality parameters
measured.
Swelling capacity
The rehydration characteristic is very important,
and the swelling power of gari was used to compare the rehydration properties of samples cooked
for different times. Results in Table 1 show that
swelling capacity increased with cooking time up
to 18 minutes, and then remained constant up to
21 minutes. Statistical analysis gave a high correlation (r 5 0.95) between cooking time and
swelling capacity.
Samples cooked for 12–21 minutes gave a
swelling capacity of between 2.7 and 3.3, which
compares favourably with the Codex Alimentarius
recommendation of a value of 3, while a range of
2.98 1 0.86 was found by Blanshard et al. (1994)
in Sierra Leone and 2.84 1 0.26 by Ajibola et al.
(1987b) in the Nigerian market. The cassava mash
cooked for nine minutes was within the recommended values of 2.7–2.9 of Chuzel et al. (1988) in
the Togo market, although a residence time of 12
minutes seems to satisfy all the various recommendations. This can however only be regarded as
a guideline as preferences vary from one country
to another. Furthermore, the required residence
time will depend on the oven and the weight of
cassava.
Colour
Variation in microwave treatment times produced
slight differences in the colour of gari (Table 1). A
greater intensity of yellow colour, indicated by
higher b*, and a darker colour indicated by lower
L* was obtained when compared with uncooked
mash. Those samples micro-waved for a longer
time of 18–21 minutes had higher b* values than
those for 9–15 minutes. The L* values increased
slightly with time, producing paler colours, and b*
also increased with time producing more yellowness. These phenomena are no doubt associated
with the greater percentage of gelatinization which
occurs at longer cooking times. The acceptable
range of colour values quoted by Codex
Alimentarius (1986), Chuzel et al. (1988) and
Sanni (1989), are: L* 80–85, b* 17–21. Cooking
times greater than 15 minutes gave colour properties which fell beyond the accepted range. Samples
cooked for 12–15 minutes would be regarded as
high quality gari in Nigeria, Ghana and most other
West African countries, where both yellow and
creamy/white types are found. In certain markets
in West Africa palm oil and other additives are
added to the roasting pan in order to manipulate
colour and flavour to suit some preferences.
The results show that microwave energy produced gari with low colour variation compared to
that of commercial gari. The L* and b* values of
gari purchased from the London Market ranged
from 77.5 to 85.8 and 18–27.5 respectively. These
cannot be regarded as representative samples but
they do represent typical quality that is available
commercially and their properties do fall outside
the recommended range. Blanshard et al. (1994)
reported observing a wide range of colours in the
gari samples purchased in Sierra Leone and no
correlation between colour and sensory data from
the consumers. It is not unknown however for
commercial suppliers to incorporate additives such
as palm oil in order to enhance both the colour
and flavour substantially.
pH and total titratable acidity
The pH and total titratable acidity varied only
slightly from 3.98 to 4.01 and 1.1–1.2% , respectively, over the cooking time periods 9–21 minutes.
This was well within the range recommended by
Codex Alimentarius Commission (1986), Chuzel
et al. (1988) and Sanni (1991), of 3.5–4.5 for pH
and 0.6–1.2% for total acidity.
This, and earlier studies, found that pH and
total titratable acidity could however vary quite
widely because they depend on the duration of the
fermentation stage, which also varies widely to suit
different tastes, and also depends on the nature of
mycofloras which occur. A previous study by
Sanni (1989) showed that the pH of market sam-
International Journal of Food Science and Technology 1999, 34, 365–370
© 1999 Blackwell Science Ltd
Quality of gari produced by microwave cooking I. Oduro & B. Clarke
ples varied from 3.9 to 7.3, with the peak between
3.5 and 4.5.
Moisture content
The results presented in Table 1 show that the
moisture content decreased from 36% to 9% as
microwave cooking time increased from nine to
21 minutes. Samples micro-waved for 9–18 minutes needed further drying to the required moisture of about 10%, while samples micro-waved
for 21 minutes were dried sufficiently. The rate of
drying may be expected to depend on temperature level, which did not vary much after reaching 100 °C, the residence time and the degree of
stirring. Statistical analysis showed a very high
correlation (r 5 20.99) between micro-waving
time and the moisture content. Longer residence
time enhances gelatinization of the starch component of the cassava mash.
Ease of treatment
The fermented cassava initially behaved in a manner which was difficult to handle, in that the
whole mass became very hard when irradiated
with microwaves. This was not impossible to
overcome by grinding as there was also nonuniformity in the treatment, with hard and soft
spots. The focal point of the microwaves
appeared to be about 3 cm from the outer surface
of the mash, which led initially to hardness in
that region with a softer outside and a softer core.
The effect of intermittent mixing was investigated, which lead to the establishment of the 45 s
mixing cycle. This is, of course, a distinct drawback for batch ovens, but discussions with a
microwave oven manufacturer have revealed the
possibility of a continuous system in the form of
a microwave tunnel with a mixer incorporated.
The effect of the intermittent cooking resulted in
some minor heat losses, but this was thought to
be of negligible importance compared to the
advantage of producing a uniform sample of gari.
The formation of lumps is of course a problem in
most garifying systems and some form of stirrer
or separation is always required.
In every other respect the gari produced was of
acceptable quality according to the recommended
standards, and it surpassed the standard of much
© 1999 Blackwell Science Ltd
commercial quality gari. With the exception of
moisture content, all the quality standards were
achieved after 12 minutes in the microwave oven.
In a commercial production system a feasibility
analysis would be needed to assess the best way
to dry the gari. It would however be quite reasonable to expect that microwave energy would
be the best way to achieve this. In the situation
tested, 20 minutes approximately would be
required to achieve both gelatinization and
drying.
The amount of energy consumed in microwave
systems is only about half that of other cooking
systems, which is one reason for making the system so popular. Furthermore the control of temperature and residence time can easily be
automated in these ovens. The level of hygiene in
such a fully enclosed system is another attractive
feature which would be important in Africa, as
elsewhere. There appears, from this study, to be
very little reason why microwave ovens should
not be used to make gari in the future.
Conclusions
The values obtained for the quality in terms of
swelling capacity, colour, pH and total titratable
acidity of samples prepared using microwave
energy compare favourably with published recommended values for gari produced by conventional processing methods. A minimum of 12
minutes cooking time is recommended to achieve
these quality standards, but additional drying
would still be required and 20 minutes, approximately, would be required, to achieve both gelatinization and drying. The main drawback is that
in a batch oven, the product has to be stirred
because otherwise it would cake harden. The
development of a continuous stirring mechanism
which would prevent caking requires further
investigation but, nevertheless, this technique
could provide a useful alternative technology for
cassava processing.
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International Journal of Food Science and Technology 1999, 34, 365–370
© 1999 Blackwell Science Ltd