Effect of microwave drying and storage on the color, breakage, dehulling and cooking quality of two red lentil varieties

The Canadian Society for Bioengineering
La Société Canadienne de Génie
The Canadian society for engineering in agricultural, food,
Agroalimentaire et de Bioingénierie
environmental, and biological systems.
La société canadienne de génie agroalimentaire, de
la bioingénierie et de l’environnement
Paper No. CSBE09-304
Effect of microwave drying and storage on the color, breakage,
dehulling and cooking quality of two red lentil varieties

Anthony Opoku, Lope Tabil and Venkatesh Meda
Department of Agricultural & Bioresource Engineering
University of Saskatchewan
57 Campus Drive
Saskatoon, Saskatchewan S7N 5A9

Written for presentation at the
CSBE/SCGAB 2009 Annual Conference
Rodd’s Brudenell River Resort, Prince Edward Island
12-15 July 2009

Abstract. Red lentil is a nutritious and healthy food which provides nutritional and health benefits to
consumers. Lentils should be harvested from the field at higher moisture content and dried to lower
storage moisture content to reduce field shattering losses during harvesting. Drying and storage
might affect the processing quality of lentils. Color characteristics, breakage susceptibility,
dehulling, and textural cooking quality of microwave dried, convection dried and undried Robin and
Impact red lentils were determined. The lentils were stored at 5oC for about 11 months. The color of
the samples was determined using Hunterlab spectrocolorimeter (Hunter Associates Laboratory
Inc., Reston, Virginia, U.S.A.). The Stein breakage test was used to determine the breakage
susceptibility of the lentils. Satake mill was used to dehull the samples at a speed of 1100 rpm for
38 s. The samples were soaked for 24 h and cooked for 2, 3, and 4 min. A texture analyzer
(Texture Technologies Corp., Scarsdale, NY) was used to determine the cohesiveness and the
hardness of the lentil samples. Storage had a marked influence on the color change of the Robin
and Impact lentils dried at 70oC. The total color change was higher for the Robin and Impact lentils
dried at 70oC compared to the microwave dried lentils. There were significant variations in the
breakage susceptibility of the stored lentils. The breakage susceptibility of the lentils was influenced
by the lentil variety as well as the drying method. The undried and microwave dried lentils had
higher dehulling efficiency compared the convection dried samples. The Impact lentils produced
higher split dehulled kernels and lower whole dehulled kernels compared to the Robin lentils.
Cooking time did not have significantly influence on the cohesiveness of most of the lentil samples.
The cohesiveness of the Impact lentils tended to be slightly higher than the Robin lentils by
comparison. The cohesiveness values for the Robin lentils ranged from 0.18 to 0.25 and the Impact
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lentils varied from 0.23 to 0.28. Cooking time did not markedly affect the hardness of most of the
lentil samples. By comparison, the Impact lentils tended to produce higher hardness values
compared to the Robin lentils. The hardness of the Robin lentils ranged from 5.38 to 16.01 N and
the Impact lentils ranged from 6.25 to 35.03 N.

Keywords. Red lentils, breakage susceptibility, color, cooking quality, texture, dehulling, hardness,
cohesiveness

Introduction
Canada is one of the leading producers of lentils and the leading exporter. On the average, Canada
earns about $240 million (Canadian) yearly from exporting lentils (Saskatchewan Pulse Growers
2005). Red lentil is a nutritious and healthy food which provides a good source of proteins,
vitamins, minerals (potassium, iron, calcium, phosphorus and zinc), and fibre and low in fat (Solanki
et al. 1999; Bhatty 1988; Savage 1988). It provides nutritional and health benefits to consumers.
Health benefits derived from consuming lentils include reduction in coronary heart disease,
preventing iron deficiency and stabilizing blood sugar (Leterme 2002).

Lentils can be swathed when about one-third of the pods start to turn yellow and the seeds
rattle in the pods. On the other hand, straight cutting of lentils can be done when the pods and
seeds are fully mature. To reduce lentil shattering losses during harvesting, it is normally
recommended to thresh lentils at around 16 to 20% wb (wet basis). The lentils should then be dried
down to 13 to 14% wb to produce high quality seeds for safe storage and to reduce subsequent
mechanical damage and breakage susceptibility during handling and processing (Tang et al.
1990b; Tang et al. 1992).
Microwave drying, compared to conventional convection drying of lentils, is energy efficient
and rapidly heats dielectric materials and it is used for drying, heating, thawing, sterilization, and
food/feed processing operations (Decareau 1986). Microwave drying is being explored as an
alternative method of drying for lentil. Microwave drying as well conventional drying might induce
stress cracks in the dried product which might affect further processing and handling of the product
(Tang et al., 1990a; Tang et al., 1991). Cracked and damaged seeds may break during dehulling
resulting in low quality product. Damaged lentil seeds may lower the grade of the product
commanding a lower price. Storage may increase the hardness of the seeds affecting the cooking
quality of the seeds.
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Lentils with dark brown color might be considered low quality so it is essential to determine
the effect of microwave drying and storage on the color. Color plays an important role in the
acceptability of a dried product by consumers. The Canadian Grain Commission (2008)
recommends the use of approved color guides for color grading of lentils. The color of lentils might
vary from light tan to brown or very dark brown. Lentils with dark brown color might be considered
low quality.
The objectives of these experiments were to determine effect of storage on the processing
characteristics of microwave and convection dried red lentil varieties, Robin and Impact. The
processing characteristics that were determined were color, breakage, dehulling, and textural
cooking qualities of the red lentils.

Materials and Methods
Materials
Two red lentil (Robin and Impact) varieties were used in these experiments. The Robin lentils were
obtained from Pure T. Organics, Regina, SK and the Impact lentils were supplied by Reisner Seed
Farm, Limerick, SK. The initial moisture content of the lentils was determined by using the method
developed by Tang and Sokhansanj (1991c). About 16 g of a sample was dried at 130oC for 20 h.
The initial moisture contents of Robin and Impact lentils were determined to be 9.85% and 6.50%
wb, respectively. The samples were placed in containers and calculated amount of water was
sprayed on the lentils to bring the moisture content around 20%. After adding water, the containers
were placed in a concrete mixer and rotated for about 5 h. The samples were then placed in walk-in
storage room maintained at 5oC to equilibrate for 1 week. The moisture contents of the conditioned
samples were determined after 1 week equilibration. The moisture contents of the conditioned
samples were determined to be 20.82% for the Robin variety and 21.86% for the Impact.

Microwave drying
A combined microwave-convective dryer, Panasonic Model NNC980W (Panasonic Canada Ltd,
Mississauga, ON) was used in drying the lentils. For microwave drying, about 700 g of lentils was
weighed and placed on 381 mm circular ceramic plate. The lentils were spread on the ceramic
plate to a diameter of 343 mm and a height of 10 mm. The lentils were dried intermittently at 3 min
intervals until the required moisture content was reached. After every 3 min of drying, the lentils
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were removed, weighed, and mixed together. Before drying the sample, the microwave oven was
preheated for 10 min by placing about 2000 g of water. Three power levels, P10 (713 W), P7 (606
W) and P4 (330 W) were used for the microwave drying. The dried samples were cooled at 5oC for
10 min before placing them in re-sealable plastic bags and storing at 5oC.

Convective oven drying
A mechanical convection oven (Model 28, Precision Scientific Group, Chicago, IL) was also used to
dry the lentil samples. The samples were dried at 70oC until the desired moisture content was
reached. Two samples of about 700 g each were placed in the oven and dried at the same time.
The samples were removed and weighed at 20 min intervals. The dried samples were cooled for 10
min before storing at 5oC.

Storage of lentils
The dried (microwave and convection) lentil samples were placed in Ziploc bags and placed in a
refrigerated storage room. The temperature in the storage room was maintained at 5oC with relative
humidity at 85%. Original Robin and Impact lentils were conditioned to 12.51% and 12.34%
moisture contents, respectively. The samples were also placed in Ziploc® double zipper bags and
placed in the storage room. The samples were stored for approximately 11 months. Before color,
dehulling, breakage and cooking quality tests the samples were brought to room temperature (22 –
24oC).

Color parameters
The color of the lentil samples was measured using Hunterlab spectrocolorimeter (Hunter
Associates Laboratory Inc., Reston, VA). After initial calibration against standard black and white
surface plates, four measurements for each sample were taken. Color change, ?L, ?a, and ?b
values were calculated. The total color change (?E) is calculated using the L, a, b color coordinates
and as defined by the Equation 1 (Maruyama et al. 2001).

2
2
2
?E =
L
? + ?a + b
?

(1)
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The base (standard) values of Hunter L, a, and b represent that of the fresh (undried) sample. The
Hunter “L” value represents the lightness or darkness of a sample on a scale of 0 to 100 (100 being
white and 0 being black). Hunter “a” value represents the greenness or redness of the sample (-50
being green and +50 being red). Hunter “b” value is also rated on a scale of -50 to +50, with -50
representing blue and +50 representing yellow. The Hunter values prefixed by a greek delta (?)
represent the difference of the base value from the dried and stored sample measured value. One-
way analysis of variance was used to test the significance of the results at 5% and Tukey’s HSD
was used to compare the means.

Breakage susceptibility of lentils
The Stein breakage tester (Model CK2-M, Fred Stein Laboratories, Inc., Atchison, KS) was used to
test the breakage susceptibility of the lentil samples. Before the tests, the samples were pre-sieved
using a 3.57 mm (9/64 in.) round-hole screen recommended by Canadian Grain Commission for
normal cleaning of small lentils (Canadian Grain Commission, 2008). About 100 g sample was
placed in a cup and the tester was run at a speed of 1725 rpm for two minutes. After the test, the
sample was collected and sieved using 3.57 mm screen. The sample remaining on the screen was
collected and weighed. The breakage susceptibility was determined as ratio of the mass passing
through the screen after the test to the original starting mass. Three replicates were conducted for
each sample.

Dehulling of lentil
A Satake grain milling machine (TM05C, Satake Engineering Co., Hiroshima, Japan) was used for
dehulling the lentil samples. The mill was run at a speed of 1100 rpm. About 30.0 g of lentil sample
was processed in the mill for 38 s. After dehulling, the product was separated into fines, whole
dehulled seeds, split dehulled seeds, undehulled seeds, and hulls. The dehulled sample was
sieved using a No. 20 mesh sieve (0.84 mm) and the fraction passing through the sieve was
defined as fines. The hulls were then removed by an aspirator (Style No. CFZ1, Carter-Day
Company, Minneapolis, MN). The dehulled seeds were then manually separated from the
undehlled seeds. Dehulled split and whole seeds were further separated using a rectangular sieve
1.98 x 19.05 mm (5/64 x 3/4 in). Dehulled seed was defined to the sample with 10% or less
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seedcoat adherence following dehulling. Dehulling efficiency (%) was defined by the percent of
dehulled whole seeds and dehulled split seeds obtained relative to the initial sample mass (Wang,
2005). Three replicated measurements were done for each sample.

Textural cooking quality of lentil
The textural quality of the lentils after cooking was determined by using a texture analyzer (Texture
Technologies Corp., Scarsdale, NY) with a load cell capacity of 25 kg. Texture Expert Exceed
software (Stable Micro Systems Ltd., Surrey, UK) was used to acquire and analyze the force, time
and displacement data. Two-cycle texture profile analysis (TPA) compression test was used. The
parameters for the compression tests were set as follows: pre-test speed =1 mm/s, test speed = 0.5
mm/s, post test speed = 1 mm/s, trigger force = 5.0 g, compression distance after trigger force =
1.8 mm. Twenty-five cooked lentil samples were used for the texture profile analysis. The lentils
were soaked at room temperature for 24 h and then they were cooked for 2, 3, and 4 min at a
boiling water temperature of 98.6oC. The compression tests were conducted at room temperature.
A cylindrical aluminum probe with a diameter of 38.1 mm (1.5 in) was used to compress the
samples on a table parallel to the probe’s compression surface.

Textural quality characteristics of the cooked lentils that were determined included hardness
and cohesiveness. Hardness is the maximum force reached during the first-cycle compression of
the product (Ma et al. 1998). Cohesiveness of the product is determined from the ratio of the
positive compression area under the second compression to the positive area under the first
compression curve up to the point of maximum compression excluding decompression area and
any adhesiveness area (Bourne and Comstock, 1981).

Results and Discussion
Lentil moisture content
The moisture content of the lentils used in the various tests is presented in Table 1. The moisture
content of the Robin lentils varied from 12.06% to 12.51% and that of the Impact lentils ranged from
12.13% to 12.50%. Statistically, there were variations in the moisture content of the samples used
for the experiments.


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Stored lentil color characteristics
Table 2 shows the color characteristics of the dried and stored red lentils. The results show that
there were significant variations in the color of the lentils samples from drying and storage. The
Robin samples became lighter and this was pronounced among samples dried at 70oC. The ?L
values for the stored Robin lentils ranged from -7.34 to -0.20 compared to the values before
storage which varied from -0.88 to1.62 (Opoku et al., 2008). The Robin samples dried with the
microwave were redder and yellowish compared to the samples dried at 70oC. The Robin samples
dried 70oC had the highest total color change of 9.73 compared to a value of 0.92 before storage.
For the Robin lentils, the total color change for P4, P7, P10, and convection dried were 0.86, 1.23,
1.29, and 9.73, respectively, compared to values of 1.72, 1.62, 1.44, and 0.92 before storage.

The Impact lentils dried 70oC showed similar color change characteristics as the Robin
samples dried at 70oC. The convection dried sample became much lighter than the microwave
dried samples. The microwave dried samples became yellowish while the convection air dried
samples became bluish. The total color change for convection dried sample was the highest. For
the Impact lentils, the total color change for P4, P7, P10, and convection dried were 1.86, 1.30,
3.78, and 9.47, respectively, compared to values of 2.41, 1.11, 2.05, and 4.45 before storage.

Breakage susceptibility of lentils
Figure 1 shows the breakage susceptibility of the lentil samples after storage. The results show that
there were significant variations in the breakage susceptibility of the stored sample. The original
Robin and Impact lentil samples produced lower breakage compared to the dried samples. The
Impact lentils had higher susceptibility to breakage than the Robin lentils. For the Robin lentils,
breakage susceptibility decreased with increasing microwave power levels. The convection dried
lentils (Robin and Impact) had lower breakage susceptibility than the microwave dried lentils. The
breakage susceptibility of the lentils may depend on the lentil variety as well as the drying method.
Tang et al. (1991a) determined the breakage susceptibility of harvested Laird lentils and reported
that at a temperature of 22oC and moisture content of 12.2%, the expected breakage susceptibility
would be 17.12%. The samples were dried at 30oC and relative humidity of 5%. Tang et al. (1991b)
determined the effect of convection drying and six-month storage on the breakage susceptibility of
Laird lentils. The samples were dried to a moisture content of 13.5%. They reported that storage
resulted in about 2 to 5% increase in breakage. Increasing the drying temperature tended to reduce
the breakage.
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Dehulling characteristics of lentil
The dehulling characteristics of the dried and the original undried lentils after storage are presented
in Table 3. The results show the method of drying affected significantly the dehulling characteristics
of the lentil samples. For the Robin lentils, the convection dried sample had the highest undehulled
seeds resulting in the lowest dehulling efficiency compared to the other samples. The undried
Robin lentil produced the highest whole dehulled and the lowest split dehulled kernels. The contrary
was produced by the Robin lentils dried at power level 4 (P4). There was no significant difference
between the dehulling efficiency of the microwave dried and undried Robin lentils.

For the Impact lentils, the convection dried sample had the highest undehulled seeds
resulting in the lowest dehulling efficiency compared to the other samples. The undried Impact lentil
produced the highest whole dehulled and the lowest split dehulled kernels. The contrary was
produced by the Impact lentils dried at 70oC. There was no significant difference between the
dehulling efficiency of the microwave dried and undried Impact lentils. The Impact lentils produced
higher split dehulled kernels and lower whole dehulled kernels compared to the Robin lentils.
Oomah and Mazza (1998) reported that microwave drying affected the dehulling of flaxseed.
Samples that were microwave-treated produced higher yields of medium and hull fractions than
untreated seeds. They indicated microwave drying proved to be a useful conditioning treatment for
achieving high yields of hulls and cotyledons.

Textural cooking quality of lentil
The cohesiveness of the cooked lentils samples is presented in Table 4. For the Robin lentils, the
cooking time did not affect the cohesiveness except the original undried samples. The undried
Robin samples cooked for 3 to 4 min had higher cohesiveness values compared to the other
samples. The cohesiveness values for the Robin lentils ranged from 0.18 to 0.25. For the Impact
lentils, the cooking time did not significantly affect the cohesiveness of the samples. The
cohesiveness values for the Impact lentils ranged from 0.23 to 0.28. The cohesiveness of the
Impact lentils tended to be slightly higher than the Robin lentils by comparison. Sareepuang et al.
(2008) reported cohesiveness values for cooked parboiled rice as 0.42 to 0.47. These values for
rice seem slightly higher than the values obtained for the cooked lentils.

The hardness values of the cooked lentils are presented in Table 5. Cooking time did not
significantly influence the hardness of the Robin lentils except the microwave dried sample at
power level 7 (P7). The hardness of values of the Robin lentils tended to slightly decrease with
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cooking time. The undried Robin lentils produced lower hardness values compared to the other
samples. The microwave dried Robin lentils had higher hardness values compared to the undried
and convection dried lentils. The hardness of the Robin lentils ranged from 5.38 to 16.01 N.
Cooking time did not significantly influence the hardness of the Impact lentils except the microwave
dried sample at power level 4 (P4). The hardness of values of the Impact lentils did not show a
clear trend with cooking time. The undried Impact lentils produced lower hardness values
compared to the other samples. The microwave dried Impact lentils had higher hardness values
compared to the undried and convection dried lentils. The hardness of the Impact lentils ranged
from 6.25 to 35.03 N. By comparison, the Impact lentils tended to produce higher hardness values
than Robin lentils. Scanlon et al. (1998) pretreated Laird lentils and cooked the samples for
different times. They reported the hardness values as 56.3, 33.3, and 26.6 N for cooking times of
30, 40, and 50 min.

Conclusions
The following conclusions can be drawn from the experiments conducted on microwave dried,
convection dried and undried lentils after storage:
1. Storage had a marked influence on the color change of the Robin and Impact lentils dried at
70oC. The total color change was higher for the Robin and Impact lentils dried at 70oC
compared to the microwave dried lentils.
Formatted: Bullets and Numbering
2. There were significant variations in the breakage susceptibility of the stored lentils. The
breakage susceptibility of the lentils was influenced by the lentil variety as well as the drying
method.
3. The undried and microwave dried lentils had higher dehulling efficiency compared to the
convection air dried samples. The Impact lentils produced higher split dehulled kernels and
lower whole dehulled kernels compared to the Robin lentils.
4. Cooking time did not have significantly influence on the cohesiveness of most of the lentil
samples. The cohesiveness of the Impact lentils tended to be slightly higher than the Robin
lentils by comparison. The cohesiveness values for the Robin lentils ranged from 0.18 to
0.25 and the Impact lentils varied from 0.23 to 0.28.
5. Cooking time did not markedly affect the hardness of most of the lentil samples. By
comparison the Impact lentils tended to produce higher hardness values compared to the
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Robin lentils. The hardness of the Robin lentils ranged from 5.38 to 16.01 N and the Impact
lentils ranged from 6.25 to 35.03 N.
6. Microwave drying produced enhanced quality lentils in terms of color and dehulling
characteristics compared to the convection air dried lentil samples.

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