EFFECTS OF BLANCHING AND SOAKING ON SOME PHYSICAL CHARACTERISTICS OF GRASS PEA (LATHYRUS SATIVUS)

EFFECTS OF BLANCHING AND SOAKING ON SOME PHYSICAL
CHARACTERISTICS OF GRASS PEA (LATHYRUS SATIVUS)

Urga K1*, Fufa H2, Biratu E2, Gebretsadik M2




Kelbessa Urga



*Corresponding author email: ddr@ethionet.et
1Department of drug Research
2Department of Food Science and Nutrition
Ethiopian Health and Nutrition Research Institute
P.O.Box 1242 or 5654, Addis Ababa, Ethiopia
E-mail: ddr@ethionet.et or ehnri@ethionet.et

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ABSTRACT

Lathyrus sativus, containing a lathyritic principle, called ß-ODAP (ß-N-Oxalyl-L-
?, ß-diaminopropionic acid), is widely grown and consumed in Ethiopia. It is a
hardy crop grown under various agro-ecological situations. The major drawback
in the use of the legume is the fact that the seeds, in common with other legumes,
are not easily rehydrated, are difficult to cook, the seed coat is difficult to remove
and long cooking time is required by traditional processing methods for
tenderization. In this study, the effect of processing methods like blanching,
soaking and cooking on water absorption, leached solids, swelling power, cooking
time and sensory qualities of grass pea seeds were investigated. The soaking
solutions used were mixed with salt solution, wood ash solution and double
distilled de-ionized water. Blanching significantly (p < 0.05) increased total water
absorbed, swelling power, leached solids and reduced cooking time compared to
unbalanced seeds. Soaking blanched seeds in salt or wood ash solution was noted
to be most effective in promoting seed softening during cooking, compared to
double distilled water. Blanched grass pea seeds soaked for 12 hours in the three
soaking solutions caused reduction of 60 %, 73 % and 68 %, respectively, in
cooking time. In contrast, unblanched grass pea seeds soaked for 12 hours in the
three soaking solutions caused reduction of 30 %, 46 % and 40 %, respectively, in
cooking time. Water absorption, swelling power and leached solids values were
negatively correlated (p < 0.05) with the cooking time. The study results indicated
that blanching and hydration of grass pea seeds improved the sensory
characteristics of cooked seeds, significantly reduced the cooking time and
increased the weight and texture of the cooked seeds. Application of blanching
preceded by soaking at village level as an integral part of traditional methods of
processing grass pea seeds, thus offers the dual advantage of saving of valuable
fuels by shortening cooking time, as well as rendering the seeds more acceptable to
consumers. In the absence of salt, wood ash suspension treatment may be a
satisfactory alternative in the production of quick-cooking grass pea seeds.

Key words: blanching, cooking, Lathyrus sativus, soaking

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EFFETS DU BLANCHISSEMENT ET DU TREMPAGE (immersion) SUR
CERTAINES CARACTÉRISTIQUES PHYSIQUES DE LA GESSE BLANCHE
(LATHYRUS SATIVUS)


RÉSUMÉ
Lathyrus sativus, qui contient un principe lathyritique appelé ß-ODAP (ß-N-Oxalyl-L-
?, ß-acide diaminopropionique), est largement cultivé et consommé en Ethiopie. C’est
une culture résistante qui pousse dans différentes situations agro-écologiques. Le plus
grand inconvénient de l’utilisation de cette légumineuse est le fait que les graines, en
commun avec d’autres légumineuses, ne sont pas facilement réhydratées, sont difficiles
à cuire, le tégument (l’enveloppe de la graine) est difficile à enlever et les méthodes de
traitement traditionnelles exigent beaucoup de temps de cuisson pour que la graine
devienne tendre. Dans la présente étude, l’effet des méthodes de traitement - telles que
le blanchissement, le trempage et la cuisson - sur l’absorption de l’eau, des solides
lessivés, le pouvoir de gonflement, le temps de cuisson et les qualités sensorielles des
graines de la gesse blanche ont fait l’objet de recherche. Les solutions du trempage ont
été des mélanges de solution de sel, de solution de cendre de bois et de l’eau
désionisée doublement distillée. Le blanchissement a considérablement (p < 0,05)
augmenté la quantité totale de l’eau absorbée, le pouvoir de gonflement et les solides
lessivés, et il a réduit le temps de cuisson par rapport aux graines déséquilibrées. Il a
été constaté que le trempage des graines dans du sel ou dans une solution de cendre
de bois est le plus efficace pour promouvoir l’adoucissement des graines au cours de la
cuisson, par rapport à l’eau doublement distillée. Les graines de gesse blanchies
trempées pendant 12 heures dans trois solutions de conservation ont causé la
réduction de 60 %, 73 % et 68 %, respectivement, du temps de cuisson. Par contre, les
graines de gesse non blanchies trempées pendant 12 heures dans les trois solutions de
conservation ont causé la réduction de 30 %, 46 % et 40 %, respectivement, dans le
temps de cuisson. Les valeurs de l’absorption de l’eau, du pouvoir de gonflement et des
solides lessivés ont eu une corrélation négative (p < 0,05) avec le temps de cuisson.
Les résultats de cette étude ont indiqué que le blanchissement et l’hydratation des
graines de gesse ont amélioré les caractéristiques sensorielles des graines cuites, ont
considérablement réduit le temps de cuisson et augmenté le poids et la couleur des
graines cuites. L’application du blanchissement précédé par le trempage, au niveau du
village en tant que partie intégrante des méthodes traditionnelles de traitement des
graines de gesse, offre ainsi le double avantage d’économiser les combustibles si chers
en raccourcissant le temps de cuisson et en rendant les graines plus acceptables aux
consommateurs. En l’absence du sel, le traitement de la suspension de la cendre de
bois peut être une alternative satisfaisante dans la production de graines de gesse
rapides à cuire.
Mots-clés: blanchissement, cuisson, Lathyrus sativus, trempage.

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INTRODUCTION

Grain legumes are important crops in Ethiopia in many ways. They play an important
role in providing the bulk of protein requirement for Ethiopians and constitute an
integral part of the staple diet. Grass pea (Lathyrus sativus) is one of the important
grain legume crops. The legume has unique adaptation to extreme harsh conditions of
dry lands. It is grown on heavy black clay soils with poor drainage where there is no
possibility for production of any other alternative pulse. Despite its tolerance to
drought, grass pea is not affected by excessive rainfall and can be grown on land,
subject to flooding [1]. Production of Lathyrus sativus ranks third among legumes. The
major growing areas are mainly concentrated in the North West, Central and Eastern
highlands [2].

Grass pea is an important food item in Ethiopia and is prepared in a wide variety of
dishes. Most often the legume is used as shiro wott (the traditional Ethiopian spiced
dish), kikh wott (decorticated split cotyledon prepared into a spiced soup-like dish),
nifro (boiled and salted whole seed), kolo (roasted whole seed), kitta (unleavened flat
bread) and diffo dabbo (sourdough bread prepared by mixing one-third of grass pea
flour with two-third of wheat flour). Both shiro and kikh wott are eaten with kitta, diffo
dabbo or injera (pancake-like fermented thin bread prepared from cereals) [3].
Moreover, grass pea provides cheap and quality fodder. After harvesting, the dried
straw and chaff are fed to farm animals.

In addition to the presence of the neurotoxin ?-ODAP in grass pea seeds, the major
drawback in the use of the legume is the fact that the seeds, in common with other
legumes, are not easily rehydrated, they are difficult to cook, the seed coat is difficult to
remove and long cooking time is required by traditional processing methods to
tenderize the grain. Extended cooking time of the legume does not encourage its
consumption - due to lack of convenience, fuel cost and nutritive value. Previous
studies have reported that grass pea seeds require 270 min for cooking into tender and
acceptable nifro [4]. A number of techniques have therefore, been utilized to render the
dry grass pea seeds quick-cooking and sufficiently tender for human consumption.
These include soaking of the seeds in different soaking solutions proceeded by
cooking. Unlike other legumes, no comprehensive study has been reported on effects
of pre-treatment methods on the cooking quality of grass pea seeds. This study reports
the effects of some soaking solutions on water absorption, level of leached solids,
swelling power, cooking time and sensory qualities of grass pea seeds.

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MATERIALS AND METHODS

Grass pea seeds: Seeds of a local variety were purchased within one month of harvest
from farmers in Ambo, Western Ethiopia. Wood ash was also obtained from the
farmers' homes. The seeds were visually inspected and any foreign material removed.
The seeds were stored in sealed plastic bags at ambient temperature, and used within
three months of harvest.

Soaking solutions
a. Mixed salt solution
Bicarbonate 1.5 % w/v
Soda ash 0.5 % w/v
Citric acid 0.75 % w/v
pH 7.0 ± 0.05
b. Wood ash solution
100 g sieved wood ash was dissolved in 1 L of de-ionized water and filtered through
Whatman No.1 filter paper. Citric acid (3.5 g) was added to the filtrate to adjust the pH
to 7.0 ± 0.05.

c. Water
Double distilled deionized water

Blanching and soaking: Aliquots (10 g) of dry seeds were blanched in 50 ml of
boiling double distilled deionized water for 5 minutes. The blanched seeds were
drained and then placed in 50 ml of double distilled deionized water, salt or wood ash
solution and left to soak at 23 ± 2 oC for 12 h. Triplicate samples of 5.0 g (weighed
accurately) were also soaked without blanching in 25 ml double distilled deionized
water, salt or wood ash solution for 12 h and used as control.

Water absorption and swelling power: For water absorption measurement, soaked
seeds were drained, rinsed with double distilled deionized water, blotted dry with paper
towel, and weighed. Absorption measurements were done after 1h, 3h, 6h, 9h, and 12h
soaking. The soaking solutions were evaporated and dried at 105 ± 2 oC in a gravity
convection oven to constant weight to determine lost solids. Water absorption for the
cooked seeds was determined in a manner similar to that for soaked grass pea seeds.
Water absorption and swelling power were calculated by the equations described by
Plhak et al. [5].
wt)
(drained
?
dry wt

(initial
?SL)
WA =
×100%
dry wt

(initial
?SL)
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(
volume)
drained
?
volume)
initial
(
SP =
×
%
100
dry wt

(initial
?

SL)
Where SL is the solids lost during soaking or cooking.

Cooking time: Blanched or unblanched (control) seeds soaked in water, salt or wood
ash solution and drained were cooked in boiling double distilled deionized water (seed:
water ratio, 1:4 w/v) until they were softened to uniform mass when pressed between
two glass slides so that no hard material was found as described by Narasimha and
Desikachar [6]. The time required to obtain such a consistently cooked product was
recorded as the cooking time.

Sensory evaluation: Sensory properties such as color, mouth feel, flavor, taste, texture
and general acceptability of pre-treated seeds were evaluated by 10 panel members.
Since women are involved in preparation of grass pea food products, female panelists
familiar with grass pea products were selected from the Institute. Freshly boiled
samples were served for sensory evaluation. A hedonic scale of; 1 = poor, 2 = fair, 3
=good and 4 = excellent, was used for color, mouth feel, flavour, taste and general
acceptability attributes. Tenderness was determined subjectively by feeling between the
fingers as done traditionally. Tenderness was scored on a 1 to 4 scale, with a rating for,
1 very hard seeds and 4, very soft seeds. The time required to achieve a rating of 4
(soft) was compared with the cooking time. Comparisons between treatments were
analyzed for significant difference (p < 0.05) by using one-way analysis of variance.
Data were analyzed using the SPSS Statistical Package (version 10, SPSS Inc, Chicago,
Il, USA).

RESULTS

Table 1 shows values of swelling power and leached solids for unblanched seeds.
Values for water absorption of seeds soaked in the 3 soaking solutions were identical
during the first hour of soaking which then varied according to both time of soaking
and soaking solution (Figure 1). Water absorption values were comparable for seeds
soaked for 6 or 12 hours in wood ash or salt solution. Un-blanched seeds soaked for 12
hours in both wood ash or salt solution exhibited equal swelling power and solids
leached which are more than 2-fold the swelling power and leached solids of seeds
soaked in water.

Water absorption of blanched seeds soaked for 12 hours in water increased by more
than 3-fold whereas, the values increased slightly in the other soaking solutions (Figure
2). None of the soaked seeds however, attained water absorption value of 100 % after
12 hours soaking in all the 3 solutions. The rate of increase in volume (swelling power)
of the blanched seeds soaked for 12 hours in salt and wood ash solutions were
comparable while seeds soaked in water for a similar length of time increased by more
than 4-fold (Table 2). Values for swelling power and leached solids of blanched seeds
soaked for 12 hours in salt and wood ash solutions were significantly higher than those
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soaked in water. Water absorption, swelling power and leached solids values were
significantly and positively correlated with the soaking time (p < 0.05).


Water
Salt
Wood ash

180
160
140

120
/kg) 100
g

(
80
A
W
60
40

20
0

1
2
3
4
5
Soaking time (hrs)
Figure 1. Water absorption (WA) of unblanched grass pea seeds. Data represents ± of S.E.M five
independent tests



Water
Salt
Wood ash

1000

800
600

400
WA (g/kg)

200
0

1
2
3
4
5
Soaking time (hrs)

Figure 2. Water absorption (WA) of blanched grass pea seeds. Data represents ±of S.E.M five
independent tests



Table 3 indicates cooking characteristics of blanched and soaked seeds. When blanched
seeds were soaked for 12 hours in water, salt and wood ash solution and then cooked to
eating ‘soft’, water absorption increased by only 18 %, 8 % and 9 %, respectively. In
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contrast, the swelling power of blanched grass peas soaked for 12 hours in water and
cooked to ‘eating soft’ increased by 54 %, 47 % and 27 %, respectively. The levels of
solids leached into the cooking water similarly increased by 17 %, 23 %, 30 %,
respectively. Unblanched grass pea seeds soaked for 12 hours in the three soaking
solutions caused reduction of 30 %, 46 % and 40 %, respectively, in cooking time
(Figure 3). In contrast, blanched grass pea seeds soaked for 12 hours in the three
soaking solutions caused reduction of 60 %, 73 % and 68 %, respectively, in cooking
time (Figure 4). For blanched seeds variation in the degree of hydration expressed as
the ratio of cooked weight to the dry weight is not significant over soaking time and
soaking solution. However, blanched seeds soaked in water for 12 hours had a
relatively lower degree of hydration than seeds soaked for a similar length of time in
salt or wood ash solutions.

Test panel scores for blanched and unblanched grass pea seeds cooked after
pretreatment in the three soak solutions are presented in Table 4. Results of the sensory
evaluation indicated that unblanched seeds exhibited less overall acceptance scores
compared to blanched seeds. The overall acceptance scores of blanched seeds soaked in
salt solution were higher, compared to blanched seeds soaked in water and wood ash
solution



Water
Salt
Wood Ash

350
.
)
n 300

250
me (mi

ng ti 200

150
Cooki
100

1
3
6
9
12
Soaking time (hrs.)
Figure 3
Cooking time of unblanched
grass pea seeds. Data represents ± of
S.E.M five independent tests

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Water
Wood Ash
Salt
250
200
150
100
50
Cooking time (min.)
0
1
3
6
9
12
Soaking time (hrs.)

Figure 4
Cooking time of blanched grass pea seeds. Data represents ± of S.E.M five independent
tests



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DISCUSSION

Water absorption values for unblanched seeds were equal during the first hour of
soaking, which then varied according to both time of soaking and soaking solution.
The unblanched seeds soaked in salt and wood ash solutions showed the highest rate of
water absorption than seeds soaked in water. Water absorption is a measure of gross
water uptake by seeds during soaking and is influenced by the integrity of the seed
coat. Water uptake by seeds is driven by several forces at various tissue levels. The
osmotic potential and matric potential contribute to the water potential of a system by
the presence of dissolved solutes and hydrocolloids, respectively. Leached solids thus
determine water uptake because of the decrease in the differential between intracellular
and extra cellular water potentials [7].

Blanching had a marked effect on the water absorption values of the seeds. For
blanched seeds, the first hour of soaking seemed to be central since a wide variation in
the imbibed water was observed as compared to the unblanched seeds. The increase in
water absorption of blanched seeds may be attributed to the change in the integrity of
the seed coat due to heat treatment resulting in alteration of the anatomical barriers of
the hilum allowing water to penetrate more rapidly. Varrian-Mariston and Jackson and
Deshpande and Cheryan observed that the hilum and micropyle were the most
important structural features influencing the initial water uptake of beans [8, 9].

Like water absorption, differences occurred in swelling power during the initial phase
of soaking due to blanching when compared to unblanched seeds. The observed
differences may be due to the effect of blanching on seed coat which increased the
imbibition of water resulting in noticeable swelling of the structural cells and swelling
power of seeds. In a similar study, it was reported that blanching markedly improved
the swelling power of winged beans [10].

Leaching of solids during soaking of the unblanched seeds was lower compared to
blanched seeds. The rate of loss of solids was approximately linear throughout the
soaking period in both cases. However, leaching losses of solids during soaking of
blanched and unblanched seeds in salt and or wood ash solutions were higher than
those soaked in water. Such variation may be related to the breakdown of intercellular
components, as well as to water taken up by the seeds because of the contribution of
leached solids to the osmotic and matric potentials of the solutions to which they are
leached. Although a higher level of leached solids in the present study may reduce
nutrient contents such as vitamins, minerals and amino acids, it also reduces the level
of anti-nutritional factors, particularly the neurotoxin ?-ODAP and may be beneficial in
preparing safe grass pea foods [11]. In a separate study, soaking and cooking treatments
reduced the contents of anti-nutritional factors of grass pea and winged bean seeds [12,
13, 14]. In the present study, a significant positive correlation was observed between
the time for soaking and swelling power, water absorption and leached solids [10].

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