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Effect of traditional cooking methods on the ascorbic acid content of some Nigerian leafy and fruit vegetables
EFFECT OF TRADITIONAL COOKING METHODS ON THE ASCORBIC ACID CONTENT OF SOME NIGERIAN LEAFY A N D FRUIT VEGETABLES O. O. KESHINRO & A. O. KET1KU
Department of Human Nutrition, University of Ibadan, Ibadan, Nigeria (Received: 26 October, 1978)
ABSTRACT
Representative samples of vegetables purchased from a local market, without the age and time of harvest being known, were analysed for ascorbic acid content. The ascorbic acid content in fresh samples was highest in Hibiscus esculentus L. with 203 mg/100 g _+51.38 and lowest in Vernonia amygdalina with 30.95 mg/lO0 g +_4.85. After laboratory preparation and cooking of the vegetables, the loss of this vitamin was significant in all the samples. The use offirewood as the source of heat and of mudpot utensils caused a higher loss of ascorbic acid in all the vegetables than cooking with gas using aluminium pots.
INTRODUCTION
L-ascorbic acid is usually considered as synonymous with vitamin C because it is the main substance in plants with vitamin C activity. Fruits are major sources of vitamin C (Boqert et al., 1966), and it is found that quite high amounts of vitamin C are contained in green and red peppers, parsley and turnip greens and that some green leafy vegetables are excellent sources of ascorbic acid, even exceeding the amounts in most fruits. There is also abundant evidence in the literature to show that ascorbic acid consumption by Nigerians has been quite good, with fruits being the major source (Kotnis & Houssain, 1964; Nicol, 1958; Oke, 1966). Oke (1968) reported that since Nigeria is rich in fruits, symptoms of vitamin C deficiency have been rare. However, fruits are now sold by producers who are mainly from the low-income group and the cost of fruit is becoming high. Therefore Nigerians depend on vegetables as their source of vitamins, especially vitamin C. The high cost of fruits in Nigeria, particularly in the urban areas, is causing their consumption to fall. 303 Fd. Chem. 0308-8146/79/0004-0303/$02.25 © Applied Science Publishers Ltd, London, 1979 Printed in Great Britain
304
o.o.
KESHINRO, A. O. KETIKU
In a survey carried out in Igbo Ora in Western Nigeria (Kotnis & Houssain, 1964) it was found that the intake of ascorbic acid by the villagers varied from 280 ~ of the recommended allowance in children 3-5 years old to about 400 ~ in pregnant women. Nicol (1958) found that lack of fresh fruits and leaves in the Northern Region of Nigeria resulted in varying degrees of dietary deficiencies of ascorbic acid. The consumption of fruits is therefore declining amongst the low-income stratum of the population which forms the greatest percentage of the community. Very little is known about the effect of cooking on some Nigerian foodstuffs, although it is universally believed that ascorbic acid is destroyed by the native way of cooking (Oke, 1967) and almost all vegetables are consumed in a cooked state. Ascorbic acid is very easily oxidised and if this oxidative process continues beyond the stage of dehydroascorbic acid it becomes irreversible; therefore the vitamin loses its activity. The enzyme oxidase facilitates rapid oxidation of the vitamin in the presence of air since it is in direct contact with ascorbic acid. Heat inactivates the oxidase enzyme but it also destroys the vitamin even in the absence of the enzyme (Davidson & Passmore, 1975). Even though some vegetables contain higher amounts of ascorbic acid than fruits (Boqert et al., 1966), Mapson (1970) points out that fruits possess an advantage over many vegetables because of the acidic media of the fruit juices compared with the nearly neutral vegetables. The fact that many fruits are eaten raw is an advantage because some loss of ascorbic acid is usually incurred during cooking (Mapson, 1970; Davidson & Passmore, 1975). Therefore it would be helpful to know the amount of this vitamin which is derived from vegetables cooked in the traditional way. It might be valuable to suggest how cooking procedures could be modified so as to retain more of the vitamin without sacrifice of economy, palatability and appearance. In order to get the body tissues well saturated with vitamin C which may help to reduce the severity of stress, heart diseases, cancer and other diseases (Anderson, 1977) the ascorbic acid loss from foods must be curtailed. In most food composition tables (e.g. Food Composition Table for Use in Africa, 1968) records of ascorbic acid composition of vegetables were given only for raw or dried samples. The various traditional treatments and cooking methods to which these vegetables are subjected differ widely from one community to another. Hewston et al. (1948) and Oser et al. (1943) reported that little information is available on the retention of vitamins and minerals by common foods cooked in family quantities. MATERIAES AND METHODS
Materials Ten of the most popular vegetables were chosen for the study: Celesia argentea (green and red types), Amaranthus chlorostachys, Crassocephalum crepidivides, Basella alba, Tallinum triangulare, Cochorus oliturius, Hibiscus esculentus L., Solanum gilo Raddi and Vernonia amygdalina.
EFFECT OF TRADITIONAL COOKING ON ASCORBIC ACID CONTENT
305
In all cases vegetables were purchased although with no knowledge of their age or the time since harvesting. Thus, it cannot be stated that the vegetables were fresh but on physical examination they were neither wilted nor damaged. This was purposely done so as to make the samples representative of what an average person would buy from the market, without any prior knowledge of the history of the vegetables. Maturity affects the ascorbic acid content of some vegetables. It was reported by Oke (1968) that the vitamin C content of the vegetable sample Cochorus oliturius varied considerably with age, i.e. an approximate 233 °/o increase, whereas, in some others, the vitamin C content was more or less constant during maturation. The vegetables were divided into three groups according to the differences in the treatment and cooking methods (see Table I). Group 1 : The leaves and the soft stems of the samples were plucked off the tough stem. They were cut into tiny pieces and washed several times to get rid of the sand and dirt. They were then parboiled for about five minutes with enough water to just cover the tops of the vegetables. The samples were drained and washed again. The traditional reason for this action is that the natural juice that comes from the vegetables, if not parboiled, has three effects: (i) it gives a sour taste to the stew made from it shortly after cooking; (ii) it gives a dark coloration to the stew; (iii) the dark colour is culturally unattractive for stews and therefore psychologically unacceptable. The vegetables are subjected next to a further cooking for about 10min in a previously prepared sauce which is boiling on the fire. Group 2: This group requires no parboiling. The edible parts are washed and cut into tiny pieces. Then the samples are added to either an estimated amount of water, as in the case of Cochorus oliturius and Hibiscus esculentus, or into pre-prepared sauces, as in the case of Basella alba and Tallinum triangulate. In this group no water is discarded, the whole product being consumed. Group 3: This group requires a special squeeze-washing process several times because of the tough nature of the vegetable fibre and to remove their bitter taste. After this process, the vegetables are parboiled as in group 1. They are then rinsed again and subjected to further cooking in pre-prepared sauce. All the experimental cookings of groups 1-3 were done in the laboratory using a gas-fire as the source of heat and various modern cooking pans. It should be noted that only a small fraction of the community has such facilities. Therefore, to make the results more meaningful, all ten vegetables were analysed after cooking in mud pots using firewood, which is used by a large percentage of the population.
Analytical method This procedure is an adaptation of the method described by Roe et al. (1966) and is based on the oxidation of ascorbic acid to dehydroascorbic acid, subsequent transformation to diketogulonic acid followed by coupling with 2,4-dinitrophenyl
(Igbo)
finger)
(green) (red)
Vernoniab amygdalina
Solarium” gilo Raddi
Tallinum lriangulare Cochorus oliturius Hibiscus esrulentus L.
Basella alba
Celesia argenrea Celesia argenlea Amaranthus chlorosrachys Crassocephalum crepidivides
Botanical name of vegetable
TABLE
1
85.80
88.87
90.18
92.93 81.61
92.50
81.70
41.14+
85.63
* 50-38 Group 3
36.75 k4.05 (10.9*0.05) 30.95 +4.85 (3,2+0,14)
203N
0
0
35.5OkO.23
29.20* 1.61 9.90 + 2.02
54.23 f 7.23
63,10+ 11.37 54.40 rf:8.08
3.8OkO.81
Group 2 62.60 + IO.85
16X10+ I.20
8.7+ 1.50 11.2&1.15
Mean ascorbic acid content of vegetable (mg/lOOgfSE)
0
0
Not applicable
Not applicable Not applicable
Not applicable
0
7.00*0
4.84+0 3.33*0
Mean ascorbic acid content in the discarded waler (mg/Wg)
5 min.
100 100
0
91.14
61.73 86.03
19.57
0
18.0&O
24.15kO.08 7,6* 1.84
50.35 + 7.53
97.83
80.32
8.1 kO.90 1,0&O
96.67 92.97
% of total loss of ascorbic acid
1.1 &O 2.7 kO.86
Final cooking for 10 min kean ascorbic acid content of vegetable (.mg/lofl g + W
LEAFY AND FRUIT VEGETABLES’
Parboiled for approx.
NIGERIAN
45.91 f 2.83
1.73
Group 1 32.95 + 1.76 38.40 + 3.00
Mean ascorbic acid content of raw sample (mg/lOOg+SE)
OF SOME COMMON
8974 89.80
Moisture content
/o
o-
ACID AND MOISTURE CONTENT
“Cooking done under laboratory conditions (i.e. use of gas fire and aluminium pots). ‘The ascorbic acid contents after the traditional squeeze-wash process are recorded in parentheses.
10. Ewuro (Bitterleaf)
9. Igbagba
7. Ewedu 8. Ila (Lady’s or Okra
5. Amunututu (sejesoro) 6. Gbure
4. Ebolo
1. Sokoyokoto 2. Sokoyokoto 3. Tete
Local name of vegetable
-
ASCORBIC
EFFECT OF TRADITIONAL COOKING ON ASCORBIC ACID CONTENT
307
hydrazine under carefully controlled conditions to give red coloured osazones which are then compared with standard ascorbic acid solutions. This method is known for its accuracy and can be used for fresh food samples. The means and standard errors were calculated from four different vegetable samples, each treated once, and the ascorbic acid content was determined.
RESULTS AND DISCUSSION
Results are given in Tables 1 and 2. Apart from the Vernonia amygdalina and Solanum gilo Raddi, which had to undergo special treatment before cooking (see group 3), all other vegetables in the raw state were good sources ofascorbic acid. The
Hibiscus esculentus L. (Lady's finger or Okra) is the richest source of ascorbic acid with about 203 _+ 50.38 mg/100g. Group 3 in Table 1 has very little ascorbic acid content left after the traditional squeeze-washing process and this is further destroyed at the end of the initial parboiling. The total loss of ascorbic acid ranged from 19"57~o in Basella alba to 100~o in Solanum gilo Raddi and Vernonia amygdalina under laboratory conditions. The percentage loss of ascorbic acid in samples cooked by traditional methods is higher than that from samples cooked by laboratory methods. The loss ranged from 73.54 ~ in Tallinum triangulate to 100 ~o in Solanum gilo Raddi, Vernonia amygdalina, and green and red types of Celesia argentea. The reasons for this are that the intensity of the fire applied under local conditions is higher than under laboratory conditions and that the mud pot retains heat for a much longer time than the aluminium pot. Secondly, the traditional method of parboiling in enough water to cover the vegetables causes more loss of vitamin C, as can be seen in groups 1 and 2 of the Tables 1 and 2. These findings correspond with those of Krehl & Winters (1950) who reported that the greatest losses of both minerals and vitamins occur as a result of cooking by the 'old fashioned' method of just sufficient water to cover. They also reported that vegetables cooked in little water, i.e. half a cupful (125 ml), or in the more rapid pressure cooker with half a cupful of water showed quite similar losses. Krehl & Winters (1950) further reported that the pressure cooker method and the waterless methods retained more of the ascorbic acid contents, probably due to the much shorter cooking time which allows for less possibility of losses due to oxidation in the case of this vitamin. Oke (1967) reported that in Nigeria the average woman is not particular about the amount of water used. She boils the vegetables in a slight excess of water and discards the water; hence the percentage of vitamin retention by this method is lowest compared with other methods, such as pressure cooking with little water or the waterless method. The discarded water also contains an appreciable amount of ascorbic acid (see Tables 1 and 2). Krehl & Winters (1950) again suggested that the factors governing the serious loss of vitamins and minerals were the leaching effect of cooking water and the influence of oxidation, particularly of
TABLE 2
0 0
0
gilo Raddi Vernonia amygdalina
10. Ewuro (Bitterleaf)
aCooking done in mud pots on local wood fire.
3-60 + 0 8.45 _+ 1.18
Group 3 0
Solanum
7. Ewedu 8. Ila (Lady's finger)
23.3_+0.37 27.0_+ 1-27 Not applicable Not applicable
Not applicable Not applicable
0.95 _+0.08
9. Igbagba (Igbo)
1.76+0.13 3.85-t-0.16 5.35+0.49
Basella alba Tallinum triangulare Cochorus oliturius Hibiscus esculentus L.
3.00 _+0.28
Group 2
Group 1
5. A m u n u t u t u (sejesoro) 6. Gbure
4. Ebolo
7-55 -+ 1.12 9.65 -+ 1.21 13.00 -+ 1.50
Parboiled for 5 mm Mean ascorbic acid Mean ascorbic acid content o f vegetable content of the (mg/1 O0 g +_SE) discarded water (mg/1 O0 g + SE)
Celesia argentea Celesia argentea Amaranthus chlorostachys Crassocephalum crepidivides
Botanical name of vegetable
1. Sokoyokoto (green) 2. Sokoyokoto (red) 3. Tete
Local name of vegetable
0
0
0 5.30 + 0.75
15.75 _ 0-14 16.70_+0.51
1-2 _+0
0 0 3.5_+0
Boiled for 10 rain Mean ascorbic acid content o f vegetable (mg/1 O0 g -+SE)
ASCORBIC A C I D C O N T E N T OF SOME C O M M O N N I G E R I A N LEAFY A N D F R U I T VEGETABLES P R E P A R E D T R A D I T I O N A L L Y
100
100
100 97.29
74-83 73.54
97.29
100 100 91.50
°/o of total loss of ascorbic acid from raw sample in Table 1
a
,.q
©
P
rn _ Z
O
EFFECT OF TRADITIONAL COOKING ON ASCORB1C ACID CONTENT
309
a s c o r b i c acid which is sensitive to o x i d a t i o n , a n d s h o w e d t h a t a p o r t i o n o f the loss was a c c o u n t e d for in the d r a i n e d liquid. D a v i d s o n & P a s s m o r e (1969) m e n t i o n e d t h a t a helping o f g o o d g a r d e n vegetables a d a y will i m p r o v e health, b u t this can o n l y be true if the m e t h o d s o f p r e p a r a t i o n a r e i m p r o v e d . Even the a s c o r b i c acid c o n t e n t o f fresh p l a n t s can be i m p r o v e d by the e n v i r o n m e n t . R i c h a r d s o n (1954) a n d Boqert et al. (1966) have b o t h suggested t h a t light was the m a i n e n v i r o n m e n t a l f a c t o r influencing the a s c o r b i c acid c o n t e n t o f fruits a n d vegetables. T h e y f o u n d t h a t there was an a v e r a g e increase o f 33 ~o in a s c o r b i c acid within seven d a y s as a result o f light intensity c o m p a r e d with p l a n t s u n d e r heavier shading. In this respect N i g e r i a , being a less i n d u s t r i a l c o u n t r y , enjoys less p o l l u t i o n o f the a t m o s p h e r e a n d therefore a b u n d a n t solar light. In c o n c l u s i o n , since there is a g r e a t difference in the a s c o r b i c acid c o n t e n t o f p l a n t s f r o m the time they are p l u c k e d on the f a r m to the time they arrive at the table r e a d y for c o n s u m p t i o n , a n d since N i g e r i a n s have to d e p e n d on vegetables for a large p r o p o r t i o n o f their a s c o r b i c acid requirements, their m e t h o d s o f p r e p a r a t i o n need to be i m p r o v e d .
ACKNOWLEDGEMENT I should like to seize this o p p o r t u n i t y to t h a n k the D i r e c t o r a n d the l a b o r a t o r y staff o f this D e p a r t m e n t w h o have c o n t r i b u t e d in no small m e a s u r e to the execution o f this project.
REFERENCES ANDERSON,T. W. (1977). New horizon for vitamin C, Nutrition Today (Jan/Feb) 12(1), 6-13. BOQERT,L. J., BRIGGS,G. M. & GALLOWAY,D. H. (1966). Nutrition and physicalfitness. W. B. Saunder Co. Philadelphia, 218-20. DAVIDSON,SXRSTANLEY& PASSMORE,R. (1975). Human nutrition and dietetics (6th ed.) 161-2 and 255. DAVIDSON,SIR STANLEY& PASSMORE,R. (1969). Human nutrition and dietetics (4th ed.) Williams and Wilkins, Baltimore, 899. FAD (1968). Food Composition Table for Use in Africa. KOTNlS, M. & HOUSSAIN,M. A. (1964). Estimated intake of calories and nutrients in relation to recommended allowances. UNICEF Fellows Report of Igbo-Ora Survey, lbadan Univ., p. 6. HEWSTON,E. M., DAWSON,E. H., ALEXANDER,L. M. & ORENT-KEILES,E. (1948). Vitamin andmineral content of certain foods as affected by home preparation. USD. Misc. Publ. No. 628, 1948. K REHL,W. A. 8£WINTERS,R. W. (1950). Effect of cooking methods on retention of vitamins and minerals in vegetables, Am. Diet Ass., 26, 966-72. MAPSON,L. W. (1970). Vitamins in fruits. In The biochemistry offruitsandtheirproducts. (Hulme, A. C. (Ed.) Academic Press, New York, 369-84. NICOL, B. N. (1958). Ascorbic acid in the diets of rural Nigerians, Personal Communication to Authors. OKE, O. L. (1966). Ascorbic acid content of some Nigerian foodstuffs, West African Phamacist., 8(5) 92-3. OKE, O. L. (1967). The ascorbic acid content of Nigerian vegetables, J. of Food Science, 32(1), 85-6. OKE, O. L. (1968). Chemical changes in some Nigerian vegetables during growth. J. ofExpl. Agric., (4) 345--9.
310
o.o.
KESHINRO, A. O. KETIKU
OSER,B. L., MELNICK, D. & OSER, M. (1943). Influence of cooking procedure upon retention of vitamins and minerals in vegetables. Food Research, 8, I 15. RICHARDSON, L. R. 0954). Effect of environment on the ascorbic acid content of plants. Influence of environment on the chemical composition o f plants. Southern Coop. Service, 36, 6-24. RoE et al. (1966). Methods of analysis. In Methods of vitamin assay. (3rd ed.). The Association of Vitamin Chemists, Inc., New York.
Department of Human Nutrition, University of Ibadan, Ibadan, Nigeria (Received: 26 October, 1978)
ABSTRACT
Representative samples of vegetables purchased from a local market, without the age and time of harvest being known, were analysed for ascorbic acid content. The ascorbic acid content in fresh samples was highest in Hibiscus esculentus L. with 203 mg/100 g _+51.38 and lowest in Vernonia amygdalina with 30.95 mg/lO0 g +_4.85. After laboratory preparation and cooking of the vegetables, the loss of this vitamin was significant in all the samples. The use offirewood as the source of heat and of mudpot utensils caused a higher loss of ascorbic acid in all the vegetables than cooking with gas using aluminium pots.
INTRODUCTION
L-ascorbic acid is usually considered as synonymous with vitamin C because it is the main substance in plants with vitamin C activity. Fruits are major sources of vitamin C (Boqert et al., 1966), and it is found that quite high amounts of vitamin C are contained in green and red peppers, parsley and turnip greens and that some green leafy vegetables are excellent sources of ascorbic acid, even exceeding the amounts in most fruits. There is also abundant evidence in the literature to show that ascorbic acid consumption by Nigerians has been quite good, with fruits being the major source (Kotnis & Houssain, 1964; Nicol, 1958; Oke, 1966). Oke (1968) reported that since Nigeria is rich in fruits, symptoms of vitamin C deficiency have been rare. However, fruits are now sold by producers who are mainly from the low-income group and the cost of fruit is becoming high. Therefore Nigerians depend on vegetables as their source of vitamins, especially vitamin C. The high cost of fruits in Nigeria, particularly in the urban areas, is causing their consumption to fall. 303 Fd. Chem. 0308-8146/79/0004-0303/$02.25 © Applied Science Publishers Ltd, London, 1979 Printed in Great Britain
304
o.o.
KESHINRO, A. O. KETIKU
In a survey carried out in Igbo Ora in Western Nigeria (Kotnis & Houssain, 1964) it was found that the intake of ascorbic acid by the villagers varied from 280 ~ of the recommended allowance in children 3-5 years old to about 400 ~ in pregnant women. Nicol (1958) found that lack of fresh fruits and leaves in the Northern Region of Nigeria resulted in varying degrees of dietary deficiencies of ascorbic acid. The consumption of fruits is therefore declining amongst the low-income stratum of the population which forms the greatest percentage of the community. Very little is known about the effect of cooking on some Nigerian foodstuffs, although it is universally believed that ascorbic acid is destroyed by the native way of cooking (Oke, 1967) and almost all vegetables are consumed in a cooked state. Ascorbic acid is very easily oxidised and if this oxidative process continues beyond the stage of dehydroascorbic acid it becomes irreversible; therefore the vitamin loses its activity. The enzyme oxidase facilitates rapid oxidation of the vitamin in the presence of air since it is in direct contact with ascorbic acid. Heat inactivates the oxidase enzyme but it also destroys the vitamin even in the absence of the enzyme (Davidson & Passmore, 1975). Even though some vegetables contain higher amounts of ascorbic acid than fruits (Boqert et al., 1966), Mapson (1970) points out that fruits possess an advantage over many vegetables because of the acidic media of the fruit juices compared with the nearly neutral vegetables. The fact that many fruits are eaten raw is an advantage because some loss of ascorbic acid is usually incurred during cooking (Mapson, 1970; Davidson & Passmore, 1975). Therefore it would be helpful to know the amount of this vitamin which is derived from vegetables cooked in the traditional way. It might be valuable to suggest how cooking procedures could be modified so as to retain more of the vitamin without sacrifice of economy, palatability and appearance. In order to get the body tissues well saturated with vitamin C which may help to reduce the severity of stress, heart diseases, cancer and other diseases (Anderson, 1977) the ascorbic acid loss from foods must be curtailed. In most food composition tables (e.g. Food Composition Table for Use in Africa, 1968) records of ascorbic acid composition of vegetables were given only for raw or dried samples. The various traditional treatments and cooking methods to which these vegetables are subjected differ widely from one community to another. Hewston et al. (1948) and Oser et al. (1943) reported that little information is available on the retention of vitamins and minerals by common foods cooked in family quantities. MATERIAES AND METHODS
Materials Ten of the most popular vegetables were chosen for the study: Celesia argentea (green and red types), Amaranthus chlorostachys, Crassocephalum crepidivides, Basella alba, Tallinum triangulare, Cochorus oliturius, Hibiscus esculentus L., Solanum gilo Raddi and Vernonia amygdalina.
EFFECT OF TRADITIONAL COOKING ON ASCORBIC ACID CONTENT
305
In all cases vegetables were purchased although with no knowledge of their age or the time since harvesting. Thus, it cannot be stated that the vegetables were fresh but on physical examination they were neither wilted nor damaged. This was purposely done so as to make the samples representative of what an average person would buy from the market, without any prior knowledge of the history of the vegetables. Maturity affects the ascorbic acid content of some vegetables. It was reported by Oke (1968) that the vitamin C content of the vegetable sample Cochorus oliturius varied considerably with age, i.e. an approximate 233 °/o increase, whereas, in some others, the vitamin C content was more or less constant during maturation. The vegetables were divided into three groups according to the differences in the treatment and cooking methods (see Table I). Group 1 : The leaves and the soft stems of the samples were plucked off the tough stem. They were cut into tiny pieces and washed several times to get rid of the sand and dirt. They were then parboiled for about five minutes with enough water to just cover the tops of the vegetables. The samples were drained and washed again. The traditional reason for this action is that the natural juice that comes from the vegetables, if not parboiled, has three effects: (i) it gives a sour taste to the stew made from it shortly after cooking; (ii) it gives a dark coloration to the stew; (iii) the dark colour is culturally unattractive for stews and therefore psychologically unacceptable. The vegetables are subjected next to a further cooking for about 10min in a previously prepared sauce which is boiling on the fire. Group 2: This group requires no parboiling. The edible parts are washed and cut into tiny pieces. Then the samples are added to either an estimated amount of water, as in the case of Cochorus oliturius and Hibiscus esculentus, or into pre-prepared sauces, as in the case of Basella alba and Tallinum triangulate. In this group no water is discarded, the whole product being consumed. Group 3: This group requires a special squeeze-washing process several times because of the tough nature of the vegetable fibre and to remove their bitter taste. After this process, the vegetables are parboiled as in group 1. They are then rinsed again and subjected to further cooking in pre-prepared sauce. All the experimental cookings of groups 1-3 were done in the laboratory using a gas-fire as the source of heat and various modern cooking pans. It should be noted that only a small fraction of the community has such facilities. Therefore, to make the results more meaningful, all ten vegetables were analysed after cooking in mud pots using firewood, which is used by a large percentage of the population.
Analytical method This procedure is an adaptation of the method described by Roe et al. (1966) and is based on the oxidation of ascorbic acid to dehydroascorbic acid, subsequent transformation to diketogulonic acid followed by coupling with 2,4-dinitrophenyl
(Igbo)
finger)
(green) (red)
Vernoniab amygdalina
Solarium” gilo Raddi
Tallinum lriangulare Cochorus oliturius Hibiscus esrulentus L.
Basella alba
Celesia argenrea Celesia argenlea Amaranthus chlorosrachys Crassocephalum crepidivides
Botanical name of vegetable
TABLE
1
85.80
88.87
90.18
92.93 81.61
92.50
81.70
41.14+
85.63
* 50-38 Group 3
36.75 k4.05 (10.9*0.05) 30.95 +4.85 (3,2+0,14)
203N
0
0
35.5OkO.23
29.20* 1.61 9.90 + 2.02
54.23 f 7.23
63,10+ 11.37 54.40 rf:8.08
3.8OkO.81
Group 2 62.60 + IO.85
16X10+ I.20
8.7+ 1.50 11.2&1.15
Mean ascorbic acid content of vegetable (mg/lOOgfSE)
0
0
Not applicable
Not applicable Not applicable
Not applicable
0
7.00*0
4.84+0 3.33*0
Mean ascorbic acid content in the discarded waler (mg/Wg)
5 min.
100 100
0
91.14
61.73 86.03
19.57
0
18.0&O
24.15kO.08 7,6* 1.84
50.35 + 7.53
97.83
80.32
8.1 kO.90 1,0&O
96.67 92.97
% of total loss of ascorbic acid
1.1 &O 2.7 kO.86
Final cooking for 10 min kean ascorbic acid content of vegetable (.mg/lofl g + W
LEAFY AND FRUIT VEGETABLES’
Parboiled for approx.
NIGERIAN
45.91 f 2.83
1.73
Group 1 32.95 + 1.76 38.40 + 3.00
Mean ascorbic acid content of raw sample (mg/lOOg+SE)
OF SOME COMMON
8974 89.80
Moisture content
/o
o-
ACID AND MOISTURE CONTENT
“Cooking done under laboratory conditions (i.e. use of gas fire and aluminium pots). ‘The ascorbic acid contents after the traditional squeeze-wash process are recorded in parentheses.
10. Ewuro (Bitterleaf)
9. Igbagba
7. Ewedu 8. Ila (Lady’s or Okra
5. Amunututu (sejesoro) 6. Gbure
4. Ebolo
1. Sokoyokoto 2. Sokoyokoto 3. Tete
Local name of vegetable
-
ASCORBIC
EFFECT OF TRADITIONAL COOKING ON ASCORBIC ACID CONTENT
307
hydrazine under carefully controlled conditions to give red coloured osazones which are then compared with standard ascorbic acid solutions. This method is known for its accuracy and can be used for fresh food samples. The means and standard errors were calculated from four different vegetable samples, each treated once, and the ascorbic acid content was determined.
RESULTS AND DISCUSSION
Results are given in Tables 1 and 2. Apart from the Vernonia amygdalina and Solanum gilo Raddi, which had to undergo special treatment before cooking (see group 3), all other vegetables in the raw state were good sources ofascorbic acid. The
Hibiscus esculentus L. (Lady's finger or Okra) is the richest source of ascorbic acid with about 203 _+ 50.38 mg/100g. Group 3 in Table 1 has very little ascorbic acid content left after the traditional squeeze-washing process and this is further destroyed at the end of the initial parboiling. The total loss of ascorbic acid ranged from 19"57~o in Basella alba to 100~o in Solanum gilo Raddi and Vernonia amygdalina under laboratory conditions. The percentage loss of ascorbic acid in samples cooked by traditional methods is higher than that from samples cooked by laboratory methods. The loss ranged from 73.54 ~ in Tallinum triangulate to 100 ~o in Solanum gilo Raddi, Vernonia amygdalina, and green and red types of Celesia argentea. The reasons for this are that the intensity of the fire applied under local conditions is higher than under laboratory conditions and that the mud pot retains heat for a much longer time than the aluminium pot. Secondly, the traditional method of parboiling in enough water to cover the vegetables causes more loss of vitamin C, as can be seen in groups 1 and 2 of the Tables 1 and 2. These findings correspond with those of Krehl & Winters (1950) who reported that the greatest losses of both minerals and vitamins occur as a result of cooking by the 'old fashioned' method of just sufficient water to cover. They also reported that vegetables cooked in little water, i.e. half a cupful (125 ml), or in the more rapid pressure cooker with half a cupful of water showed quite similar losses. Krehl & Winters (1950) further reported that the pressure cooker method and the waterless methods retained more of the ascorbic acid contents, probably due to the much shorter cooking time which allows for less possibility of losses due to oxidation in the case of this vitamin. Oke (1967) reported that in Nigeria the average woman is not particular about the amount of water used. She boils the vegetables in a slight excess of water and discards the water; hence the percentage of vitamin retention by this method is lowest compared with other methods, such as pressure cooking with little water or the waterless method. The discarded water also contains an appreciable amount of ascorbic acid (see Tables 1 and 2). Krehl & Winters (1950) again suggested that the factors governing the serious loss of vitamins and minerals were the leaching effect of cooking water and the influence of oxidation, particularly of
TABLE 2
0 0
0
gilo Raddi Vernonia amygdalina
10. Ewuro (Bitterleaf)
aCooking done in mud pots on local wood fire.
3-60 + 0 8.45 _+ 1.18
Group 3 0
Solanum
7. Ewedu 8. Ila (Lady's finger)
23.3_+0.37 27.0_+ 1-27 Not applicable Not applicable
Not applicable Not applicable
0.95 _+0.08
9. Igbagba (Igbo)
1.76+0.13 3.85-t-0.16 5.35+0.49
Basella alba Tallinum triangulare Cochorus oliturius Hibiscus esculentus L.
3.00 _+0.28
Group 2
Group 1
5. A m u n u t u t u (sejesoro) 6. Gbure
4. Ebolo
7-55 -+ 1.12 9.65 -+ 1.21 13.00 -+ 1.50
Parboiled for 5 mm Mean ascorbic acid Mean ascorbic acid content o f vegetable content of the (mg/1 O0 g +_SE) discarded water (mg/1 O0 g + SE)
Celesia argentea Celesia argentea Amaranthus chlorostachys Crassocephalum crepidivides
Botanical name of vegetable
1. Sokoyokoto (green) 2. Sokoyokoto (red) 3. Tete
Local name of vegetable
0
0
0 5.30 + 0.75
15.75 _ 0-14 16.70_+0.51
1-2 _+0
0 0 3.5_+0
Boiled for 10 rain Mean ascorbic acid content o f vegetable (mg/1 O0 g -+SE)
ASCORBIC A C I D C O N T E N T OF SOME C O M M O N N I G E R I A N LEAFY A N D F R U I T VEGETABLES P R E P A R E D T R A D I T I O N A L L Y
100
100
100 97.29
74-83 73.54
97.29
100 100 91.50
°/o of total loss of ascorbic acid from raw sample in Table 1
a
,.q
©
P
rn _ Z
O
EFFECT OF TRADITIONAL COOKING ON ASCORB1C ACID CONTENT
309
a s c o r b i c acid which is sensitive to o x i d a t i o n , a n d s h o w e d t h a t a p o r t i o n o f the loss was a c c o u n t e d for in the d r a i n e d liquid. D a v i d s o n & P a s s m o r e (1969) m e n t i o n e d t h a t a helping o f g o o d g a r d e n vegetables a d a y will i m p r o v e health, b u t this can o n l y be true if the m e t h o d s o f p r e p a r a t i o n a r e i m p r o v e d . Even the a s c o r b i c acid c o n t e n t o f fresh p l a n t s can be i m p r o v e d by the e n v i r o n m e n t . R i c h a r d s o n (1954) a n d Boqert et al. (1966) have b o t h suggested t h a t light was the m a i n e n v i r o n m e n t a l f a c t o r influencing the a s c o r b i c acid c o n t e n t o f fruits a n d vegetables. T h e y f o u n d t h a t there was an a v e r a g e increase o f 33 ~o in a s c o r b i c acid within seven d a y s as a result o f light intensity c o m p a r e d with p l a n t s u n d e r heavier shading. In this respect N i g e r i a , being a less i n d u s t r i a l c o u n t r y , enjoys less p o l l u t i o n o f the a t m o s p h e r e a n d therefore a b u n d a n t solar light. In c o n c l u s i o n , since there is a g r e a t difference in the a s c o r b i c acid c o n t e n t o f p l a n t s f r o m the time they are p l u c k e d on the f a r m to the time they arrive at the table r e a d y for c o n s u m p t i o n , a n d since N i g e r i a n s have to d e p e n d on vegetables for a large p r o p o r t i o n o f their a s c o r b i c acid requirements, their m e t h o d s o f p r e p a r a t i o n need to be i m p r o v e d .
ACKNOWLEDGEMENT I should like to seize this o p p o r t u n i t y to t h a n k the D i r e c t o r a n d the l a b o r a t o r y staff o f this D e p a r t m e n t w h o have c o n t r i b u t e d in no small m e a s u r e to the execution o f this project.
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KESHINRO, A. O. KETIKU
OSER,B. L., MELNICK, D. & OSER, M. (1943). Influence of cooking procedure upon retention of vitamins and minerals in vegetables. Food Research, 8, I 15. RICHARDSON, L. R. 0954). Effect of environment on the ascorbic acid content of plants. Influence of environment on the chemical composition o f plants. Southern Coop. Service, 36, 6-24. RoE et al. (1966). Methods of analysis. In Methods of vitamin assay. (3rd ed.). The Association of Vitamin Chemists, Inc., New York.