J . Sci. Food Agric. 1984, 35,1112-1119

Composition and Nutritive Value of Cormels of Colocasia esculenta (L.) Schott

Muhammed Hussain," Grenville Norton and Roger J. Nealeb

Department of Applied Biochemistry and Food Science, University of Nottingham, School of Agriculture, Sutton Bonington, Loughborough, Leicestershire LE12 5RD (Manuscript received 5 January 1984)

The nutritional quality of the cormels of Colocasia esculenta (L.) Schott cultivar 'Guavir' was determined chemically and biologically in feeding experiments with weanling rats. The contents of starch and unavailable carbohydrates (cellulose, hemicellulose and pectin) were 54 and 15% of the dry matter respectively. Lipid accounted for less than 1% of the dry matter. Palmitic, oleic and linoleic acids were the main fatty acids in the lipids, with linoleic acid predominating. The content and fatty acid composition of the individual lipids was examined. Crude protein amounted to 10.4% of the dry matter. This protein was low in the sulphur- containing amino acids and tryptophan, but contained adequate levels of the other essential amino acids. Diets containing raw cormel meal gave severely depressed net protein utilisation (NPU) and total nitrogen and carbohydrate digestibility when fed ad libitum to weanling rats. Both NPU and nitrogen and carbohydrate digestibility improved when the cormel meal was cooked prior to inclusion in the diets but wch diets still did not support growth in weanling rats. Keywords: Colocasia; cormels; composition; nutritional value.

1. Introduction Aroids make a significant contribution both as root crops and vegetables in the diets of people in Bangladesh, particularly in rural areas where they are freely available, but the crop has not realised its full potential because of the palatability problems associated with its high content of oxalates in the form of raphides. Generally, aroids are consumed widely only as subsistance or emergency foods in times of famine. Of the tropical aroids, Colocasia esculenta (L.) Schott is probably the most important as a food source.' Many different forms and cultivars of this species are grown in different areas and it is likely that these differ in composition and nutritional value. Data on the chemical composition of a few cultivars are available.24 Splittstoesser et ~ 1 . ~ reported that the average protein content of four cultivars of Colocasia esculenta ranged from 1.75 to 11.72%, and that the essential amino acid content, apart from the S-containing amino acids, was satisfactory compared with the F A 0 reference protein. Later work revealed that the tryptophan content was 0.9g 1OOg-' proteim6

Preliminary investigations with rats and broiler chickens revealed low utilisation of cormel meals which were unable to support Inadequate processing of the dasheen and taro meals has been assumed to be responsible for their poor performance.

As foods containing 5% energy as good quality protein can maintain nitrogen balance if consumed in sufficient q ~ a n t i t y , ~ some of the aroids may be marginally adequate as protein sources. Colocasia esculenta cv. Guavir contains 12% crude protein in the dry matter.2 The present study was therefore initiated to determine the nutritive value of the cormels of cv.

a Present address: Department of Biochemistry, Bangladesh Agricultural University, Mymensingh, Bangladesh.

To whom all correspondence should be addressed.

1112

Nutritive value of Colocasio esculenta 1113

Guavir, because they may provide a useful food in Bangladesh apart from their use in emergencies.

2. Experimental 2.1. Chemical Analysis Freshly harvested mature cormels of Colocasia esculent& cv. Guavir grown in the locality of Mymensingh were peeled, sliced, sun-dried and ground to a fine meal. Such meals contained about 10% moisture. Chemical analysis was performed according to the flow diagram (Figure 1). Soluble amino acids, sugars and organic acids were separated by ion exchange chromatography on Dowex 50x4 (H' form 60 mesh) and Dowex 1x8 (formate form) and identified qualitatively by paper chromatography. Free amino acids were estimated by the ninhydrin procedure and results expressed as leucine equivalents. lo Soluble sugars were estimated by enzymic procedures using commercially available kits (Boehringer, Mannheim). Total reducing sugars were deter- mined by the Nelson-Somogyi method. '' Starch was determined following hydrolysis with amyloglucosidase according to the method of Macrae and Armstrong12 and Macrae13 using Agidex (Glaxo Research Ltd, Greenford, Middlesex) or the purified enzyme. Unavailable carbohydrates were isolated according to the method outlined by Southgate. l4 Each component of the fraction was hydrolysed and the individual sugars identified by paper chromatography prior to quantification. l5 Galacturonic acid and pectin were determined by the carbazole method16 with suitable correction for neutral sugars present. Both hot water-soluble and oxalate-extracted pectins were hydrolysed by a pectolytic enzyme preparation (Ultrazyme 100) which had been dialysed overnight against distilled water prior to use.17 The overall scheme for carbohydrate analysis is presented in Figure 1.

Fresh cormel material was used for lipid analysis. This was homogenised in chloroform- methanol (2: 1 by vol) and the extracts purified according to Christie." Lipids were separated by means of preparative t.1.c. on silica gel G using chloroform-methanol-acetic acid-water (85: 15: 10:3.2 by vol) as s ~ l v e n t . ' ~ Each lipid band was scraped off the plate, and the fatty acid methyl esters prepared according to the procedure of Allen and Good.20 Quantitative analysis was carried out using a Pye series 104 gas chromatograph fitted with dual columns packed with 3% SP-2310/2% SP-2300 on 100/120 Chromosorb Q (Supelco Inc, USA). Detection was by flame ionisation detection, and argon was used as carrier gas (20mlmin-'). Peaks were integrated using a 3373 BHP integrator.

Proteins were hydrolysed according to Boulte?' and amino acid analysis performed on a Locarte Mark 4A Analyser.22 Cysteine and cystine were determined as cysteic acid23 and tryptophan was estimated by a spectrofluorimetric method.24 Protein, moisture, ash and fat were determined according to AOAC methods.25

2.2. Biological evaluation Three-week-old Wistar rats specific pathogen free (Joint Animal Breeding Unit, School of Agriculture, Sutton Bonington) were used in groups of four for feeding trials. The animals were housed individually in stainless steel metabolism cages (Associated Crates Ltd, Stockport, UK) in a random arrangement. Six experimental diets containing 10% crude protein and a protein-free diet were fed to rats which were matched for their initial weights (Table 1). All diets were adequately supplied with minerals and vitamins and animals were provided with food and water ad libitum. Daily food intake was recorded, taking into account any spilt food. Faeces were also collected daily. Each experiment was of 10 days duration following a 3 day preliminary adjustment period. Initial and final weights of each rat over the 10 day experimental period were recorded.

The nutritional value of the crude protein of cormel meals was estimated by determining true protein digestibility (TD) and net protein utilisation (NPU) according to the method of Miller and Bender.26 Carbohydrates in the diet and faeces were calculated by subtracting protein, fat,

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Nutritive value of Coloeasiu esculentu 1115

Table 1. Composition of experimental diets used to determine nutritive value of aroid meals

Ingredients

Diets (g kg-’)

Protein free (PF)

A B C D Eb F G diet

Raw aroid powder Aroid powder (dry) autoclaved for 10 min at 110°C Aroid powder autoclaved in water (4: 1 wt/vol) for 15 min at

Aroid powder steeped overnight and autoclaved in water

Maize starch Glucose Arachis oil (ml) with vitamins A, D, E and K Mineral mix” Vitamin B m i f Casein (76.6% protein) Whole egg (52% protein)

110°C and freeze-dried

(4: 1 wt/vol) for 1 h at 120°C and freeze-dried

834 - -

- 5

100 50 11 -

-

834 -

- 5

100 50 11 -

417 - -

207 150 100 50 11 65

382 150 100 50 11 97.6 -

497 150 100 50 11

192 -

-

- 834

- 5

100 50 11 -

834

-

5 100 50 11 -

689 150 100 50 11

~ ~~

a Composition as described by Payne and Stewart”; reference diet.

ash and moisture from the total food ingested and the freeze-dried faeces as suggested by Shuhpalekar et aL2’ Apparent carbohydrate digestibility was calculated.

3. Results and discussion 3.1. Chemical composition The proximate analysis of cormels of the cultivar employed in this study (Table 2) has been reported elsewhere.’ The starch content was considerably lower than that reported previously. This discrepancy may be caused by the different analytical procedures employed, as in the earlier investigation some of the unavailable carbohydrates may have been released during acid hydrolysis. Unavailable carbohydrate accounted for 14.7% of the dry matter. Almost 70% of this fraction was hemicellulose, with 17% pectin and 13% cellulose. Comparable data on aroid composition is lacking, but much lower pectin levels (0.1-0.49% dry matter) have been reported for dasheen taro.29

The fatty acid spectrum of fresh cormels of Guavir resembled that reported earlier for similar tissue3’ although linoleate (18.2), the major fatty acid of all lipids, in both cases, was slightly

Table 2. Chemical composition of cormels of Colocasiu esculentu (L.) Schott cv. Guavir. Values

are means f s.e. (mean) of triolicate analvses

g 100 g-’ dry matter

Soluble solids Starch Free sugar

Glucose Fructose Sucrose

Free amino acid Lipid Unsaponifiables Unavailable carbohydrates

Hemicellulose Cellulose Pectin

Protein (Nx6.25)

9.2f0.1 54.0f1.0

0.33+0.02 0.43f0.02 5.0820.33 0.09f0.001 0.8820.003 O.llfO.01

10.2+0.8 1.88f0.19 2.56f0.51 10.4+0.2

1116 M. Hussain et 01.

Table 3. Fatty acid composition of the total lipid extract and major lipid classes of fresh cormels of Colocusia esculenru cv. Guavir. Values are meansf SEM, of triplicate analyses

Fatty acid composition (mg 100 g-’ fresh sample)

16.0 18.0 18.1 18.2 18.3

Total lipid

Lysophosphatidylcholine Phosphatidylcholine Diacyldigalactosylglycerol Phosphatidylethanolarnine Sterolglycoside Diacylgalactosylglycerol

37.0f0.4 3.53t0.15

2.26 0.08 1.46 0.18 1.47 0.32 2.07 0.06 0.72 0.04 0.26 0.03

(25.0)” (2.4) 32.1t0.81

(21.4) 1.20 2.48 1.63 0.90 0.49 0.51

73.8f1.0 3.59f0.15

1.60 Trace 3.30 0.06 1.67 Trace 2.48 0.07 0.07 Trace 1.74 0.12

(49.0) (2.4)

Figures in parentheses are % mass composition of total fatty acid.

higher in the material used in the present study (Table 3). The lipids of aroid cormels had a different fatty acid composition from those of potato tubers.” Potato tuber lipids have a higher content of linolenate (18.3) and a lower oleate (18.1) content than the cormels. It was suggested that in cormel tissue grown at higher temperatures and frequently subjected to anaerobic conditions, the formation of 18.1 is favoured while the interconversion to 18.3 is prevented.” Qualitatively, the lipid composition of the cormels resembles that of potato.

Compared with the FA0 reference amino acid pattern,32 the essential amino acid content was adequate except for the sulphur-containing amino acids, tryptophan and lysine (Table 4). Generally, the essential amino acid composition of Guavir meal was superior to those of other c~ l t iva r s .~ Earlier, Coursey’ reported that aroids of Colocusia spp. were richer in S-containing amino acids than other aroid species. The cormels of Guavir are also richer in isoleucine than other cultivars. Generally, the amino acid composition of the cormels of this cultivar were comparable with, or even better than, that of potato protein.33 In view of the relatively higher ly2ine content of the cormels, the material has considerable potential as a supplement to rice-based diets.

Table 4. Amino acid composition of cormels of Colocasiu esculento (L.) Schott cv. Guavir

g 16 g-’ N

FAOAKHO recommended

Experimental score

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His

Met+Cysteic acid

GlY

TYr

LY s

‘4%

Try

17.6 5.5 7.4

12.9 5.2 6.5 5.7 6.2 4.4

10.0

6.1 4.8 2.7 7.9 1.4 0.324

4.0

5.0 4.0 7.0

6.0

5.5

3.5 1.0

Nutritive value of Colocasia esculenta 1117

3.2. Biological evaluation Diets containing raw cormel meal as the sole source of the protein and major source of gross energy (diet A) failed to support growth and the animals deteriorated rapidly and were near to death from inanition after 10 days (Table 5). This was primarily because of the extremely low digestibility of the carbohydrate. When dry heat-processed meal was used to replace the raw meal, the animals did not lose as much weight. The TD was slightly improved. Autoclaving with water added to the level found in fresh cormel tissue (80% moisture) for 15 min at 110°C in diet F produced a further slight improvement in NPU, TD and carbohydrate digestibility compared with the dry heat-processed meal (diet B). Food intake was much lower than for diets A and B but weight loss was reduced. Nevertheless, diets B and F were grossly inferior to the reference protein diet (diet E).

Table 5. NPU and TD of protein and apparent digestibility of total carbohydrate of diets containing cormel meal of Colocasia esculenta fed to rats for 10 days

Food intake Weight change' Digestibility of Diet" g/rat/lO days g/rat/lO days NPU TD carbohydrate

A 220 B 165 C 75 D 54 E 110 F 44 G 89 PF 54

-13.3 -18.5 (3.7)' 0.17 (0.17) 39.5 (3.4) -10.9 3.4 (9.2) 0.25 (0.03) 39.6 (1.8)

0 '"42.5 (5.4) 0.52 (0.01) ***59.3 (2.6) 3.7 ***73.2 (11.5) *0.79 (0.03) **'88.3 (2.1)

-7.1 *'*8.6 (1.3) 0.51 (0.01) 47.3 (1.4) -3.6 ***57.8 (7.4) 0.77 (0.01) ***92.1 (0.1)

30.2 "'83.5 (11.7) **0.95 (0,001) -

- - - -11.7

a See Table 1 for details of the diets. ' Values are the average of four rats per group.

Levels of significance compared to diet A are indicated thus: ***. P<O.001; **, P<O.Ol; Values are means with standard errors in parentheses for four rats per group.

*, P<0.05.

Diet G was formulated according to Payne et When the meal was allowed to stand overnight with water (4: 1 wthol) prior to autoclaving ( l h at 15psi and 120°C), NPU, TD and carbohydrate digestibility were markedly improved. Compared with diet F (meal autoclaved with water), more food was consumed and weight loss was lower than on diet F.

A microscopic examination of freeze-dried faeces from animals fed diets A, B and F revealed considerable amounts of undigested starch granules, often situated in intact cells. Raw potatoes are also poorly digested. Whittemore ef showed that the inclusion of raw potato in mixed diets impaired the digestibility of nitrogen and other ingredients. Similarly El-Harith et

reported reductions in NPU and food efficiency values when raw potato starch was fed to rats. The inferiority of diets B and F compared with G is apparently due to the unavailability of starch and possibly protein.

The slight weight loss of rats on diet G compared to the large weight gains with whole egg (E) is no doubt partly due to the low level of the S-containing amino acids and tryptophan in the aroid proteins in association with the high level of unavailable carbohydrate (dietary fibre). The role of unavailable carbohydrate (dietary fibre) in these studies is not known, but complex interactions are known to occur between this and available carbohydrate, fats and protein. The presence of free oxalic acid may also render some minerals unavailable, and as a consequence impair growth. The cultivar used in this investigation is known to contain 0.28% oxalate.'

The high apparent digestibility of carbohydrate in diet G apart from the effect on starch may be explained by the fact that considerable digestion of dietary fibre may have occurred in the gut. Recently Cummings et d3' reported that dietary fibre was mainly metabolised, and dietary pectin completely metabolised in the gut.

1118 M. Hussain el al.

In view of the favourable protein-energy ratios of aroids of Colocasia compared with other foods, this material may be a useful food for humans and monogastric animals, provided it is adequately cooked and precautions are taken to remove undesirable components from the tubers.

Acknowledgements The authors acknowledge the technical assistance of Mr J. Harris, Mr David Bozon and Mr David Howson and one of the authors (MH) acknowledges the financial assistance from the Association of Commonwealth Universities.

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