Major constraints associated with the use of taro (Colocasia esculenta) flour as raw material for the preparation of achu




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FoodAfrica: Improving Food Systems in sub-Saharan Africa: Responding to a Changing Environment


Major constraints associated with the use of taro (Colocasia esculenta) flour as raw material for the preparation of achu

Njintang Y.N.1, Mbofung C.M.F.2, Abdou bouba A.2, Aboubakar2, Parker M3, Faulk C.3 Smith A.3, Graham M.3 Bennett R.3 and Waldron W.K.3

1 Department of Biological Sciences, University of Ngaoundéré, Cameroon.

2 Department of Food Science and Nutrition, School of Agro-industrial Sciences, University of Ngaoundéré, Cameroon.

3 Institute of Food Research, Norwich Research Park, UK.

Abstract


Achu is a highly digestible food, commonly consumed in Cameroon and traditionally made by pounding cooked taro corms in a mortar. This study was carried out to determine the major drawbacks that limit the utilisation of taro flour for the preparation of achu. Taro corms of six cultivars Ekona red, Ngaoundéré red, Ngaoundéré white, Ekona white, Ngaoundéré yellow and Sosso from Chad were processed into flour and analysed for their phenolic compounds, amino acids and reducing sugar contents. Flour was reconstituted into achu and the physicochemical characteristics, browning and proanthocyanidin levels werecompared to that obtained by the traditional method. The results indicated significant cultivar differences in total phenols, proanthocyanidins and amino acids content . In addition, a significant correlation was observed between the measured characteristics and the browning behaviour of the flours. Cultivar Ngaoundéré yellow exhibited the highest browning tendency while cultivars Ngaoundéré red and Ekona red had the lowest. Apart from browning which was the major problem, achu obtained from taro flour was observed to be very soft as suggested by creep-recovery analysis. The water-absorption characteristics, bulk density and iodine affinity were significantly higher for reconstituted achu than for that traditionally produced . Oxalate, the acridity factor of taro, was not observed in RIN and RIE varieties suggesting their high potential for the production of taro flour.

Key words: Taro, achu, browning, rheology, oxalate


Introduction


Taro (Colocasia esculenta) is a tropical tuber crop widely cultivated in Cameroon for its leaves, corms and cormels. Despite its nutritional, industrial and health importance, taro has not gained sufficient research attention to enhance its potential. Taro post harvest technology is very limited. In addition, taro has a high post harvest perishable rate and, as such, it has a poor position on the food security profile of the country. Agbor andandRickard (1991) estimated at over 60%, the losses that occur during the storage of Colocasia spp. for a short period of 5 to 6 months. Foods produced from taro suffer from the presence of acrid factors, which causes itchiness, and considerable inflammation of tissues to some consumers. As a result of a high rate of post harvest loss and lack of proper scientific attention to this problem, a reduction in annual production estimated at more that 70% has been reported in Cameroon (Minagri, 1981/1999). Development of post harvest processing and utilisation techniques could certainly resolve some, if not all, of the problems that affect the consumption and utilisation of taro as well as go a long way in increasing labour efficiency, productivity, income of farmers, shelf life of products, marketing opportunities and upgrade nutrition of consumers while substantially contributing to food security.

Unfortunately, no studies have so far been carried out in this respect. In Cameroon, taro is generally used in the preparation of a much cherished food, known as achu, obtained by pounding in a mortar cooked taro corms/cormels into a paste. One of the goals of our current research is to develop a taro flour that could be used as a raw material for the production of achu. Since taro flour would normally keep longer than the corms or cormels, its use as a raw material for the preparation of achu would significantly increase its consumption and availability. However, the conditions under which this can be feasible need to be closely studied and established.



The primary objective of the study was to determine the feasibility of reconstituting raw taro flour into achu.

Materials and Methods

Samples of 6 different cultivars of taro (Colocasia esculenta (L) Schott) were obtained from 3 agro-ecological zones. In Cameroon, three cultivars, Ngaoundéré white, Ngaoundéré red and Ngaoundéré yellow were obtained from a farm in Ngaoundéré while two others described as Ekona white and Ekona red cultivars were obtained from Ekona. In Chad, a white cultivar was obtained, locally called Sosso.



Taro flour sample: The corms of each of the cultivars were transported to the laboratory, abundantly washed with tap water and peeled using a stainless steel knife. Peeled corms were cut into 0.5 cm thick slices, dried to a constant weight in an oven set at 452°C before milling into flour using a Culatti grinder (Polymix, Kinematica AG, Germany) fitted with a 500-µm mesh sieve. Flours obtained were packed in polyethylene bags and stored at 4°C to be used for analysis.

Achu preparation: Achu was prepared following two different methods. Firstly following the traditional method, which entails the cooking and pounding of taro corms to obtain traditional achu and secondly by reconstituting taro flour into achu. For traditionally prepared achu, the corms from the six cultivars were cooked until soft, peeled and pounded in a mortar to obtain a smooth achu. For the preparation of achu from taro flour, the flours were mixed with distilled water in the ratio of 1:3 (w/v) and cooked while stirring to obtain a paste. During cooking, water was added occasionally and the paste gently mixed until it was ready.


Chemical analyses: Analyses were carried out on the raw taro flour and achu samples. Reducing sugars were ethanol-extracted from samples by the modified method of Agbo et al. (1985) and estimated by the DNS (dinitrosalicylic acid) spectrophotometric method of Bernfeld (1955). Total phenols, as equivalent of gallic acid, was evaluated in the ethanol-water extract using the Folin-Ciocalteu reagent method following the Swain and Hillis (1959) method. Free amino compounds were analysed according to the modified method of Devani et al. (1989). For proanthocyanidin analysis, raw taro flour, traditionally prepared achu and reconstituted flour achu samples were extracted in pure methanol for 1h. Analyses of monomers and oligomers of catechin and epicatechin were performed using a Luna Silica normal-phase column and high performance liquid chromatograph model HP1100 system with sample passing through a fluorescence detector (FID with excitation and emission set at 276 and 316 nm, respectively. Proanthocyanidin contents were expressed as equivalent of catechin.

Browning behaviour: During the preparation of achu from taro flour, samples were taken at varying intervals of time (10, 20, 30, 40 50 and 60 min) and the colour assessed by a panel of 12 members using suitable taro paste standards ranging from 0to 100% as earlier described by Njintang (2003).

Rheological analysis: The load-unload (creep-recovery) behaviour of reconstituted and traditional achu was recorded on a rheometer-controlled stress (KS10, UK) at 25C and 10 Pa shear stress, in torsional flow between a stationary and a rotating plate.

Functional properties: The traditional and reconstituted achu were converted into flour by drying at 40°C before milling into flour using a Culatti grinder fitted with a 500 µm mesh sieve. The raw and cooked flours were analysed for water-absorption capacity, blue value indice, bulk density of the flour following the procedure earlier described by Njintang et al. (2001) and Njintang and Mbofung (2003).

Light microscopy of oxalate and achu texture: Taro sections 1.5 or 2 µm thick were cut with an ultramicrotome (Reichert Ultracut E), stained with 1% toluidine blue in 1% borax, pH 11 and examined with an Olympus BX60 microscope (Olympus Optical Ltd, Japan). The results obtained were and recorded digitally with AcQuis Bio software (Synoptics Ltd, Cambridge, UK). Calcium oxalate crystals were observed as druses and raphides. The overall distribution of cells in achu sections was also evaluated.


Statistical analysis: Differences between cultivars and treatments were statistically evaluated using the analysis of variance followed by the Duncan multiple range test as installed in the SPSS (1993) statistical package. Relationships between browning behaviour and independent variables such as reducing sugars, total phenols free amino compounds and proanthocyanidins were evaluated using linear models.

Results


Biochemical characteristics: Table 1 shows the variation in some biochemical characteristics of the different taro cultivars studied. Significant differences were observed in the free amino groups, total phenols and proanthocyanidins components of taro flour. Ngaoundéré white cultivar presented the highest free amino compounds (102 mg/100g) as opposed to the Ngaoundéré red, which showed the lowest (59 mg/100g). For total phenol contents Ngaoundéré yellow showed the highest while Ekona red showed the lowest. No significant differences were observed in the reducing sugar contents of taro cultivars studied. However, significant differences were observed between proantocyanidin contents of the cultivars with Ngaoundéré white showing the highest and Ekona red the lowest. In addition, achu prepared traditionally and reconstituted from taro flour were significantly lower in proantocyanidin levels as compared to that of raw taro flour.

Table 1: Cultivars differences in some biochemical characteristics of taro flour.




Cultivars

Parameters

Ekona white

Ekona red

Ngaoundéré white

Ngaoundéré red

Ngaoundéré yellow

Sosso

Free amino compounds

(g N/100g)



78.12.8ab

69.112.8b

102.32.1a

59.35.9b

80.110.9ab

89.79.6a

Total phenols (mg/100g)$

43.23.3cd

35.65.3d

53.75.2bc

36.60.8d

70.27.0a

63.31.2ab

Reducing sugars (g/100g)

2.00.1ab

1.30.1b

1.990.4ab

1.60.1ab

1.70.3ab

2.30.6a

Proathocyanidins

(mg/100g)*





















Raw taro flour

7.3

7.0

11.0

7.3

9.8

11.2

Traditional achu

4.2

3.6

1.9

nd

1.4

3.7

Reconstituted Achu

1.2

1.7

0.9

1.3

0.7

0.8

$=gallic acid equivalent; *catechin equivalent

Means SD;

Figures in a row followed by different superscripts indicate significantly (P<0.05) different values determined by Duncan’s Multiple Range Test.

Browning behaviour: Figure 1 shows the browning of taro flour during reconstitution into achu. Generally, the different cultivars behaved differently with respect to browning. Ngaoundéré yellow howed the highest level of browning while Ngaoundéré red had the least tendency. Significant correlations were observed between browning and total phenols (R=0.86), free amino compounds (R=0.58) and proanthocyanidin (R2=0.83) contents. More important reduction in proanthocyanidins during reconstitution was highly correlated (R2=0.89; P<0.05) to the browning behaviour.

Rheological characteristics of reconstituted and traditional achu: Table 2 shows the variations in the rhelogical characteristics of traditional and reconstituted achu. The traditional and reconstituted achus of the 6 cultivars had similar creep-recovery curves showing an instantaneous deformation, a steady state period where the deformation did not vary with time and a high recovery. In general, reconstituted achu had a higher deformation than the traditional.

Functional properties: Table 3 presents the water-absorption capacity, blue value index and bulk density of different cultivars as affected by the treatments. Generally, the functional characteristics of traditional achu were significantly higher than those of reconstituted achu. In addition, significant differences were observed between the cultivars studied.



Figure 1: Browning behaviour of taro flours during reconstitution

Table 2: Comparative visco-elastic characteristics of traditional and reconstituted-flour achu

Cultivars 




Parameters

Creep viscosity (104 Pas)

Creep steady state (10-1)

Creep compliance

(10-3 Pa-1)



Recovery compliance

(10-3 Pa-1)



Creep instantaneous compliance

(10-4 Pa-1)



Recovery instantaneous compliance (10-4 Pa-1)

Ekona red

TFA

89.129.6

1.70.6

0.110.01

0.120.02

0.460.01

0.460.0

 

TTA

210.555.6

1.40.4

0.070.01

0.080.02

0.430.02

0.460.03

Ekona white

TFA

109.749.9

1.60.7

0.100.01

0.100.00

0.450.00

0.450.02

 

TTA

76.99.6

2.20.2

0.100.01

0.100.01

0.430.01

0.450.02

Ngaoundéré white

TFA

662.055.2

2.00.2

0.130.02

0.150.03

0.580.07

0.570.05

 

TTA

117.436.3

1.50.2

0.100.01

0.110.01

0.460.00

0.470.01

Ngaoundéré yellow

TFA

64.215.3

3.20.7

0.070.01

0.080.01

0.440.01

0.460.01

 

TTA

169.516.3

1.60.2

0.060.00

0.080.00

0.390.02

0.400.00

Sosso

TFA

122.741.5

2.10.5

0.060.00

0.070.00

0.470.01

0.460.01

 

TTA

301.364.4

1.50.2

0.040.01

0.040.00

0.250.08

0.240.05

TFA=reconstituted achu; TTA=traditionally prepared achu

Table 3: Functional characteristics of traditional and reconstituted achu flour




Cultivars

Parameters




Ekona white

Ekona red

Ngaoundéré white

Ngaoundéré red

Ngaoundéré yellow

Sosso

Water-absorption capacity g/100g)

TFA

606.07.0bc

597.316.6c

617.412.6 bc

692.510.6 a

637.94.5 bc

661.111.4 ab

TTA

220.626.4

215.010.0

318.615.9 ab

306.615.9 ab

293.310.0 b

351.78.8

P

0.05

0.05

0.05

0.05

0.05

0.05

Blue value index (eqDO/100g)

TFA

300.93.0c

791.720.8 a

780.228.3 a

324.825.6 bc

462.70.7 b

624.733.1 a

TTA

275.019.5 b

300.020.1 b

381.325.4 ab

441.032.8 a

414.56.1 a

376.025.8 ab

P

ns

0.05

0.05

0.05

0.05

0.05

Bulk density

(g/ml)


TFA

0.840.02ab

0.830.01 ab

0.810.01 bc

0.850.01 a

0.800.01c

0.710.01d

TTA

0.770.01 a

0.650.01 d

0.590.01e

0.670.01c

0.730.01b

0.720.01b

P

0.05

0.05

0.05

0.05

0.05

ns

P=level of significant differences between traditionally prepared achu (TTA) and reconstituted achu (TFA); ns=no significant differences. Means in a row sharing a common superscript are significantly (P<0.05) different.

Microscopic observations of taro oxalate and achu texture: Calcium oxalate crystals, in the form of raphides and druses, were observed only in taro corms (Photograph 1). In this study, no oxalate crystals were found in Ekona white and Ekona red cultivars. The overall observation of achu structure showed that traditional achu is mostly made up of undisrupted cells while reconstituted achu contains highly destroyed cells (Photograph 2).

Calcium oxalate raphide

Calcium oxalate druses



Photograph 1: Oxalate calcium Crystal in the form of raphides and druses


Traditional achu



Reconstituted achu


Photograph 2: Cross section of traditional and reconstituted achu

Discussion

Browning constraints: Browning was the most important factor limiting the use of raw taro flour for achu production. As suggested by Nip (2001), the browning reaction that occurs in taro is dependent on the variety. Other studies on taro processing have not reported browning as a problem. This could be due to the fact that the taro cultivars used in those studies did not either have the browning trait or the quality of the products resulting from the dehydration did not present browning (Moy et al., 1977). Results obtained in this study indicated that, irrespective of cultivars, browning occurred during the reconstitution of untreated taro flour and the degree varied with cultivars. Browning that occurs during reconstitution is of the non-enzymatic type. It may be due to the polymerization of flavan-3-ols to form its equivalent trimer and tetramer, which then undergo reaction with proteins and sugars. In their work on phenolic composition and browning susceptibility of various mature apple and pear varieties, Amiot et al. (1992; 1993) reported that the occurrence of browning was linked to the quantity and quality of phenols present. Earlier achu studies have showed that consumers preferred a white colour for achu (Njintang et al., 2000). On the basis browning susceptibility, Ekona white and Ekona red cultivars are better suitable for the production of taro flour than the other cultivars studied.

Textural constraints: Texture is the second limiting factor in achu production. A number of evidences tended to confirm the soft consistency of paste resulting from the reconstitution of untreated taro flour. The physicochemical properties (water-absorption capacity, blue value index and the bulk density) of reconstituted achu are higher than those of traditional achu. The linear relation of the increase in these properties with the degree of gelatinisation has been reported for various foods systems (Abbey and Ibey, 1988; Bencini, 1986; Tagodoe and Nip, 1994). The high gelatinisation of achu prepared from flour is further emphasized by the distribution of cells in the paste as visualized under light microscopy. In addition the compliance of the achu prepared from flour is generally higher than that of traditional achu, suggesting its liquefaction. During cooking, taro flour tends to liquefy, giving rise to a soft texture paste. This is probably due to taro flour starch damage occurring during processing as compared to that in the corms or cormels. The high degree of browning and gelatinisation indicate that raw taro flour produces a paste different to that of achu. To keep the whole cell undisrupted, cooking of corms before milling could be envisaged, provided the milling procedure is suitable. Ongoing studies in this aspect tend to confirm this hypothesis.

Oxalate constraint: The irritation caused by taro corms reduced the potential of flour as raw material in the preparation of achu. Irritation, related to the presence of oxalate crystal needles, tends to reduce its utilization due to the injection of the needles out of the idioblast into the body when the corms are crushed. According to Nip (1997), boiling of corms is the only treatment that reduces irritation. However, idioblast cells are not found in some irritating taro cultivars. Generally, cultivars containing high levels of calcium oxalate take longer to boil so as to reduce acridity. Hence, Ekona red and white cultivars will cook faster than those containing the acrid principle.

Conclusion

In this experiment, the performance of taro flour as an ingredient for the preparation of achu is limited by the browning reaction that takes place during reconstitution and the very soft texture of achu. Browning results from the reaction between proanthocyanidins and sugars or proteins to form brown insoluble matter. The very soft achu texture obtained from taro flour could be due to starch damage. Ekona red and Ngaoundéré red present a high potential for taro flour production due to their low oxalate crystal contents and their low degree of browning. The soft texture of achu will probably be resolved by cooking whole corms before drying and milling to obtain flour that will be reconstituted in hot water. Future studies need to address the effects of drying and milling.



Acknowledgements

This study was supported through a grant offered to the first author by the Committee on the Scientific and Technological Co-operation of the Organization of Islamic Conference (COMSTECH), Islamabad, Pakistan and the International Foundation for Science (IFS), Stockholm, Sweden.



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