Advances in Agriculture
Volume 2016 (2016), Article ID 2658983, 7 pages
http://dx.doi.org/10.1155/2016/2658983

Research Article

Influence of Cultivar on the Postharvest Hardening of Trifoliate Yam (Dioscorea dumetorum) Tubers

Christian Siadjeu,1,2 Emmanuel Akdowa Panyoo,² Gabriel Mahbou Somo Toukam,1,3 Joseph Martin Bell,¹ Benoit Nono,⁴ and Gabriel Nama Medoua²

¹Department of Plant Biology, Faculty of Science, University of Yaounde 1, P.O. Box 812, Yaounde, Cameroon
²Centre for Food and Nutrition Research, IMPM, P.O. Box 6163, Yaounde, Cameroon
³Institute of Agricultural Research for Development (IRAD), P.O. Box 2067, Yaounde, Cameroon
⁴Appropriate Development for Africa Foundation (ADAF)/“Ferme Ecole de Baham” (FEBO), P.O. Box 11646, Yaounde, Cameroon

Received 31 March 2016; Revised 20 May 2016; Accepted 31 May 2016

Academic Editor: Claus A. Soerensen

Copyright © 2016 Christian Siadjeu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The influence of cultivar on the postharvest hardening of Dioscorea dumetorum tubers was assessed. 32 cultivars of D. dumetorum tubers were planted in April 2014, harvested at physiological maturity, and stored under prevailing tropical ambient conditions (19–28°C, 60–85% RH) for 0, 5, 14, 21, and 28 days. Samples were evaluated for cooked hardness. Results showed that one cultivar, Ibo sweet 3, was not affected by the hardening phenomenon. The remaining 31 were all subject to the hardening phenomenon at different degree. Cooked hardness increased more rapidly in cultivars with many roots on the tuber surface compared to cultivars with few roots on the tuber surface. When both the characteristics flesh colour and number of roots on tuber surface were associated, cooked hardness in cultivars with yellow flesh and many roots increased more rapidly than in cultivars with white flesh and many roots, whereas cooked hardness in cultivars with yellow flesh and few roots increased more slowly than in cultivars with white flesh and few roots. Accessions collected in high altitude increased more rapidly compared to accessions collected in low altitude. The cultivar Ibo sweet 3 identified in this study could provide important information for breeding program of D. dumetorum against postharvest hardening phenomenon.

1. Introduction

Trifoliate yam (Dioscorea dumetorum) is the most nutritious species of the eight yam species commonly grown and consumed in West and Central Africa [1]. Its tubers are rich in protein (9.6%), fairly balanced in essential amino acids (chemical score of 0.94), and its starch is easily digestible [2, 3]. This yam species is high-yielding (40 t/ha) and may not require staking, thus saving on labour [4]. Unlike other yam species, the storage of D. dumetorum is restricted by a severe postharvest hardening phenomenon characterized by loss of the ability to soften during cooking [5]. Tuber hardening begins within 24 h after harvest and it is manifested by the loss of culinary quality due to a combination of factors resulting from the normal but inadvertently deleterious reactions leading to textural changes [6, 7]. Microstructural studies showed that lignification and thickening of cell walls are one of the characteristic features of the hardening process [8].

Several studies were focused on the biochemical modifications related to postharvest hardening and the understanding of hardening mechanism [1, 3, 6–9]. In contrast, only few studies [8, 10] reported the variation in the postharvest hardening phenomenon among D. dumetorum cultivars. Trèche [10] noted that 3% of D. dumetorum tubers are not subject to the hardening phenomenon after one month of storage, and Sefa-Dedeh and Afoakwa [1] observed that the hardening phenomenon of D. dumetorum varies between yellow and white cultivars. This suggests that some cultivars of D. dumetorum may not suffer from the hardening phenomenon and could therefore be used in breeding programs of this species. To the best of our knowledge, few or no studies have been performed so far to evaluate the influence of cultivar on the postharvest hardening phenomenon of D. dumetorum tubers. The present study was therefore carried out to close this gap.

2. Material and Methods

2.1. Materials

Thirty-two cultivars of trifoliate yam (Dioscorea dumetorum) tubers were collected from the main yam growing regions in Cameroon and Nigeria (Table 1) and then planted in April 2014 at the “Ferme Ecole de Boukué” (FEBO) in the West region of Cameroon. Tubers were randomly harvested at physiological maturity, 9 months after planting (December 2014), from 3 different plants of each cultivar. These were immediately transported to the laboratory and stored under prevailing tropical ambient conditions (19–28°C, 60–85% RH) for a period of 28 days. During storage, yam samples were collected at fixed time intervals (days 0, 5, 14, 21, and 28). They were thoroughly washed with water, peeled, and chopped into chips of 1 cm thickness. No growth of mould was observed during the storage period. The sprouting of most tubers was observed after 28 days of storage.

Table 1: D. dumetorum accessions used for hardness characterization.

Accessions code
Area of collection
Roots on the tuber surface
Tuber flesh colour
Altitude (m)
Latitude (N)
Longitude (E)

1
Babadjou
West
Few
Yellow
1395
05°37.488
010°15.668

2
Babungo
Northwest
Few
Yellow
1182
06°03.984
010°26.823

3
Baigon
West
Few
Yellow
1120
05°34.183
010°40.489

4
Bambalang
Northwest
Few
Yellow
1185
05°53.644
010°30.814

5
Bambui
Northwest
Few
Yellow
1262
06°00.797
010°13.635

6
Bamendjou 1
West
Few
Yellow
1647
05°20.063
010°22.118

7
Bamendjou 2
West
Many
Yellow
1647
05°20.063
010°22.118

8
Bamokonbou
West
Many
Yellow
1414
05°28.781
010°25.197

9
Bana
West
Few
Yellow
1167
05°09.365
010°10.147

10
Banga bakundu sweet
Southwest
Few
White
62
04°17.314
009°24.451

11
Bangang 1
West
Few
Yellow
1776
05°34.303
010°09.133

12
Bangang 2
West
Many
Yellow
1776
05°34.303
010°09.133

13
Bangou
West
Many
Yellow
1350
05°08.379
010°31.406

14
Batibo
Northwest
Few
Yellow
1127
05°50.144
009°53.467

15
Bayangam 1
West
Few
Yellow
1560
05°17.930
010°26.446

16
Bayangam 2
West
Many
Yellow
1560
05°17.930
010°26.446

17
Country yam
Southwest
Few
White
62
04°17.314
009°24.451

18
Dschang
West
Few
Yellow
1337
05°26.637
010°03.404

19
Fenkam-Foto
West
Few
Yellow
1414
05°28.781
010°25.197

20
Fongo-Tongo
West
Many
Yellow
1460
05°30.118
009°59.976

21
Fonkouankem
West
Many
Yellow
1167
05°09.365
010°10.147

22
Fundong
Northwest
Few
Yellow
1554
06°16.790
010°17.075

23
Guzang
Northwest
Many
Yellow
1233
05°49.983
009°55.278

24
Ibo sweet 1
Southwest
Few
White
56
04°24.103
009°26.522

25
Ibo sweet 2
Southwest
Many
White
56
04°24.103
009°26.522

26
Ibo sweet 3
Southwest
Few
Yellow
56
04°24.103
009°26.522

27
Kumbo
Northwest
Few
Yellow
1722
06°12.386
010°40.478

28
Lysoka sweet
Southwest
Many
Yellow
60
04°11.306
009°18.745

29
Mabondji sweet white
Southwest
Few
White
80
04°33.745
009°11.806

30
Mankon
Northwest
Few
Yellow
1253
05°58.172
010°08.541

31
Muyuka
Southwest
Few
White
62
04°17.314
009°24.451

32
Nkwen
Northwest
Few
Yellow
1251
05°57.717
010°10.078

These accessions cultivated in southwest of Cameroon are originated from Nigeria.

2.2. Hardness of Cooked Tubers and Edibility

Tuber samples of 1 cm thickness and about 5 cm diameter were cooked in boiling water for 60 min (this time was selected during preliminary studies as it allowed the yam tuber to soften completely) on a hot plate and made to cool completely at room temperature. Hardness was assessed with a PCE-PTR 200 digital penetrometer (PCE Instruments, France EURL) using an 8.83 mm probe. Six measurements were made for each cultivar and the data were expressed in Newton.

Coarseness feeling in the mouth during eating of cooked tubers was used to evaluate the edibility of yam. This parameter was used to assess hardness value above which the tuber is not consumable. For this purpose, a sensory evaluation of cooked yam tubers was conducted with nine trained panellists. Panellists were trained to evaluate edibility using fresh tubers as reference. Samples (D. dumetorum tubers stored during 0, 1, 2, 3, 4, 5, 7, 14, 21, and 28 days) were cooked as described above and then evaluated for edibility on a 9-point hedonic scale quality analysis with 9 being liked extremely, 8 being liked very much, 7 being liked, 6 being liked mildly, 5 being neither liked nor disliked, 4 being disliked mildly, 3 being disliked, 2 being disliked very much, and 1 being disliked extremely.

2.3. Microstructural Studies

Yam tubers were cut into 0.04 mm thickness using a laboratory scale sledge microtome (Shibuya Optical Co., Ltd., Tokyo, Japan). Cut tissues were then fixed by soaking them in carmine green iodine for 5 min, after soaking in diluted sodium hypochlorite for 15 min. Clearing was done by a series of washing processes in distilled water with acetic acid added in the last step.

2.4. Statistical Analyses

All measurements were carried out at least in triplicate. Statistical analyses of data were performed using SPSS IBM statistics 20 (SPSS Inc., Chicago, Illinois, USA) and STATISTICA 6 (Data Analysis Software System, StatSoft, Inc., USA). Analysis of variance (ANOVA), Duncan multiple range test, Pearson’s correlations, principal component analysis (PCA), and hierarchical cluster analysis (HCA) were performed. Statistical significance was defined at .

3. Results and Discussion

Hardening during storage of D. dumetorum is characterized by a rough and fluffy surface of peeled tubers as opposed to the smooth and moist surface of freshly harvested ones. This could be explained by microstructural changes linked with tuber’s structure. As mentioned in previous studies [8], microstructural observation of tubers parenchyma showed very thin and flexible cell membranes in fresh tuber (Figure 1(a)) and thick membranes in hardened tuber (Figure 1(b)). The hardening phenomenon of D. dumetorum tubers is manifested by the coarseness in the mouth of cooked tubers, making them unsuitable for human consumption. Table 2 displays the results of hardness and sensory evaluation scores of D. dumetorum cooked tubers. Cooked hardness varied from  N for fresh tubers to  N for tubers stored during 28 days, and these fresh and 28-day-stored tubers were, respectively, extremely liked and very much disliked by panellists. Significant correlation (, ) was found between cooked hardness and sensory evaluation. Similar results were reported in previous studies [11, 12]. From the results of sensory evaluation, D. dumetorum tubers were not eatable at cooked hardness values above 8.83 N.

Table 2: Hardness and sensory evaluation scores of D. dumetorum cooked tubers.

Tuber, storage period
Cooked hardness (N)‡
Sensory evaluation scores†

0
3.98 ± 0.30
8.56 ± 0.53

1
6.73 ± 0.78
7.56 ± 0.73

2
8.83 ± 0.43
7.00 ± 0.71

3
11.32 ± 0.53
3.44 ± 1.24

4
13.23 ± 0.85
2.67 ± 1.22

5
14.92 ± 1.53
2.56 ± 1.13

7
17.75 ± 0.77
2.33 ± 1.22

14
23.63 ± 1.31
2.11 ± 1.05

21
26.15 ± 1.62
1.67 ± 0.71

28
27.23 ± 1.66
1.56 ± 0.53

Mean ± SD.

.

†9 (liked extremely) to 1 (disliked extremely), .

Means with the same letter in each column are not significantly different (), according to Duncan’s multiple rage test.

(a)

(b)

Figure 1: Parenchyma microstructure of fresh (a) and hardened (b) tubers ×400.

Among the 32 accessions evaluated, one, Ibo sweet 3, was not affected by the hardening phenomenon. The remaining 31 were all subject to the hardening phenomenon at different degree. In general, and as already noted in previous studies [7], cooked hardness significantly increased with the storage time following a first-rate-order kinetic according to empirical relation of the type , where is the rate constant, t is the storage time, and a is an empirical parameter (Figure 2). The magnitude of this effect was significantly affected () by the area of collection, the altitude, the tuber’s flesh colour, and the number of roots on the tuber’s surface. In this regard, varimax rotated loading factors representing correlations between principal components (PC) and original data (Table 3) showed that PC1 and PC2 representing 77.83% of total variance in the data can be enough to restitute the maximum information contained in the original data; PC1 describing 57.24% of the total variance in D. dumetorum cooked hardness had as main contributor the area of collection, the altitude, and the tuber’s flesh colour, while PC2 described 20.59% of the total variance with the number of roots on the surface of the tuber as main contributor. Nonsignificant correlation was noted between the tuber’s weight and cooked hardness, suggesting that cooked hardness was independent of the weight of tuber as previously reported by Trèche [10].

Table 3: Loading for varimax rotated principal components.

Variable
PC1
PC2
PC3
PC4
PC5

Tuber weight
0.55
0.48
−0.67
−0.11
−0.02

Area of collection
0.90
−0.10
0.28
−0.25
−0.20

Tuber flesh colour
0.90
0.02
0.03
0.43
−0.06

Roots on the tuber surface
−0.25
0.89
0.39
0.02
−0.02

Altitude
−0.94
−0.02
−0.20
0.10
−0.25

Eigenvalue
2.86
1.03
0.73
0.28
0.11

% total variance
57.24
20.59
14.53
5.51
2.14

Cumulated eigenvalue
2.86
3.89
4.62
4.89
5.00

Cumulated variance (%)
57.24
77.83
92.35
97.86
100.00

Loading with absolute values higher than 0.67 represents a significant contribution.

Figure 2: Development of hard-to-cook defect during storage of D. dumetorum tubers; values of cooked hardness are the means of all 31 cultivars subjected to the hardening phenomenon.

The effect of some environmental and tuber’s characteristics on the development of hard-to-cook defect during storage is displayed in Table 4 and Figure 3. Cooked hardness increased more rapidly (a) in cultivars with white flesh colour compared to cultivars with yellow flesh colour, (b) in cultivars with many roots on the tuber surface compared to cultivars with few roots on the tuber surface, and (c) in cultivars collected in low altitude compared to cultivars collected in high altitude. When flesh colour and number of roots on tuber surface were associated, it was noted that cooked hardness increased more rapidly in cultivars with yellow flesh and many roots compared to other cultivars; the development of cooked hardness was more rapid in cultivars with white flesh and many roots than in accession with white flesh and few roots, whereas cooked hardness in cultivars with yellow flesh and few roots increased more slowly than in other cultivars.

Table 4: Rate constants of the hard-to-cook defect development during storage of D. dumetorum tubers according to some environmental and tuber’s characteristics.

Variables
Flesh colour
Number of roots on the tuber
Combined factors
Altitude

Yellow
White
Few
Many
Yellow-few
Yellow-many
White-few
White-many
Low
High

(day⁻¹)
0.11 ± 0.05
0.16 ± 0.05
0.09 ± 0.04
0.19 ± 0.07
0.07 ± 0.04
0.19 ± 0.07
0.15 ± 0.05
0.17 ± 0.07
0.17 ± 0.06
0.11 ± 0.05

23.99 ± 3.56
25.02 ± 2.13
24.01 ± 4.46
26.25 ± 2.17
24.99 ± 8.09
26.39 ± 2.23
25.08 ± 2.08
24.77 ± 2.58
24.61 ± 1.99
23.83 ± 3.52

0.91
0.94
0.92
0.93
0.91
0.92
0.95
0.91
0.94
0.91

750
180
600
330
450
300
150
30
210
720

0.0127
0.0057
0.0099
0.0084
0.0129
0.0091
0.0050
0.0128
0.0060
0.0123

: constant rate; : semiempirical parameter; : number of samples; mean ± standard error.

(a)

(b)

(c)

(d)

Figure 3: Effect of some environmental and tuber’s characteristics on the development of the hard-to-cook defect during storage of D. dumetorum tubers. Symbols represent experimental data points and lines indicate predictions of first-order kinetic model.

These variations of cooked hardness with morphological characteristics of tubers or the area of collection could be explained by genotype and environmental factors, as it is known that phenotype results from the expression of an organism’s genes as well as the influence of environmental factors and the interactions between the two. In this regard, it was reported that the hardness of wheat endosperm is controlled by genes and varied between accessions [13].

Results of HCA were displayed in Figure 4. In this dendrogram, there were three main groups for the similarity 0.7 (similarity index = ). The first group was composed of 15 cultivars (Babadjou, Fundong, Mankon, Bamendjou 1, Fenkam-Foto, Fonkouankem, Nkwen, Banga bakundu sweet, Kumbo, Bambui, Batibo, Dschang, Baigon, Bayamgan 1, and Bangang 1), the second group comprised 16 cultivars (Babungo, Ibo sweet 2, Bambalang, Bamokonbou, Bayamgan 2, Country yam, Fongo-Tongo, Mabondji sweet white, Lysoka sweet, Muyuka, Bana, Bamendjou 2, Bangou, Bangang 2, Ibo sweet 1, and Guzang), and the third group was formed with only Ibo sweet 3. Besides, this last cultivar showed a net separation from the other D. dumetorum cultivars at all the levels of the similarity index. In fact, Ibo sweet 3 was the only cultivar showing no significant variations in cooked hardness during storage. It was characterized by yellow flesh and few roots on the tuber surface.

Figure 4: Dendrogram showing hierarchical clusters of 47 D. dumetorum accessions based on Ward’s method and squared Euclidean distance using cooked hardness (peak force).

4. Conclusion

In general, cooked hardness of D. dumetorum was affected by some environmental and tuber’s characteristics. The number of roots on the tubers surface, the flesh colour, and the altitude of the area of collection all affected the development of cooked hardness during the tuber’s storage. One cultivar, Ibo sweet 3, was not subject to the postharvest hardening phenomenon and could therefore be used to provide information for the breeding of D. dumetorum against posthardening.

Competing Interests

The authors declare no competing interests.

Acknowledgments

The authors are grateful to Dr. Paul Kammogne Fokam and Dr. Justin Bomda, for practical and financial support through ADAF (Appropriate Development for Africa Foundation).

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