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

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 (19–28C, 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 surfacewere 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.


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][7][8][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 2 Advances in Agriculture 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.

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. 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.

Hardness of Cooked Tubers and
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.

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.

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 4.0 ± 0.3 N for fresh tubers to 27.2 ± 1.7 N for tubers stored during 28 days, and these fresh and 28-daystored tubers were, respectively, extremely liked and very much disliked by panellists. Significant correlation ( = −0.867, < 0.01) 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. 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 = (1 − − ), 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 ( < 0.05) 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 4 Advances in Agriculture  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].
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. 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 Advances in Agriculture 5  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].

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.