Yam tubers

In an earlier report (J>), the decay of healthy yam tubers during storage was shown to be a result of catabolism of its proteins by an active a-glutamyl transpeptidase. There is also some alkaline proteolytic activity in the yam tuber (6), but little information is available on individual enzymes of the purine degradative pathway and on the properties of an alkaline proteinase that may function in yams during storage. This report describes the interrelation of five enzymes of ureide metabolism in fresh and stored yams, the release of ammonia in vitro by three of the enzymes that may provide an environment for alkaline proteinase activity in vivo, and the in vitro properties of an... 

Yam tubers of Dioscorea alata (Umudike cultivar), D. rotundata (asukwu and obiaturugo cultivars)" and D. cayenensis (water yam and Nkokpu cultivars) were obtained from the National Root Crops Research Institute, Umudike, Nigeria. Some tubers were stored 6 or 12 months at room temperature (25-27 °C), some in vacuum dessicators over a suitable dessicant, and some in paper bags placed in a dark cabinet (absence of circulating air). Fresh tubers were peeled by carefully scraping away the cork layer to minimize loss of outer tissue since much of the protein is concentrated here ( ). They were then cut into 2 cu. cm. pieces, quickly frozen with solid CO2 in 50 9 portions in plastic bags, and stored in a freezer until needed. 

A summary of the proximate analyses of these yam tubers is shown in Table 1. On a fresh weight basis, protein is quite low because of the high water and starch contents of yams (dry weight protein content is 4-8%). Yams are rarely consumed alone but,... 

Table I. Summary of Proximate Analyses of Fresh Yam Tubers U.

Table 2. Effects of 12 Months Storage on Relative Activities of Ureide Enzymes in Yam Tubers.

Alkaline Proteinase Activity in Yams. The release of ammonia at several stages during ureide metabolism suggested a potential for alkaline conditions in yam tubers, rather than the usual neutral or acid conditions generally found in seeds and plants. 

Figure 1. Heat Stability of Yam Tuber Alkaline Proteinase at 60°(with azocasein as substrate).

Figure 2. Photograph of Yam Tubers, Fresh (A) and after 4 Months Storage (B) Showing a Typical Sprout that Occurs in Stored Tubers.

Table 4. Substrate Specificity of Yam Tuber Alkaline Proteinase (Activity mg protein hydrolyzed)/10 mg enzyme/2hr).

Figure 3. Dehydrated Flakes Prepared from Fresh Peeled Yam Tubers by the SRRC-Sweet Potato Flaking Process.

Osuji, G. 0. Umezurike, G. "TFe Biochemistry and Technology of the Yam Tuber" ASUTECH Press, Enugu, Nigeria, 1965. 

Gelatinization of sweet potato, tania, and yam tuber starches. Starch/Starke. 47, 298-306. Varavinit, S., Shobsngob, S., Varanyanond, W., Chinachoti, P., and Naivikul, O. (2003). Effect of amylose content on gelatinization, retrogradation and pasting properties of flours from different cultivars of Thai rice. Starch/Starke 55,410-415. 

Dioscorea cirrhosa L. D. hispida Dennst. D. japonica Thunb. Shu Liang (Dyeing yam) (tuber) Tannins, mucus 33 Hemostatic, increases platelet aggregation, increases uterine contraction. 

Morse, S., Acholo, M., McNamara, N., and Oliver, R. 2000. Control of storage insects as a means of limiting yam tuber fungal rots. J. Stored Prod. Res. 36, 37-45. 

Arakane, Y., H. Hoshika, and N. Kawashima et al. 2000. Comparison of chitinase isozymes from Yam tuber-enzymatic factor controlling the lytic activity of chitinases. Biosci. Biotechnol. Biochem. 64 723-730. 

Somewhat like the potato in nutritional value but yam tubers contain 50% more calories and about the same amount of protein. Yam are a fair to good source of calcium, phos phorus, and iron and a fair source of vitamii C, but they contain little carotene. 

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