Nutrient and Antinutrient Content of an Underexploited Malawian Water Tuber Nymphaea petersiana (Nyika)

Nymphaea petersiana Klotzsch (Nyika) is an important wild tuber eaten in some districts of Malawi. The tubers were processed by boiling/freeze-drying(BFD) and sun-drying(USD). The tuber's nutrient and antinutrient composition was determined to produce a preliminary nutrient data base for use in sub-Saharan Africa.

There was no significant difference (P > 0.05) in protein content of BFDand USD samples. Sun-dried samples were significantly (P < .05) higher in ash than boiled samples while boiled samples were significantly higher (P < .05) in crude fat and total carbohydrate. The protein content of the tubers (8.0 and 8.1 %) was higher than that of the staple maize (7.9%), African millets (unspecified) (7.5%), and polished rice (7.0%), but lower than sorghum (10.7%). Protein content was higher than tubers like cassava (1.3%), potato (2%), sweet potato (1.6%), yams (1.5%) and N. lotus (5.2). Nyika tubers have a well balanced amino acid content, limiting only in lysine.

There were no significant (P > 0.05) differences in the mineral content of BFD and USD samples, except for iron, which was lower in the boiled samples. Nyika tubers have a higher calcium (1376 and 946 ug/g) and phosphorus (2250 and 2883 ug/g ) content than wild and domesticated cassava, potatoes, sweet potatoes and wild and domesticated yams. Sun-dried tubers have a higher iron content (88ug/g) than maize (20ug/g). The zinc content of tuber was higher (21and 25ug/g) than that of boiled maize flour, boiled sorghum flour, rice, cassava, and sweet potato. The predominant fatty acids in the tubers were oleic (47%), linoleic (32%), palmitic (21%) and linolenic (7%) acids. Ascorbic acid content was very low, only 0.1 and 0.003 mg/100g.

Tannin content was lower (1 and 1.7 %) in the tubers than in Vulgare Pers. sorghum, DeKalb sorghum from U.S. and Kabale sorghum from Uganda. There was a significantly (P < 0.05) lower content of phytate in boiled (3.9ug/g) than in sun-dried tubers (6.0 ug/g). Phytate content of the tubers was lower than that of cooked maize flour, unrefined maize flour, cassava and sweet potato. Trypsin inhibitor activity in the tubers was reduced from 463 to 55 TIU/g tuber and chymotrypsin inhibitor activity was reduced to 50 from 267 CIU/g tuber by cooking.

Nyika is a good source of iron and quality protein limiting only in lysine. Protein is comparable to staple maize and higher than root crops consumed in Malawi. It is not a good source of fat and ascorbic acid. Tannin, phytate, trypsin, and chymotrypsin inhibitor content lower than most food crops consumed in Malawi.