Effect of cooking time on selected metals , oxalate and phytate contents of the raw and cooked lettuce from five farms in Ethiopia

ISSN: 2410-9649 Lakew et al / Chemistry International 4(1) (2018) 15-23 iscientic.org. 16 www.bosaljournals/chemint/ editorci@bosaljournals.com in plant foods, particularly Ca, Fe and Zn, than on the macronutrients (Mohite et al. 2013; Ayele et al., 2015). Phytate has been recognized as an anti-nutrient due to its adverse effects. It reduced the bioavailability of minerals and caused growth inhibition. It forms insoluble complexes with Cu2+, Zn2+, Fe3+ and Ca2+ and as a result reduces the bioavailability of these essential minerals. Especially Zn and Fe deficiencies were reported as a consequence of high phytate intake (Greiner and Konietzny, 2006; Greiner et al., 2006). Phytate, zinc, and calcium contents of 30 East African foods and their phytate:Zn, Ca:phytate, and [Ca][phytate]/[Zn] molar ratios have been reported by Ferguson et al. (1988). The study also reported that cooking had no effect on the phytate content of cereals, but milling and fermentation reduced both the phytate and zinc contents of maize flour. Effect of local food processing on phytate levels in cassava, cocoyam, yam, maize, sorghum, rice, cowpea, and soybean has been reported by Marfo et al. (1990). Oxalic acid forms an insoluble calcium salt with a 1:1 molar stoichiometry. The calcium oxalate may also precipitate around soft tissues such as the kidney, causing kidney stones (Umaru et al., 2007). Thus a fraction of dietary Ca is rendered unavailable for absorption. Soaking and cooking of foodstuffs high in oxalate reduce the oxalate content by leaching (Noonan and Savage, 1999). Boiling may cause considerable skin rupture and facilitate the leakage of soluble oxalate into cooking water (Bhandari and Kawabata, 2004; Poeydomenge and Savage, 2007). Mosha et al. (1995) have reported the effect of blanching on the antinutritional content in cabbage, turnip, collard, sweet potato and peanut leaves. Levels of tannic acid and phytic acid were significantly reduced by conventional and microwave blanching methods while oxalic acid levels were not significantly reduced in most of the treatments by either of the blanching methods. According to Yadav and Sehgal (2002), blanching of green leafy vegetables induces moderate losses of 5-15% of phytate in vegetables. Furthermore, it is desirable to establish optimum time and temperature to process food in order to derive maximum health benefits. Ayele et al. (2015) have reported the effect of cooking temperature on mineral content and antinutritional factors of yam and taro grown in southern Ethiopia. The study showed significant decrease in the phytate and oxalate contents of yam and taro on cooking and increase in the bioavailability of minerals in the cooked samples. Recently some studies have been carried out on the mineral contents of plant-based food: kocho and bulla (Atlabachew and Chandravanshi, 2008), enset (Debebe et al., 2012), vegetables (Weldegebriel et al., 2012), cactus pear (Aregahegn et al., 2013a), yam (Aregahegn et al., 2013b), white lupin (Akalu and Chandravanshi 2014), linseed (Mekebo and Chandravanshi 2014) and ginger (Wagesho and Chandravanshi 2015) used in Ethiopia but not on the antinutritional factors and bioavailability of minerals except on yam and taro (Ayele et al., 2015). The objectives of this research were to assess the cooking effect on minerals, oxalate and phytate in the lettuce cultivated in Ethiopia and to assess the inhibitory effect of oxalate and phytate and effect of cooking on the bioavailability of calcium, iron and zinc in lettuce produced with irrigated rivers of Akaki, Bulbula and Ziway Lake of Ethiopia. Therefore total content and bioavailability of metals, phytate and oxalate were determined in lettuce plants, before and after cooking. MATERIAL AND METHODS