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Boiling versus Roasting Can Make a Nutritional Difference

When it comes to the cooking of sweet potatoes and yams, it's better to roast than boil if you want to preserve as much zinc and iron as possible. That conclusion was reached by researchers at University of West Indies in Mona, Jamaica, who studied the impact of boiling and roasting on tubers (including yams and sweet potatoes). While zinc and iron were two of the key minerals studied, the researchers also studied three anti-nutrients (anti-nutrients is a term used to describe compounds in food that might block digestion or absorption of nutrients). The anti-nutrients studied by these Jamaican researchers were trypsin inhibitor, cyanoglucosides, and phytic acid.

For both yams and sweet potatoes, roasting did a better job of deactivating trypsin inhibitor than did boiling. It also did as good a job or better in lowering cyanoglucoside levels. Roasting also did a far better job at preserving the amounts of zinc and iron in both sweet potatoes and yams.

However, there was one finding that seemed to favor boiling over roasting—reduction of phytic acid. Boiling did a better job at reducing phytic acid than did roasting. Since phytic acid can block the absorption of both zinc and iron, this superior reduction of phytic acid would have made the researchers consider boiling to be of superior benefitwere it not for the fact that boiling also removed more of the zinc and iron from the yams and sweet potatoes. In other words, even though boiling helped remove one of the nutrient-damaging factors (phytic acid), it also helped removed the nutrients (zinc and iron) that needed protection from that damaging factor. From our perspective, this combination of circumstances clearly gives the nod to roasting over boiling when it comes to optimizing zinc and iron nourishment.

It's important to note that a wet-heat cooking method (boiling) did a better job at reducing phytic acid content in these yams and sweet potatoes. At the World's Healthiest Foods, we often recommend steaming as a preparation method for vegetables, since steaming can expose a vegetable to high heat for a very short period of time, produce a delicious texture and flavor, and still do a good job preserving nutrients. Steaming avoids submerging the entire vegetable and its surface area into water, which would increase leeching of water-soluble nutrients from the vegetable. Although our wet-heat cooking method (steaming) would not be expected to lower anti-nutrients like phytic acid to the same extent as boiling (another wet-heat cooking method), the research findings in this study suggest that wet versus dry heating methods may make a difference in reduction of phytic acid content. Based on the results of this study, it's very possible that steaming may achieve a unique balance in reducing anti-nutrients while still preserving desirable nutrients when it comes the cooking of certain foods. We look forward to more research in this area.

Reference

Omoruyi FO, Dilworth L, and Asemota HN. Anti-nutritional factors, zinc, iron and calcium in some Caribbean tuber crops and the effect of boiling or roasting. Nutr and Food Sci 2007, 37(1): 8-15.

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