Effect of cooking on mineral and polyphenol levels in roots vegetables

<p>In this study, with the objective of collecting data for selecting cooking methods according to the situation of the eaters, we focused on root vegetables, which are frequently consumed by Japanese people, and investigated the effects of different cooking methods on the mineral and polyphenol contents. Radish, burdock root and lotus root were used as samples and manipulated using the cooking process of simmered dishes as a model. Radish was cut into half-moon slices, without preparatory boiling, preparatory boiled in water or rice water, and then boiled in water. The total boiling time was 15 min., with or without preparatory boiling. Burdocks were sliced diagonally or roughly, soaked in water or vinegar water, and boiled in water for 15 min. The lotus root was cut into pieces, soaked in water or vinegar water and then boiled in water or vinegar water for 10 min. After boiling the samples were measured for mineral content by atomic absorption spectrophotometer and for polyphenol content by UPLC. In addition, the tissue structure was observed by scanning electron microscopy. In all cases, the mineral and polyphenol content decreased after boiling. The amounts of Na and K in radish decreased when the radish was preparatory boiling. The amounts of K and 1,5-dicaffeoylquinic acid in burdock root decreased when the burdock root was cut into pieces with a larger surface area than when it was cut diagonally. In burdock, greater thermal decomposition of the tissue was observed in the outside of the burdock than in the inside. The amounts of K, Ca and Mg in bricks were reduced when boiled in vinegar water, whereas the total polyphenol content was reduced when boiled in water. In bricks, it was observed that there was less separation between cells when boiled in vinegary water compared to boiled in water.</p>