If the kelp soaked in water is not tough as if it was boiled. Is already spoiled. Based on your description of the problem, it is no longer edible.

If the kelp is soaked for too long, all the nutrients in the kelp will dissolve in the water and the nutritional value will be reduced. Correct should be soaked for about 6 hours can be.

The nutritional value of kelp is very high, rich in protein, fat, carbohydrates, dietary fiber, calcium, phosphorus, iron, carotene, vitamin B1, vitamin B2, niacin, as well as iodine and other trace elements.

Dry kelp its surface has a white powdery along with the iodine and mannitol contained in kelp, especially mannitol is a white powder attached to the surface of the kelp, there is no any white powder kelp quality is poor. Secondly, observe the kelp to leaf width and thickness, color green or purple in the yellowish, no yellow leaves for the top grade. In addition, the kelp after processing and bundling should be selected without sediment impurities, neat and clean without mold, and feel non-sticky for the best.

Reference Source: Kelp_Baidu Encyclopedia