Effects of the use of mineral waters on the components of kombu soup stock

Abstract

  We prepared kombu soup stock using some types of mineral waters that differ in hardness, and evaluated their pH, mineral contents, and contents of umami ingredients. The following results were obtained:
1) Water absorption by kombu during extraction for the preparation of soup-stock was lower using hard compared to soft water. The mineral waters were weakly alkali and neutral, but the kombu soup stock was weakly acid.
2) The Na content of the kombu soup stock reached a peak 1-6 hours after the initiation of extraction, reaching equilibrium thereafter using mineral water with each hardness. The K content rapidly increased 1 hour after the initiation of extraction, slightly increased or reached a peak thereafter, and then slightly decreased. These results showed the extraction of Na and K from kombu to soup stock. Using soft water, Ca extraction into kombu soup stock was low, and its amount negligibly changed even with an increase in the extraction time. However, using hard water, the Ca content in kombu soup stock decreased 1 hour after the initiation of extraction, and adsorption of Ca originally contained in mineral water to kombu occurred. Mg extraction from kombu was observed immediately after the initiation of extraction, and thereafter adsorption of Mg to kombu occurred.
3) The glutaminic acid content in kombu soup stock increased with the extraction time, but the increase did not differ between hard and soft water.
  When mineral water is used for cooking, water appropriate for each type of cooking should be used.