Mechanism of selective hydrolysis of alginates under hydrothermal conditions

Mechanisms of selective hydrolysis of alginates under hydrothermal conditions were investigated by comparing reactivities of sodium alginate(Na-ALG,960 ku)solutions and calcium alginate(Ca-ALG)gels as substrates.Under hydrothermal conditions(150°C),hydrolysis of Na-ALG gave product molecular weights of 223,66,26 and 17 ku while those of Ca-ALG gave product molecular weights of 340,102,45 and 31 ku for reaction times of 10,20,30 and 60 min,respectively.The ratios of mannuronic acid(M)to guluronic acid(G)varied only slightly(from 1.3 to 1.2)for Na-ALG over the range of reaction times at 150°C,while ratios(M/G)for Ca-ALG exhibited a remarkable decrease(from 1.1 to 0.8).Diad sequence of alginate products obtained for Na-ALG were 17%,23%,27%and 31%(GG);30%,32%,36%and 38%(MM);and 53%,46%,37%and 32%(GM+MG);while for Ca-ALG they were 18%,22%,24%and 33%(GG);26%,23%,26%and 18%(MM);and 56%,54%,50%and 48%(GM+MG).Reaction mechanisms are proposed for hydrolysis of alginate solutions and alginate gels under hydrothermal conditions;de-polymerization of alginates into monomers and monomeric sequences can be controlled not only by hydrothermal conditions,but also by varying the physical state(solution,gel)of the starting materials.