Studies on “sugar-reactivity” of agars extracted from some Indian agarophytes

Sugar reactivity was observed within the sugar–agar complexes in presence of sucrose and glucose with agars of Indian agarophytes viz. Gelidiella acerosa, Gracilaria edulis, Gracilaria crassa and Gelidium pusillum. The sugar reactivity was more pronounced in presence of sucrose than glucose. Oxoid agar was used as the reference material. Control agar gel contained 1.12% agar (w/w) in water. Sucrose–agar and glucose–agar gels in water consisted of 50% (w/w) sucrose and 50% (w/w) glucose, respectively along with 1.12% (w/w) agars of the four seaweeds mentioned above. Addition of sucrose resulted in increase (ca. 25–45%) in gel strength; increase (2–3 °C) in gelling and melting temperatures was observed in the gels prepared with agars from all the agarophytes and Oxoid agar. On the other hand, addition of glucose resulted in increase (19–34%) in the gel strength and gelling and melting temperatures of the agar gels of Oxoid as well as of all other agars decrease (2–3 °C). Maximum sugar reactivity was observed with the 50% level of sucrose and glucose in agar gels. Rheological and thermogravimetric characteristics of these gel samples were studied. The latter showed two patterns e.g. control agar gel of Oxoid agar was thermally less stable than the four control agar samples studied; in sugar–agar gel samples it followed a reverse pattern. To our knowledge, this is the first report of “sugar reactivity” of agar of Indian agarophytes. Sugar reactivity of agar in presence of glucose is also reported for the first time. The results of this study will be useful in bioprospecting as well as in exploring new applications.