Preparation, characterization, and application of high-whiteness agar bleached with hydrogen peroxide

Agar that has been extracted via traditional alkaline pretreatment always appears yellowish because it contains pigments. This characteristic adversely affects the food and biotechnological applications of agar. In this work, agar was bleached with hydrogen peroxide. The whiteness of high-whiteness agar reached 74.4%, which was 32% higher than that of raw agar (56.48%). The high-whiteness agar bleached with hydrogen peroxide exhibited several other excellent qualities over raw agar. These characteristics included, low sulfate content (0.67%), low ash content (1.01%), and high transparency (64.4%). In particular, the transparency of the high-whiteness agar was 13% higher than that of raw agar (56.8%). By contrast, the gel strengths of the two agars did not significantly differ. The structures of raw and high-whiteness agar were characterized by using thermogravimetric (TG)-differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). TG-DSC analysis indicated that the high-whiteness agar had better thermal stability, hygroscopicity, and water-holding capacity than raw agar. SEM images showed that the surface of the high-whiteness agar had imperfections or fissures that were caused by the oxidative degradation of hydrogen peroxide. The diversified application of the high-whiteness agar was explored on the basis of the promotion of agar quality. Results indicated that the visual appearance of the jelly prepared with the high-whiteness agar was better than that of the jelly prepared with raw agar. Moreover, the high-whiteness agar medium was easier to observe and use for colony counting than biochemical medium.

Automated summary

The study demonstrates that bleaching with hydrogen peroxide can reduce pigment content in agar, increase whiteness, and improve transparency and quality characteristics. High-whiteness agar has lower sulfate and ash content, higher transparency, better thermal stability, and water-holding capacity. It is superior to raw agar in terms of jelly preparation and microbial counting.