From pudding to Petri Dish

In the late 19th century, Lina Hesse, the American wife of a German scientist, recalled the advise of friends who had lived in Asia and made agar jellies and puddings that stayed solid despite the summer heat. Her husband relayed the message to his boss, microbiologist Robert Koch, who then used agar to isolate the causative agent of tuberculosis, Mycobacterium tuberculosis.

Agar gels are used as an invaluable tool in the microbiology laboratory. Scientists make agars containing various nutrients that help grow and differentiate bacteria. Agar is helpful to microbiologists because

• very few bacteria can digest the agar carbohydrates, so agar gels remain intact and bacteria colonies separate. Bacteria readily digest the proteins in gelatin and many liquify the gel.
• Agar gels remain solid at the ideal growth temperature for most bacteria, around 100 degrees F/38 degrees C. Gelatin melts at this temperature.

Agar

Agar is a mixture of several different carbohydrates and other materials extracted from red algae. Today it is manufactured primarily by boiling the seaweeds, filtering the liquid, and freeze-drying it in strands. Solid agar pieces can be consumed raw in salads; it can be used in many sauces as a thickening agent; or used to gel flavorful mixtures of fruit juices and sugar, stews, meats, and vegetables. Agar is consumed as a jellied sweet in Japan.

Agar forms gels at much lower concentrations than gelatin. Where commercial gelatin concentration is usually >3%, agar concentrations work well under 1% by weight. The jelly is somewhat opaque, as opposed to the clear gelatin, and it has a more crumbly texture. Formed agar melts at 185 degrees F/85 degrees C, therefore it does not have the "melt-in-your-mouth" quality of gelatin. Agar gels must be chewed. The higher melting point is often desirable for cool treats that do not melt in the summer heat as well as hot dishes.