Vol. X · Emulsions, Foams and Gels Plate 9 of 11agar, pectin, jam, low-methoxyl pectin, starch gel, 凉粉, 琼脂, setting, sugar, acid, calciumMT

Agar, Pectin and Starch Gels.

An agar jelly stands firm on a hot summer table; a gelatin one slumps. Jam sets only when its sugar and acid are right.

Agar (琼脂, kanten) is extracted from red seaweeds. It must be boiled to dissolve, sets as it cools to about 35 °C, and then does not melt again until about 85 °C: a gel that keeps its shape in warm weather and can be served warm, and that proteases cannot cut. It is firmer and more brittle than gelatin, and vegetarian. Pectin is the cement between plant cell walls (V.4), plentiful in citrus peel, apples and quinces. Ordinary (high-methoxyl) pectin forms a gel only when sugar has taken up most of the water and acid has quieted its charges: jam needs about 60 % sugar and a pH near 3.

Low-methoxyl pectin sets with calcium instead of sugar, which makes low-sugar jams possible. Starch gels as it cools after gelatinising (VII.1, VII.3): mung-bean starch for Chinese 凉粉, cornflour for puddings, rice flour for Korean and Chinese rice cakes. Each gel has its own conditions, and the workshop tests them.

Below: whether each gel sets, how firm it is, and where it sets and melts, for the gelling agent, concentration, sugar and pH you choose (colloid bench; teaching thresholds from the published conditions).

What you will learn
Workshop
Colloid bench, teaching thresholds from published conditions: agar sets above about 0.2 % at about 32–36 °C and melts near 85 °C; high-methoxyl pectin needs at least about 55 % sugar and pH 3.5 or below; low-methoxyl pectin needs calcium; starch gels above about 4 % on cooling. Firmness is a relative teaching scale.

Different gels, different rules

A 0.5 % agar jelly sets near 32 °C and does not melt until about 85 °C, against about 29 °C for a 1.5 % gelatin one: agar jellies stand on a summer table and can be poured over warm fruit. For jam, pectin with 60 % sugar at pH 3.2 sets in this model; the same with only 40 % sugar does not, nor does it at pH 3.8. That is why jam recipes give exact sugar weights and add lemon juice to low-acid fruits such as strawberries.

Practical notes. Agar must be boiled for a minute or two to dissolve; 1 g sets about 200–300 mL firmly; acid weakens it, so add lemon juice off the heat. Test jam by the wrinkle test: a drop on a cold saucer wrinkles when pushed once the set point (about 104–105 °C at sea level) is reached. For low-sugar jam use low-methoxyl pectin with its calcium. For 凉粉, whisk one part mung-bean starch into about seven parts water, cook until clear and thick, and let it set; it slices cleanly and keeps its shape in chilli oil.

At the stove

Set the same juice two ways, with 1 % gelatin and with 0.5 % agar, and leave both on the counter on a warm day. One slumps; the other stands.

Agar was discovered in Japan in the 1650s, so the story goes, when an innkeeper threw out leftover seaweed jelly that froze and thawed into a dry, spongy block — the first kanten. In 1882 Fanny Hesse suggested it to Robert Koch's laboratory as a culture medium.

Terms

Agar
A seaweed gel that sets near 35 °C and melts near 85 °C.
Pectin
The cement between plant cells; gels with sugar and acid.
Low-methoxyl pectin
A pectin that gels with calcium instead of high sugar.
凉粉
A Chinese starch jelly, usually of mung-bean starch.
Agar sets near 35 °C and melts near 85 °C; pectin jam needs about 60 % sugar and pH near 3; starch gels as it cools.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.