Vol. X · Emulsions, Foams and Gels Plate 8 of 11gelatin, gel, bloom strength, sheets, powder, panna cotta, jelly, melting, setting, pineapple, 皮冻MT

Gelatin.

A panna cotta melts on the tongue because gelatin gels melt at about 30 °C. Fresh pineapple in a jelly stops it setting at all.

Gelatin is collagen (VI.6) that has been unwound and broken into shorter chains by long heating. Dissolved in warm liquid, the chains move freely. As the liquid cools below about 20–25 °C they begin to wind back, in short stretches, into the triple helices of the original collagen, linking chain to chain into a network that holds the water: a gel. Warm it to about 30 °C and the helices unwind again; the gel melts — just below body temperature, which is why jellies and panna cotta melt in the mouth.

Gelatin is sold as powder or sheets of different strengths, graded by Bloom: the force needed to press a standard plunger 4 mm into a standard gel. Stronger gelatin needs less weight for the same set. Fresh pineapple, kiwi, papaya, figs and ginger contain proteases that cut the chains (V.9); cooked or canned fruit has had them destroyed. Chinese 皮冻 and the jellied stock under a roast are the same gel made from skin and bones. The workshop sets a jelly.

Below: the setting and melting temperatures and the firmness of a gelatin gel against concentration (colloid bench), and the weight of each grade of gelatin that gives the same set as your recipe.

What you will learn
Workshop
Colloid bench: melting point about 27 + 4 ln(c) °C for c % gelatin, setting about ten degrees lower; firmness rises with the square of concentration (teaching model of the published trend). Conversion between grades: mass × √(Bloom of the recipe's gelatin ÷ Bloom of yours), the rule used in the trade.

Set in the fridge, melt in the mouth

A jelly of 1 % gelatin melts at about 27 °C in this model; one of 3 %, at about 31 °C — firmer and slower to melt, but rubbery. Panna cotta is made near 1 %, so it barely holds its shape and melts at once in the mouth; a jelly that must stand turned out of a mould needs nearer 2 %; a mousse that must be sliced, more. Because firmness rises with the square of concentration, small changes matter.

Five hundred millilitres at 1.5 % needs 7.5 g of powder; in gold sheets, about 8.0 g, in silver, 8.9 g — the square-root rule. Soak sheets in cold water first (and squeeze them) or sprinkle powder over cold liquid to swell before warming; never boil gelatin, which weakens it. Allow at least four hours in the fridge for a full set: the helices go on forming for a day. And cook fresh pineapple, kiwi or papaya before setting them in a jelly, or use agar (X.9), which proteases cannot cut.

At the stove

Make two small jellies from the same juice, one with fresh pineapple pieces and one with canned. After four hours in the fridge only one has set.

The Bloom test was devised in 1925 by Oscar T. Bloom of the Swift meat company; the grades on gelatin packets still carry his name.

Terms

Gelatin
Collagen broken into shorter chains that dissolve in warm water and gel when cool.
Bloom
A measure of gel strength: the force to press a standard plunger into a standard gel.
Triple helix
The three-stranded coil of collagen that gelatin partly re-forms as it sets.
皮冻
A Chinese jelly made by long-simmering pork skin.
Gelatin sets below about 20 °C and melts near 30 °C; convert grades by the square root of the Bloom ratio; fresh pineapple and kiwi stop it setting.

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.