Vol. V · The Molecules of Food Plate 5 of 11proteins, amino acids, folding, denaturation, weak bonds, hydrogen bondsM

Proteins From Amino Acids.

A protein is a chain of amino acids folded into a shape. The chain is strong; the folding is held by weak bonds that heat breaks easily.

Proteins are long chains of amino acids — twenty kinds, joined end to end in a sequence set by a gene. The chain folds into a particular shape, held by many weak bonds: hydrogen bonds, attractions between charges, and the tendency of oily parts to hide from water. That shape is what lets a protein do its job in the living animal or plant, and it is fragile. The chain itself (the "peptide bonds") survives cooking; the folding does not.

As temperature rises, the weak bonds shake loose and the protein denatures: it unfolds. Because each protein is held by a different set of bonds, each unfolds at its own temperature — fish muscle proteins in the forties, beef myosin in the fifties, egg white's ovotransferrin near 62 °C, its ovalbumin near 80 °C. Unfolded proteins then tangle and bond with each other, and the food sets (VI.2). The workshop shows the unfolding curves.

Below: the fraction of each protein unfolded after ten minutes at each temperature (course kinetics with midpoints from calorimetry), and where your temperature falls.

What you will learn

Describe a protein as a folded chain of amino acids.
Explain denaturation as unfolding.
Compare unfolding temperatures of food proteins.
Connect unfolding to setting.

Workshop

Fraction unfolded after ten minutes held at each temperature, for six food proteins (course kinetics: midpoints from differential scanning calorimetry, cited in the sources).

Each protein its own temperature

At 50 °C only 1 of these seven proteins is more than half unfolded — fish myosin, which is why fish cooks so much faster and at lower temperatures than meat (VI.8). At 60 °C, 4 are; at 70 °C, 6. The spread, from the forties to about 80 °C, is the reason a cook can choose a texture: a steak at 55 °C has set its myosin but not its actin; an egg at 63 °C has set part of its white but not all of it.

Heat is not the only thing that unfolds proteins. Acid (as in ceviche, XIX.6), salt (in brines and cures), alcohol, and mechanical work (whipping egg white, VI.1, X.6) do it too, by disturbing the same weak bonds. What heat adds is speed: the warmer it is, the faster the unfolding, so time matters as well as temperature.

Linus Pauling and Robert Corey worked out in 1951 how protein chains coil and fold through hydrogen bonds — the α-helix and β-sheet — the shapes that cooking undoes.

Terms

Amino acid: One of twenty building blocks of proteins.
Peptide bond: The strong link between amino acids in a chain; cooking does not break it.
Folding: The three-dimensional shape a protein chain takes, held by weak bonds.
Denaturation: Unfolding of a protein by heat, acid, salt, alcohol or mechanical work.

Proteins are folded chains; heat breaks the weak bonds of the folding, and each protein unfolds at its own temperature.

Sources

Differential scanning calorimetry of muscle: myosin denatures around 54–58 °C, collagen shrinks around 60–67 °C, actin around 70–80 °C (A. Wright, I. B. Leach and P. Wilding, "Differential scanning calorimetric studies of muscle and its constituent proteins", J. Sci. Food Agric. 28, 1977; H. McGee, On Food and Cooking, 2004). The course uses midpoints within these ranges.
Egg proteins: ovotransferrin sets near 62 °C, ovalbumin near 80 °C, the yolk thickens near 65 °C and sets near 70 °C (H. McGee, On Food and Cooking, 2004, ch. 2; P. Barham, The Science of Cooking, 2001).