Vol. VI · Proteins and Heat Plate 1 of 11denaturation, calorimetry, DSC, thermogram, egg white, meat proteins, acidMH

Denaturation.

Warm a protein slowly and at one temperature it soaks up extra heat as it unfolds. Each peak on the chart is one protein letting go.

Food scientists watch proteins unfold with a differential scanning calorimeter: it warms a sample at a steady rate and measures how much heat it absorbs beyond what warming needs. Unfolding takes energy, so each protein shows as a peak at its own temperature. Egg white gives two big peaks, ovotransferrin near 62 °C and ovalbumin near 80 °C; beef muscle gives three, myosin in the mid-fifties, collagen in the low sixties, actin in the seventies.

The peak temperatures are the cook's map. They say where a little more heat makes a large change in texture, and where it makes almost none. Other agents shift them: acid and salt lower the temperature at which many proteins unfold, sugar raises it. The workshop draws a thermogram for egg white and for beef, and shows the effect of acid.

Below: heat absorbed by unfolding as egg white or beef is warmed at 1 °C per minute (computed from the course's protein kinetics), with the peaks labelled.

What you will learn

Read a thermogram.
Name the denaturation peaks of egg white and meat.
Explain how acid and sugar shift them.
Use peaks to choose cooking temperatures.

Workshop

Rate of unfolding (a thermogram's shape) as each protein is warmed at 1 °C per minute, from the course kinetics with midpoints from DSC studies. Acid lowers the midpoints by about 1.5 °C per pH unit below 7 in this teaching model.

Peaks on the map

Egg white shows peaks at 62.0 and 80.0 °C; beef at 55.0 and 63.0 °C, with actin above that. Between the peaks, a few degrees change little; across a peak, they change a great deal. That is why an egg held at 64 °C has a soft, barely set white, and at 70 °C a firm one, while 66 and 68 look much alike; and why a steak changes so much from 55 to 62 °C.

Acid moves the peaks down: at pH 4 the egg-white peaks sit at 57.5 and 75.5 °C in this model, which is part of why a squeeze of lemon or a dash of vinegar helps poached eggs set quickly (XVII.3), and why acid alone can "cook" fish (XIX.6). Sugar does the reverse: it protects proteins and raises the setting temperature, which is why a sweet custard sets higher and more gently than an unsweetened one (XVII.5).

Differential scanning calorimetry was developed in the 1960s at Perkin-Elmer; food scientists quickly adopted it to map the denaturation temperatures of meat, fish and egg proteins.

Terms

Thermogram: A chart of heat absorbed against temperature, with peaks where proteins unfold.
DSC: Differential scanning calorimetry, the instrument that draws thermograms.
Denaturation temperature: The peak temperature at which a protein unfolds most rapidly.
Stabiliser: A substance such as sugar that raises the temperature at which proteins unfold.

Each protein unfolds around its own peak temperature; acid lowers the peaks, sugar raises them.

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).