Vol. V · The Molecules of Food Plate 9 of 11enzymes, papain, bromelain, polyphenol oxidase, tenderising, blanching, inactivationMH

Enzymes.

Fresh pineapple keeps a jelly from setting; tinned pineapple does not. The difference is an enzyme, and the heat that killed it.

Enzymes are proteins that make particular reactions run fast at ordinary temperatures. Food is full of them, left from the living tissue. Some help the cook: enzymes in meat tenderise it as it ages (XVIII.1); papain from papaya, bromelain from pineapple and actinidin from kiwi break down muscle protein and are used in marinades and meat tenderisers. Some hinder: polyphenol oxidase turns cut apples and avocados brown (VIII.6); bromelain digests gelatin, so fresh pineapple ruins a jelly.

Because enzymes are proteins, they behave like proteins in heat. Up to a point, warmth speeds them — so a marinade with papaya works fastest just before it gets hot. Then their folding fails and they stop, permanently. That is the logic of blanching vegetables before freezing (XVI.2), of cooking pineapple before adding it to gelatin, and of the mushy surface a papaya marinade leaves if the meat sits too long. The workshop shows the activity of an enzyme over ten minutes at each temperature.

Below: the relative work done in ten minutes by three enzymes at each temperature — rising with warmth, then collapsing as heat unfolds them (a teaching model with course inactivation rates).

What you will learn

Describe enzymes as protein catalysts.
Explain the rise-then-fall of enzyme activity with heat.
Use and stop enzymes in cooking.
Explain why fresh pineapple stops gelatin setting.

Workshop

Teaching model: activity rises with temperature (activation energy 50 kJ/mol) and is lost as the enzyme unfolds (course inactivation rates for browning enzyme, papain and bromelain). Relative to activity at 20 °C.

Speed up, then stop

Held for ten minutes, the pineapple enzyme bromelain does most work near 56 °C in this model; hotter, it unfolds faster than it works. Two minutes at 70 °C leaves only 0.0 % of it active, and anything hotter destroys it almost at once, which is why cooked or tinned pineapple sets in jelly and fresh does not. The browning enzyme of apples peaks near 60 °C and is destroyed by a brief blanch — the reason vegetables are blanched before freezing, so their enzymes do not slowly spoil colour and flavour in the freezer.

Use enzymes with a light hand. A papaya, pineapple or kiwi marinade works on the surface only (XVIII.5) and can turn it to paste within hours; an hour or two is plenty. Enzymes that do good slowly — the ones that age beef, the koji enzymes that make soy sauce and miso (XIV.7), the rennet that curdles milk (XIV.6) — are given time on purpose. Cold slows all of them; heat ends them.

At the stove

Make two small bowls of gelatin with pineapple: one with fresh pineapple pieces, one with pieces simmered for two minutes first. Chill both. The fresh one stays liquid around the fruit; the cooked one sets.

Eduard Buchner showed in 1897 that a cell-free extract of yeast could ferment sugar — proof that the work was done by molecules, later named enzymes from the Greek for "in yeast".

Terms

Enzyme: A protein that speeds a particular chemical reaction.
Protease: An enzyme that breaks down proteins, such as papain or bromelain.
Inactivation: Permanent loss of an enzyme's activity, usually by heat unfolding it.
Blanching: A brief boil to inactivate enzymes before freezing (XVI.2).

Enzymes speed up with warmth, then stop for good when heat unfolds them. Use them briefly, or kill them with heat.

Sources

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