Enzymatic Browning.
A sliced apple browns in minutes on the counter and stays pale for hours in lemon water. Neither heat nor sugar is involved: this is an enzyme at work.
Plant cells keep an enzyme, polyphenol oxidase (PPO), apart from the phenolic compounds it acts on. Cut, bruise or bite the tissue and they meet, with oxygen from the air: the enzyme oxidises the phenols to quinones, which join into brown pigments. This is browning without heat, and it has nothing to do with the Maillard reaction. Apples, pears, bananas, avocados, potatoes, aubergines, mushrooms and lotus root all do it.
Each of the reaction's needs is a way to stop it. Acid: PPO works poorly below about pH 4, and lemon juice adds vitamin C, which turns quinones back before they brown. Cold slows it (V.10). Keeping air out — under water, under a cling film pressed on the surface — starves it of oxygen. Salt water inhibits it, as Japanese cooks know when they dip cut apples. Heat destroys it: blanching (XVI.2). The workshop browns a cut apple surface under each treatment.
Below: browning of a cut apple surface over two hours on the counter under each treatment (teaching rates, with Arrhenius cooling and the course enzyme-inactivation rate for blanching).
- Explain enzymatic browning.
- Distinguish it from Maillard browning.
- Choose a way to prevent it.
- Recognise where it is wanted, as in tea.
Take away one thing it needs
After an hour on the counter an untreated apple slice has browned to 0.78 on this scale, one tossed in lemon juice only 0.16. Blanched for a minute at 85 °C, the enzyme is essentially destroyed and the slice stays pale; at 80 °C 5 % of its activity survives and at 70 °C 66 %, which is why blanching water must be at a full boil to protect vegetables before freezing (XIII.3). The cost of blanching is a cooked texture, so for raw fruit acid, cold and keeping air out are the tools.
Practical rules. Cut apples, pears and bananas just before serving or toss them in lemon or lime juice; hold peeled potatoes and lotus root under cold water until they are cooked; press cling film onto the surface of guacamole rather than stretching it over the bowl; store cut fruit cold. A stainless knife and a clean board help: iron and copper ions speed the reaction. And where the browning is wanted — black tea, oolong, raisins, cocoa, the dark 普洱 — the producers encourage exactly this enzyme.
Slice an apple into five pieces. Leave one bare, put one in salt water, one in lemon water, one under plain water and one in the fridge. Compare them after an hour.
Terms
- Polyphenol oxidase (PPO)
- The enzyme that browns cut fruit and vegetables by oxidising phenols.
- Quinones
- Reactive products of the enzyme that join into brown pigments.
- Antioxidant
- A substance such as vitamin C that reverses or prevents oxidation.
- Blanching
- Brief heating to destroy enzymes (XVI.2).
Sources
- Enzymatic browning by polyphenol oxidase: inhibited by acid (activity falls steeply below about pH 4), by cold, by excluding oxygen and by heat; blanching inactivates it (M. Y. Lee and J. R. Whitaker, eds., Enzymatic Browning and Its Prevention, ACS Symposium Series 600, 1995). The course's rates are teaching values.
- A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.