How Good Cooks Keep Green Veggies from Going Brown
As a culinary scientist, I’m constantly straddling the border between two worlds. My career is predicated on marrying two disparate skill sets and ideologies—those of the chef and those of the biochemist—to reinforce the strengths and cover the weaknesses of each side. There are gaps in this hybrid armour, however—situations where science, technology and culinary creativity have yet to catch up with our ambition. At times, these puzzles can appear maddeningly simple. For instance, we have yet to discover an effective method for manufacturing a good bottled beverage containing naturally green ingredients.
Chemically, the preservation of green is all about coddling chlorophyll molecules. At the centre of all members of the chlorophyll family is a chlorin ring, containing an array of aromatic hydrocarbon rings anchored at the centre by a magnesium ion. These arrays are precisely tuned to interact with certain wavelengths of light, so any change in the structure of chlorophyll can potentially change its colour. The most fragile component of the array is the magnesium ion, which can easily be displaced by hydrogen ions in aqueous solutions, turning the pigment from vibrant green to dull brown or yellow.
This effect is enhanced in acidic environments, which cooks have long combated by using neutral or alkaline water for blanching green vegetables, thus minimising the number of hydrogen ions vying for a place at the centre of the chlorin ring. While the two most common culinary chlorophylls, chlorophyll a and b, have a long hydrocarbon chain attached to the chlorin ring, rendering the pigment virtually insoluble in water in its native form, in most situations heating can accelerate acid or alkaline hydrolysis of that hydrocarbon tail, turning the chlorin ring highly soluble—and vulnerable in a sea of eager hydrogen ions.
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