the vibrant colors of fruits and vegetables are a sign of their freshness. Alas, no sooner have avocados, salsifies, apples, pears, and mushrooms been sliced or chopped than they turn brown. Can this degradation be avoided? Can fresh-squeezed apple juice make it from the kitchen to the table without turning dark? Cooks have long recommended using lemon, whose juice they believe prevents the appearance of colors associated with overripe, damaged, or rotten organic matter. Is this recommendation sound?
Let's put it to the test. If we compare avocado slices exposed to the oxygen in the air with slices that have first been sprinkled with lemon juice, the difference is plain after a few hours. This confirms the wisdom of customary culinary practice but does not explain why lemon juice has a protective effect. If acidity were responsible, it ought to be possible to substitute vinegar. But this is easily disproven by experiment.
And so? Lemons contain ascorbic acid, or vitamin C, an antioxidant compound. Pure ascorbic acid of the kind one finds in tablet form at the pharmacy ought to be more effective than lemon juice, and experiments show that this is indeed the case. By investigating the role of oxygen in the darkening of vegetables, modern food science has been able to add to the empirical list of remedies that cooks have compiled, which includes not only the juice of certain citrus fruits (lemons, oranges, limes) but also various salty brines.
The darkening of vegetables is caused by enzymes called polyphenol oxidases, which alter the structure of the polyphenol molecules of fruits and vegetables. These molecules have a benzene center surrounded by six carbon atoms at the apices of a hexagon, with either a hydrogen atom or a hydroxyl (-oh) group associated with each carbon atom. In the presence of oxygen the polyphenol oxidase enzymes replace the hydroxyl groups with oxygen atoms, producing quinones whose reaction generates brown pigments of the same family as melanin (the pigment that is formed in our skin when it tans under the sun). This enzymatic darkening, thought to defend plants against ravaging birds and insects, is observed in most fruits, leaves, and many mushrooms that have been cut; it is not commonly found in uncut vegetables because the enzymes and polyphenols are separated by membranes.
Various methods are used to prevent the darkening of vegetables and fruits that have been sliced or chopped—often their lot in the kitchen. Freezing and refrigeration slow but do not prevent the action of enzymes. Pasteurization, a more radical procedure that inactivates the enzymes, cannot be applied to all fruits and vegetables, for it often degrades their texture and color. Finally, vacuum packing—sealing fruits and vegetables in containers from which the oxygen has been drawn out—prevents the appearance of brown compounds; alternatively, nitrogen and carbon dioxide atmospheres sometimes are used in the food processing industry.
Inhibitors are also found in nature that, in minute proportions, work to prevent enzymatic darkening. For example, a very weak dose of salicylhydroxamic acid completely inhibits the formation of polyphenol oxidases in apples and potatoes. Bentonite, a protein-absorbent clay, reduces the activity of enzymes as well. Gelatin, activated charcoal, and polyvinyl pyrrolidone can also be used also extract soluble phenols from wines and beers, but they modify the properties of these beverages.
Sulfites are used in the food processing industry to prevent darkening because they bond with quinones and form colorless sulfoquinones. Sulfur dioxide and sodium metabisulfite are commonly used by wine producers, for example, but they too have secondary effects that worry health authorities (in addition to migraine headaches caused by excessively sulfited wines, asthma
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attacks and outbreaks of hives, nausea, even anaphylactic shock have been reported). Research into other equally effective but less harmful inhibitors therefore is needed.
Other inhibitors have been discovered or synthesized whose safety remains to be proved. One that is now being studied is cysteine—an amino acid containing sulfur—and its derivatives, as well as natural compounds (found in honey, figs, and pineapple) and synthetic ones.
Jacques Nicolas, a researcher at the Conservatoire National des Arts et Métiers in Paris, is exploring the use of cyclodextrins. Working with researchers at the Institut National de la Recherche Agronomique station at Montfavet, Nicholas first analyzed the antidarkening effect of these molecules in trial solutions containing one or two phenols and some polyphenol oxidases from the Red Delicious apple. For the time being, until commercial applications of these results are developed, if you want to make apple juice without resorting to inert atmospheres you will need to clarify the juice. Polyphenol oxidases in the cellular chloroplast of apples form solid fragments that darken, aggregate, and fall to the bottom of the liquid as sediment. So let the juice rest awhile, and then decant the clear amber liquid.
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