"Warren C. Liebold" wrote
> Second, air and water cause chemical changes in the tea, not storage which
> restricts exposure to air. Once tea, or many other materials, is steeping
> in hot water, substances are diffusing from the leaves at varying rates,
and
> those substances change over relatively short periods of time.
Concentration
> allows some of these substances to dissipate, increases the chemical
> reactions among others and adds a preservative or two to inhibit spoilage.


An important question is: How did they get all that water out in the first
place? There are only a few practical ways to do this. The main group all
involve evaporation - from boiling to wiped-film stills to freeze-drying and
spray-drying. Some of them involve heat, which can cause chemical reactions.
All involve removal of volatiles in addition to water, which means many of
the delicate aroma notes. Lower pressure permits lower evaporation
temperature, which avoids cooking but at the expense of less discrimination
between water and desirable compounds. Instant coffee producers have been
fighting this battle for years, and losing. They even pump recovered
volatiles back into the can or jar so there's at least one burst of
almost-real aroma!

There are other ways to do it. Freezing out the water (the way apple jack is
made in th winter from hard cider) would be best, but no-one does that
commercially AFAIK. Ultrafiltration or reverse osmosis would also likely be
better for tea than evaporation, but again they are more expensive.

So it's probably the concentration step itself that does the most damage.

-DM