Water, which is most of it
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The largest ingredient in almost every drink, the one nobody specifies, and the one that makes a product taste different in a different town.
Ninety percent or more
A soft drink, a tea, a kombucha and most juices are overwhelmingly water.
Which means water quality is a formulation variable rather than a utility, and it means the shipping economics are mostly about moving water.
What is in tap water
Chlorine or chloramine, which is there deliberately to suppress microbial growth, and which will do the same to a live culture. A kombucha made with untreated tap water can ferment sluggishly or stall for exactly this reason.
Hardness, which affects extraction from tea and coffee markedly, and mouthfeel.
Minerals and off-flavors, which vary by supply and by season.
Metal ions, which catalyze oxidation and shorten shelf life.
All four vary geographically, which is why the same recipe makes a different drink in two towns.
Treating it
Carbon filtration removes chlorine and most taste and odor compounds, and is the minimum for anything fermented or tea-based.
Reverse osmosis or deionization gives a blank slate, which is reproducible and can taste flat, so some producers build minerals back in to a specification, which is the professional answer.
Whatever you use, put it on the formula sheet. "Water" is not a specification.
And keep treated water clean: a storage tank of filtered water is a microbial culture if it stands.
Water sold as water is a different regime
Worth knowing before anybody bottles a flavored water.
Bottled water has its own standard of identity and quality under 21 CFR 165.110, with defined terms and quality requirements that do not apply to other beverages.
Which means a product presented as water is regulated differently from a product presented as a drink, and what you call it decides which.