Apparently, the beverage I had just ingested was curado, or cured pulque, in this case mixed with oatmeal, which explained the slight sweetness.
BBC: Mexico??s ancient drink makes a comeback
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Revueltas, the tall, pony-tailed owner, makes a mean peanut curado.
BBC: The return of Mexico??s national nectar
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Pulque curado, which is mixed with any number of flavours but usually fruit, became popular in the early 19th Century, perhaps as a way to make the beverage palatable to more people.