From the rooftop patio of his growing brewery in the southwestern elbow of Brooklyn, Shane C. Welch, the founder and “chief visionary” of Sixpoint Brewery, appeared to have staked a nice corner of an intensely competitive beer market.

But the constant worry of being crowded out by the big brands, like Anheuser-Busch InBev, the world’s largest brewer, intensified this month when it was revealed that ZX Ventures, an incubator operated by A.B.I., had purchased a minority stake in a popular beer review website, RateBeer.com.

For smaller breweries like Sixpoint, RateBeer had become an invaluable barometer for the marketability of their products, reaching a global audience of like-minded enthusiasts. There they could promote new items or events, follow trends and interact with customers in forums.

Now, Mr. Welch worries that the playing field has shifted.

“It purports to be an objective, consumer-driven site,” Mr. Welch said. “But there is a massive conflict of interest.”