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"It has long been known that adding hops can control the growth of bacteria in beer," said Matt McCarroll, professor of chemistry and biochemistry and director of the Fermentation Science Institute at Southern Illinois University. Although these antibacterial properties are usually not required for modern brewing, the popularity of mixed-fermented beer has reawakened interest in this little-known property of hops.

In 2015, McCarroll started a study at the Fermentation Science Institute of his university to study the effects of hops on bacteria such as Lactobacillus and Pediococcus in a mixed fermentation environment. Using in-house cultures from Scratch Brewing in Illinois, Fonta Flora Brewery in North Carolina, and Jester King Brewery in Texas, McCarroll brewed the same batch of wort from each culture and added different levels of hops. To understand how fermentation is affected. He brewed another batch with commercial brewer's yeast as a control.

For some time, the institute has been working informally with the nearby Scratch Brewing, which inspired the idea of ​​conducting this research.

"One of the factors they use to control fermentation is the level of hops added," McCarroll explained. "The interesting thing about Scratch is that they use hops and their family culture to adjust the type of beer they brew. This led to the idea of ​​a controlled study."

Therefore, McCarroll divided each individual wort in the study into three batches for a total of twelve batches (including controls), and repeated verifications. The first one didn't jump at all, the second jumped to 12 IBUs with Styrian Golding pellets, and the third jumped to 25 IBUs.

"In all cases, beer with zero IBU will become sour in all cases," McCarroll explained, which proves the presence of acid bacteria in all three indoor cultures. But in some hop batches, the bacteria were significantly inhibited.

These findings are not surprising to Marika Josephson, the brewer and co-owner of Scratch, who fermented most of the beer with homemade yeast originally developed for pizza and bread. The culture mainly contains yeast and Lactobacillus. Scratch has long known that the acidity of the bacteria in the culture can be partially controlled by hops.

"Once our culture became healthy, we started experimenting with sour and non-sour beers," Josephson explained. "At that time we started experimenting to see what happens when we add hops to see if it helps or limits bacteria. Hops seems to play a very important role in whether we get acid fermentation. Only 5 IBUs Helped [control] our bacterial fermentation."

McCarroll looked at the different samples in his study and said that the most consistent change he saw was the attenuation level, which refers to the sugar content of each wort that was successfully fermented into alcohol. "We drank a beer with an alcohol concentration ranging from 4.94% to 5.87%, from low to high IBU, and the control had a degree of fermentation within 10%." This finding indicates that the lactic acid-producing bacteria fermented the unhopped hops. Some sugar in the group, but in the hop example where these bacteria cannot reproduce, more sugar is metabolized by yeast into ethanol.

Professor Christopher Curtin of Oregon State University said that his laboratory focuses on the role of microorganisms in the alcoholic beverage industry, and hops can limit the growth of bacteria for specific reasons. "When hops are extracted into beer, iso-alpha acids enter bacterial cells and destroy cell functions." Hops acid prevents the absorption of nutrients in bacterial cells, causing the cells to starve and eventually die.

However, not all bacteria will be affected in this way. McCarroll's research also proves this. Some samples do not seem to be affected by the addition of hops.

"We just looked at the pH and saw a really significant difference in fermentation," McCarroll said. "They behave differently. Some cultures have a [medium acid] pH under all IBU conditions, and some cultures are only acidic under zero IBU conditions."

Why are certain types of bacteria suppressed and other types of bacteria not?

"Bacteria that don't respond to hops have genes that allow them to pump out these alpha acids," Curtin explains. In addition, cultures containing these hop-resistant strains can spread this immunity through plasmid transfer, a process by which bacteria share genetic information. However, this gene exchange appears to be limited. "The hop resistance gene in lactic acid bacteria has only been shown to be transferable to other lactic acid bacteria," Curtin added. But in theory, over time, this may make the entire culture resistant to hops and have a profound impact on breweries that harvest and reproduce mixed cultures batch by batch.

"If you start with a culture that contains 90% of hop intolerant bacteria, you will eventually change this ratio," Curtin explains. "If they do have copies of these resistance genes and they have been exposed to hops, they can be mild to very tolerant. You can start with a culture that contains lactic acid bacteria that are not very resistant to hops, and eventually Obtain a culture that is mainly resistant to hop bacteria."

At Fonta Flora Brewery, co-founder and brewer Todd Boera has seen his family culture change over time to reflect more lactic acid characteristics, even though the hop experiment aims to curb this trend. The brewery uses two different mixed strains to make the sour series. Affectionately named OG and Dandy, both strains have changed over time due to the microbial community introduced by the brewery using forage plant ingredients after boiling. "This kind of culture requires about ten generations of brewing to produce more acidity," Boeira said. "It may have been evolving."

"Dandy was originally made as a clean Saison mixed with some Brettanomyces drei," he explained. "After we [added] the local dandelion flower, this culture began to change. My experience with Dandy and OG so far is that they have not been particularly affected by the increased IBU. We did notice that with With the increase of hops, the acidity will decrease slightly, but we will never be able to make pure tasting mixed culture beer just by increasing IBU."

McCarroll's research includes genetic analysis of all three cultures, but hop resistance is not necessarily at a clear boundary between bacterial species, so it is difficult to predict the outcome based on the composition of a given culture. (For various reasons, the beer in this experiment has never been poured and has been in the barrel for five years. McCarroll hopes to complete their new sensory and laboratory analysis as soon as possible to update the research results.)

"I suggest that each brewery play with their culture on a small scale under different conditions, just to see what they do," McCarroll suggested. He found all the possibilities very tempting. "It’s a bit sexy to be able to brew two or three distinct beers from the same culture."

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