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A new method for producing yogurt has been developed by a research team at the DTU National Food Institute, and it all began with a straightforward question.

"We've studied lactic acid bacteria and their energy metabolism for years, but one day it struck us: what if we let the bacteria acidify without allowing them to grow? It was something of a eureka moment," says Associate Professor Christian Solem, who has researched lactic acid bacteria for more than 25 years.

The method has been tested on a smaller scale in the laboratory at the DTU National Food Institute, and the researchers see no reason why it cannot be immediately implemented by dairies. It requires no new technology—only an adjustment of the temperature control during production.

"In principle, dairies could adopt the method as early as tomorrow. We have not observed any drawbacks, apart from the process taking an hour or two longer—and you will have a more stable and sustainable product," says Solem.

The research holds great potential, as it is estimated that, on average, each person globally consumes nearly 12 kilograms of yogurt annually. The research is in the journal Food Bioscience.

A two-step fermentation technique

Traditional yogurt production involves adding a relatively large amount of yogurt starter culture to milk, which is then fermented at 42°C for 4–6 hours. The desired acidity is then reached, and the yogurt is cooled. However, the process is known to present several challenges: high costs for starter culture, limited shelf life, and so-called "post-acidification," where the yogurt continues to acidify during storage, affecting both quality and longevity.

With the new method, researchers use just 20% of the usual amount of starter culture. The milk is first fermented at 42°C, as per standard procedure, and then the temperature is raised to 51°C for a few hours. At this elevated temperature, the bacteria cease to divide, but continue to produce lactic acid.

"It's like putting the bacteria on a treadmill—they're not going anywhere, but they're still working. This allows us to control the acidification while avoiding undesirable post-acidification," explains Solem.

The 51°C step also acts almost as a mild pasteurization phase. "We eliminate up to 99.9% of yeast cells and mold spores, which would otherwise significantly shorten the yogurt's shelf life," says Solem.

Goodbye to post-acidification—hello to extended shelf life

Post-acidification is among the most serious quality concerns in yogurt production. It causes the product to become overly sour, bitter, and prone to phase separation—characteristics often associated with spoiled yogurt.

"We've tested the method on three different starter cultures, including some particularly prone to post-acidification. In all cases, the issue was resolved," says Postdoctoral Researcher Shuangqing Zhao, corresponding author of the study.

The exact extension of shelf life has not yet been determined, but according to the researchers, it is expected to far exceed the current 3–4 weeks. This could have significant implications for both long-distance transport and food waste reduction.

Substantial benefits for dairies

At present, dairies use up to 0.18 grams of starter culture per liter of milk. Given the scale of production, the cost of bacterial cultures can be considerable.

"We reduce the use of starter culture by a factor of five. That's a substantial saving, and it matters in an industry where profit margins are tight," says Solem.

Moreover, the extended shelf life improves planning and logistics:

"Yogurt is typically produced in large batches, and each production run requires cleaning and reconfiguration. With longer shelf life, larger and perhaps even more continuous production becomes feasible," he adds.