Cost-effective method for manufacturing yoghurt

With global consumption of yoghurt estimated to be nearly 12 kg per person annually, researchers in Denmark have now developed a simple method that requires no new technology and could transform how the food industry manufacturers dairy-based yoghurt. 

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

Now researchers have developed a simple yet powerful method that has the potential to reduce the use of expensive bacterial cultures by up to 80%, while also extending shelf life. The new yoghurt manufacturing method was developed after a straightforward question was posed by the research team at the DTU National Food Institute — what if we let the bacteria acidify without allowing them to grow?

“It was something of a eureka moment,” said Associate Professor Christian Solem, who has researched lactic acid bacteria for more than 25 years and is one of the authors of the article about the new yoghurt production method, which is published in the scientific journal Food Bioscience. 

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.

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,” Solem said.

The new two-step fermentation technique is described by Solem as “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,” Solem said.

The 51°C step also acts almost like a mild pasteurisation phase.

“We eliminate up to 99.9% of yeast cells and mould spores, which would otherwise significantly shorten the yoghurt’s shelf life,” Solem said.

Goodbye post-acidification, hello extended shelf life

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

Associate Professor Christian Solem with a test tube with yoghurt in the laboratory at the DTU National Food Institute. Image credit: Lene Koss.

“We’ve tested the method on three different starter cultures, including some particularly prone to post-acidification. In all cases, the issue was resolved,” said postdoctoral researcher Shuangqing Zhao, corresponding author on 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. 

At present, dairies use up to 0.18 g of starter culture per litre 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,” Solem said.

The extended shelf life also improves planning and logistics: “Yoghurt 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,” Solem said.

Top image credit: iStock.com/Cylonphoto