For decades, milk with an unexpected blueberry-like off-flavour has puzzled both dairies and farmers in Sweden. Now a research project at Swedish University of Agricultural Sciences (SLU) has identified the explanation behind the phenomenon. The project, which ran from 2019 to 2024 and was partly funded by Kampradstiftelsen, investigated the cause of the flavour deviation in collaboration with dairies, the advisory organisation Växa Sverige and farmers across Sweden. The unusual taste has often been described as blueberry-like and has in some cases led dairies to reject milk deliveries from individual farms. “A small flavour deviation could spoil large volumes of milk at dairies. Many investigations had previously been carried out, but no one had been able to explain the cause. That was the reason we decided to tackle the issue,” says Mårten Hetta, researcher at SLU, in a press release. The breakthrough came when researchers stopped focusing solely on a “blueberry flavour”. In international literature the problem was instead described as “fruity”, which provided new clues. Experiments with both feed and milk quickly revealed a pattern: when cows had consumed feed with a high sugar content, the taste of the milk could change. The explanation lies in a chemical process. High sugar concentrations in feed can be converted into ethanol. When ethanol reacts with butyric acid in the milk, esters are formed – including ethyl butanoate – which produce a fruity aroma. Humans can detect this compound even at extremely low concentrations. The researchers also found that the high sugar levels in feed often occur when crops are harvested very early in the spring, when their nutritional value is also at its peak. According to the study, the problem affects only a small number of farms each year. According to the researchers, the issue can often be resolved by replacing the silage responsible for the flavour deviation. The use of additives that inhibit yeast growth in feed may also allow farmers to continue harvesting early without causing flavour problems. The research team will now investigate why not all cows in a herd are affected, even when they have eaten the same feed.