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Food & Drink

The Soy Milk Challenge

Love your morning coffee but struggling to make that perfect cup with soy milk? Dr. Karen Johnston is a lecturer in Chemical and Process Engineering at the University of Strathclyde and has been vegan for almost 5 years. Here, she discusses some of her research into the soy milk-coffee curdling process, in addition to her ‘Soy Milk Challenge’ science outreach activity, which she launched following the success of the laboratory work. Most importantly, based on her research findings, Karen shares some valuable tips on how best to avoid curdling when getting your daily dose of caffeine!


Most vegans are probably familiar with the challenge of mixing soy milk into coffee – it often results in curdling. This doesn’t always take place, but when it does it is not very appetising! So, how do we stop it? The first step towards understanding how and why the curdling takes place is to find out when it happens and when it doesn’t. The curdling depends on many factors, some of which include: the brand of soy milk, the coffee type, the ratio of coffee to soy milk, and the temperature of the mixture.

I work in the Chemical and Process Engineering department at the University of Strathclyde and therefore have access to lab equipment, enabling me to conduct research into the soy milk-coffee curdling process. The research project began during the summer of 2016, when my colleague Jan Sefcik and a summer student began heating soy milk and coffee mixtures in the lab, and using a webcam to take photos at 5 second intervals. This allowed us to observe whether or not curdling took place, in addition to how the curdling progressed with time (see sequence of images taken at 30 second intervals). For each experiment carried out, we recorded the concentration of soy milk, the temperature of the mixture, and whether or not curdling took place, allowing us to assess the impact of these factors.

What we found from our experiments was that the soy milk-coffee mixture tended to curdle when soy milk was present in lower concentrations, and also when the mixture was at higher temperatures. More surprisingly, we found that the curdling process is reversible, meaning you can get rid of the curdling by either cooling the mixture or, if you prefer hot coffee, adding more soy milk (and then stirring)!

By analysing the curdling with time, together with another colleague, Mark Haw, we were able to explain the basic curdling mechanism of the soy milk in coffee. Soy milk is a suspension of protein and fat particles, and under curdling conditions these particles start to aggregate (as can be seen after the first image). When these aggregates get big enough, they sediment, which can be clearly seen in the last three images.

After the success of the lab experiments, I developed an outreach activity for our Really Small Science team where members of the public can do the experiments themselves and create a phase diagram (see photos below). This is a ‘map’ that tells us when curdling does and does not take place, which requires making a lot of cups of coffee under different conditions – a great chance to enrol the public in some ‘crowdsourced science’! We have already presented the Soy Milk Challenge three times. The first was to celebrate Earth Day in Glasgow Science Centre, the second time was again in the Science Centre for a Science Late on Taste, and in September we presented it in the Riverside Museum as part of Explorathon European Researchers’ Night. The activity also highlights the lower carbon and water footprints of soy milk compared to those of dairy milk, and also the food engineering involved in soy milk production.

So, based on our research, all you need to do the next time your soy milk coffee curdles is simply add more soy milk, stir, and enjoy!

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The phase diagram, showing the ‘map’ that tells us when curdling happens.