soy milk curdling coffee

A barista’s guide to dairy alternatives: how to stop your soy milk curdling in coffee

Vitasoy Café Ambassador and 2019 Australian Barista Champion Matthew Lewin on the legacy of soy milk, where it stands in the market now, and what to do if your soy milk curdles in coffee.

Soy was the most requested plant-milk for more than a decade, only to be overtaken by almond in the last couple of years. It’s hard to imagine what cafés would look like today without the pivotal role soy milk has played in introducing people into the world of plant-milk based coffees.

I’ve talked before about how oat highlights fruity notes while almond pairs beautifully with chocolatey coffees, but for me, soy milk isn’t about flavour: it’s about the experience. The more I drink soy milk, the more I come to appreciate that its strongest suit is its texture. When soy milk is done really well it becomes quite a decadent mouthfeel.

This suits a particular style of coffee that’s darker in nature, maybe roasted for a little bit longer with dark chocolate notes and lots of body. You put those two together, you get this bold-textured, luscious style milk coffee that can speak to the everyday coffee drinkers who are used to those stronger flavours.

Matthew Lewin is the 2019 Australian Barista Champion.
Matthew Lewin is the 2019 Australian Barista Champion.

I often compliment oat milk for its neutrality, but many soy drinkers stick with it because they’ve gotten used to the soft soy notes and luscious experience. It’s something they still look for when they order their coffees today and Vitasoy has fantastic recognition with consumers, having been making soy milk since 1940 and launched Café for Barista Soy Milk in 2008.

People are also looking more for locally made products and companies like Vitasoy champion that ‘support local’ mentality, sourcing 100 per cent Australian grown soy beans for their Café for Baristas Soy Milk. While soy milk’s longevity makes it popular with general consumers, particularly older customers, it brings with it some preconceived notions.

Café’s use of soy milk goes back further than not only almond and oat, but the very idea of plant milks being crafted to work with coffee. This means a lot of those who have been in coffee for a long time have memories of soy milk being tricky to work with, curdling in coffee, and many failed soy lattes going down the drain. Almond and oat don’t have the same reputation because they blossomed in a time where plant milk suppliers saw the need to develop specific recipes to be paired with coffee. Soy milk now has to compete in a more diverse marketplace, and companies like Vitasoy have made sure the quality of their café-oriented soy milks is in the same league as their almonds and oats.

That being said, soy milk is still its own product with unique needs, and there are definitely steps a barista can take to bring the best out of it.

PRO TIP: Steam the soy milk to a lower temperature – about 55°C to 60°C – and introduce less air at the beginning. Soy milk tends to stratify if left to sit, so begin pouring as soon as you finish texturing.

Part of why soy milk requires a slightly different approach is the type of proteins it contains compared to dairy milk, which are impacted by the heat during steaming and acidity of the coffee. Oat and almond milks contain low levels of protein, hence the use of other ingredients to create the foam emulsion. 

There is a large pH difference between plant-based milks – soy ranges around seven to eight – and coffee, which differs from shot to shot and bean to bean usually is about 4.5. The proteins in soy milk are sensitive to the lower pH of the coffee shot which is why the proteins can ‘denature’ and look curdled. So how can we overcome this?

PRO TIP: Don’t be afraid to work the milk and espresso together. I like to start with a small pour, only about 10 millilitres, and give it a swirl to acclimate the espresso to the higher pH before I pour the rest of the soy milk into the cup.

This PRO TIP is a short answer to a big question, and I’ve done my own research into how to best marry soy milk and espresso to avoid curdling.

In the training room at Ona Coffee Melbourne, I trialled many different espresso extractions with soy milks textured to different levels and temperatures to find a recipe with the most consistent structure and highest quality. I used the same coffee blend, roasted for milk with a traditional flavour profile, for each experiment and started with a standard 20 grams in, 40 millimetres out espresso extraction. 

I steamed the soy milk to different temperatures, starting high at 65°C, working my way down, and found there were challenges when combining hot soy milk with coffee (which wasn’t really a surprise). At the same time, the customer doesn’t want a lukewarm coffee, so I found 55°C to be a good cross point between what will work for the customer and the barista.

I also thought about how we could increase the pH level of the espresso so there was less of a ‘shock’ when we introduced the soy milk. The longer you extract coffee, the more acids you will bring out, but at the same time, you’ll extract more of everything else and end up with a diluted shot with less pH overall. So, I tried introducing small amounts of different liquids – only five millilitres – to the espresso before pouring the textured soy milk.

First was cold tap water, which has a pH level around 7.5. The soy milk blended well with the coffee, and when we tasted it, the coffee was OK, but it was also a tad diluted and a little cool. It was promising though, so we tried hot water from the espresso machine and again it worked quite well. It was warmer than the first one, but was still a little bit weak.

To similarly raise the pH before adding the textured soy, I thought it might be worth trying to add a dash of soy milk to the espresso before pouring. As mentioned earlier in my PRO TIP, I already start with a small dollop of textured soy, but what if we used even more stable, untextured cold soy milk?

After extracting my shots, I added five millilitres of cold soy milk – or exactly two lid-fulls using Vitasoy Café for Baristas soy milk cartons – and the results were phenomenal. It was still a little weak, so rather than a ‘20 in, 40 out’ balanced shot, I made it stronger with 20 grams in and 30 millimetres out, like you’d do for a ristretto or a magic. I added five millilitres of cold soy to each shot then quickly added the steamed soy at 55°C and the result was a delicious coffee.

I tried this recipe a few times and it kept yielding an incredible tasting coffee that looked amazing in a latte glass or flat white cup with no curdling. If you put them up on a coffee bar, the server wouldn’t be able to tell the difference between them and a dairy milk coffee.

This exact recipe might need a bit more testing before I recommend doing it on the bar yourself, but it shows there’s ways of tailoring your coffee to your plant-based milk, rather than only the other way around. Oat and almond may be on-trend in the market, but soy will undoubtedly stick around in cafés for a long time. It is our job as baristas to make the best use of soy milk to provide our customers with the greatest coffee experience possible. 

For more information, visit vitasoycafe.com.au, or contact your local distributor or the Bega Dairy and Drinks Customer Service team on 1800 000 570.

This article appeared in the October 2021 edition of BeanScene. Subscribe HERE.

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