Wayne Turner, of Dunedin, asks :-

Why does rice, almond, and soy milk have a tendency to curdle in hot drinks whereas dairy milk doesn't?

Matt Golding, a food colloidist at Massey University's School of Food & Nutrition, responded.

Curdling tends to be particularly problematic when added to coffee - and this provides an indication as to the cause of the issue.

The pH of black coffee is about 5, which means that coffee is slightly acidic. At neutral conditions, the proteins in these milks have a negative charge, which provides a repulsive force between protein molecules and helps keep them stable in the milk - even through high temperature treatments such as UHT. However, introduction into coffee which is more acidic, starts to screen out this repulsive negative charge allowing the proteins to stick together, resulting in curdling. High temperatures can accelerate this problem, which is why it tends to be more pronounced in hot drinks.

However, this doesn't necessarily explain why we don't see this effect for cow's milk, which can also coagulate under acidic conditions (which is where we get yogurt from). To answer this, there may be some additional subtle effects taking place that can contribute to the curdling of milks such as soy.

One consideration is the differences in buffering between different types of milk. Buffering solutions are those that can resist changes to pH as a consequence of acid (or alkali) addition. Proteins are generally quite good at buffering, so if we consider that the proteins in cow's milk are more effective buffers than the plant proteins then, when added to coffee, the cow's milk may not decrease in pH as much for soy or rice proteins, which will help to keep the cow's milk proteins stable. When considering it from this approach, it can be helpful to add coffee to milk, rather than the other way round, as the change in pH is less pronounced when adding coffee to milk.

Another consideration is that some plant based milks (soy milks in particular) are very sensitive to the presence of calcium, which can also cause the protein molecules to stick together. This is essentially the basis for making tofu. For hot beverages prepared with hard water, i.e. water with a high mineral content, the calcium in the water may contribute to the curdling of the milk by causing the proteins to stick together. Cow's milk is a bit less sensitive to the presence of calcium under these conditions, and less likely to coagulate as a result.

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