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Every recipe I have found for making traditional Spanish churros or porras calls for adding the flour to boiling or near-boiling water (or, less commonly, milk).

These are very simple pastries, usually there are only three ingredients: flour added to boiling salted water, then piped into hot oil and fried until golden.

When I tried using room temperature water, the churros ended up heavy and greasy.

Edit: Some answers have maintained that this dough is a Choux pastry. Choux pastry is also made with boiling water or milk, however, it is always made with eggs and butter, and sometimes milk. This seems to be a huge difference, especially when talking about so few ingredients, so I find it dubious that this is even "a variant" of Choux.

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    I found this video helpful for visually comparing properties of dough with boiling water (compared to room temperature and yeasted dough): youtube.com/watch?v=Vj9lM6iBo1U
    – The Hagen
    Dec 21, 2021 at 20:31

3 Answers 3

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Churro dough is a variant of choux pastry. While yeasted doughs rise through the expansion of gas created by the yeast, and "quick bread" batters rise through gas created by chemical leavening agents, choux pastry rises simply by pockets of water boiling into steam.

For that to work properly, the dough needs to have a high hydration yet also be cohesive enough to trap the steam. That's achieved by adding boiling water to the flour, causing some of the starch to gel.

If room temperature water is used, gelation does not occur. For a given quantity of water/flour, the resultant mixture is less cohesive, and it does not expand as effectively during cooking (the steam boiling out rather than being trapped within).

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Dough that is created using boiling water is known as choux pastry.

There is no special reason why churros are made using choux pastry rather than other types of dough. It is simply that the definition of churros is that they are deep-fried long pieces of simple choux. You could ask just as well why stuffed peppers are made with peppers - you could make them with other vegetables too, you simply wouldn't call the result "stuffed peppers".

There is no general rule that only choux pastry can be deep-fried successfully. There are many other examples of deep-fried dishes made with other types of dough, such as donuts or mekizas.

Your observation that a mixture of room temperature water and flour didn't make good churros can have different reasons. One is that not every random combination of flour and liquid creates a good, workable, tasty dough. Humanity has spent a lot of time in creating optimal doughs, and when you experiment randomly, your result will almost certainly be worse than if you follow a well-known process. Another one is that different doughs require a suitable preparation technique. Even if your water-flour mixture had the potential to produce something tasty, apparently a frying process which works well for choux pastry doesn't work for that type of dough and would have to be adjusted.

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If you start frying with a hot dough, cooking will start far more quickly than if you start cold. In particular the outside (crust) will cook almost instantly when it hits the oil, so will absorb less oil, and the quicker cooking will mean less time to absorb oil.

All the recipes I've seen have baking powder added as well, and from having eaten them I'd say that's essential - you need the gas bubbles it makes

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    According to this recipe (pequerecetas.com/receta/como-hacer-churros), porras use baking powder, while churros do not, although I have also seen recipes with baking powder for churros. But considering that the dough is left to sit 10 minutes before frying, I do not think that the reason to use boiling water is to fry the dough while hot. It would be more efficient to heat the dough after mixing if that were the case.
    – kevins
    Dec 21, 2021 at 11:52
  • @kevins it's still warm, even if not hot, but also at the start of the rest it's 83°C (calculated from the masses at your link and heat capacities, assuming the flour is at 20°C). That's already hot enough to start cooking the dough, especially gluten is affected by such temperatures
    – Chris H
    Dec 21, 2021 at 12:35

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