Mezclamos la masa madre con la harina de fuerza.

It can mean either, because mezclamos is the same form for:

• present indicative: we mix
• preterite: we mixed

In this sentence, the context usually makes it clear. In a recipe, it normally means we mix.

Spanish recipes often use the first person plural like this as an instruction style:

• Mezclamos... = We mix...
• Añadimos... = We add...
• Horneamos... = We bake...

In English, this often feels like an instruction, even though it literally says we.

Spanish recipes commonly use several styles for instructions, and first person plural is one of the most common:

• Mezclamos la masa madre... = instructional we mix
• Mezcla la masa madre... = singular informal command, mix...
• Mezclen la masa madre... = plural/formal command
• Mezclar la masa madre... = infinitive style, like mix...

The nosotros style sounds natural and standard in recipes and cooking instructions. It is not necessarily saying that a group is literally doing it together; it is just a conventional recipe style.

Literally, yes:

• masa = dough
• madre = mother

But as a fixed expression, masa madre means sourdough starter.

So you should learn it as one unit of meaning, not translate it word by word every time. In baking Spanish, masa madre is the normal term for the fermented starter used to make bread.

The article la agrees with masa, which is a feminine noun:

• la masa

In masa madre, the main noun is masa, and madre is part of the expression describing what kind of dough it is. So the whole phrase stays feminine singular:

• la masa madre
• una masa madre
• esta masa madre

Harina de fuerza is a baking term meaning strong flour, often close to bread flour in English.

It refers to flour with a higher protein/gluten content, which helps give bread structure.

Breakdown:

• harina = flour
• de fuerza = literally of strength

But idiomatically, you should understand harina de fuerza as a set phrase meaning strong flour / bread flour.

Because harina de fuerza is the standard fixed expression in Spanish baking vocabulary.

Even though fuerte means strong, Spanish does not normally say harina fuerte for this baking meaning. Instead, it uses the noun phrase:

• harina de fuerza

This is very common in Spanish: sometimes English uses an adjective, but Spanish prefers a noun + de + noun structure.

Spanish uses the definite article more often than English does.

So:

• la masa madre
• la harina de fuerza

In English, depending on context, you might say:

• Mix sourdough starter with strong flour or
• Mix the sourdough starter with the strong flour

In Spanish, using la here sounds natural because the recipe is referring to specific ingredients already being used in the process.

Con means with.

The structure is:

• mezclar X con Y = to mix X with Y

So:

• Mezclamos la masa madre con la harina de fuerza = We mix the sourdough starter with the strong flour

This is the normal verb pattern with mezclar.

Yes, absolutely.

Both are grammatically correct:

• Mezclamos la masa madre con la harina de fuerza
• Mezclamos la harina de fuerza con la masa madre

The meaning is basically the same: you are combining the two ingredients. Sometimes the first item mentioned is simply the one the writer wants to focus on first.

No. Spanish usually drops subject pronouns when they are not needed.

So:

• Mezclamos la masa madre...

already tells you the subject is we from the verb ending -amos.

You could say nosotros mezclamos if you want emphasis or contrast, but in a recipe that would normally sound unnecessary.

In standard Spain Spanish, it is pronounced roughly like:

• meth-KLA-mos

More exactly:

• the z in mezclamos is pronounced like the th in thin in most of Spain
• the stress falls on kla: mez-CLA-mos

So in Spain:

• mezclamos ≈ meth-KLA-mos

In many parts of Latin America, the z would sound more like an s.

Yes, functionally it describes the type of flour, but grammatically it is not an adjective. It is a prepositional phrase:

• de = of
• fuerza = strength

So instead of using an adjective, Spanish uses:

• harina de fuerza

This is a very common pattern in Spanish vocabulary:

• zumo de naranja = orange juice
• cuchillo de cocina = kitchen knife
• harina de fuerza = strong flour / bread flour

Yes. It sounds very natural in Spain, especially in cooking or baking instructions.

Two things make it especially typical:

• the recipe-style mezclamos
• the baking term harina de fuerza

Also, in Spain the pronunciation of z in mezclamos is the distinctive th sound.