La fermentación en bloque empieza después del amasado.

Because fermentación is a feminine singular noun, so its article is la.

Spanish also uses the definite article more often than English. In a sentence like this, la fermentación refers to the process as a known step, so using la sounds natural.

• la fermentación
• el amasado

The article has to match the noun’s gender and number.

Nouns ending in -ción are almost always feminine in Spanish.

So:

• la fermentación
• la conversación
• la preparación

That is why you get la fermentación, not el fermentación.

Because empieza is a conjugated verb, while empezar is the infinitive.

The subject is La fermentación en bloque, which is third person singular, so the verb must also be third person singular:

• empieza = it starts / begins

So the structure is:

• La fermentación en bloque = subject
• empieza = verb
• después del amasado = time phrase

Because empezar is a stem-changing verb. In the present tense, the e in the stem changes to ie in most forms.

So:

• yo empiezo
• tú empiezas
• él/ella empieza
• nosotros empezamos
• vosotros empezáis
• ellos empiezan

In your sentence, the subject is singular, so you need empieza.

En bloque is a phrase that modifies fermentación. In baking, it refers to bulk fermentation.

Grammatically, it is a fixed expression, so Spanish normally says fermentación en bloque, not fermentación en el bloque.

This is similar to how Spanish often uses en in set phrases without an article.

Because de + el contracts to del.

So:

• de + el amasado → del amasado

This contraction is required in standard Spanish.

Compare:

• después del amasado
• antes del horneado

But note that de + la does not contract:

• después de la fermentación

Both are possible, but they mean the idea in slightly different ways.

• después del amasado = after the kneading
  Here amasado is a noun.
• después de amasar = after kneading
  Here amasar is an infinitive verb.

Your sentence uses the noun phrase el amasado, which sounds very natural in technical or recipe language.

Here it is a noun.

Although amasado can also be a past participle in other contexts, in this sentence el amasado means the kneading.

So:

• amasar = to knead
• el amasado = the kneading

Spanish often turns actions into nouns this way, especially in cooking, baking, and technical language.

Yes. Comienza and empieza both mean begins/starts here.

So these are both natural:

• La fermentación en bloque empieza después del amasado.
• La fermentación en bloque comienza después del amasado.

Empezar is often slightly more everyday, while comenzar can sound a bit more formal, but in this context both are fine.

In standard Spain Spanish, z is pronounced like the th in thin.

So empieza is approximately:

• em-PYE-tha

Very roughly:

• em as in English them without the th
• pie as in pye
• za with a Spain Spanish th sound

Also, the stress falls on pie: em-PIE-za.

The accent marks show the correct stressed syllable.

• fermentación → stress on the final -ción
• después → stress on the final syllable

These accents are required by Spanish spelling rules.

They are important because they tell you how to pronounce the word correctly, and sometimes they also distinguish words that would otherwise look similar.