Опара перекисла, оттого что закваска была слишком активной.

They are related, but not the same thing.

• Опара is a preferment or sponge: a mixture of flour and liquid that is left to ferment before making the final dough.
• Закваска is the starter culture itself, often a sourdough starter in baking contexts.

So here, the опара is what went wrong, and the закваска is the reason why.

Перекисла comes from the idea of becoming sour.

• киснуть = to turn sour
• prefix пере- often adds the idea of too much, over-, or excessively

So перекисла means something like:

• became too sour
• over-soured
• over-fermented

In a bread-making context, it suggests that the preferment fermented too far and became too acidic.

In the past tense, Russian verbs agree with the subject in gender and number.

The subject here is опара, and опара is a feminine singular noun.
So the verb must also be feminine singular:

• masculine: перекис
• feminine: перекисла
• neuter: перекисло
• plural: перекисли

That is why the sentence has опара перекисла.

Because after an expressed form of быть in the past or future, Russian often uses the instrumental case for a predicate adjective or noun.

So:

• закваска была слишком активной = the starter was too active

This is the standard pattern learners should use:

• он был уставшим
• она была довольной
• закваска была активной

You may sometimes hear nominative forms like была активная in colloquial speech, but была активной is the safer and more standard choice here.

Because it agrees with закваска in:

• gender: feminine
• number: singular
• case: instrumental

The dictionary form is активный.
Its feminine instrumental singular form is активной.

So the ending is there because of grammatical agreement, not because the word has a different basic meaning.

Оттого что means because.

It is very close to потому что, and in many sentences they can be swapped:

• Опара перекисла, потому что закваска была слишком активной.
• Опара перекисла, оттого что закваска была слишком активной.

The difference is mostly stylistic:

• потому что = the most neutral and common everyday because
• оттого что = a bit more literary, bookish, or expressive

So a learner should definitely understand it, but потому что is probably the one you will use more often in conversation.

Here it is part of the fixed conjunction оттого что, meaning because.

So in this use, оттого is written as one word.

This is different from от того, written separately, when it literally means from that or because of that specific thing.

For this sentence, it is best to learn оттого что as a set expression meaning because.

Because оттого что закваска была слишком активной is a subordinate clause of reason.

Russian normally puts a comma before conjunctions that introduce subordinate clauses, including:

• что
• потому что
• оттого что
• если
• когда

So the comma here is completely standard.

Not exactly in the human sense of energetic.

In baking, активная закваска means the starter is:

• fermenting strongly
• producing gas well
• rising vigorously
• affecting the dough quickly

So слишком активной means it was too vigorous or too strong in fermentation, which caused the preferment to over-ferment.

Because the sentence describes a completed result.

Перекисла is perfective: it tells us the preferment ended up over-soured.

That fits the meaning:

• first, the starter was too active
• as a result, the preferment over-fermented

If you used an imperfective form like перекисала, it would sound more like an ongoing process or background action, not a simple finished outcome.

Yes. Russian word order is flexible.

This sentence gives the result first and the reason second:

• Опара перекисла, оттого что закваска была слишком активной.

You could also say:

• Оттого что закваска была слишком активной, опара перекисла.

That version puts more emphasis on the cause. The basic meaning stays the same.