Клейковина стала крепче.

Because клейковина is a feminine singular noun. In the Russian past tense, the verb agrees with the subject in gender and number:

• стал = masculine singular
• стала = feminine singular
• стало = neuter singular
• стали = plural

So Клейковина стала... is correct because клейковина is feminine.

Крепче is the comparative form of крепкий.

So:

• крепкий = strong / firm
• крепче = stronger / firmer

Russian often uses a one-word comparative like this instead of a separate word like more in English.

Other examples:

• быстрый → быстрее = faster
• сильный → сильнее = stronger
• мягкий → мягче = softer

Russian very often expresses change of state with:

стать + comparative

So:

• стать лучше = to become better
• стать хуже = to become worse
• стать сильнее = to become stronger
• стать крепче = to become stronger / firmer

This pattern emphasizes that something changed relative to how it was before.

So Клейковина стала крепче means that the gluten is now stronger than it was earlier.

Yes, you could, but it is not exactly the same.

• стала крепче = became stronger
• стала крепкой = became strong / firm

The sentence with крепче focuses on comparison: there was an increase in strength.

The version with крепкой describes the new quality more absolutely: it became strong.

In many situations, especially when talking about a process or gradual improvement, стала крепче sounds more natural.

That is a very good question.

Yes, стать often takes the instrumental with nouns and full adjectives:

• стать врачом = to become a doctor
• стать крепкой = to become strong

But крепче is a comparative form, and comparatives do not take the instrumental here.

So:

• стала крепкой = became strong
• стала крепче = became stronger

Both are correct, but they use different structures.

Клейковина is in the nominative case because it is the subject of the sentence.

You can think of the structure like this:

• Клейковина = the thing being talked about
• стала = became
• крепче = stronger

So клейковина is the subject, and subjects are normally in the nominative.

No. In this kind of sentence, крепче does not change for gender, number, or case.

Compare:

• Клейковина стала крепче.
• Тесто стало крепче.
• Нити стали крепче.

Even though the subjects are feminine, neuter, and plural, крепче stays the same.

That is different from full adjective forms like крепкий / крепкая / крепкое / крепкие, which do change.

Yes, Russian can form a comparative in two ways:

1. Simple comparative: крепче
2. Analytical comparative: более крепкая or более крепкой depending on the structure

But in a short sentence like this, стала крепче is much more natural and idiomatic.

The simple comparative is very common in everyday Russian, especially after стать:

• стало лучше
• стало хуже
• стало легче
• стало крепче

The adjective крепкий has a fairly broad meaning in Russian. Depending on context, it can mean:

• strong
• firm
• sturdy
• solid
• potent

In the context of клейковина, крепче usually means something like:

• stronger
• firmer
• more developed
• more elastic/resistant

So the exact English wording depends on the situation, but grammatically the Russian is simply saying that the gluten has become more strong/firm than before.

Because Russian has no articles.

So Клейковина can mean:

• gluten
• the gluten

The exact interpretation depends on context.

This is normal in Russian. Articles like a and the are not expressed with separate words.

Yes, Russian word order is more flexible than English, but Клейковина стала крепче is the most neutral order.

This order is:

subject + verb + result/state

Other orders are possible, but they usually add emphasis or sound more marked, for example in speech or stylistic writing.

For a learner, Клейковина стала крепче is the best default pattern to remember.