Ons hou die suurdeeg vars deur dit koud te hou.

Because the sentence has two linked actions:

• Ons hou die suurdeeg vars = We keep the sourdough fresh
• deur dit koud te hou = by keeping it cold

So the first hou is the main verb of the sentence, and the second hou is part of the infinitive phrase te hou (to keep).

Afrikaans often does this, just like English can do something similar:

• We keep it fresh by keeping it cold.

Hou can mean different things depending on context. Here it means keep.

Some common uses are:

• hou van = like / love
  • Ek hou van koffie. = I like coffee.
• hou = keep
  • Hou dit warm. = Keep it warm.
• sometimes it can also mean hold

In your sentence, hou ... vars clearly means keep ... fresh.

Yes, deur can mean door as a noun, but here it is a preposition meaning by in the sense of by doing something.

So:

• deur dit koud te hou = by keeping it cold

This is very common in Afrikaans:

• Hy leer Afrikaans deur baie te lees. = He learns Afrikaans by reading a lot.

So the meaning depends on the role of the word in the sentence.

Dit means it, and it refers back to die suurdeeg.

Afrikaans repeats the object here:

• Ons hou die suurdeeg vars = We keep the sourdough fresh
• deur dit koud te hou = by keeping it cold

In English, we also usually repeat the object:

• We keep the sourdough fresh by keeping it cold.

So dit is not extra or optional in normal usage here; it tells you exactly what is being kept cold.

Because te is the infinitive marker, similar to English to.

• hou = keep
• te hou = to keep

After deur when Afrikaans expresses by doing something, you commonly get a structure like:

• deur ... te + verb

So:

• deur dit koud te hou = by keeping it cold

Literally, it is something like:

• through/by it cold to keep

That literal order sounds strange in English, but it is normal in Afrikaans.

Afrikaans often puts the infinitive verb at the end of this kind of phrase.

So the structure is:

• deur = by
• dit = it
• koud = cold
• te hou = to keep

Together:

• deur dit koud te hou = by keeping it cold

A useful pattern is:

• deur + object + description + te + verb

For example:

• deur die brood warm te hou = by keeping the bread warm
• deur die venster oop te laat = by leaving the window open

So the word order may feel backwards to an English speaker, but it is very normal in Afrikaans.

Because vars is not directly describing the noun inside the noun phrase. It is describing the result or state caused by the verb hou.

Compare:

• die vars suurdeeg = the fresh sourdough
  Here vars comes before the noun because it is a normal attributive adjective.

• Ons hou die suurdeeg vars = We keep the sourdough fresh
  Here vars comes after the noun because it is part of the pattern hou something + adjective.

This is very similar to English:

• We keep the sourdough fresh not
• We keep the fresh sourdough
  Those mean different things.

In this sentence it stays vars because it is used predicatively, after the noun and linked to the verb hou.

Afrikaans adjectives often behave differently depending on position:

• vars brood = fresh bread
• Die brood is vars. = The bread is fresh.
• Ons hou die brood vars. = We keep the bread fresh.

So in sentences like this one, vars stays in its basic form.

Suurdeeg literally means sourdough. Depending on context, it can refer to:

• sourdough as a bread type, or
• a sourdough starter

In a sentence about keeping it fresh and keeping it cold, it most likely means sourdough starter or the sourdough mixture being maintained.

So a natural English understanding would be:

• We keep the sourdough starter fresh by keeping it cold.

Die is the normal definite article in Afrikaans, meaning the.

Unlike English learners may expect from languages like German or Dutch, Afrikaans does not change the for gender in ordinary usage. So die works for many nouns regardless of gender.

Examples:

• die brood = the bread
• die vrou = the woman
• die kind = the child

So die suurdeeg simply means the sourdough.

Usually not in Afrikaans. One of the simpler features of Afrikaans is that verbs generally do not change much for person.

So you get:

• Ek hou = I keep
• Jy hou = you keep
• Ons hou = we keep
• Hulle hou = they keep

That is why ons hou looks the same as other forms. This is much simpler than English in some ways, and much simpler than Dutch or German.

Yes, it is natural and grammatical.

A speaker might also say similar things like:

• Ons hou die suurdeeg vars deur dit in die yskas te hou.
  We keep the sourdough fresh by keeping it in the fridge.

• Ons hou dit koud sodat dit vars bly.
  We keep it cold so that it stays fresh.

But your original sentence is perfectly normal. It clearly expresses the method:

• first the result: we keep the sourdough fresh
• then the method: by keeping it cold