Hello, again!!!
I came across the following sentence in a newspaper:

"The brown soy sauce is produced by crushing a mixture of soybeans and wheat that then undergoes yeast fermentation in saltwater for several months."

I believe the "that" in bold is a nonrestrictive relative pronoun. However, as far as I know, relative "that" is not usually used in nonrestrictive use. I wonder if this is a sort of exception and if so, I would like to know if there are any regularities in this use. Please be so kind as to help me.

Thanking always....
Moon

This message has been edited. Last edited by: moon,

There are relative clauses that can be perceived as either nonrestrictive or restrictive; your example sentence is one of them.

The sentence is correct as it is, with the restrictive relative clause beginning with that. In this case, the mixture is identified by describing the yeast fermentation process that it will undergo.

The sentence is also correct with a nonrestrictive clause beginning with which. In this case, the mixture is perceived as already identified – the mixture of soybeans and wheat – and the information that it will undergo a fermentation process is additional. In this case, of course, a comma follows wheat, and if the sentence is spoken, there is a pause at the comma.
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Michael Swan* states:

"The distinction between identifying [restrictive] and non-identifying[nonrestrictive] clauses is most clear when they modify definite noun phrases like the car, this house, my father, Mrs. Lewis. After indefinitenoun phrases like a car, some nurses or friends, the distinction is less clear, and both kinds of clause are often possible with slight differences of emphasis.

He's got a new car that goes like a bomb.
or
He's gota new car, which goes like a bomb.

We became friendly withsome nurses that John had met in Paris.
or
We became friendly withsome nurses, whom John had met in Paris.

In general, 'identifying' clauses are used when the information they give is felt to be central important to the overall message. When this is not so, non-identifying clauses are preferred."

According to Michael Swan, then, your construction with the relative clause as non-identifying – that is, nonrestrictive – is preferred.

Rachel
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*Practical English Usage, by Michael Swan. Oxford University Press. 1995