Which or That?
as told here
If the sentence doesn’t need the clause that the word in question is connecting, use which. If it does, use that.
consider the following example on the link.
Our office, which has two lunchrooms, is located in Cincinnati.
Our office that has two lunchrooms is located in Cincinnati.
These sentences are not the same. The first sentence tells us that you have just one office, and it’s located in Cincinnati. The clause which has two lunchrooms gives us additional information, but it doesn’t change the meaning of the sentence. Remove the clause and the location of our one office would still be clear: Our office is located in Cincinnati.
The second sentence suggests that we have multiple offices, but the office with two lunchrooms is located in Cincinnati. The phrase that has two lunchrooms is known as a restrictive clause because another part of the sentence (our office) depends on it. You can’t remove that clause without changing the meaning of the sentence.
Now, Regarding your question, It is proven that certain food that are ingested aid the learning processes by releasing natural memory enhancers called CCKs, also known as cholecystokinin, which are composed of tryptophan, choline and phenylalanine.
The word "which" should have been correctly used.