"The ingredient is a jam."
Translation:L'ingrediente è una marmellata.
Depending on the fruit you use for making it there are many different jams. The ingredient is one of many possible jams >> a jam.
Is the ingredient of this cake a fruit? No, the ingredient is a jam, a red fruit jam. It may be a strawberry jam or a raspberry jam. These are legitimate, and not very unusual English sentences
I have been studying English for 10 years and I'm telling you that saying "a jam" is COMPLETELY wrong!!! You can't say one jam, two jams, three jams and that means that the word "jam" is an UNCOUNTABLE noun. Uncountable nouns don't take a/an in front of them.
The cupboard has three jams in it - an apricot jam, a strawberry jam and a raspberry jam.
I see what pgmbuddy is saying. For instance, I could reasonably say, "Strawberry jam is a jam." The fact that it's a type of jam is implied, but it doesn't need to be specifically stated, similar to how you can say, "The hammer is a tool."
English is my native language and I'm doing an undergraduate in it. 'A jam' is not wrong. It's not particularly common, but there's nothing incorrect in it :)