zlacker

My best guess: this prevents situations like selling jars of "strawberry jam" which is only 10% strawberry.

Which is partly backed up by 21 CFR § 150.160 (e)(1):

(e)

>(1) The name of each preserve or jam for which a definition and standard of identity is prescribed by this section is as follows:...

>>vharuc+(OP)
Well, no, the section you quoted is what prevents you from selling "Strawberry jam" that has fruits other than strawberry. The section about number of fruits seems to just be about arbitrarily restricting the amount of any fruit present to at least one fifth of the total (except pineapple for some reason). I don't see why that's necessary, but you could at least write the law as

>(2) The following combinations of fruit ingredients may be used:

>(i) Any combination of such fruits in which the weight of each is not less than one-fifth of the weight of the combination, or in which the weight of each part is no less than half the weight of any other part; except that the weight of pineapple may be not less than one-tenth of the weight of the combination.

I'm not quite sure what the right legal wording is for or such that you can comply with whichever half works for you is, but that would both allow for everything the existing law allows and allow for arbitrary numbers of fruit without requiring that they be mixed in exactly equal ratios.