Are both "foodstuff" and "foodstuffs" valid?

I suppose that I could probably summarize that to "Is foodstuff a mass or count noun?" but I wasn't entirely certain if that would get me the wrong answer. I've always used "foodstuffs" when referring to actual components for recipes, but I could see an argument only using the singular since, after all, it's a noun which covers all of the components at once. I thought of submitting it to "The Word Detective" but it seems like more of a case of usage.

Dictionaries seem to indicate it in the singular without noting pluralization, but there are also many example sentences which use "foodstuffs" such as the Tarzan quotes I found here: "The she was for Tarzan--all that he desired was to bury his snout in the foodstuffs of the Tarmangani." and "They appeared to be raiding parties, for they drove goats and cows along with them and there were native porters laden with grain and other foodstuffs."

Various dictionaries (OxfordDictionaries, Merriam-Webster) don't specifically list foodstuffs as the plural of foodstuff, but then proceed to give examples using foodstuffs!

Take this entry from M-W Unabridged:

foodstuff, noun : a substance with food value: as a : the raw material of food before or after processing {a bountiful crop of cereal foodstuffs} b : an element of nutrition (as protein, carbohydrate, vitamin) {the sponge obtains its necessary foodstuffs from the plankton}

They're both valid -- foodstuff in the singular, foodstuffs in the plural.