GG5: milk sugar; sugar of milk; lactose
中辞典: lactose; milk sugar
ルミナス: milk sugar, lactose

It can be a bit of a fine line. 
We generally assign [food] when the term is a recognized foodstuff such as you would buy in a grocery, see in recipes, etc. Also, we usually don't stray too far from the field classifications in other dictionaries. In this case the 新和英大辞典第５版 tags it as [化] and not [料], so we're in the same ballpark there. I see that reference also has [化] for クエン酸 although you'll see that lists of contents too.
Making the "milk sugar" a parenthesized expansion might work, but I suspect 乳糖 is probably a bit wider than just lactose. Koj/Daij, etc. are fairly broad on this and the JEs (quoted) have both as separate glosses.

I'm not sure I understand why this is not a food term? I saw it on the ingredients list of a pack of oreos, as in they directly added lactose.

In a technical sense "milk sugar" covers a breadth of sugars other than lactose (doi:10.1007/978-1-4615-7050-9_1) although obviously lactose is the main one. 乳糖 term seems to be 
used to refer to lactose specifically (デジタル大辞泉 contains the molecular formula https://dictionary.goo.ne.jp/word/乳糖/)

This seems to be relevant from the editorial policy: "where the English meaning is an obscure technical term, add a short explanation in lay terms after it in parentheses. Do not add 
such explanations where the English meaning should be clear to a literate user (this is not an English dictionary.)" Perhaps the entry could be "lactose (the primary sugar in milk)"

gg5

Not really a food term.

@@ -12,2 +12 @@
-<pos>&adj-no;</pos>
-<field>&food;</field>
+<field>&chem;</field>

No. Same thing.

Are these two different senses?

@@ -12,0 +13,2 @@
+<field>&food;</field>
+<gloss>lactose</gloss>
@@ -14 +15,0 @@
-<gloss>lactose</gloss>