trace mineral

Reader's Digestには必ず健康や食べ物に関する記事があり、2月号にはEveryday Superfruitsについて書かれていました。その中の次の項目に私が知っている意味では通じない単語がありました。
Blueberries: One cup (150g) of blueberries supplies 24% of your daily vitamin C needs and about 14% of your fibre needs. It also contains vitamin K and the trace mineral, manganese - all for only 250kJ.
"trace mineral" の "trace" がその単語ですが、後に "manganese" があるのがヒントです。マンガンは稀少金属ではなかったと思うが、いずれにしてもブルベリーにはほんの僅か入っているだけですよね。辞書を見た方が早い。
・Collins Dictionary: any of various chemical elements, such as iron, manganese, zinc, copper, and iodine, that occur in very small amounts in organisms and are essential for many physiological and biochemical processes
・Oxford English Dictionary: A trace element required for nutrition: selenium and other trace minerals are vital to good health: Nutritionally, mushrooms are rich in potassium and selenium, a trace mineral known for its antioxidant properties and tested recently for its ability to combat prostate cancer and cardiovascular disease.
なるほど、それなら "trace" に微量、わずかなの意味があるはずなので、"trace" の項目も見ます。
・Oxford English Dictionary: A very small quantity, especially one too small to be accurately measured: his body contained traces of amphetamines: [as modifier]: trace quantities of PCBs
・Merriam-Webster's Online Dictionary: a very small amount of a substance, especially when it is too small to see clearly or measure accurately: trace of: Traces of blood were found in the bathroom.