买回来的香蕉皮上有很薄一层白色粉，呈小斑块状，像泼溅上的白灰点子，不知是何。

There is a thin layer of white powder on the hull of the banana just bought, spotty and just like some lime spattered on, i did not konw why first.

刚刚查了一下，说现在常用的香蕉催熟方法是乙烯利催熟法：
“乙烯利的化学名是2-氯乙基磷酸，对水果的成熟有明显的促进作用。乙烯利含量为40%。香蕉的催熟浓度为0.2%(2000ppm)，即500公斤水兑配40%乙烯利为2.5公斤。乙烯利为微酸性，须在微碱性情况下才能起作用，因此，须在乙烯利溶液里加入0.05%的洗衣粉。先将洗衣粉溶解后再按比例加入乙烯利。香蕉数量少可以直接放入乙烯利溶液里浸泡一分钟，沥去药液，装入塑料袋里放入适宜的室温下。香蕉数量多可用喷淋的方法，最后用塑料布覆盖香蕉，使其产生乙烯，才能起到催熟的作用。1～2天把塑料布揭除，2～3天即黄熟。”　

Search on line, reading that the common way of ripen banana is ethephon method(these words are hard to translate well for me, the main meaning is use some chemical materials to ripen the fruit, and the material is basically ethephon. and also some launder powder used in the method.)

所以，估计香蕉皮上的白色粉是洗衣粉，或类似的碱性物质。
分析得对否，请教大家！

So i just guess the white powder should be the launder powder or other alkalescent chemical.

 

right or not, maybe you have more idea?