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Since when did Japanese people start eating polished white rice?
Literal
When [from-から] Japanese-people [topic-は] polished-white-rice [object-を] eat-[habitual-ようになった] [explanatory-のですか]?
~ようになる ('come to X, start X-ing as a habit') is the companion of ~ようにする ('try to X'), with なる for natural development rather than する for deliberate action. Past form ~ようになった = 'came to do / started doing (as a new habit).' 精白米 ('polished white rice') is a formal technical term — 白米 is the casual equivalent.