题目：
白酒自然老熟的机理是什么？
答案：
参考答案：
1）物理变化：
A.挥发作用：新酒中硫化氢、硫醇等挥发性的硫化物，丙烯醛、丙醛醇、丁烯醛等刺激性挥发物经贮存后自然挥发。
B.氢键缔合：乙醇和水分子通过氢键缔合，加强了乙醇分子的束缚力，降低了乙醇分子的活度，使白酒口感变得柔和。
2）化学变化：通过醇类的氧化、醛类的氧化、醇和酸的酯化、醇和醛的缩合，减少了白酒的辛辣味和形成柔和的香味。
白酒自然老熟的机理是什么？
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Weak dollar or no, $ 46,000-the price for a single year of undergraduate instruction amid the red brick of Harvard Yard-is (1) But nowadays cost is (2) barrier to entry at many of America’s best universities. Formidable financial-assistance policies have (3) fees or slashed them deeply for needy students. And last month Harvard announced a new plan designed to (4) the sticker-shock for undergraduates from middle and even upper-income families too.
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