萎凋目的及变化机制
   2.酶活性与内含物转化:伴随着鲜叶水分的散失以及体内呼吸代谢活动的进行,叶细胞膜透性及酶活性渐趋增强。特别是水解酶活性有较显著地增强,促进了叶内含大分子不可溶性的物质降解转化,形成分子量较小、溶解度大的物质。同时也有一定的氧化变化作用,致使青叶在萎凋阶段其可溶物有不同程度增多,这对增进乌龙茶品质有着积极的意义。

   3.气味:随着萎凋时间的推移,离体叶渐趋衰亡,叶组织细胞严密的整体结构趋于解体,许多结合态的芳香成分变成游离态,容易向外扩散挥发。率先挥发的多为具有青臭气的低沸点的组分。因此,萎凋阶段青叶多显示有强烈的青臭气特征。同时,萎凋阶段,由于物质的转化作用,也会部分地形成新的芳香成分。据测定,类似乌龙茶萎凋作用的红茶萎凋过程,芳香成分可由鲜叶的40多种增至60多种。萎凋阶段,对于乌龙茶品质香气的构成也是不可缺少的环节之一。

  4.色泽:鲜叶色泽在萎凋阶段,由于色素组成的改变,如黄旦,毛蟹品种的叶绿素含量分别为0.262%(占干量)和0.363%(占干量)而其萎凋叶分别减为0.249%和0.281%,多酚类氧化产物的形成,失水后细胞汁液浓度的加大等,由富有光泽的鲜绿色渐转为暗绿色,且失去光泽特征。

  总之,萎凋突出的变化机制是失水。在失水条件下,和呼吸代谢的带动下,鲜叶的一系列物理和化学特性也随之改变。这一转化为后续工序的进行作好了鲜叶在理化性状上必要的准备。对整个工艺而言,它是开路先锋,类似于一支曲子的第一个定音音符,虽然整支曲子的模式不是由于这个音符可以确定,但是,由它定下的调门,有着贯穿一曲终了的深远的影响作用。

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