气味是一种挥发性化合物，人类和其他动物通过嗅觉感知。气味也被称为芳香或香料，如果它们令人不快，则称为臭味、恶臭和臭味。产生气味的分子类型称为芳香化合物或气味剂。这些化合物体积小，分子量小于300道尔顿，由于蒸汽压高，容易分散在空气中。嗅觉可以检测到浓度极低的气味。

气味的工作原理

具有嗅觉的生物体通过称为嗅觉受体（OR）细胞的特殊感觉神经元来检测分子。在人类，这些细胞聚集在鼻腔后部。每个感觉神经元都有延伸到空气中的纤毛。在纤毛上，有与芳香化合物结合的受体蛋白。当结合发生时，化学刺激在神经元中启动电信号，将信息传输至嗅觉神经，嗅觉神经将信号传输至大脑中的嗅球。嗅球是边缘系统的一部分，也与情绪有关。一个人可能识别出一种气味并将其与情感体验联系起来，但可能无法识别气味的特定成分。这是因为大脑不能解释单个化合物或它们的相对浓度，而是将化合物的混合物作为一个整体来解释。研究人员估计，人类可以区分10000到一万亿种不同的气味。

气味检测有一个阈值限制。一定数量的分子需要结合嗅觉受体来刺激信号。一种单一的芳香化合物可以与几种不同的受体结合。跨膜受体蛋白是金属蛋白，可能涉及铜、锌和锰离子。

芳香与芳香

在有机化学中，芳香族化合物是指由平面环状或环状分子组成的化合物。在结构上最像苯。虽然许多芳香化合物确实有香气，但“芳香”一词指的是化学中一类特定的有机化合物，而不是有香味的分子。

从技术上讲，芳香化合物包括可与嗅觉受体结合的低分子量挥发性无机化合物。例如，硫化氢（H2S）是一种无机化合物，具有独特的臭鸡蛋气味。元素氯气（Cl2）有一种辛辣的气味。氨（NH3）是另一种无机加臭剂。

有机结构致香化合物

有机加臭剂分为几类，包括酯类、萜烯类、胺类、芳香类、醛类、醇类、硫醇类、酮类和内酯类。下面列出了一些重要的芳香化合物。有些是自然发生的，而另一些是合成的：

Among the "smelliest" of the odorants are methyl phosphine and dimethyl phosphine, which can be detected in extremely low amounts. The human nose is so sensitive to thioacetone that it can be smelled within seconds if a container of it is opened hundreds of meters away.

The sense of smell filters out constant odors, so a person becomes unaware of them after continuous exposure. However, hydrogen sulfide deadens the sense of smell. Initially, it produces a strong rotten egg smell, but binding of the molecule to odor receptors prevents them from receiving additional signals. In the case of this particular chemical, the loss of sensation can be deadly, as it is extremely toxic.

Aroma Compound Uses

Odorants are used to make perfumes, to add odor to toxic, odorless compounds (e.g., natural gas), to enhance the flavor of food, and to mask undesirable scents. From an evolutionary standpoint, a scent is involved in mate selection, identifying safe/unsafe food, and forming memories. According to Yamazaki et al., mammals preferentially select mates with a different major histocompatibility complex (MHC) from their own. MHC can be detected via scent. Studies in humans support this connection, noting it's also affected by the use of oral contraceptives.

Aroma Compound Safety

Whether an odorant occurs naturally or is produced synthetically, it may be unsafe, especially in high concentrations. Many fragrances are potent allergens. The chemical composition of fragrances is not regulated the same from one country to another. In the United States, fragrances in use before the Toxic Substances Control Act of 1976 were grandfathered in for use in products. New aroma molecules are subject to review and testing, under the oversight of the EPA.

Reference

• Yamazaki K, Beauchamp GK, Singer A, Bard J, Boyse EA (February 1999). "Odortypes: their origin and composition."​ Proc. Natl. Acad. Sci. U.S.A. 96 (4): 1522–5.
• Wedekind C, Füri S (October 1997). "Body odour preferences in men and women: do they aim for specific MHC combinations or simply heterozygosity?". Proc. Biol. Sci. 264 (1387): 1471–9.  

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Helmenstine, Anne Marie, Ph.D. "Aroma Compounds and Their Odors." ThoughtCo, Aug. 1, 2021, tl80.cn/aroma-compounds-4142268. Helmenstine, Anne Marie, Ph.D. (2021, August 1). Aroma Compounds and Their Odors. Retrieved from https://www.tl80.cn/aroma-compounds-4142268 Helmenstine, Anne Marie, Ph.D. "Aroma Compounds and Their Odors." ThoughtCo. https://www.tl80.cn/aroma-compounds-4142268 (accessed September 3, 2021).

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