气味来源于本身具有很难闻气味的不同的化学物质，这些气味化学物质有些是本身存在于自然界中，有的是要在特定的环境和条件下形成的。这里我们要讨论的是最常见，也是最容易引起气味问题的微生物活动气体的形成。我们知道，微生物总体分为有氧和无氧两种。它们的活动都能给我们的环境产生气味问题，但无氧生物活动引起的气味更加复杂和难以对付，成为我们生活中的主要的气味源。Odor comes from the distinct offensive smell of different chemical materials. Some of them exist in nature constantly, but some of them are formed in special environment and condition. Hereon, we would like to discuss the most common odor materials that are resulted from microbial activities. We know, all of the microbes are divided into two groups that are aerobic and anaerobic microbes. Both aerobic and anaerobic bio degradation can generate mal odor, but anaerobic bioactivities are the main sources of unpleasant odors.
在无氧生物活动中，“无氧呼吸”和“发酵”是两大无氧生物活动分类Anaerobic respiration and fermentation are two distinct forms of oxygen-independent energy metabolism.
在“发酵”活动中，虽然它的最终活动产物是 甲烷和二氧化碳，但在整个的发酵活动过程中会产生很多的有很强气味的中间产物，因为它是在一系列的微生物种群的有顺序的相互配合下完成的一个相当复杂的微生物活动。请看下面发酵的几大步骤: In fermentation, although the final product is methane, carbon dioxide, it generate lots of odor components during the process, please see the whole process of fermentation.
• 第一阶段微生物分泌 “酶”来水解大分子聚合物，使之变成较小的单体分子物质，如葡萄糖和氨基酸等等，它们又再而被转化成高挥发性脂肪酸 VFAs(如丙酸和丁酸,它们具有很强的变质陈腐味道)；The first group of microorganisms secrete enzymes which hydrolyze polymeric materials to monomers such as glucose and amino acids, which are subsequently converted to higher volatile fatty acids (such as propionic and butyric acids. These acids create odors that smell like vinegar, Swiss cheese, and rancid butter, respectively.);
• 第二阶段中，产生氢气的产乙酸菌把高挥发性脂肪酸 VFAs如丙酸和丁酸等转化成氢气，二氧化碳和和乙酸.微生物，In the second stage, hydrogen-producing acetogenic bacteria convert the higher volatile fatty acids e.g., propionic and butyric acids, produced, to H2, CO2, and acetic acid;
• 第三阶段中，产甲烷细菌把氢气和醋酸转化成甲烷和二氧化碳。Finally, the third group, methanogenic bacteria convert H2, CO2, and acetate, to CH4 and CO2.
在“无氧呼吸”活动中，相应的细菌在一定的温度湿度和其它条件下，把氮氧化物，硫化物还原成氮气和硫化氢气体(具有臭蛋的气味) 。Anaerobic respiration plays a major role in the global nitrogen, sulfur and carbon cycles through the reduction of the oxyanions of nitrogen, sulfur and carbon to form molecular nitrogen gas, Hydrogen sulfide.
下面举几个生物气体气味的例子Some example cases are explained below:
• 硫化氢气体是最众所周知的无氧生物活动产生的气味物质，而其它气味物质如硫醇，有机酸，有机硫化物，胺类， 醛类和酮类有更强的气味，因为它们是以微量元素数量释放出来，仅仅是无氧生物气味的很少的几个百分比而已。Hydrogen sulfide (H2S) with a “rotten-egg” smell is the most known odor-causing compound generated during anaerobic biotransformation. While other odorous compounds such as mercaptans, organic acids, organic sulfides, amines, aldehydes and ketones have more odor potential than H2S, they are generally emitted in trace amounts relative to the amount of H2S, which may comprise up to a few percent of the biogas from anaerobic digesters
• 气味物质分类有挥发性脂肪酸，酚类，氮衍生物，和硫化合物等等。在这些物质中，挥发性脂肪酸 VFAs是一个主要的气味物质，它与动物农场，农业操作和废物处理系统有关。从上面的理论分析得知，挥发性脂肪酸 VFAs是无氧微生物在发酵过程(把碳水化合物和蛋白质降解成甲烷和二氧化碳)的中间产品。丁酸是被公认的主要引起气味的挥发性脂肪酸，因为它达到 “气味阀值”时的气味强度或浓度非常的低，只有0.001 ppm，与硫化氢气体和丙酸比，丁酸要显得更臭。Odorous substances are categorized into classes such as volatile fatty acids, phenols, nitrogen derivatives and reduced sulfur compounds. Among these substances, volatile fatty acids (VFAs) are known as the major components of odors, associated with livestock farming, agricultural operations and waste treatment system. Short chain VFAs formed as intermediate fermentation products during anaerobic microbial degradation of carbohydrates and proteins. They have unpleasant, rancid, and vinegar-like odor characteristics. Butyric acid is recognized as the major volatile fatty acids as it presents a very low odor threshold value of 0.001 ppm. Butyric acid is known to be more offensive compared to hydrogen sulfide and priopionic acid.
• 在气味物质中，有机硫化合物也是一大恶臭源，因为它们较低的“气味阀值”浓度。据研究论文报道，有氧和无氧微生物都能在降解蛋白质中的含硫氨基酸时，产生甲硫醚，而另一种含硫气味物质,甲硫醇, 却只能在无氧的条件下形成。所以它们都可是无氧生物降解活动的中间产物。Among the odor causing compounds, the organic sulfur compounds are the most odorous due to their low threshold concentrations. It has been reported in the literature that dimethyl sulfide (CH3SCH3) forms under both anaerobic and aerobic conditions during the microbial decomposition of sulfur-containing amino acids found in proteins, while another sulfur-containing compound methyl mercaptanor Methanethiol (CH3SH) is formed under anaerobic conditions. They also are intermediate fermentation products.
• 大便的气味是由无氧细菌分解的产物所致，产生气味的主要成分为吲哚、粪臭素、硫化氢、胺类, 乙酸、丁酸等。其中产生粪便恶臭的是吲哚、粪臭素 For example, odor of the human’s waste result from the decomposition of anaerobic bacteria. The primary components are Indole, Skatole, Hydrogen Sulphide, amine, Butyric acid, etc. The fecal nauseating odor mainly come from Indole and statole.