Nature-identical compounds, aroma chemicals

Phenylethanol has a rose-like odour and makes the chemically produced compound the most used fragrance chemical in perfume and cosmetics, with a world market of about 7,000 t year [107, 108]. 2-Phenylethanol is also found in many foods as a characteristic flavour compound rounding off the overall aroma, especially in foods obtained by fermentation, such as wine, beer, cheese, tea leaves, cocoa, coffee, bread, cider and soy sauce [109]. In food applications, natural 2-phenylethanol is preferred rather than its nature-identical counterpart from chemical synthesis and it has a market volume of 0.5-11 year . This product is sold at market prices of up to US 1,000 per kiklogram and is mainly produced by yeast-based bioprocesses since its isolation from natural sources, e.g. rose oil, would be too costly [109]. 

Aroma chemicals are an important group of organic molecules used as ingredients in flavor and fragrance composition. Aroma chemicals consist of natural, nature-identical, and artificial molecules. Natural products are obtained directly from the plant or animal sources by physical procedures. Nature-identical compounds are produced synthetically, but are chemically identical to their natural counterparts. Artificial flavor substances are compounds that have not yet been identified in plant or animal products for human consumption. 

The onset of the industrial production of essential oils can be dated back to the first half of the 19 century. After the importance of single aroma chemicals was recognised in the middle of the century, efforts were started to isolate such compounds from corresponding natural resources for the first time. This was soon followed by the synthesis of aroma chemicals. In this context, the most important pioneers of synthetic aroma chemicals have to be mentioned, such as methyl salicylate [1843], cinnamon aldehyde [1856], benzyl aldehyde [1863] and vanillin [1872], as they constitute the precursors of a rapidly growing number of synthetically produced (nature-identical) aroma chemicals in the ensuing years. 

Production Even today many aroma chemicals are still isolated from essential oils, others are prepared semisynthetically from the components of the oils or from other suitable oiganic compounds. The proportion of natural and nature identical aroma chemicals used in the perfume industry is ca. 70%. The aroma chemicals not occurring in nature are often structural analogues of natural products that are difficult to synthesize, e.g., the sandalwood aroma chemicals prepared from campholene aldehyde or camphene and guaiacol as substitutes for the expensive sandalwood oil or santalols. 

The aroma substances that comprise flavors are found in nature as complex mixtures of volatile compounds. A vast majority of volatile chemicals that have been isolated from natural flavor extracts do not provide aroma contributions that are reminiscent of the flavor substance. For instance, n-hexanal is a component of natural apple flavor (1) however, when smelled in isolation, its odor is reminiscent of green, painty, rancid oil. Similarly, ethyl butyrate has a nondescript fruity aroma although it is found in strawberries, raspberries, and pears, it does not uniquely describe the aroma quality of any of these individual fruits. It has long been the goal of flavor chemists to elucidate the identity of pure aroma chemicals that have the distinct character impact of the natural fruit, vegetable, meat, cheese, or spice that they were derived from. Often, these are referred to as character impact compounds (2). 

The ultimate goal of flavor research in the food industry is to identify and classify unique aroma chemicals that contribute to the characteristic odor and flavor of foods. Having this knowledge enables flavor duplication through nature-identical or biosynthetic pathways and can facilitate better quality control of raw materials by screening of the appropriate analytical target compounds. 

---