# Natural Compounds in the Battle against Microorganisms--Linalool
## 1 Introduction  
Linalool (C\({}_{10}\)H\({}_{18}\)O, 3,7-dimethyl-1,6-octadier-3-ol) is a monoterpene acyclic tertiary alcohol with a global production market valued at USD 9980 million in 2019 and expected to reach USD 12,300 million in 2024 [1]. It is mainly obtained by chemical synthesis [2]. As a fragrance compound, it has found numerous applications in the production of perfumes, lotions, soaps, and shampoos as well as in non-cosmetic household products such as detergents and cleaners. An interesting option in recent years is the possibility of using linalool as an active packaging ingredient, for example, by injecting it into the space above the contents of food packaging or even by applying it to food coatings and films. The problem of its high volatility and effect on the contents of the package can be solved by encapsulating it in micro- or nano-emulsions [3]. In turn, in the pharmaceutical industry, it is used as a precursor for the synthesis of vitamin E [4]. Linalool is the most widely used monoterpene in food as its presence has been reported in 23 foods, albeit in varying amounts. In orange and bergamot juices, the content of linalool was 6740-34,430 ug/L and in apricots, grapes, nectarines, peaches, strawberries, and tomatoes, it ranged from less than 10 ug/kg to over 600 ug/kg, while in lemon, melon, and raspberries, it was less than 10 ug/kg. It is worth noting that the addition of jasmine to green tea significantly increases the content of linalool in the range from 30,500 to even 1,315,000 ug/kg [5]. Linalool and its acetate are the main flavors in Earl Gray teas [6]. Linalool is also found in many otherplants, being a key component of their volatile fractions. Table 1 contains an overview of plants with a linalool content of more than 10% in the volatile fraction. Linalool represents about 70% of the floral fragrances [20] and exists as two enantiomers: (_3S_)-(+)-linalool (coriandrol) and (_3R_)-(\(-\))-linalool (licareol). It is worth noting that both enantiomers have different fragrance profiles: coriandrol is perceived as sweet, floral, herbal and petitgrain-like with citrus and fruity notes, while licareol has a woody, lavender aroma. The odor threshold of the (_3R_)-(\(-\))-enantiomer is approximately nine times lower than that of the (_3S_)-(+)-linalool (0.8 vs. 7.4 ppb). It may also exist in racemic form (e.g., passion fruit and apricots). The racemate is also a product of the fermentation processes involved in the production of foodstuffs. In plants, linalool is made from geranyl pyrophosphate (GPP)--a universal monoterpene precursor, which is produced by the condensation of isopentenyl pyrophosphate (IPP) and dimethylallyl diphosphate (DMAPP) by GPP synthase [21]. It may also be a byproduct of the biosynthetic pathways of other terpenids, e.g., geraniol and nerol. Linalool accumulates in the compartmentalized secretory structures of glandular hairs or is directly emitted into the environment [22]. Thanks to the tools of molecular biology, linalool can also be produced by microorganisms. _Escherichia coli_ strains were able to produce (_3R_)-(\(-\))-linalool by overproducing its precursor, mevalonate. On the other hand, the use of bacterial linalool/nerolidol synthase (bLinS) from _Streptomyces clavuligerus_ allowed linalool to be obtained in an amount of up to 500 mg/L. However, bLinS also accepts farnesyl diphosphate as a substrate that is naturally produced by _E. coli_; therefore, _trans_-nerolidol was formed as a by-product (\(\sim\)30% of the total product mix), making this technology difficult for commercial applications [4]. The (_3S_)-(+)-linalool synthase from _Actinidia arguta_ was in turn introduced into _Saccharomyces cerevisiae_ and _Yarrowia lipolytica_. In contrast, in _Synchocystis_ spp. PCC 6803 cyanobacteria, the co-expression of the above (_3S_)-(+)-linalool synthase and _E. coli_ farnesyl pyrophosphate synthase mutant S80F acting as GPP synthase was used.