**6. Conclusion**

Although several groups have attempted to produce β-ionone using yeast or *E. coli*, their yields are relatively low. Simkin et al. firstly engineered *E. coli* cells to synthesize β-ionone but with only detectable trace amounts being reported [89]. Beekwilder et al. engineered *Saccharomyces cerevisiae* for the production of β-ionone; however, the titer achieved was only 0.22 mg/L [87]. López et al. inserted extra copies of geranylgeranyl diphosphate synthase gene and CCD1 gene from the plant *Petunia hybrid*, which enabled their *S. cerevisiae* strain to produce about 6 mg/L of β-ionone when grown in a bioreactor [90]. To date, the best-reported β-ionone strain was from our laboratory, where the engineered *E. coli* strain produced 500 mg/L of

**titer (mg/g DCW)**

**Culture conditions**

bioreactors

bioreactors

bioreactors

**References**

[90]

[19]

[19]

**5. Challenges and potential for the commercialization of microbial** 

In general, the chief challenge for commercializing microbial production of chemicals is relatively high cost. The cost depends mainly on titer, rate (or productivity) and yield (or 'TRY') [91]. Hence, researchers are inventing and exploring different approaches to engineer microbes to obtain TRY figures of merit. Until then, it would not be cost effective or competitive to other sources (such as chemical synthesis). The good news is that carotenoids and apocarotenoids are high-value specialty chemicals; thus, their requirements for commercialization are less stringent as compared to fuels and commodity chemicals. For example, the current processes of β-carotene production in microalga *Dunaliella* [21] and the fungus *Blakeslea trispora* [2] are already profitable. Many recent cases of microbial production of carotenoids have reached

**production of carotenoids and apocarotenoids**

**Table 3.** Microbial production of retinol, α- and β-ionones in literature.

**No. Hosts Apocarotenoids Titer (mg/L) Specific** 

 *Escherichia coli* Retinol 54 6.3 2–3 days in flasks [85] *Escherichia coli* Retinol 76 9.8 2–3 days in flasks [86] *Escherichia coli* Retinol 28 10.0 2 days in flasks [19] *Saccharomyces cerevisiae* β-Ionone 0.22 / 2–3 days in flasks [87]

7 *Escherichia coli* α-Ionone 340 ng/L / 2 days in flasks [88]

5 *Saccharomyces cerevisiae* β-Ionone 6 1.0 2–3 days in

6 *Escherichia coli* β-Ionone 500 16.0 2 days in

8 *Escherichia coli* α-Ionone 480 7.0 2 days in

β-ionone [19] (**Table 3**).

96 Progress in Carotenoid Research

Amid diverse natural products, carotenoids and apocarotenoids are particularly interesting. This is not only due to their bright color and pleasant fragrances but also their light-harvesting capability, the electron/energy transferring ability, the potent anti-oxidant properties, the hormone function, vitamin A activity and numerous other health benefits to both human and other life forms on the Earth. Increasingly, clinical studies have supported the concept that the regular uptake of carotenoids can prevent many serious diseases. The list of benefits and applications keeps growing and with the market for commercial exploitation it can be confidently expected to increase. In light of this and the extremely low levels found in plant materials, it is urgent to find solutions enabling these valuable molecules to be supplied in a sustainable and cost-effective manner. In the past decade, the metabolic engineering of microorganisms has progressed remarkably for the production of carotenoids and apocarotenoids. Some of these processes are being commercialized already but the scope to further extend this family of molecules is high, adding an increasingly solicited pipeline of natural products to compete with chemical synthesis.
