**5. Challenges and potential for the commercialization of microbial production of carotenoids and apocarotenoids**

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 TRY figures [46, 47, 71] higher than existing processes. It is not surprising that some of them will be translated into more cost-effective industrial processes. More importantly, scientists and engineers are working together to continue improving microbial strains and fermentation processes. Breakthrough by innovation and collective knowledge will markedly reduce product cost and make it more competitive. In addition, the recent trend of consumers' preference to 'natural' or 'bio-based' ingredients will make microbial-derived carotenoids and apocarotenoids more appealing.
