11.4.1 ^-Carotene

The biomass of certain microalgae could find application as food supplements due to their nutritional content other than proteins. The P-carotene content of Dunaliella salina, a halotolerant green microalga, can reach up to 14% of dry weight (Metting, 1996). P-Carotene, a component of the photosynthetic reaction center, accumulates as lipid globules in interthylakoid spaces of chloroplasts of alga (Vorst et al., 1994). It contributes to light harvesting and protects the alga from oxidative damage dur­ing excessive irradiance by quenching the triplet-state chlorophyll or by reacting with singlet oxygen (1O2), thus preventing the formation of reactive oxygen species (Demming-Adams and Adams, 2002; Del Campo et al., 2007; Raja et al., 2007; Telfer, 2002). The beneficial effects of P-carotene on human health are attributed to its antioxidant properties (Guerin et al., 2003; Higuera-Ciapara et al., 2006; Hussein et al., 2006), and several studies have indicated that adequate intake of carotenoids has the ability to prevent degenerative diseases (Astorg, 1997; Demming-Adams and Adams, 2002; Krinsky and Johnson, 2005). P-Carotene also has the ability to act as provitamin A (Garcia-Gonzalez et. al., 2005; Gouveia and Empis, 2003). Because of these properties, P-carotene has found applications as a food supplement and colorant.

The extent of P-carotene accumulation in Dunaliella biomass is a function of high salinity, temperature stress, high light intensity, and nitrogen limitation. Being an extremophile, by virtue of its ability to grow at high salinity, it is possible to grow Dunaliella biomass in open-pond cultivation systems in photo-autotrophic mode. The production ponds are typically located in areas that could provide high solar irradi — ance, warm temperatures, and hypersaline waters (Ben-Amotz, 1999). Commercial cultivation facilities of Dunaliella are located in Australia, Israel, China, and the United States (Del Campo et al., 2007), with global production estimates at about

I, 200 MT y-1 (Pulz and Gross, 2004). The open-pond cultivation systems used are either very large ponds (without mixing) of up to 250 ha or paddle-mixed raceway ponds of about 3,000 m2 surface area (Del Campo, 2007). Commercial producers are offering Dunaliella biomass directly as a powder for application as an ingredient in human dietary supplements and functional foods (Spolaore et al., 2006).

Downstream processing of Dunaliella biomass is carried out to extract P-carotene for use as a natural food colorant and food supplement. The natural P-carotene from Dunaliella must compete with cheaper synthetic P-carotene in the marketplace. Synthetic P-carotene is dominated by all-trans-P-carotene (Von Laar et al., 1996), whereas natural P-carotene from Dunaliella contains more than 50% 9-cis-P-carotene (Johnson et al., 1996). Therefore, although more expensive, natural P-carotene provides the natural isomers in their natural ratio (Guerin et al., 2003; Garcia-Gonzalez et al., 2005; Spolaore et al., 2006), and the natural isomer of P-carotene is accepted as superior to the synthetic all-trans-isomer (Radmer, 1996; Vilchez et al., 1997; Lorenz and Cysewski, 2000; Becker, 2004; Spolaore et al., 2006). Although not yet cost compared to synthetic P-carotene, production of natu­ral P-carotene from Dunaliella has been reported as an economically viable and growing industry (Singh et al., 2005; Chisti, 2006). The algal meal of Dunaliella after extraction of P-carotene is reported to contain about 40% protein and therefore could find application in fish and poultry feed (Iwamoto, 2004).