To increase production rates of secondary metabolites, different in situ extraction procedures have been applied to plant cell and tissue cultures. The extraction phases can either be water-immiscible liquids, such as w-hexa-decane or Miglyol™, or solid adsorbents such as the hydrophobic polysty-rene-divinylbenzene resins (e.g., Amberlite XAD). Zeolites have also been successfully applied for the specific accumulation of essential oil compounds from liquid fermentation media (70). By provision of an external accumulation site, further metabolism and degradation of products in the medium as well as cytotoxic product concentrations may be impeded. Moreover, and especially for highly volatile flavors, the danger of physicochemical losses of product is minimized by addition of an appropriate adsorbent. In combination with further optimization attempts, such as immobilization or elic-
itation, the productivity of a cell culture process can be vitally enhanced (for reviews see Refs. 24,71). Superior production rates of cell culture systems compared with field-grown material are sought by numerous efforts to improve bioreactor design. Current apparatus proposals for suspension cultures as well as for organized cultures were summarized by Scragg (9). Design equations for oxygen transfer rates in large-scale root culture reactors have been discussed by Tescione et al. (72).
The tomato flavor enzyme system (lipoxygenase and hydroperoxide lyase) was harnessed as a crude enzyme preparation in a hollow-fiber reactor to produce hexanal from linoleic acid. At exogenous substrate (linoleic acid) concentrations of 16 mmol L~', hexanal production rates of about 5 /xg mirf 1 were achieved. The reactor system proved to be stable over an operation period of 5 days, indicating that flavor production with immobilized membrane-associated enzymes in a hollow-fiber reactor is a promising technique. It allows retention of the enzyme system and substrate, with concomitant removal of the product (73).
For the synthesis of extracellular metabolites the immobilization of intact plant cells provides several processing advantages, such as protecting cells from mechanical stress, enhanced productivity, high stability, and facilitated product recovery (10). Several immobilization agents (polyurethane foam, carrageenan, alginate, pectate, polyphenyleneoxide) for cell cultures of Solanum aviculare and Dioscorea deltoidea did not affect the biotransformation course of (—)-limonene to (Z)- and (£>carveol and to carvone but significantly changed product ratios (64).
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