10 Joseph H.Hulse Trends in Food Science Technology 15(2004)3-18 larly.metabolites may be synthesized by isolated, fractionation.Supercritical gas/liquid extractions (SGE) immobilized enzymes. are useful for substances sensitive to organic solvents or Plant cell culture begins by propagation of a callus,a susceptible to oxidation.At pressures between 10,000 mass of undifferentiated cells.To derive a new plant and 40,000 kPa,carbon dioxide is a benign solvent for with shoot and root,cells from the callus must be cul- essential oils.oleoresins,natural terpenoids,caffeine. tured in different media.Desirable metabolites can be and other sensitive biochemicals.Unlike many organic extracted from a callus without progression to a shoot solvents.SGE leaves no toxic residues. and root.Plant cell culture seems better suited to Membrane processing,reverse osmosis,ultrafiltra- synthesis of metabolites useful in foods,biopesticides tion,microfiltration,nanofiltration and electrodialysis and cosmetics than for biomedicals. are among other industrial fractionation technologies. Mammalian and insect cell cultures offer more inter- Chromatographic systems include gel filtration,ion- esting opportunities for biomedical applications.Sour- exchange and affinity separations that use binding ces of mammalian cells include kidneys from aborted interactions between proteins and packing materials, embryos and ovarian cells from Chinese hamsters which with various ligands coupled into hydrophilic support replicate relatively rapidly.Insect cell cultures,in com- matrices. bination with GM virus vectors,produce recombinant proteins and viral insecticides.The baculo virus,which Preservation and sterilization infects insect cells,genetically modified yields specific Foods are for healthy nourishment;drugs to diag- proteins in high-density insect cell culture. nose,prevent or cure disease.It is critical that all foods Mammalian cells generate metabolites of greater pur- and drugs be free from organisms that may cause insult ity,potency and complexity than most microbial cul- or injury to those who consume and use them.Gen- tures but,being highly sensitive,require careful culture erally speaking,biological materials such as foods and in relatively small bioreactors.Means to expand mam- pharmaceuticals can be preserved by any process that malian cell culture in batch or continuous systems pre- (1)inhibits,destroys or removes and prevents re-entry sents an interesting challenge to bioengineers. of pathogenic and microorganisms that cause spoilage; Monoclonal antibodies,plasminogen activators,hor- (2)restricts adverse biochemical and biophysical mones to stimulate blood cell growth and Factor VIIl change. to control blood clotting are among products from Degradation of food and other biological materials mammalian cell culture.In association with specific can be restricted by packaging under inert atmosphere, viruses mammalian cells will produce viral vaccines and by reducing water activity and thermal sterilization. recombinant proteins used in gene therapy. Freeze-drying effectively lowers water activity in sensi- In 1997,Human embryonic stem cells (HESC)were tive biologicals.Rapid freezing in liquid nitrogen prior isolated from discarded human embryos.It is postulated to freeze-drying (lyophilisation)restricts cell disruption that pluripotent stem cells may be cultured into different by slow-growing ice crystals. cells with the capacity to replace or repair cellular tis- sues in various human organs.Whether embryo stem Thermal processes and alternatives cells will realise their hypothetical potential seems to Thermal processing in hermetic containers (cans,bot- depend as much on legislation influenced by religious tles,laminated plastics)takes a long time for heat to be belief as on bioscience. conducted throughout the material.Excessive heating of foods and other biologicals can cause adverse change in Downstream processing critical functional properties,nutritional quality,fla- 'Downstream'relates to all that follows bioreactor vour,physical structure and texture.The higher the synthesis:the isolation.purification and sterilization of temperature,the longer the time,the greater the degree end products.Downstream processes are estimated to of biochemical and biophysical change absorb ca 80%of production costs,indicating an urgent Existing processes that reduce heat damage include need for more economical downstream technologies and spray-drying,tubular and scraped surface heat exchan- bioengineers competent to design and operate them. gers,and steam injection followed by aseptic packaging Several alternative means of preservation are in various Isolation stages of investigation and development. Synthesized substances are isolated from various bioreactor fractions:insulin from harvested cells,some Irradiation vaccines from supernatant fluids.intra-cellular metabo- Ionising radiations can inactivate microorganisms lites are released by mechanical,chemical or enzymatic and kill insects.Radiation sources for food and phar- rupture of cell walls;antibiotics by liquid:liquid extrac- maceuticals include gamma rays from radioisotopes tion;tolerant volatile substances by fractional distilla- Co60 or Ces137.X-rays or electrons generated by tion;heat-sensitive enzymes by aqueous phase liquid machines.Absorbed radiation is measured in Grays orlarly, metabolites may be synthesized by isolated, immobilized enzymes. Plant cell culture begins by propagation of a callus, a mass of undifferentiated cells. To derive a new plant with shoot and root, cells from the callus must be cul￾tured in different media. Desirable metabolites can be extracted from a callus without progression to a shoot and root. Plant cell culture seems better suited to synthesis of metabolites useful in foods, biopesticides and cosmetics than for biomedicals. Mammalian and insect cell cultures offer more inter￾esting opportunities for biomedical applications. Sour￾ces of mammalian cells include kidneys from aborted embryos and ovarian cells from Chinese hamsters which replicate relatively rapidly. Insect cell cultures, in com￾bination with GM virus vectors, produce recombinant proteins and viral insecticides. The baculo virus, which infects insect cells, genetically modified yields specific proteins in high-density insect cell culture. Mammalian cells generate metabolites of greater pur￾ity, potency and complexity than most microbial cul￾tures but, being highly sensitive, require careful culture in relatively small bioreactors. Means to expand mam￾malian cell culture in batch or continuous systems pre￾sents an interesting challenge to bioengineers. Monoclonal antibodies, plasminogen activators, hor￾mones to stimulate blood cell growth and Factor VIII to control blood clotting are among products from mammalian cell culture. In association with specific viruses mammalian cells will produce viral vaccines and recombinant proteins used in gene therapy. In 1997, Human embryonic stem cells (HESC) were isolated from discarded human embryos. It is postulated that pluripotent stem cells may be cultured into different cells with the capacity to replace or repair cellular tis￾sues in various human organs. Whether embryo stem cells will realise their hypothetical potential seems to depend as much on legislation influenced by religious belief as on bioscience. Downstream processing ‘Downstream’ relates to all that follows bioreactor synthesis: the isolation, purification and sterilization of end products. Downstream processes are estimated to absorb ca 80% of production costs, indicating an urgent need for more economical downstream technologies and bioengineers competent to design and operate them. Isolation Synthesized substances are isolated from various bioreactor fractions: insulin from harvested cells, some vaccines from supernatant fluids. intra-cellular metabo￾lites are released by mechanical, chemical or enzymatic rupture of cell walls; antibiotics by liquid:liquid extrac￾tion; tolerant volatile substances by fractional distilla￾tion; heat-sensitive enzymes by aqueous phase liquid fractionation. Supercritical gas/liquid extractions (SGE) are useful for substances sensitive to organic solvents or susceptible to oxidation. At pressures between 10,000 and 40,000 kPa, carbon dioxide is a benign solvent for essential oils, oleoresins, natural terpenoids, caffeine, and other sensitive biochemicals. Unlike many organic solvents, SGE leaves no toxic residues. Membrane processing, reverse osmosis, ultrafiltra￾tion, microfiltration, nanofiltration and electrodialysis are among other industrial fractionation technologies. Chromatographic systems include gel filtration, ion￾exchange and affinity separations that use binding interactions between proteins and packing materials, with various ligands coupled into hydrophilic support matrices. Preservation and sterilization Foods are for healthy nourishment; drugs to diag￾nose, prevent or cure disease. It is critical that all foods and drugs be free from organisms that may cause insult or injury to those who consume and use them. Gen￾erally speaking, biological materials such as foods and pharmaceuticals can be preserved by any process that (1) inhibits, destroys or removes and prevents re-entry of pathogenic and microorganisms that cause spoilage; (2) restricts adverse biochemical and biophysical change. Degradation of food and other biological materials can be restricted by packaging under inert atmosphere, by reducing water activity and thermal sterilization. Freeze-drying effectively lowers water activity in sensi￾tive biologicals. Rapid freezing in liquid nitrogen prior to freeze-drying (lyophilisation) restricts cell disruption by slow-growing ice crystals. Thermal processes and alternatives Thermal processing in hermetic containers (cans, bot￾tles, laminated plastics) takes a long time for heat to be conducted throughout the material. Excessive heating of foods and other biologicals can cause adverse change in critical functional properties, nutritional quality, fla￾vour, physical structure and texture. The higher the temperature, the longer the time, the greater the degree of biochemical and biophysical change. Existing processes that reduce heat damage include spray-drying, tubular and scraped surface heat exchan￾gers, and steam injection followed by aseptic packaging Several alternative means of preservation are in various stages of investigation and development. Irradiation Ionising radiations can inactivate microorganisms and kill insects. Radiation sources for food and phar￾maceuticals include gamma rays from radioisotopes Co60 or Ces137, X-rays or electrons generated by machines. Absorbed radiation is measured in Grays or 10 Joseph H. Hulse / Trends in Food Science & Technology 15 (2004) 3–18