General References

A. J. Bailey and N. D. Light, Connective Tissue in Meat and Meat Products, Elsevier Applied Science, London, United Kingdom, 1989.

M. Glicksman, Gum Technology in the Food Industry, Academic

Press, New York 1969. G. Stainsby, Recent Advances in Gelatin and Glue Research, Pergamon Press, New York, 1958. A. Veis, The Macromolecular Chemistry of Gelatin, Academic

Press, New York, 1964. A. G. Ward and A. Courts, The Science and Technology of Gelatin, Academic Press, New York, 1977.

Bernard Cole Leiner David Gelatin Krugersdorp, South Africa

GENETIC ENGINEERING: ANIMALS. See Genetic engineering: animals in the Supplement section.

GENETIC ENGINEERING: FOOD FLAVORS

Traditional food biotechnologies are frequently characterized by the formation of ethanol, acetic acid, propanoic acid, and lactic acid. The resulting products, though different from the original fruit must, flour paste, milk, or meat in many ways, were experienced not only as having no adverse effects on well-being (if consumed in moderation), but they even possessed new, attractive sensory properties. Based on these very roots of biotechnology (1), a great number of nonvolatile flavor compounds, such as acidulants, amino acids, 5'-nucleotides, and certain carbohydrates are now manufactured on an industrial scale (2). Although the enzymatic origins of the volatile flavor fraction of many foods are clearly recognized (3), aroma-producing microorganisms and enzymes thereof have, for a long time, been rather regarded as laboratory oddities (4). Today, the increasing number of papers and patents on the biotechnology of aromas indicates a strong trend in both academic and industrial flavor research (5-26). Increasing confidence in the future of biotechnology (27) and in the so-called soft chemistry approach has merged together with "all-natural" marketing claims into the recent development of pilot and production scale bioprocesses for aroma chemicals (Table 1).

Table 1. Industry Driving Forces

Technical Push

High selectivities or specificity (chemo-, regio-, stereo-) High reaction rate at low molar fractions Environmentally compatible, renewable substrates and mild reaction conditions Multistep syntheses including cofactor regeneration (whole cell)

Business Pull

Health- and nutrition-conscious lifestyles demand "natural" products

Classical plant sources suffer from microbial or insect infestation, sociopolitical instability and depend on seasonal variation, fertilizers, etc Character impact components may possess additional bioactivities and, therefore, often occur in traces only in their plant sources

Volatile flavors and fragrances command a growing (currently ca $10 billion) market (29). The market of food flavors in the United States alone is estimated at over $1 billion and is continuously increasing (21). The resulting demand of the industry for natural ingredients suggests that the agricultural sources with their inherent instabilities will no longer satisfy the need. The physiologically most active constituents of natural flavors, the so-called character impact components, are often present in plants in traces and in bound forms, an additional obstacle for an economic isolation. Recent advances in cell biology and in bioengineering—and particularly the ability to alter the biocatalyst's properties genetically—possess great potential for the flavor industry, as the legal definitions of the Food and Drug Administration and of the Council of the European Communities classify fermentation flavors (with few restrictions) as "natural" compounds. From a chemical viewpoint, the use of a chiral (bio)catalyst would offer the most adequate synthetic approach anyway: The sensory properties of many flavors strongly depend on the chiral structure of the molecules, and the enantiomeric forms of many terpenes, lactones, and so on may exhibit different, sometimes even opposite, sensory characteristics (30).

An estimated number of less than 100 flavor compounds and building blocks are now available on the market, among them carboxylic fatty acids, esters and lactones derived thereof, cheese and yeast flavors, methyl ketones, benzaldehyde, vanillin, cinnamates, and some other character impact components. Particular progress has been made using lipases and certain other hydrolases, whereas the improvement of cell based processes appears more difficult. According to the increasing structural complexity of the biocatalyst, this article highlights some recent achievements, but also current problems of the biotechnology of volatile flavors using enzymes, microbial and fungal cells, and plant cells.

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