Plant Catalysts

Most of the volatile flavors processed by the food industry are directly or indirectly derived from plant metabolism. Pressing, distillation, extraction, and chromatography are the common techniques for obtaining raw materials of natural flavors from intact plants.

Crude Enzymes and Homogenates

Crude enzyme preparations from fruits and vegetables were able to partly restore the fresh odor impression in processed food by converting nonvolatile odorless precursors to volatile flavors. Developed in the late fifties, this so-called flavorese-enzyme concept failed from economic and biogenetic reasons. Successors used by-products of food processing as a less expensive source of enzymes. Following up earlier work with apple peel, a Hungarian group developed a preparative scale generation of apple flavor (84). A vigorous formation of the volatile C6-compounds typical of plant lipoxygenase/hydroperoxide lyase systems occurred in homogenates of some apple cultivars and grass species. The yield of the main volatile compound, (2E)-hexenal (pleasant fresh-green note), reached some 100 mg kg"1 h"1 (85,86).

Progress toward understanding how fatty acid peroxides are obtained from lipoxygenase action (87), how the products can be analyzed (88), and how these precursors are further transformed by a cytochrome-type lyase (89)

has culminated in the development of immobilized biocat-alysts (90,91). Patents describe, for example, the generation of (3Z)-hexenol by subjecting linolenic acid and watermelon foliage to mechanical shearing in the presence of a yeast (92), or the generation of several green note compounds using a lyase from guava (93). This biosynthetic principle has been extended to the cooxidation of carrot oleoresin (rich in carotenoids) by fatty acid hydroperoxides to obtain oxidative breakdown products, such as ionones (fruity-violet odors) or dihydroactinidiolide (fragrant-woody notes) (94). The genetic engineering approach holds a lot of promise for these bioprocesses, because the single enzymes involved are well known. The operational instability of plant lyases may be improved by enzyme engineering in the future.

Callus and Suspension Culture

Derived from a wounding-induced tissue (primary callus), a continuously growing sterile culture of plant cells can be established by mechanical separation and further subcul-turing on a synthetic medium (callus culture). After transfer to a liquid medium, cells of a macroorganism are finally reduced to the single-cell level (suspension culture). According to its origin, the somatic cell is provided with a complete genom and able to develop into a whole plant again. This totipotency was translated into the idea that all genetic functions, including, for example, essential oil and aroma formation, would be expressed in such cells (95). However, numerous disappointing experiments have caused the industry to almost completely refrain from further investments into this kind of plant biotechnology. The absence of flavor compounds in cell cultures of flavor-yielding plants has been attributed to a lack of morphologically differentiated structures (eg, oil glands, resin ducts, etc), or to a rapid catabolism of the presumed intermediates. Detailed studies on the synthesis of lower terpenes in callus cultures of essential oil bearing plants showed all species to possess high prenyltransferase and pyrophosphate isomerase activities, and most of the cultures to possess further activities of the mevalonate pathway, although a significant accumulation of products did not occur.

A physical stimulus to secondary metabolism in plant cells has been neglected for a long time: light. Although not always sufficient, the cytodifferentiation from heterotrophic to phototrophic cells induced the formation of leaf-typical constituents, such as pigments, quinones, and essential oils. Chlorophyll formation in the presence of light is tied to a preceding formation of phytol; this diterpenoid alcohol in turn can only be formed if the initial steps of the mevalonate pathway are operating. The accumulation of the intermediate lower terpenoids along this route is then to be expected. In addition to light a fine-tuning of the phy-toeffector concentrations in the growth medium is mandatory to support both growth and aroma formation, as was shown with illuminated callus cultures of Citrus (96). These cells accumulated a full spectrum of volatile terpenes, and the best yields were about 5% of the volatiles isolated from a mature tissue of the mother explant (grapefruit peel; Table 8). Addition of precursors resulted in con-

Table 8. Some Volatile Constituents of in vitro cells of

Citrus paradisi

Table 8. Some Volatile Constituents of in vitro cells of

Citrus paradisi

Compound, mg kg"1 fresh wt

Low phytoeffector conc

High phytoeffector conc


Aromatherapy Ambiance

Aromatherapy Ambiance

Aromatherapy, a word often associated with calm, sweet smelling and relaxing surroundings. Made famous for its mostly relaxing indulgent  feature, using aromatherapy has also been known to be related to have medicinal qualities.

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