Metabolism volatile products

Ethylene coordinates the expression of genes responsible for enhanced respiratory metabolism, chlorophyll degradation, carotenoid synthesis, conversion of starch to sugars, increased activity of cell wall-degrading enzymes, aroma volatile production, and so on. All these events stimulate a series of biochemical, physiological, and structural changes making fruits mature and attractive to the consumer. 

At the present time, the generation of volatiles by microorganisms is more of an academic curiosity than a commercial reality. Although the metabolic events leading to volatile production have been described in many microorganisms, few processes have been scaled-up to commercially practiced processes. This is due to a variety of technical difficulties ... 

Figure 5 shows the influence of harvest time and storage on total volatiles production. Since the production of many but not all of the aroma volatiles is linked to amino acid precursors it may be expected that the total volatiles behavior may reflect that of the amino acids especially those which supply many of the carbon skeletons for the esters found in melons. The pattern of behavior for the total volatiles is generally in accord with that of these amino acids and does very clearly illustrate the profound influence of harvest time on the generation of the aroma profile. Fruit harvested only two days before folly ripe develops only about one quarter of the total volatiles concentration shown a few days postharvest, by a folly ripe sample. This rather dramatic difference may reflect the inability of prematurely harvested fiuit to accumulate sufficient concentrations of required volatiles substrates because certain metabolic responses have not been activated. 

Bakers yeast (Saccharomyces cerviseae) has also been used in combination with mass spectrometry thereby assaying volatile products from cell metabolism (39). The basic concept here is that the living cells are kept in a flow system continuously fed with buffer or substrate and that any volatile reactant passes through a semi permeable membrane into the evacuated room containing an ionizer and a quadrepole mass spectrometer (Fig. 12)... 

The increased formation of AA by C02-treated fruits is probably due to dark CO2 fixation that can contribute to higher volatile production via fixation of CO2 to malic acid by malic enzyme (Pesis and Ben Arie 1986). Plant tissue can easily metabolize applied malate or pyruvate to produce AA (Clijsters 1965 Pesis and Ben Arie 1986). In addition, it has been shown that under an anaerobic CO2 atmosphere there is a stimulation of pyruvate decaboxylase (PDC) activity in sweet potato roots and the levels of PDC are higher than under an N2 atmosphere (Chang et al. 1983). 

Antibiosis Inhibition or lysis of an organism mediated by metabolic products of the antagonist these products include lytic agents, enzymes, volatile compounds, and other toxic substances. 

N-Nitrosamines are formed during processing and smoking of tobacco products. Proteins, agricultural chemicals and alkaloids in tobacco products serve as major precursors for volatile, nonvolatile, and tobacco-specific nitrosamines (Figure 1). In this review we will summarize the progress achieved in respect to tobacco nitrosamines since the last ACS symposium in Boston in June of 1978 (J ). Additional papers will review the metabolism of cyclic N-nitrosamines, including that of N -nitrosonornicotine 1) and the correlation between tobacco and alcohol consumption and cancer of the upper alimentary tract (J ). 

Commercial mixtures of surfactants consist of several tens to hundreds of homologues oligomers and isomers. Their separation and quantification is complicated and a cumbersome task. Detection, identification and quantification of these compounds in aqueous solutions, even in the form of matrix-free standards, present the analyst with considerable problems. The low volatility and high polarity of some surfactants and their metabolites hamper the application of gas-chromatographic (GC) methods. GC is directly applicable only for surfactants with a low number of ethylene oxide groups and to some relatively volatile metabolic products, while the analysis of higher-molecular-mass oligomers is severely limited and requires adequate derivatisation. 

Off-gas analysis is widely used in many industrial fermentation plants to determine the cellular activity of growing cultures by monitoring respiration. One can measure oxygen uptake and CO2 production rates and thus measure metabolic activity/9 In addition, off-gas analysis is also used for monitoring other volatiles, the synthesis of which are strongly dependent on cultivation conditions 10 and product formation. 11 Off-gas estimation and control therefore serves as an indirect method for process analysis and control. 

Closed flow-through laboratory-scale systems appear to have the greatest potential for analogy to current terrestrial metabolism chambers. While obviously more complex than static aquaria, they allow closer control of environmental variables such as oxygenation and volatilization, adaptability to a wide range of species, maximum freedom from remetabolism, and improved collection of waste products. Dosing can be accomplished through immersion or, beforehand, by injection or intubation where appropriate. On the other hand, none of the four types of metabolism system is ideal, and the most complete data probably will be derived from a combination of methods. 

An example of beneficial metabolism is illustrated by the conversion of toluene, a volatile chemical present in petroleum products and readily absorbed through the lungs, to benzoic acid, as shown ... 

Gases and highly volatile chemicals can move out of the blood into the lungs, and be exhaled. Carbon dioxide, for example, is a metabolic product of many chemicals and also derives from the metabolism of essential molecules it is excreted from the body through the lungs. 

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