Pasteur point

The next stage in the development of the atmosphere began when formation of oxygen exceeded its consumption (Mason, 1971). This occurred in the period between 1.7 and 1.2 b.y. ago at about 1 b.y., at the boundary of the Proterozoic and Paleozoic, the amounts of Oj reached the level of the Pasteur point ( 1% of the present oxygen content), as a result of which more complex organisms could appear in the biosphere. The CO2 content at this stage was controlled by carbonate-silicate equilibria in the hydrosphere. 

Fig, 35. Stages in the evolution of animals (after Fisher) I = animals live in total respiratory dependence on the number of plants the atmosphere is practically devoid of free oxygen. II = the atmosphere reaches the Pasteur point , animals can leave plants but seek water-air interface. Ill = the atmosphere and water contain a large amount of oxygen, animals are widespread. 

When O2 is lowered below the Pasteur point, anaerobic metabolism induces accumulation of AA and ethanol, which can lead to development of off-flavours (Fidler and North 1971). 

We only briefly mentioned alkaline stabilization, but in reality this is a variation of sludge pasteurization. The basic process uses elevated pH and temperature to produce a stabilized, disinfected product. The two alkaline stabilization systems most common in the U.S. are a lime pasteurization system and a cement kiln dust pasteurization system. The lime pasteurization product has a wet-cake consistency, while the kiln dust pasteurization has a moist solid like consistency. Both products can be transported to agricultural areas for ultimate use. Literature studies show that the kiln dust product can capture a marketable value of 6.60/Mg ( 6.00/ton) to offset hauling costs, while the lime product does not appear to be able to capture financial credits for product revenues at this point in time. The reasons for this are not entirely clear. 

Today, we would describe Pasteur s work by saying that he had discovered enantiomers. Enantiomers, also called optical isomers, have identical physical properties, such as melting point and boiling point, but differ in the direction in which their solutions rotate plane-polarized light. 

Demott, B. J. 1967. The influence of vacuum pasteurization upon the freezing point and specific gravity of milk. Milk Food Technol. 30, 253-255. 

All of the threonine stereoisomers 19-22 are chiral substances that is, they are not identical with their mirror images. However, it is important to recognize that not all diastereomers are chiral. To illustrate this point, we return to the tartaric acids mentioned previously in connection with Pasteur s discoveries (Section 5-1C). 

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