Product-Limiting Microbial Fermentation

Chapter 28 Microbial Fermentation—Introduction and Overall Picture 1623 Chapter 29 Substrate Limiting Microbial Fermentation /630 Chapter 30 Product Limiting Microbial Fermentation /645... 

Traditional fermentation using microbial activity is commonly used for the production of nonvolatile flavor compounds such as acidulants, amino acids, and nucleotides. The formation of volatile flavor compounds via microbial fermentation on an industrial scale is still in its infancy. Although more than 100 aroma compounds may be generated microbially, only a few of them are produced on an industrial scale. The reason is probably due to the transformation efficiency, cost of the processes used, and our ignorance to their biosynthetic pathways. Nevertheless, the exploitation of microbial production of food flavors has proved to be successful in some cases. For example, the production of y-decalactone by microbial biosynthetic pathways lead to a price decrease from 20,000/kg to l,200/kg U.S. Generally, the production of lactone could be performed from a precursor of hydroxy fatty acids, followed by p-oxidation from yeast bioconversion (Benedetti et al., 2001). Most of the hydroxy fatty acids are found in very small amounts in natural sources, and the only inexpensive natural precursor is ricinoleic acid, the major fatty acid of castor oil. Due to the few natural sources of these fatty acid precursors, the most common processes have been developed from fatty acids by microbial biotransformation (Hou, 1995). Another way to obtain hydroxy fatty acid is from the action of LOX. However, there has been only limited research on using LOX to produce lactone (Gill and Valivety, 1997). 

The second limitation in the use of wood is that microbial fermentation of wood products is slow. While it has been shown that Solka Floe (trade mark of the Brown Paper Co., Montreal, Quebec) an filter paper powder can be completely digested if exposed to rumen fluid for 72 hours, the particle size of these is such that they are likely to pass from the... 

Methionine was first reported from casein in 1922 by Mueller. It is a limiting amino acid in the monogastric s feed and the addition of synthetic methionine in animal feed started from the 1950s. The addition of amino acids in the feed increases the nutritional quality and conversion efficiency of low protein feed and hence lowers the feed cost. Methionine is commercially produced by either chemical synthesis, enzymatic methods or microbial fermentation. Methionine has an advantage that it can be supplied to animal feed as a chemically produced racemate or a racemic mixture as the mammals are able to convert it to utilizable form with a methionine racemase enzyme. Chemical production uses harmful chemicals and production from protein hydrolysates requires several separation steps. Chemical synthesis produces a racemic mixture and is acetylated to produce L-methionine. Microbial fermentation overcomes these difficulties and has added advantages over the racemate that it helps optimal nutrient utihzation. 

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