Growth And Product Formation

Anaerobic growth and product formation are often investigated together when the product is growth associated. In anaerobic fermentations, adenosine triphosphate (ATP) needed for growth is often formed as a result of substrate phosphorylation. Products such as ethanol are produced because the organism is growing and needs ATP for synthesis of microbial cell mass. Mass balances and yield parameters that describe growth and product formation can be written in terms of the mass of substrate, biomass, and product or in terms of the equivalents of available electrons of each of these.

Consider the anaerobic production of a simple product, such as ethanol, by anaerobic fermentation. The chemical balance equation is as follows:

where CHmOl, CHpO„Nç, and CHrOsNt give the elemental compositions of the carbon, hydrogen, oxygen, and nitrogen in the substrate, biomass, and extracellular product, respectively. For glucose as substrate, m = 2 and l = 1; for ethanol as product, r = 3, s = 0.5, and t = 0. For the valences C = 4, H = 1, O = —2, and N = —3, the available electron balance (2,9) is yc£H + z LE = 1.0

where y is the reductance degree (2,9) and the subscripts s, b, and p refer to substrate, biomass, and product, respectively.

Luedeking and Piret (10,11) reported that product formation kinetics and growth kinetics were related; product or

Table 3. Fermented Food Products and Associated Microorganisms

Product

Microorganisms

Reference

Yogurt Sauerkraut

Cucumber fermentation (pickles) Sausage

Bread and sourdough bread

Cheddar cheese Swiss cheese

Blue cheese

Camembert cheese

Cottage cheese

Acidophilus milk Kefir

Buttermilk and sour cream

Wine

Beer

Soy sauce

Miso

Tempeh

Streptococus thermophilus Lactobacillus bulgaricus Enterobacter cloacae Erwinia herbicola Leuconostoc mesenteroides Lactobacillus brevis Lactobacillus plantarum Pediococcus cerevisiae Lactobacillus brevis Lactobacillus plantarum Pediococcus cerevisiae Leuconostoc mesenteroides Lactobacillus Leuconostoc Pediococcus

Saccharomyces cerevisiae Lactobacillus Candida milleri Leuconostoc mesenteroides Saccharomyces cerevisiae Streptococcus (Lactococcus) cremoris Streptococcus (Lactococcus) lactis Lactobacillus bulgaricus Streptococcus thermophilus Propionibacterium shermanii Streptococcus (Lactococcus) lactis Streptococcus (Lactococcus) cremoris Penicillium roquiforti Streptococcus (Lactococcus) lactis Streptococcus (Lactococcus) cremoris Penicillium camemberti Streptococcus (Lactococcus) lactis Streptococcus (Lactococcus) cremoris Lactobacillus acidophilus Streptococcus (Lactococcus) lactic Streptococcus (Lactococcus) cremoris Torula or Candida yeast

Streptococcus (Lactococcus) lactis, (including subspecies diacetylactis) Streptococcus (Lactococcus) cremoris Leuconostoc cremoris Saccharomyces cerevisiae Saccharomyces uvarum Other Saccharomyces Saccharomyces cerevisiae Succharomyces uvarum Asperigillus oryzae Saccharomyces rouxii Torulopsis versatilis Pediococcus soyae Pediococcus halophilus Saccaromyces rouxii Torulopsis

Streptococcus faecalis Rhizopus oligosporus

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