80 ◾ Fundamental Food Microbiology
capable of conserving the free energy as biochemical energy. Some of these are derivatives of
phosphoric acid (nucleotide triphosphates, acylphosphates, and inorganic polyphosphates) and
derivatives of carboxylic acids (acetyl-coenzyme A). The most important of these is ATP (adenosine triphosphate), which is formed from ADP (adenosine diphosphate) in coupled reactions
through either oxidative phosphorylation or substrate-level phosphorylation.
The energy-liberating oxidation reactions of the substrates generate electrons (H 2 → 2H + +
2e − ), which are then accepted by the oxidizing agents. This aspect is briefly discussed under redox
potential in Chapter 6. In a food system, the substrates are mainly the metabolizable carbohydrates, proteins, and lipids. Microorganisms important in foods are heterotrophs (i.e., they require
organic carbon sources, substances more reduced than CO 2 ) and chemoorganotrophs (i.e., they
use organic compounds as electron donors to generate energy). On the basis of the nature of the
terminal electron acceptors, there are different energy-generating reactions: aerobic respiration,
which requires molecular oxygen as the electron acceptor; anaerobic respiration, in which inorganic compounds act as the electron acceptors; and fermentation, which uses organic compounds
as electron acceptors. 1,2 The methods used for electron transfer by different groups of microorganisms important in food can be illustrated by the following scheme:
Organic compounds
Inorganic compounds (NO 3 , SO 4 )
1
5
2
6
3
4
H 2 O 2 or O 2
Substrates (2H:NAD/FD) cytochrome(s) cyt.a O2
7
Aerobes (some Bacillus spp., Pseudomonas, molds, and yeasts): 1, 2, 3, 4
Anaerobes
a. Cyt. independent, for example, some Clostridium spp.: 1, 5
b. Cyt. dependent, for example, Desulfotomaculum spp.: 1, 2, 6
Facultative anaerobes
a. Cyt. independent, for example, lactic acid bacteria, some yeasts
i. Aerobic: 1, 7
ii. Anaerobic: 1, 5
b. Cyt. dependent, for example, Enterobacteriaceae group
i. Aerobic: 1, 2, 3, 4, or 1, 7
ii. Anaerobic: 1, 5 or 1, 2, 6
The energy-generating metabolic pathways also produce (from the substrates) many metabolic
products that the microbial cells either use for the synthesis of cellular components or release into
the environment. The nature of these metabolites differs greatly and depends on the nature of the
substrates, the type of microorganisms with respect to their aerobiosis nature, and the oxygen
availability (more correctly, redox potential) of the environment. The metabolism and growth of
capable of conserving the free energy as biochemical energy. Some of these are derivatives of
phosphoric acid (nucleotide triphosphates, acylphosphates, and inorganic polyphosphates) and
derivatives of carboxylic acids (acetyl-coenzyme A). The most important of these is ATP (adenosine triphosphate), which is formed from ADP (adenosine diphosphate) in coupled reactions
through either oxidative phosphorylation or substrate-level phosphorylation.
The energy-liberating oxidation reactions of the substrates generate electrons (H 2 → 2H + +
2e − ), which are then accepted by the oxidizing agents. This aspect is briefly discussed under redox
potential in Chapter 6. In a food system, the substrates are mainly the metabolizable carbohydrates, proteins, and lipids. Microorganisms important in foods are heterotrophs (i.e., they require
organic carbon sources, substances more reduced than CO 2 ) and chemoorganotrophs (i.e., they
use organic compounds as electron donors to generate energy). On the basis of the nature of the
terminal electron acceptors, there are different energy-generating reactions: aerobic respiration,
which requires molecular oxygen as the electron acceptor; anaerobic respiration, in which inorganic compounds act as the electron acceptors; and fermentation, which uses organic compounds
as electron acceptors. 1,2 The methods used for electron transfer by different groups of microorganisms important in food can be illustrated by the following scheme:
Organic compounds
Inorganic compounds (NO 3 , SO 4 )
1
5
2
6
3
4
H 2 O 2 or O 2
Substrates (2H:NAD/FD) cytochrome(s) cyt.a O2
7
Aerobes (some Bacillus spp., Pseudomonas, molds, and yeasts): 1, 2, 3, 4
Anaerobes
a. Cyt. independent, for example, some Clostridium spp.: 1, 5
b. Cyt. dependent, for example, Desulfotomaculum spp.: 1, 2, 6
Facultative anaerobes
a. Cyt. independent, for example, lactic acid bacteria, some yeasts
i. Aerobic: 1, 7
ii. Anaerobic: 1, 5
b. Cyt. dependent, for example, Enterobacteriaceae group
i. Aerobic: 1, 2, 3, 4, or 1, 7
ii. Anaerobic: 1, 5 or 1, 2, 6
The energy-generating metabolic pathways also produce (from the substrates) many metabolic
products that the microbial cells either use for the synthesis of cellular components or release into
the environment. The nature of these metabolites differs greatly and depends on the nature of the
substrates, the type of microorganisms with respect to their aerobiosis nature, and the oxygen
availability (more correctly, redox potential) of the environment. The metabolism and growth of