Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL METABOLISM OF FOOD COMPONENTS 85
amount of monosaccharides, type of microorganisms, and redox potential of the
system. Although all microorganisms important in foods can metabolize glucose,
they differ greatly in their ability to utilize fructose, galactose, tetroses, and pentoses.
Fermentable monosaccharides are metabolized by five major pathways, and many
microbial species have more than one pathway. They are the Embden–Meyerhoff–Parnas
(EMP) pathway, the hexose monophosphate shunt (HMS) or pathway,
the Entner–Doudroff (ED) pathway, and two phosphoketolase (PK) pathways (pentose
phosphoketolase and hexose phosphoketolase). Pyruvic acid produced via these
pathways is subsequently metabolized by microorganisms in several different pathways
through fermentation, anaerobic respiration, and aerobic respiration. 1–5
D. Fermentation
Monosaccharides are fermented by anaerobic and facultative microorganisms by the
five major pathways mentioned previously. 1–3 In addition, several other pathways
are used, especially for the metabolism of pyruvate, by some specific microbial
species and groups. In general, the terminal electron acceptors are organic compounds,
and energy is produced at the substrate level. The overall reactions of the
five main pathways and the end products are briefly listed in Table 7.1 (also in Table
8.1). 1–3 The metabolic pathways are discussed in more detail in Chapter 11.
1. EMP Pathway
The EMP pathway is used by homofermentative lactic acid bacteria, Enterococcus
faecalis, Bacillus spp., and yeasts.
\
Glucose phosphate Æ Fructose diphosphate Æ 2 Pyruvate
Lactate fermentation: Pyruvate Æ Lactate
Alcohol fermentation: Pyruvate Æ Acetaldehyde Æ Ethanol
Ø
CO 2
Table 7.1 End Products of Carbohydrate Metabolism by Some Microorganisms
Microbial Type Fermentation Pattern Major End Products
Yeasts Alcohol Ethanol, CO 2
Lactic acid bacteria Homofermentative Lactate
Heterofermentative Lactate, acetate, ethanol,
CO 2, diacetyl, acetoin
Bifidobacteria Bifidus (hexose ketolate) Lactate, acetate
Propionibacteria Propionic acid Propionate, acetate, CO 2
Enterobacteriaceae Mixed acid Lactate, acetate, formate,
CO 2, H 2, succinate
Bacillus, Pseudomonas Butanediol Lactate, acetate, formate,
2,3-butanediol, CO 2, H 2
Clostridium Butyric acid Butyrate, acetate, H 2, CO 2,
butanol, ethanol, acetone,
isopropanol