Food Microbiology &
Helen Billman-Jacobe
Microbial metabolism
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Ray and Bhunia Ed 5 Ch 8
Todar http://textbookofbacteriology.net/structure_8.html
Intended learning outcomes
Give examples of substrates that microorganisms can use for generating energy and cellular components Explain why some metabolites are desirable and others are not desirable in food
Identify the cellular compartment where monosaccharides are degraded
Name the three types of carbohydrate metabolism (fermentation, anaerobic respiration and aerobic respiration)
Microbial metabolism
Bacterial growth in food occurs though the metabolism of food components or nutrients
• occurs in the cytoplasm and cytoplasmic membrane
• involves the transport of nutrients from the environment through the cell wall and cell membrane and into the cytoplasm
• the breakdown of nutrients generate energy and provide building blocks for growth
• the release of unusable end products into the environment
http://textbookofbacteriology.net/structure_8.html
Lactose Phosphotransfer system
Out permease
In Lactose
Lactose-6-P
Galactose-6-P P
Glucose glycolysis
Generating energy
Microorganisms synthesise energy and cellular materials during growth in food The energy producing reactions are oxidation reactions
The sequence of reactions are called metabolic pathways
The metabolic pathways generate energy from an organic substrate
Energy liberating oxidation reactions generate electrons
H2 2H+ + 2e- The electron are accepted by oxidising agents
Generating energy
Microorganisms synthesise energy and cellular materials during growth in food The energy producing reactions are oxidation reactions
The sequence of reactions are called metabolic pathways
The metabolic pathways generate energy from an organic substrate
Energy liberating oxidation reactions generate electrons
H2 2H+ + 2e- The electron are accepted by oxidising agents
Generating energy
H2 2H+ + 2e-
The electron are accepted by oxidising agents
Different energy generating reactions have terminal electron acceptors Aerobic respiration = e- acceptor is oxygen
Fermentation = variety of different organic compounds can act as e- acceptors
Microbial metabolic products
Metabolites
Energy generating metabolic pathways produce cellular components and compounds that are released into the environment
The type of metabolites produced varies greatly and depends on the substrate and oxygen availability
Consequences of microbial metabolites in food Undesirable
spoilage ( loss of flavour, texture, colour or appearance; organoleptic properties) toxin production ( bacterial toxins, mycotoxins)
Enzymes for food processing
Bio preservatives ( bacteriocins and acids) Flavours (diacetyl, acetoin, lactic acid, acetic acid) Alcohol
Polysaccharides
Microbial metabolic endproducts
Substrates
Microorganisms use the organic carbon sources are growth substrates Growth substrates available in food
Carbohydrates Proteins
Meat and fish:
Molluscs, milk:
rich in carbohydrates, some (soy) rich in protein, some rich in oil rich in proteins, low in carbohydrates
rich in protein and carbohydrates
Processed food can have all the nutrients in sufficient quantities to support microbial
Microbes generally metabolise carbohydrates before proteins or lipids Metabolism of carbohydrates produces acid
Microbial metabolic endproducts
Carbohydrates
Polysaccharides (starch, glycogen, cellulose, pectin)
• converted into monosaccharides, disaccharides, and trisaccharides outside the cell.
• Mono-, di-, and tri- separates are transported into the cell
• hydrolysed into monosaccharides in the cytoplasm
Monosaccharides are metabolised by aerobic, anaerobic and facultative anaerobic microorganisms are different pathways and produce different metabolic endproducts
All microorganisms can metabolise glucose Microorganisms differ in their ability to use other sugars
Yeast using different sugars
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