Food Microbiology & Safety
Helen Billman-Jacobe
Factors influencing microbial growth in food : Intrinsic factors
Copyright By PowCoder代写 加微信 powcoder
Ray and Bhunia Ch 6
Intended learning outcomes
Distinguish between intrinsic, extrinsic and implicit factors
List the categories of intrinsic factors in food that affect microbial growth
Describe the intrinsic factors of samples of food
Explain how intrinsic factors affect the growth and survival of microorganisms in food
Implicit, Intrinsic and Extrinsic factors
The ability of microbes to grow or multiply in food is determined by their biology (implicit factors), the food environment (intrinsic factors) as well as the environment in which it is stored(extrinsic factors).
Intrinsic factors are factors relating to the food itself
• Nutrients
• Growth factors
• Inhibitors
• Water activity
• Oxidation-reduction potential
Extrinsic factors are factors relating to the environment of food
• Temperature
• Gasous atmosphere • Humidity
Implicit factors are properties of the microbe
• Growth temp
• Oxygen requirement
• Hydrolytic capability
• Ability to compete
• Sensitivity to physical and
chemical damage
optimal growth preferences
Ability to compete with other microbes
Sensitivity to physical and chemical damage
pH and buffering Redox potential
Antimicrobial factors Nutrient content
Metabolic capacity
Implicit factors
Growth rate
Factors affecting microbial growth and survival in food
Extrinsic factors
Intrinsic factors
Gaseous atmosphere
temperature
Water activity
FOOD MICROBES
• • • • • •
Factors influencing microbial growth in food
Intrinsic factors: food related
nutrients,
growth factors,
inhibitors
water activity (Aw)
oxidation-reduction potential (Eh)
Extrinsic factors: environment related
temperature
relative humidity gaseous environment
Intrinsic factors: Nutrients
Microbes derive their nutrients from their immediate environment
Microbes growing in food derive their nutrients from that food e.g. carbohydrates, protein, lipids, minerals, vitamins
Microorganisms present in food in their nutrient requirements and ability to extract nutrients from food
The microbes will need to breakdown complex molecules using extracellular enzymes so they can absorb smaller molecules
E.g. proteases break down proteins into amino acids and peptides
Nutrients: carbohydrates
Carbohydrates present in different foods can be grouped on the basis of their chain length or complexity
Polysaccharides: polymers of glucose units i.e starch, glycogen, cellulose Oligosaccharides polymers of fructose units i.e. inulin
raffinose=glucose+ fructose+ galactose
stachyose =glucose+ fructose+ galactose Disaccharides
lactose= galactose+ glucose sucrose= fructose+ glucose maltose= glucose+ glucose
: glucose, fructose, mannose, galactose Pentoses: xylose, arabinose, ribose, ribulose, xylulose
Intrinsic factors: Nutrient uptake
• Digestion of complex polymeric carbohydrates is often difficult for many microbes (e.g. starch, cellulose) but molds are usually more capable of degrading these.
• In this example the intrinsic factor is the presence of cellulose and the implicit factor is the ability of the fungi to degrade the cellulose
“The archive of the Universidad de maintains a nineteenth-century French collection of drawings and lithographs in which the biodeterioration by fungi is rampant.”
Coronado-Ruiz, Carolina, et al. “Two new cellulolytic fungal species isolated from a 19 th-century art collection.” Scientific reports 8.1 (2018): 1-9.
Composition of wheat
International Journal of Food Science & Technology, Volume: 50, Issue: 12, Pages: 2509-2518, First published: 14 November 2015, DOI: (10.1111/ijfs.12935)
Nutrient uptake: proteins
Proteins are polymers of amino acids
~ 15-18% nitrogen
e.g. albumen (in eggs), globulins (milk), gluten (cereal)
•Bacteria/fungi usually transport short peptides/amino acids into cells
•Many secrete proteases to break down proteins in foods – can be desirable (cheese) or undesirable (spoilage)
•Metabolism of proteins can:
• release ‘off flavour’ compounds like ammonia, H2S • produce toxins like histamine (from histidine)
Intrinsic factors: Nutrients Lipids
Examples of lipids are phospholipids, glycerides, fatty acids, sterols
Generally a less preferred substrate for microbial growth than carbohydrates
Microbes can produce lipases to break down phospholipids to smaller molecules like fatty acids and glycerol, and transport these into the cell for metabolism for carbon and energy
Cell lysis can release intracellular lipases that lead to spoilage
Some plant-based foods are rich in lipids e.g. nuts, oil-seeds, coconuts, olives, avocados Foods rich in cholesterol are typically from animal products
Composition of plant-based oils
Fats and Lipids in various foods
Zock et al 2006 14
Nutrients: Growth factors, minerals and vitamin C
Microbes need small amounts of certain minerals and vitamins
The small amounts are usually present in foods
Not usually a restriction to microbial growth
Most microbes can make organic factors like vitamins (that we can’t!)
Not possible or practical to try and limit growth by restricting the availability of one or more nutrients
Intrinsic factors:
Stimulators and inhibitors of growth
These are compounds are naturally present substances that either stimulate or inhibit microbial growth
They affect the growth of microbes but are not nutrients.
Inhibitors are more common
Lysozyme (high levels in egg white) – this is an enzymes breaks the peptidoglycan of bacterial cell walls. It is a natural defence.
Euganol (cloves). This is an essential oil in the spice. It is a natural defense of the plant against insect attack and microbial attack
Agglutinin, lactoperoxidase (raw milk)
Common antimicrobial plant chemicals
Rosemary α-pinene
Garlic allicin
Mint Menthol
Thyme Thymol
. Microbiol. Rev. 1999; doi:10.1128/CMR.12.4.564
Cloves Eugenol
To be continued
程序代写 CS代考 加微信: powcoder QQ: 1823890830 Email: powcoder@163.com