food microbiology n.
Skip this Video
Loading SlideShow in 5 Seconds..
Food Microbiology PowerPoint Presentation
Download Presentation
Food Microbiology

Loading in 2 Seconds...

play fullscreen
1 / 67

Food Microbiology - PowerPoint PPT Presentation

  • Uploaded on

Food Microbiology . Dr M.Altamimi. Is it a big deal?. I ndian police have arrested the headteacher of the school where 23 children died after eating food contaminated with a pesticide last week. World's best restaurant Noma (Denmark) gives 70 customers food poisoning.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
Download Presentation

PowerPoint Slideshow about 'Food Microbiology' - angie

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
food microbiology

Food Microbiology

Dr M.Altamimi

is it a big deal
Is it a big deal?

Indian police have arrested the headteacher of the school where 23 children died after eating food contaminated with a pesticide last week.

World's best restaurant Noma (Denmark) gives 70 customers food poisoning

history and development of food microbiology
History and Development of FoodMicrobiology

Food Fermentation

  • 1822 C.J. Person named the microscopic organism found on the surface of wine during vinegar production as Mycodermamesentericum.
  • Pasteur in 1868 proved that this organism was associated with the conversion of alcohol to acetic acid and named it Mycodermaaceti.
  • In 1898, MartinusBeijerinck renamed it Acetobacteraceti.
  • 1837 Theodor Schwann named the organism involved in sugar fermentation as Saccharomyces (sugar fungus).
  • 1838 Charles Cogniard-Latour suggested that growth of yeasts was associated with alcohol fermentation.
  • 1860 Louis Pasteur showed that fermentation of lactic acid and alcohol from sugar was the result of growth of specific bacteria and yeasts, respectively.
  • 1883 Emil Christian Hansen used pure cultures of yeasts to ferment beer.

Food Spoilage

  • 1804 Francois Nicolas Appert developed methods to preserve foods in sealed glass bottles by heat in boiling water. He credited this process to LazzaroSpallanzani(1765), who first used the method to disprove the spontaneous generation theory.
  • 1819 Peter Durand developed canning preservation of foods in steel cans.
  • Charles Mitchell introduced tin lining of metal cans in 1839.
  • 1870 L. Pasteur recommended heating of wine at 145 F (62.7°C) for 30 min to destroy souring bacteria. F. Soxhlet advanced boiling of milk for 35 min to kill contaminated bacteria. Later, this method was modified and named pasteurization, and used to kill mainly vegetative pathogens and many spoilage bacteria.
  • 1895 Harry Russell showed that gaseous swelling with bad odors in canned peas was due to growth of heat-resistant bacteria (spores).

Foodborne Diseases

  • 1820 Justin Kerner described food poisoning from eating blood sausage (due to botulism). Fatal disease from eating blood sausage was recognized as early as A.D. 900.
  • 1849 John Snow suggested the spread of cholera through drinking water contaminated with sewage.
  • In 1854, FilippoFacini named the cholera bacilli as Vibrio cholera, which was isolated in pure form by Robert Koch in 1884.
  • 1856 William Budd suggested that water contamination with faeces from infected person spread typhoid fever and advocated the use of chlorine in water supply to overcome the problem.
  • In 1800, G. de Morveau and W. Cruikshank advocated the use of chlorine to sanitize potable water.

1885 Theodor Escherich isolated Bacterium coli (later named Escherichia coli) from the feces and suggested that some strains were associated with infant diarrhea.

  • 1888 A.A. Gartner isolated Bacterium (later Salmonella) enteritidisfrom the organs of a diseased man as well as from the meat the man ate.
  • In 1896, Marie von Ermengem proved that Salmonella enteritidiscaused a fatal disease in humans who consumed contaminated sausage.
  • 1894 J. Denys associated pyogenic Staphylococcus with death of a person who ate meat prepared from a diseased cow.
  • 1895 Marie von Ermengem isolated Bacillus botulinus(Clostridium botulinum) from contaminated meat and proved that it caused botulism.

Microbiology Techniques

  • 1854 Heinrich Schröder and Theodore von Dusch used cotton to close tubes and flasks to prevent microbial contamination in heated culture broths.
  • 1876 Car Weigert used methylene blue (a synthetic dye) to stain bacteria in aqueous suspensions.
  • 1877 Ferdinand Cohn showed heat resistance of Bacillus subtilisendospores.
  • 1878 Joseph Lister isolated Streptococcus (now Lactococcus) lactisin pure culture by serial dilution from sour milk.
  • 1880s Robert Koch and his associates introduced many important methods that are used in all branches of microbiology, such as solid media (first gelatin, then agar) to purify and enumerate bacteria, Petri dish, flagellarstaining, steam sterilization of media above 100 C, and photography of cells and spores.
  • 1884 Hans Christian Gram developed Gram staining of bacterial cells.
current status

Food Spoilage

Food Fermentation



Foodborne Diseases


your role as food microbiologist

• Determine microbiological quality of foods and food ingredients by using appropriate techniques

• Determine microbial types involved in spoilage and health hazards and identify the sources

• Design corrective procedures to control the spoilage and pathogenic microorganisms in food

• Learn rapid methods to isolate and identify pathogens and spoilage bacteria from food and environment


• Identify how new technologies adapted in food processing can have specific microbiological problems and design methods to overcome the problem

• Design effective sanitation procedures to control spoilage and pathogen problems in food-processing facilities

• Effectively use desirable microorganisms to produce fermented foods

• Design methods to produce better starter cultures for use in fermented foods and probiotics

• Know about food regulations.

• Understand microbiological problems of imported foods

characteristic of predominant microorganisms in food
Characteristic of predominant microorganisms in food

Bacteria, yeasts, molds, and viruses are important in food for their ability to:

  • cause foodborne diseases.
  • food spoilage.
  • produce food and food ingredients.


With nucleus

Without nucleus



Eubacteria (bacteria)


Gram stain

Protein profile

Base composition C+G % (10%)

DNA RNA hybridization (90%)

Nucleotide sequence.



Metabolic patterns

yeast vs molds
Yeast vs Molds

Yeasts are widely distributed in nature. The cells are oval, spherical, or elongated, about 5–30 X 2–10 µm in size They are nonmotile. The cell wall

contains polysaccharides (glycans), proteins, and lipids. The wall can have scars, indicating the sites of budding. The membrane is beneath the wall.. The nucleus is well defined

with a nuclear membrane.

Molds are nonmotile, filamentous, and branched . The cell wall is

composed of cellulose, chitin, or both. A mold (thallus) is composed of large numbers of filaments called hyphae. An aggregate of hyphae is called mycelium. A hypha can be

vegetative or reproductive. The reproductive hypha usually extends in the air and form exospores, either free (conidia) or in a sack (sporangium).

Shape, size, and

color of spores are used for taxonomic classification.

  • unicellular, most ca. 0.5–1.0 X 2.0–10 µm in size.
  • have three morphological forms: spherical (cocci), rod shaped (bacilli), and curved (comma).
  • The ribosomes are 70S type and are dispersed in the cytoplasm.
  • The genetic materials (structural and plasmid DNA) are circular, not enclosed in nuclear membrane, and do not contain basic proteins such as histones.

On the basis of Gram-stain behavior, bacterial cells are grouped as Gram-negative Gram-positive.

  • Gram-negative cells have a complex cell wall containing an outer membrane (OM) and a middle membrane (MM).
  • OM is resistant to many antibiotics, hydrophilic compounds and enzymes.
important microorganisms in food

1- Molds

Fusarium. Many types are associated with rot in citrus fruits, potatoes, and grains.

They form cottony growth.

Aspergillus. It is widely distributed and contains many species important in food. Able to grow in low Aw such as grains,

causing spoilage. They are also involved in spoilage of foods such as jams, and fruits and vegetables. Produce aflatoxin, some can be food additives.

Rhizopus. They cause

spoilage of many fruits and vegetables. Rhizopusstolonifer is the common black

bread mold.

Penicillium. form conidiophores on a blue-green,brushlike conidia head. Some species are used in food production, such as Penicilliumroquefortii

and Pen. camembertii in cheese.

2 yeast genera
2-Yeast Genera

Saccharomyces. Cells are round, oval, or elongated. Saccharomycescerevisiae

are used in baking for leavening bread and in alcoholic fermentation. They also

cause spoilage of food, producing alcohol and CO2.

Zygosaccharomyces. Cause spoilage of high-acid foods, such as sauces, ketchups,

pickles, mustards, mayonnaise, salad dressings, especially those with less acid and

less salt and sugar (e.g., Zygosaccharomycesbailii).

Candida. Many species spoil foods with high acid, salt, and sugar and form pellicles

on the surface of liquids. Some can cause rancidity in butter and dairy products

(e.g., Candida lipolyticum).

3 viruses
  • Hepatitis A,B,C.
  • Enteric virus.
  • Poliovirus.
  • Bacteriophage.
4 g bacteria
4-G (-) bacteria.

Brucella. Coccobacillii, mostly single; nonmotile. Different species

cause disease in animals, including cattle, pigs, and sheep. They are also human pathogens and have been implicated in foodborne brucellosis. Brucellaabortus

causes abortion in cows.

Campylobacter. Two species, Campylobacter jejuni and Cam. coli, are foodbornepathogens. microaerophilic, helical, motile cells found in the

intestinal tract of humans, animals, and birds.

Escherichia. Straight rods; motile or nonmotile; mesophiles. Found in the

intestinal contents of humans, warm-blooded animals, and birds. Many strains nonpathogenic, but some strains pathogenic to humans and animals and involved in foodborne diseases. Used as an indicator of sanitation: Escherichia


Pseudomonas. aerobes; motile rods; psychrotrophs(grow at low temperatures).

Acetobacter. Ellipsoid to rod-shaped, occur singly or in short chains; motile or nonmotile; aerobes; oxidize ethanol to acetic acid; mesophiles. Cause souring of alcoholic beverages and fruit juices and used to produce vinegar.


Salmonella. Medium rods ; usually motile; mesophiles. There are over

2000 serovarsand all are regarded as human pathogens. Found in the intestinal

contents of humans, animals, birds, and insects. Major cause of foodborne diseases.

Species: Salmonella enterica ssp. enterica.

Vibrio. Curved rods ; motile; mesophiles. Found in freshwater and marine environments. Some species need NaCl for growth. Several species are pathogens and have been involved in foodborne disease (Vibriocholerae,Vib.

parahaemolyticus, and Vib. vulnificus), whereas others can cause food spoilage

(Vib. alginolyticus).

Shigella. Medium rods; nonmotile; mesophiles. Found in the intestine of humans and

primates. Associated with foodborne diseases. Species: Shigelladysenteriae.

5 g bacteria
5- G(+) bacteria

Enterococcus. Spheroid cells; occur in pairs or chains; nonmotile; facultative

anaerobes; some strains survive low heat (pasteurization); mesophiles. Normal habitat is the intestinal contents of humans, animals, and birds, and the environment. Can establish on equipment surfaces. Used as an indicator of sanitation.

Important in food spoilage. Species: Enterococcusfaecalis.

Staphylococcus. Spherical cells ; occur singly, in pairs, or clusters; nonmotile; mesophiles; facultative anaerobes; grow in 10% NaCl. Staphylococcus aureus strains are frequently involved in foodborne diseases. Sta. carnosus is used for processing some fermented sausages. Main habitat is skin of humans, animals,

and birds.

Streptococcus. Spherical or ovoid ; occur in pairs or chains; nonmotile; facultative

anaerobes; mesophiles. Streptococcus pyogenes is pathogenic and has been

implicated in foodborne diseases; present as commensals in human respiratory


Str. thermophilus is used in dairy fermentation; can be present in raw milk;

can grow at 50 C.


Lactococcus. Ovoid elongated cells ; occur in pairs or short chains; nonmotile; facultative anaerobes; mesophiles, but can grow at 10 C; produce lactic acid. Used to produce many bioprocessedfoods, especially fermented dairy

foods. Species: Lactococcuslactis subsp. lactis and subsp. cremoris; present in

raw milk and plants and several strains produce bacteriocins, some with a relatively wide host range against Gram-positive bacteria and have potential as food biopreservatives.

Leuconostoc. Spherical or lenticular cells; occur in pairs or chains; nonmotile; facultative

anaerobes; heterolacticfermentators; mesophiles, but some species and

strains can grow at or below 3o C. Some are used in food fermentation. Psychrotrophic strains are associated with spoilage (gas formation) of vacuum-packaged refrigerated foods. Found in plants, meat, and milk. Species: Leuconostocmesenteroidessubsp. mesenteroides, Leu. lactis, Leu. carnosum. Leu. Mesenteroidessubsp. dextranicum produces dextran while growing in sucrose. Several strains produce bacteriocins, some with a wide spectrum against Gram-positive bacteria,

and these have potential as food biopreservatives.


Pediococcus. Spherical cells; form tetrads; mostly present in pairs; nonmotile;

facultative anaerobes; homolacticfermentators; mesophiles, but some can grow at 50C; some survive pasteurization. Some species and strains are used in food

fermentation. Some can cause spoilage of alcoholic beverages. Found in vegetative

materials and in some food products. Species: Pediococcusacidilactici and Ped.

pentosaceus. Several strains produce bacteriocins, some with a wide spectrum

against Gram-positive bacteria, and they can be used as food biopreservatives.

spore forming
Spore forming

Bacillus. Rod-shaped, straight cells; (thick or thin); single or in chains; motile or nonmotile; mesophiles or psychrotrophic; aerobes or facultative anaerobes; all

form endospores that are spherical or oval and large or small (one per cell), spores are highly heat resistant. Includes many species, some of which are

important in foods, because they can cause foodborne disease (Bacillus cereus)

and food spoilage, especially in canned products (Bac. coagulans, Bac. stearothermophilus).

Enzymes of some species and strains are used in food bioprocessing (Bac. subtilis).

Present in soil, dust, and plant products (especially

spices). Many species and strains can produce extracellular enzymes that hydrolyze

carbohydrates, proteins, and lipids.

G positive


Clostridium. Rod-shaped cells that vary widely in size and shape; motile or nonmotile; anaerobes (some species extremely sensitive to oxygen); mesophiles or psychrotrophic; form endospores (oval or spherical) usually at one end of the cell, some species sporulate poorly, spores are heat resistant. Found in soil, marine sediments, sewage, decaying vegetation, and animal and plant products.

Some are pathogens and important in food (Clostridium botulinum, Clo. perfringens) and others are important in food spoilage (Clo. tyrobutyricum, Clo. saccharolyticum,

Clo. laramie).

Some species are used as sources of enzymes to hydrolyze carbohydrates and proteins in food processing.


Gram-Positive, Non sporulating Regular Rods

G negative spore forming

Lactobacillus. Rod-shaped cells that vary widely in shape and size, some are very long whereas others are coccobacilli, appear in single or in small and large chains;

facultative anaerobes; most species are nonmotile; mesophiles (but some are psychrotrophs); can be homo- or heterolacticfermentors. Found in plant sources, milk,

meat, and feces. Many are used in food bioprocessing (Lactobacillus delbrueckii

subsp. bulgaricus, Lab. helveticus, Lab. plantarum) and some are used as probiotics

(Lab. acidophilus, Lab. reuteri, Lab. casei subsp. casei). Some species can grow at low temperatures in products stored at refrigerated temperature (Lab. sake, Lab.


Several strains produce bacteriocins, of which some having a wide spectrum can be used as food biopreservatives.

Desulfotomaculum. One species important in food is Delsufatomaculumnigrificans.

The medium-sized cells are rod shaped, motile, thermophilic, strictly anaerobes,

and produce H2S. Endospores are oval and resistant to heat. Found in soil.

Cause spoilage of canned food.


Gram-Positive, Nonspore forming Irregular Rods

Corynebacterium. Slightly curved rods; some cells stain unevenly; facultative anaerobes;

nonmotile; mesophiles; found in the environment, plants, and animals. Some

species cause food spoilage. Corynebacteriumglutamicum is used to produce

glutamic acid

Bifidobacterium. Rods of various shapes; present singly or in chains; arranged in V

or star-like shape; nonmotile; mesophiles; anaerobes. Metabolize carbohydrates to

lactate and acetate. Found in colons of humans, animals, and birds. Some species

are used in probiotics(Bifidobacteriumbifidum, Bif. infantis, Bif. adolescentis).

Why ?

An understanding of the sources of microorganisms in food is important to

develop methods to control access of some microorganisms in the food, develop

processing methods to kill them in food, and determine the microbiological quality

of food, as well as set up microbiological standards and specifications of foods and

food ingredients.

How ?

Plants (Fruits and Vegetables)

Proper methods used during growing (such as use of treated sewage or other types of fertilizers), damage reduction during harvesting, quick washing with good qualitywater to remove soil and dirt, and storage at low temperature before and after processing can be used to reduce microbial load in foods of plant origin.

Animals, Birds, Fish, and Shellfish

Prevention of food contamination from these sources needs the use of effective husbandry of live animals and birds, which includes good housing and supply of uncontaminated feed and water. Also, testing animals and birds for pathogens and culling the carriers are important in reducing the incidence of pathogenic microorganisms in foods.



Microbial contamination of food from the air can be reduced by removing the potential sources, controlling dust particles in the air (using filtered air), using positive air pressure, reducing humidity level, and installing UV light.


Removal of soil (and sediments) by washing and avoiding soil contamination can reduce microorganisms in foods from this source.


To reduce incidence of microbial contamination of foods from sewage, it is better not to use sewage as fertilizer. If used, it should be efficiently treated to kill the pathogens. Also, effective washing of foods following harvesting is important.



Chlorine-treated potable

water (drinking water) should be used in processing, washing, sanitation, and as an ingredient

To overcome the problems, many food processors use water, especially as an ingredient, that has a higher microbial quality than that of potable water.

Food Ingredients

The ingredients should be produced under sanitary conditions and given antimicrobial treatments. In addition, setting up acceptable microbial specifications for the ingredients will be important in reducing microorganisms in food from this source.



Proper training of personnel in personal hygiene, regular checking of health, and maintaining efficient sanitary and aesthetic standards are necessary to reduce contamination from this source.


Proper cleaning and sanitation of equipment at prescribed intervals are important

to reduce microbial levels in food. In addition, developing means to prevent or reduce contamination from air, water, personnel, and insects is important. Finally, in designingthe equipment, potential microbiological problems need to be considered.

discussion scenario 1
Discussion scenario 1
  • A batch of turkey rolls (10 lb — about 4.5 Kg — each) were cooked to 73.8⁰cinternal temperature in bags, opened, sliced, vacuum-packaged, and stored at 4.4⁰c. The product was expected to have a refrigerated shelf life of 50 days. However, after 40 days, the packages contained gas and 10 7bacterial cells/g of meat. The bacterial species involved in the spoilage was found to be Leuconostoccarnosum, which is killed at 73.8⁰ c. What could be the sources of the bacterial species in this cooked product?
normal microbiological quality of food
Normal microbiological quality of food

understanding of the microbial types (and their levels where possible) that can be expected under normal conditions in different food groups.


Normally, carcasses contain an average of 101–3 bacterial cells/in

Salmonella serovars, Yersiniaenterocolitica,

Campylobacter jejuni, Escherichia coli, Clostridium perfringens, and Staphylococcus aureus,


Ground meat can have 104–5 microorganisms/g Salmonella can be present at 1 cell/25 g.

As indicated before, the frequency of the presence of Salmonella is higher in chicken than in red meats.

  • If the products are kept under aerobic conditions, psychrotrophic aerobes will grow rapidly, especially Gram negative rods, such as Pseudomonas.
  • Under anaerobic packaging, growth of psychrotrophic facultative anaerobes and anaerobes (e.g., Lactobacillus, Leuconostoc,)

Heat treatment, especially at an internal temperature of 73o C or higher, kills most microorganisms, except some thermodurics, which include Micrococcus, some Enterococcus, and maybe some Lactobacillus and spores of Bacillus and Clostridium.

  • The microbial level can be 101–2/ g.


  • The predominant types from inside a healthy udder are Micrococcus, Streptococcus, and Corynebacterium.
  • Normally, raw milk contains <103 microorganisms/ml.
  • During refrigerated storage (at dairy farms and processing plants) before pasteurization, only psychrotrophs can grow in raw milk. They include Pseudomonas,coliforms and Bacillus spp.

In the U.S., the standard plate counts of raw milk for use as market milk are 1–3 X 105/ml, and for use in product manufacturing are 0.5–1 X 106/ml.

  • Grade A pasteurized milk can have standard plate counts of 20,000/ml and ≤10 coliforms/ml.


107 bacteria

Pseudomonas, Esc. coli, Enterobacter, Enterococcus, Micrococcus,

and Bacillus. They can also have Salmonella from fecal contamination. Infected

ovaries of laying hens can be the source of Salmonella Enteritidis in the yolk.



  • Muscles of fish and shellfish are sterile, but scales, gills, and intestines harbor microorganisms.
  • 103–8 bacterial cells/g.
  • Pseudomonas,Enterococcus, Micrococcus, coliforms.
  • Fish and shellfish harvested from water polluted with human and animal waste can contain Salmonella, Shigella, Clo. perfringens, Vib. cholerae, and hepatitis A and Norwalk-like viruses.
  • Following harvest, microorganisms can grow rapidly in fish and crustaceans because of high Aw and high pH of the tissue.


  • Generally, vegetables have 103–5 microorganisms/cm2 or 104–7/g.
  • lactic acid bacteria, Corynebacterium, Micrococcus, Enterococcus, and sporeformers.
  • Alternaria, Fusarium, and Aspergillus.
  • pathogenic protozoa and parasites.
  • In general, fruits have microbial populations 103–6/g.


Unprocessed products (grains) may contain high bacterial levels (aerobic plate count @104/g, coliform @102/g, yeasts and molds @103/g).



  • The products with a pH of 4.6 or above are given heat treatments to obtain commercial sterility, but those with a pH below 4.6 are given heat treatments 100oC.
  • In canned products stored at 30oC or below, thermophilic spores do not germinate to cause spoilage. However, if the cans are temperature-abused to 40oC or higher, the spores germinate.
  • If the canned products are given lower heat treatment (100oC), spores of mesophilic bacteria that include both spoilage (Bac. coagulans, Bac. licheniformis, Clo. sporogenes, Clo. butyricum) and pathogenic types (Bac. cereus, Clo. perfringens, Clo. botulinum), along with the spores of thermophiles, survive.

In low-pH products, particularly in tomato products, Bac. coagulans spores can germinate and cells can multiply and cause spoilage. Other sporeformers can germinate and grow in high-pH products


Sugar can have thermophilic

spores of Bac. stearothermophilus, Bac. coagulans, Clo. thermosaccharolyticum,

as well as mesophilic bacteria (e.g., Lactobacillus and Leuconostoc), yeasts, and molds


When sugars are used as ingredients in food products, the spores can survive and cause spoilage of products. Pathogens are not present in refined sugar unless contaminated. In liquid sugar, mesophiles can grow.

  • Refinedsugar, used in canned products or to make liquid sugar, has strict microbiological standards (for spores).


pH of 2.5 to 4.0. Fruit juices (100%) have a pH of 4.0 or below. Vegetable juices (e.g., tomato) can have a pH of 4.5 or above.


molds, yeasts, lactic acid bacteria, and acetic acid bacteria, can multiply.

  • In carbonated beverages, some yeasts being microaerophilic can grow; in beverages with fruit juices, Lactobacillus and Leuconostocspecies can grow.
  • In noncarbonated beverages, molds(Geotrichum) and Acetobacterand Gluconobacterspp. can also grow.

Bottled water should not contain more than 10 to 100 bacteria and >10 coliforms/100 ml.

mayonnaise and salad dressings
  • pH between 3.5 and 4.0.
  • Microorganisms are introduced into the products through ingredients, equipment, and air.
  • Normally, their numbers should not exceed 10/g.


may contain microorganisms as high as 106–7/g.

Spores of molds, Bacillus, and Clostridium spp. Also, micrococci, enterococci, yeasts, and several pathogens such as Salmonella spp., Sta. aureus, and Bac. cereus have been found.

microbial growth increased number of cells
Microbial Growth= increased number of cells.
  • Binary fission
  • Spores.
  • Asexual and sexual.
  • Generation time, for bacteria 18-20 min. then yeast and moulds.
  • Vibrio parahaemolyticus, under optimum conditions can have a generation time as low as 10 to 12 min.
  • microbial population can be calculated mathematically using logarithmatics (log10)
optimum growth
Optimum Growth
  • Many environmental parameters of food, such as storage temperature, acidity (pH), water activity (Aw), oxidation–reduction (O–R) potential, and nutrients, influence microbial growth rate.
  • If the growth rate is fastest (or generation time is shortest) at a certain temperature. This temperatureis referred to as the optimumgrowthtemperature.
  • The area under the two points on both sides of an optimum growth condition where minimum growth occurs is the growth temperature range.
growth curve
Growth Curve
  • Cell mass, optic density OD, 600 nm
  • Cell count, every 1 hour.
  • Cell componenet, DNA, RNA
  • Cell products, acid, gas, metabolites.
  • Growth curve= cell(s)/hour.
nature of microbial growth in food
  • Mixed Population
  • Depending on the environment, which includes both the food environment (intrinsic) and the environment in which the food is stored (extrinsic)some of the species or strains can be in optimum or near-optimum growth condition.
  • If a food contains among the mixed population, two species initially present in equal numbers and both growing optimally under the specific intrinsic and extrinsic environments of the food, but one having a shorter generation time than the other. After a storage period, the one with shorter generation time becomes predominant.

Sequence of Growth

  • Among the different microbial types normally present in a food, different species (strains) can become predominant in sequence during storage.
  • Initially, depending on the environment, one or two types may grow optimally and create an environment in which they can no longer grow rapidly.
  • If a food is packaged in a bag with a little air (e.g., ground meat), the aerobes grow first and utilize the oxygen. The environment then becomes anaerobic, in which the anaerobes (or facultative anaerobes) grow favourably.

Growth in Succession or DiauxicGrowth

  • Microorganisms that can metabolize two or more nutrients in a food, one preferred over the other and present in limiting concentrations, show growth in stages separated by a short lag phase.
  • Initially a bacterial strain grows by utilizing the preferred nutrient and after a short lag of adaptation grows by utilizing the other nutrient.
  • During each stage, the growth curve has exponential and stationary phases with the lag phase in-between.

An example is the growth of certain bacterial strains (such as some lactic acid bacteria and Gram-negative bacteria) in fresh meat.

A strain grows initially by utilizing the limiting concentrations of carbohydrate present, followed by utilization of nonprotein nitrogenous (NPN; such as amino acids) substances


Symbiotic Growth

  • Synergistic Growth For example, both Str. thermophilusand Lab. delbrueckiisubsp. bulgaricus, when growing in milk independently, produce ca. 8 to 10 ppm acetaldehyde, the desirable flavour component of yogurt. However, when growing together in milk, 30 ppm or more of acetaldehyde is produced,
  • Antagonistic Growth
intrinsic factors or food environment
  • Nutrients.
  • These nutrients include carbohydrates, proteins, lipids, minerals, and vitamins.
  • Water is not considered a nutrient.
  • Microorganisms normally present in food vary greatly in nutrient requirements, with bacteria requiring the most, followed by yeasts and molds.
  • All microorganisms normally found in food metabolize glucose, but their ability to utilize other carbohydrates differs considerably

Food carbohydrates are metabolized by microorganisms principally to supply energy.

  • Proteins differ greatly in their solubility, which determines the ability of microorganisms to utilize a specific protein.
  • Soluble proteins are more susceptible to this hydrolytic action than are the insoluble proteins. Hydrolysis of food proteins can be either undesirable(texture loss in meat) or desirable (flavor in cheese).
  • Microorganisms can also metabolize different NPN compounds found in foods. Amins, urea, a.a.

Production of specific metabolic products is used for the laboratory identification of microbial isolates from food.

  • An example of this is the ability of Escherichia coli to produce indole from tryptophan, whichis used to differentiate this species from non- indole-producing related species (e.g., Enterobacterspp.).

Lipids are, in general, less preferred substrates for the microbial synthesis of energy and cellular materials.

  • Some microorganisms produce extracellular lipid oxidases, the action of these enzymes is associated with food spoilage (such as rancidity)
  • Some beneficial intestinal microorganisms, such as Lactobacillus acidophilus strains, can metabolize cholesterol and are believed to be capable of reducing serum cholesterol levels in humans.

Growth Factors and Inhibitors in Food.

  • Foods can also have some factors that either stimulate growth or adversely affect growth of microorganisms.
  • An example is the growth factors in tomatoes that stimulate growth of some Lactobacillus species.
  • Some of the natural inhibitors are lysozyme in egg, agglutinin
  • in milk, and eugenol in cloves.

Water Activity and Growth.

Water activity (Aw) is a measure of the availability of water for biological functions.

The Aw of food ranges from ca. 0.1 to 0.99.

  • cereals, crackers, sugar, salt, dry milk, 0.10 to 0.20;
  • noodles, honey, chocolate, dried egg, <0.60;
  • jam, jelly, dried fruits, parmesan cheese, nuts, 0.60 to 0.85;
  • fermented sausage, dry cured meat, sweetened condensed milk, maple syrup, 0.85 to 0.93;
  • evaporated milk, tomato paste, bread, fruit juices, salted fish, sausage, processed cheese, 0.93 to 0.98;
  • fresh meat, fish, fruits, vegetables, milk, eggs, 0.98 to 0.99.

Each microbial species (or group) has an optimum, maximum, and minimum Aw level for growth.

  • the minimum Aw values for growth of microbial groups are as follows:
  • most molds, 0.8,
  • most yeasts, 0.85,
  • most Gram-positive bacteria, 0.90;
  • Gram-negative bacteria, 0.93.

Some exceptions are growth of Staphlococcusaureus at 0.85 and halophilicbacteria at 0.75.


pH and Growth.

  • pH indicates the hydrogen ion concentrations in a system and is expressed as –log [H+], the negative logarithm of the hydrogen ion or proton concentration. It ranges from 0 to 14, with 7.0 being neutral pH.
  • high-acid foods (pH below 4.6).
  • low-acid foods (pH 4.6 and above).
  • Most fruits, fruit juices, fermented foods (from fruits, vegetables, meat, and milk), and salad dressings are high-acid (low-pH) foods,
  • most vegetables, meat, fish, milk, and soups are low-acid (high-pH) foods.

Each species has an optimum and a range of pH for growth.

  • In general, molds are able to grow at lower pH 1.5 to 9.0; for yeasts, 2.0 to 8.5.
  • Gram-negative bacteria 4.5 to 9.0.
  • G - are more sensitive to low pH than are Gram-positive bacteria (pH 4.0 to 8.5).

Redox Potential, Oxygen, and Growth.

  • The redox or oxidation–reduction (O–R) potential measures the potential difference in a system.
  • The redox potential of a food is influenced by its chemical composition, specific processing treatment given, and its storage condition.
  • The presence or absence of oxygen in or around food determine the growth capability of a particular microbial group in a food and the specific metabolic pathways used during growth to generate energy and metabolic by-products.