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.
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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
1885 Theodor Escherich isolated Bacterium coli (later named Escherichia coli) from the feces and suggested that some strains were associated with infant diarrhea.
• 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
Bacteria, yeasts, molds, and viruses are important in food for their ability to:
Base composition C+G % (10%)
DNA RNA hybridization (90%)
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.
On the basis of Gram-stain behavior, bacterial cells are grouped as Gram-negative Gram-positive.
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
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.
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).
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.
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
Shigella. Medium rods; nonmotile; mesophiles. Found in the intestine of humans and
primates. Associated with foodborne diseases. Species: Shigelladysenteriae.
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,
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.
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.
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,
Some species are used as sources of enzymes to hydrolyze carbohydrates and proteins in food processing.
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.
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
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).
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
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.
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.
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.
understanding of the microbial types (and their levels where possible) that can be expected under normal conditions in different food groups.
RAW AND READY-TO-EAT MEAT PRODUCTS
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.
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.
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.
SHELL EGG AND LIQUID EGG
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.
CEREAL, STARCHES, AND GUMS
Unprocessed products (grains) may contain high bacterial levels (aerobic plate count @104/g, coliform @102/g, yeasts and molds @103/g).
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
SUGARS AND CONFECTIONERIES
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.
SOFT DRINKS, FRUIT AND VEGETABLE DRINKS, JUICES, AND BOTTLED WATER.
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.
Bottled water should not contain more than 10 to 100 bacteria and >10 coliforms/100 ml.
SPICES AND CONDIMENTS
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.
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
Food carbohydrates are metabolized by microorganisms principally to supply energy.
Production of specific metabolic products is used for the laboratory identification of microbial isolates from food.
Lipids are, in general, less preferred substrates for the microbial synthesis of energy and cellular materials.
It is not possible or practical to control microbial growth in a food by restricting nutrients.
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.
Each microbial species (or group) has an optimum, maximum, and minimum Aw level for growth.
Some exceptions are growth of Staphlococcusaureus at 0.85 and halophilicbacteria at 0.75.