Dr.

Dharmalingam K
B.Tech., M.Tech., PGDIS, PhD.
Food Biotechnology
Assistant Professor
Department of Biotechnology
Chaitanya Bharathi Institute of
Unit - 3
Technology
Hyderabad – 500075
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UNIT- III
Food Spoilage And Food Microbiology: Shelf life of food, Microbes found in
raw materials and foods that are detrimental to quality, Factors that influence the
development of microbes in food, Food spoilage by bacterial agents
(Clostridium, Salmonella, Vibrio and Shigella), Non-bacterial agents (Protozoa,
Algae, Fungi and Viruses)
 Shelf life of food

➢ Shelf-life is the length of time a food can be kept under stated storage
conditions while maintaining its optimum safety and quality.

➢ Shelf life of a food begins from the time the food is manufactured and is
dependent on many factors such as its manufacturing process, type of
packaging, storage conditions and ingredients.

It is an offence to sell goods past their use by date as they may not be
safe for consumption.
 High-risk Foods
• High-risk foods are ready-to-eat foods and often contain the ideal conditions for bacterial
growth such as moisture and a high protein content.

• These foods are often stored under refrigeration and are marked with a ‘use-by’ date.

• ‘Use by’ dates are used to indicate that a food could become harmful after that date.

• These dates are determined and verified through microbial testing.

Examples of high-risk foods include:
• Cooked meat and poultry.
• Smoked salmon.
• Prepared salads and vegetables.
• Dairy products, such as milk, cream and cheese.
• Meat gravies, sauces, pâté and meat pies.
• Foods made with uncooked egg, such as mousse and
mayonnaise.
• Seafood, such as cooked shellfish, prawns and oysters.
• Cooked rice and pasta.
• Freezing makes the pathogens dormant and does not kill them.

• That is why it is not recommended to refreeze products, and mishandling a

frozen product will affect the shelf life.
 Low-risk Foods

• Low-risk foods are often stable at ambient temperatures due to preservation

methods such as dehydration or acid fermentation. These foods have a ‘best
before’ date.

• ‘Best before’ dates apply to foods which will deteriorate in quality after the
specified date, but which would not become harmful to health.
Examples of low-risk foods include:
• Sweets.
• Pickles.
• Honey.
• Jam and preserves.
• Syrups.
• Vinegar.
• Flour and dried pasta.

✓ The exemptions from date marking include drinks over 10% alcohol, sugar, salt,
and products that are generally expected to be eaten on the day of purchase such as
fresh fruit & vegetables and bakery products.
 What are the Factors Affecting Shelf Life?
Intrinsic factors are inherent within the food and cannot be controlled, such as:
• Water activity.
• Moisture content.
• pH.
• Salt content.
• Sugar content.
• Nutrient content.
• Oxidation potential.
Extrinsic factors are the shelf-life factors which can be controlled or changed, for example:
• Time.
• Temperature
• Modified Atmospheric Packaging (MAP) and packaging materials.
• Processing methods.
• Chemical preservatives.
 Processing Methods

✓ Processing methods are often used to kill bacteria and make the product
uninhabitable for pathogens.

For example:
• Acid fermentation, such as kimchi.
• Curing and smoking, such as cured meats and smoked salmon.
• Thermal processes, such as UHT milk or juice pasteurization within the packaging
to ensure a long shelf life.
 Packaging

Packaging can be used to extend the shelf life of products, examples include:
• Modified Atmospheric Packaging, such as adding carbon dioxide to ready-to-eat
ham slices.
• Vacuum packing, for example, when steaks are vacuum packed to remove oxygen
from the packaging.
• Sealed plastic, such as the plastic packaging in cereal boxes, to prevent the cereal
from going soft.
• Canning.
• Foods, microorganisms, and humans have had a long and interesting association that
developed long before the beginning of recorded history.

• Foods are not only of nutritional value to those who consume them but often are ideal
culture media for microbial growth.

• Foods, because they provide nutrients for us, also are excellent environments for the
growth of microorganisms.
• If a food consists primarily of carbohydrates, spoilage does not result in major odors.
Ex: Foods such as breads, jams, and some fruits first show spoilage by fungal growth.

• In contrast, when foods contain large amounts of proteins and/or fats (for example,
meat and butter), spoilage can produce a variety of foul odors.

• Think of rotting eggs: This proteolysis and anaerobic breakdown of proteins that yields
foul- smelling amine compounds is called putrefaction.

• Degradation of fats ruins food as well. For example, the production of short-chained fatty
acids from fats renders butter rancid and unpleasant.
• The pH of a food also is critical because a low pH favors the growth of yeasts and molds.
• In neutral or alkaline pH foods, such as meats, bacteria are more dominant in spoilage
and putrefaction.
• The presence and availability of water also affect the ability of microorganisms to colonize
foods. Simply by drying a food, one can control or eliminate spoilage processes.
• When large quantities of salt or sugar are added to food, most microorganisms are dehydrated
by the hypertonic conditions and cannot grow. Even under these adverse conditions, osmophilic
and xerophilic microorganisms may spoil food.
• Osmophilic microorganisms grow best in or on media with a high osmotic concentration,
whereas xerophilic microorganisms prefer a low aw environment and may not grow under
high aw conditions
• Many foods contain natural antimicrobial substances, including complex chemical
inhibitors and enzymes. Coumarins found in fruits and vegetables exhibit antimicrobial
activity. Cow’s milk and eggs also contain antimicrobial substances. Eggs are rich in the
enzyme lysozyme that can lyse the cell walls of contaminating gram-positive bacteria.

• Herbs and spices often possess significant antimicrobial substances; generally, fungi are
more sensitive than most bacteria. Aldehydic and phenolic compounds are found in
cinnamon, mustard, and oregano. These compounds inhibit microbial growth. Other
important inhibitors are garlic, which contains allicin, and cloves, which have eugenol.

• Unfermented green and black teas also have well documented antimicrobial properties
because of their polyphenol contents, which apparently are diminished when the teas are
fermented. Such teas are active against bacteria, viruses, and fungi and may have
anticancer properties.
• Temperature and RH are important extrinsic factors in determining whether a food will
spoil. At higher RHs microbial growth is initiated more rapidly, even at lower
temperatures.

• When dried foods are placed in moist environments, moisture absorption can occur on the
food surface, eventually allowing microbial growth.

• The atmosphere in which the food is stored also is important. This is especially true with
shrink-packed foods because many plastic films allow oxygen diffusion, which results in
increased growth of surface-associated microorganisms.
• Excess CO2 can decrease the solution pH, inhibiting microbial growth.
• Storing meat in a high CO2 atmosphere inhibits gram-negative bacteria,
resulting in a population dominated by the lactobacilli. With a carbon dioxide
content of 60% or greater (MAP) in the atmosphere surrounding a food, spoilage
fungi will not grow, even if low levels of oxygen are present.

• Some oxygen is kept because if all the oxygen is removed, the psychrophile
Clostridium gasigenes can grow. This organism can produce gases in 14 days at
2°C, which leads to swollen food packages.
Microbes found in raw materials and foods –
Good and harmful bacteria
• Microorganisms associated with foods can be categorized as "spoilage," "pathogenic,"
or "useful.‘’
• Spoilage microorganisms are those that can grow in a food and cause undesirable
changes in flavor, consistency (body and texture), color, or appearance.
• Also bacterial enzymes may effect slow deterioration of frozen or dried foods during
long-time storage.
• These changes diminish the quality characteristics of foods and may render them
ultimately unfit for human consumption.
• For example, refrigerated perishable foods such as milk, fresh meat, poultry, fish, fruits,
and vegetables lose some quality characteristics during normal storage and ultimately
spoil, due in part to the activity of microorganisms capable of growth at refrigeration
temperatures.
• Usually, extensive microbial growth (millions of organisms per g or cm2) occurs before
quality losses are perceptible.
• These changes, when perceived by the consumer, serve as an alert that extensive
microbial activity has taken place.
• Pathogenic microorganisms can render foods harmful to humans in a variety of ways.

• Foods may serve as the vehicle for the introduction of infectious microorganisms into
the gastrointestinal tract, e.g., Salmonella and Shigella.

• Multiplication of certain microorganisms in foods prior to consumption may result

in production of toxins, e.g., Clostridium botulinum, Staphylococcus aureus,
and Bacillus cereus.

• Foods may also be the vehicle for microorganisms that form toxins in vivo,
e.g., Clostridium perfringens and certain pathogenic Escherichia coli.
• With some foods, conditions are chosen to favor the development of useful

microorganisms such as lactic acid bacteria and yeasts, which are either

naturally present or added intentionally.

• Such foods as cheeses, yogurt, breads, pickles, and fermented sausages offer

desirable organoleptic properties and shelf-life.

 Food as a Selective Environment
• Microbial activities in foods can be viewed from the perspective of the food as a "selective
environment," despite the diversity of microorganisms that contaminate the surfaces of the
raw materials.

• The selectivity is imposed by the physical-chemical characteristics of the food, the additives
it contains, the processing techniques, the packaging material, and the storage conditions.

• Microbiological shelf-stability of many foods is related to storage conditions. For example,

dried and frozen foods are microbiologically shelf-stable as long as they remain dry or
frozen.

• Dried beans are shelf-stable because they contain insufficient moisture to permit
microbial growth.

• Mayonnaise is shelf-stable because it contains sufficient quantities of acetic acid in the

moisture phase of the product to prevent growth of contaminating organisms.
 Food spoilage by bacterial agents
(Clostridium, Salmonella, Vibrio and Shigella)

• A total of 66% of foodborne diseases is caused by bacteria.

• Major diseases include botulism caused by C. botulinum, gastroenteritis
caused by E. coli strains, Salmonellosis and Staphylococcal poisoning.
• Moreover, B. cereus and V. cholerae are bacteria frequently reported as
causative agents of toxicoinfection by food
➢ Food poisoning or food borne disease (FBD) is one of the main problems in public
health worldwide.

➢ According to the WHO, 1 out of 10 becomes ill after consuming contaminated food.

➢ Among all these people, 420,000 die, including 125,000 children under 5 years of
age, due to the vulnerability of this population to develop a diarrheal syndrome, about
43% of FBDs occur in these patients.
✓ Among the microorganisms causing FBDs are bacteria that have different virulence
factors that give them the ability to cause a disease.
✓ Among these factors, we can find toxins that can be produced in food or once the
pathogen has colonized the digestive tract.

✓ The major difference between food intoxication and food infections lies in the origin of
the illness-causing toxins. While food intoxication/poisoning occurs when you eat food
that contains toxins produced by bacteria, food infections occurs when you eat food that
contains a harmful microorganism, such as a virus, bacteria, or parasite. The
microorganism can invade your intestinal lining and multiply, or it can release toxins
while in your intestinal tract
 Types of bacterial toxins

✓ A bacterial toxin is a macromolecule mainly of protein origin, which can cause toxic
damage in a specific organ of the host. Toxins can be divided in endotoxins and
exotoxins:

✓ Endotoxins or lipopolysaccharides (LPS): These are the components of the outer

membrane of the Gram-negative bacteria; they are considered the most important
antigen of the bacteria; they are released into the medium after different processes
such as lysis and cell division. This endotoxin is capable of causing endotoxic shock
and tissue damage.

✓ Exotoxins: These are the macromolecules of protein origin, which are produced
and later released to the medium by the microorganism.

✓ Exotoxins causes Food intoxication or food infection?

Examples of exotoxin producing bacteria are: Examples of endotoxin-producing bacteria are:
Clostridium botulinum (Botox), Salmonella typhi (Typhoid),
Clostridium tetani (Tetanus), Vibrio cholerae (Cholera).
Corynebacterium diphtheriae (Diphtheria). Streptococcus pneumoniae (sepsis )
Shigella (shigellosis)
 Clostridium botulinum
• C. botulinum is a spore-forming microorganism; these spores can remain viable for
long periods of time when the environmental conditions are absolutely unfavorable
for the development of the microorganism.

• Gram-positive, rod-shaped, anaerobic, spore-forming, motile bacterium with the

ability to produce the neurotoxin botulinum
 What is Botulism Food Poisoning?
Botulism is a life-threatening food-borne disease that causes neuroparalysis from the
consumption of botulinum neurotoxins (BoNTs). Botulism is a rare disease but has a high
fatality rate even a small amount of toxin can kill millions.

• Clostridium botulinum causes disease ingesting C. bot contaminated foods which

produce BoNTs in the intestine.
• There are seven distinct botulinum toxins (Type A to G) that cause disease in humans
and animals.
• The clinical-epidemiological forms of botulism are food-borne botulism, infant
botulism and wound botulism.
• Botulinum toxins Type A, B and E are associated with food-borne botulism in
humans.
• The lethal dose of BoNTs for humans is 0.2 µg to 2 µg per kg body weight.
• Botulinum toxin is considered a hazardous biological substance in the environment.
 Source of Clostridium botulinum contamination
• Due to the nature of the bacterium to grow in wide distribution, its spore can contaminate
foods where it colonizes and produces toxins.
• C. botulinum spores contaminating honey and syrup which are the major source of
infant botulism.
• Poorly processed canned foods are also the main source of contamination as the spores
can tolerate high heating temperatures during processing.

• Other food sources include various types of lake and sea fishes, smoked, salt-dried and
fermented meat products.

• Type A and B BoTNs are also commonly present in soil and biofertilizers, therefore, they
are likely to contaminate vegetables and fruits.

• Improperly cooked vegetable products such as beans, baked potatoes, corn, celery,
mushrooms, onions and olives.
 Clostridium botulinum neurotoxin production
• The BoNTs are extracellular proteins produced by C. botulinum that weighs around
150kDa.

• Based on the physiology and phylogenetic of bacteria, BoNTs have seven distinct
serotypes from Type A to G.

• All seven toxins are categorized into four groups based on proteolytic and non-
proteolytic activity: Group I, II, III and IV.

• Group I proteolytic Type A, B and F can proliferate on temperature range from 10 to 20°C
similarly Group II non-proteolytic Type B, E and F at 2.5 to 3°C.

• Group III includes non-proteolytic Type C and D while Group IV has Type G.
• C. botulinum is strict anaerobes and can resist high temperatures where they form
spores and when consumed produce toxins in the intestine and cause intoxication.
 Clinical manifestation of Botulism Food Poisoning
• Symptoms are visible within 18 to 38 hours of ingestion that affect the gastrointestinal
tract at an initial stage.
• Common symptoms include abdominal pain, nausea, vomiting, diarrhea and sometimes
constipation.
• After the intoxication, neurological symptoms appear that include flaccid paralysis,
symmetric descending weakness, slurred speech, difficulty in swallowing, dizziness,
blurred vision, dry mouth and extraocular muscle weakness.

• In infant botulism, babies under 1 year are mostly affected being a poorly developed gut
microflora where the toxins are formed in the intestine.
 Clinical manifestation of Botulism Food Poisoning
• In infants, symptoms like a weak hoarse cry, poor suck, inability to hold the head upright
and weak muscles may appear.

• In the case of wound botulism, the organism colonizes in the infected part like wound,
abscess, pus where the Clostridium spores germinate and produce toxins.

• The symptoms of wound botulism are hypotension, respiratory problems, eye muscle
paralysis, ataxia and dilated fixed pupils.

• Death usually occurs within 3 to 6 days of intoxication in case of fatality.

 Control measures of Botulism Food Poisoning

• Proper heat treatment (above 121°C) and refrigeration (below 4°C) of foods.

• Acidifying agents such as citric acid reduces the chance of spore formation in

canned products.

• Avoid food that you suspect is undercooked or not well reheated.

 Rod-shaped gram-negative
What is Salmonella? bacteria of the family
Enterobacteriaceae

• Salmonella is one of the major food-borne pathogens that cause systemic or enteric
infection affecting approximately 2 million people worldwide each year.

• The first organism, Salmonella choleraesuis was isolated from a pig intestine by a
scientist named Dr. Daniel Salmon.

• Salmonella belongs to a member of the Enterobacteriaceae family and only two

species of Salmonella namely S. enterica and S. bongori are pathogenic and cause
illness to humans.
 What is Salmonellosis?
• Salmonellosis is a serious disease that has a clinical spectrum of gastroenteritis and typhoid
fever.

• S. typhi and S. paratyphi A is responsible for causing typhoid fever and is usually
common in South and Southeast Asia.

• S. typhimurium and S. enteritidis are non-typhoidal Salmonellae and cause invasive

bacterial disease most common in sub-Saharan Africa.
• The fatality rate of Salmonellosis is less than 1% but malnourished children,
immunocompromised patients, adults with HIV are at high risk.
 Salmonella Characteristics
• Gram-negative bacteria
• Facultative anaerobes
• Motile, rod-shaped bacteria
• Non-spore former
• Non-capsulated (except S. typhi and S. paratyphi)
• Growth temperature ranges from 5 to 45°C (Optimum temperature – 35 to 37°C)
• Broad pH range 3.8 to 9.5
• Resists bile salts
• Produces H2S
 Sources of transmission of Salmonellosis
• Poultry is the major reservoir of Salmonella and is transmitted through the fecal-oral route.
• It is present in the GI tract of many animals, birds and reptiles.
• Salmonella was also isolated from the intestine of turkey flocks and chicken broiler flocks.
• The water and foods get contaminated from the feces and/or urine of the infected humans or
animals.
• Consuming contaminated water and food infects the healthy individual and cause infections.
• The person-to-person also get infected through fecal-oral transmission and/or by handling
contaminated food products or utensils and poor hygiene practices.
• Raw meats, eggs, milk and milk products, salad dressing, cake desserts and topping, cocoa,
peanut butter and chocolate are the possible source of Salmonella infection.
 Clinical manifestations of Salmonellosis

• The process of Salmonellosis starts after 6 to 48 hours of ingestion in case of gastroenteritis

(food poisoning).
• The incubation period depends on the dose of bacterial cells ingested from contaminated
food and water.
• Common symptoms of gastroenteritis are nausea, vomiting, diarrhea, abdominal cramp,
myalgia, headache, fever and chills.
• The infection lasts only for 2 to 7 days and is self-limiting.
• In the case of enteric fever i.e, typhoid fever, the complication begins after 10 to 14
days of ingestion of bacterial cells.
• The symptoms are non-specific at the first week of infection with constipation, headache
and mild fever.
• In the second week, the patient starts to develop myalgia, continuous high fever, severe
headache, abdominal distension, watery diarrhea and sometimes foul green-yellow stools.
• If untreated, patients may die from toxemia, myocarditis and intestinal hemorrhage.
 Treatment and Control of Salmonellosis
• Antibiotics are not recommended for Salmonella entero-colitis but in case of enteric fever
and typhoid fever, antibiotics such as Ciprofloxacin, Azithromycin, Ceftriaxone.

• The use of antibiotics in uncomplicated cases shortens the illness but may increase the
antibiotic-resistant strains.
• Typhoid vaccines are recommended for travelers, but the vaccine is ineffective
against S. paratyphi A, B and C.

• Consume properly cooked foods, pasteurized milk and milk products and refrigerate foods at
an adequate temperature in terms of storage are the preventive measures recommended.
Vibrio cholerae Food Poisoning- Cholera toxin (CT)
• Vibrios are inhabitants of the marine environment and are aquatic bacteria that can be
found as planktons.

• They are normally present in aquatic animals and seafood products and only a
few Vibrio species are pathogenic to humans and cause illness.

• Vibrio species are categorized into two groups: Choleragenic which includes only one
species Vibrio cholerae and non-choleragenic type namely, Vibrio parahaemolyticus,
Vibrio vulnificus, Vibrio fluvialis, and Vibrio metschnikovii.
• When contaminated water or food is ingested, Vibrio cholera causes serious
diarrheal illness resulting in watery diarrhea and severe dehydration.
• V. cholerae has two serotypes of toxigenic strains O1 and O139.
• Non-choleragenic bacteria cause mild and less serious illnesses and do not cause
epidemics.
• Choleragenic bacteria also cause septicemia when a wound or cuts on the skin of a
person comes in contact with contaminated water or sea products.
• The mortality rate of cholera is high as compared to other food-borne illnesses
and often causes an outbreak in developing countries.
• Vibrio food poisoning is rare in high-income and developed countries which might
be due to proper sanitation practices.
• Each year it is estimated that 91,000 death are reported due to cholera infection
which is of high concern for the public health commission on controlling the
disease.
Characteristics of Vibrio cholerae
• Gram-negative bacteria
• Curved-rod or comma-shaped
• Non-spore forming
• Aerobes or facultatively anaerobes
• Motile with a single polar flagellum
• Temperature range from 14 to 40°C
• Can survive NaCl concentration of 6% Photomicrograph of Vibrio cholerae

• pH range 6.5 to 9
Sources of contamination
• Fresh, brackish, and marine waters are the major reservoir of Vibrio and are
found in aquatic animals, seabirds, sewage water, sediments, and inanimate
surfaces.

• They also live within chitinous zooplankton, bivalve shellfish such as clams,
oysters, and mussels.

• Cholera is transmitted from one person to another from water contaminated with
feces and bacteria.

• Food poisoning is relatively high during the summer season as bacterial

growth is favored during this month but cannot survive low temperatures
(10°C).

• It is also present in water salinity of 5-30%.

Clinical Symptoms
• Cholera infection starts appearing after 12 hours to 15 days of consumption.
• The severity of illness depends on the biotypes strains of V. cholerae that have been
ingested.
• Classical biotypes cause mild illness whereas biotype EI Tor strain caused pandemics
recently.
• If the case is more severe, then the illness increases rapidly during the 24 hours of
infection.
• Rice watery diarrhea with fishy odor, pale grey stool with severe abdominal
cramp, and high fever are the symptoms of cholera gravis.
• If the patient fails to receive medical treatment, the illness leads to severe
dehydration, loss of electrolytes, circulatory collapse, and death.
• Dehydrated patients appear to have sunken eyes, dry and dull skin, low urine
volume, low pulse rate, low body weight, and are not able to eat and drink.
• Cholera patients may suffer from be hypoglycemiac i.e, low blood glucose level.
• Prolonged hypotension may lead to acute renal failure.
Vibrio cholerae Food Poisoning Pathogenesis
• The pathogenicity of V. cholerae is mediated by the virulence factors CT and TCP
which are long filamentous pili that help in the attachment and colonization of bacterial
cells in the human intestine.
• The tcpA gene of the Vibrio pathogenicity island (VPI) – I act as a receptor that adheres
to the host cell.
• Cholera toxin (CT) is the most important virulence factor that targets the cell by
producing a protein called adenosine diphosphate (ADP)-ribosylation of guanosine
triphosphate (GTP)- binding protein.
• This protein locks the adenylate cyclase enzyme and changes its conformation resulting
in increased activation of the enzyme.
• This leads to the decrease of intracellular cAMP level that disrupts the epithelial
cell membrane and causes massive fluid loss and watery diarrhea.
• Cholera is mediated by non-invasive enterotoxin CT produced by the strains of V.
cholerae O1 and O139.
• These strains cause sporadic cases that cause invasive extraintestinal illness like fatal
bacteremia and are different from CT illness.
Detection methods

• Isolation of organism from stool sample with the use of selective media such
as thiosulfate-citrate-bile salt-sucrose (TCBS) agar, cellobiose polymyxin B colistin
(CPC), and mannitol-maltose agar.
• The organism can be confirmed by the ‘shooting star’ movement in cholera stool
when observed under a dark field microscope.
• The dipstick ELISA method also detects and differentiates the toxigenic and non-
toxigenic V. cholerae with high sensitivity and specificity.
• PCR-based assays are also helpful in detecting the virulence recA genes of the
pathogen.
• This method uses unique oligonucleotide primers and gives fast and specific results.
What is Shigella and how does it cause food poisoning?

Recognized worldwide as the most common cause of dysentery, the Shiga

bacillus—or Shigella—is a facultatively anaerobic, non-motile gram negative
rod belonging to the family Enterobacteriaceae, so named because many of its
members live in the intestines of humans and warm-blooded animals.
The Shiga bacillus is an antigenically diverse pathogen that comprises four species (also
called groups or subgroups):

• Shigella flexneri: Also known as Group B Shigella, it is the leading cause of endemic
diarrhea in low-income and middle-income countries.

• Shigella sonnei: Also known as Group D Shigella, S. sonnei accounts for over three-
fourths of shigellosis in the United States. It is the leading cause of endemic diarrhea in
high income countries.

• Shigella dysenteriae: Serotype 1 causes pandemics precipitated by natural disaster and

social disruption, and associated with high attack rates of severe disease in all age groups;
the remaining serotypes are uncommon.

• Shigella boydii: Relatively uncommon

The Discovery and Naming of Shigella

• The subgroups of Shigella bacteria have been named after the scientists or
investigators who discovered each one.

• The first bacterium discovered, Shigella dysentariae, was named after Kiyoshi
Shiga, a Japanese scientist who discovered it in 1896 while investigating a large
epidemic of dysentery in Japan.

• The genus was first termed Shigella in the 1930 edition of Bergey's Manual of
Determinative Bacteriology.
What foods are associated with Shigella?

• Salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk
and dairy products, and poultry can carry Shigella bacteria.

• Water contaminated with human waste and unsanitary handling by food

handlers are the most common causes of contamination in these food products.

• Common pets, farm animals, and wild animals do not spread these germs; only
monkeys and people can.
What are the symptoms?

• Shigella species are bacteria that cause a foodborne illness called shigellosis. One of the
symptoms of Shigellosis is diarrhea. Severe diarrhea can cause dehydration for the very
young or the chronically ill.

• The most common symptoms are diarrhea, fever, nausea, vomiting, stomach cramps,
and straining to have a bowel movement. The stool may contain blood, mucus, or pus.

• The bacteria must be swallowed to cause disease. They are often spread when people do
not wash their hands with soap and warm water after using the toilet or changing a diaper.

• People who get the germs on their hands can infect themselves by eating, smoking, or
touching their mouths. They can also spread the germs to anyone or anything they touch,
making others sick.
How to control this pathogen ?

1. Wash your hands with warm soapy water before and after handling raw foods.
2. Always wash your hands after using the toilet and after changing a baby's diaper.
3. Prepare food for yourself but not others if you are ill with diarrhea.
4. Wash fruits and vegetables before eating them.
5. Knives, cutting boards, and food preparation surfaces should be washed with hot
water and soap after contact with raw foods.
Non-bacterial agents (Protozoa, Algae, Fungi and Viruses)
Food spoilage by protozoa
Food spoilage by protozoa is relatively uncommon compared to other microorganisms like
bacteria, fungi, and molds. Protozoa are single-celled eukaryotic microorganisms that
can cause deterioration in certain food products, particularly those with high moisture
content.
Characteristics of Protozoa:
1.Protozoa are unicellular eukaryotic microorganisms.
2.They can move and feed using structures like cilia or pseudopodia.
3.Protozoa are often found in water and moist environments.
How Protozoa Spoil Food:
Protozoa can spoil food through several mechanisms:
• Off-flavors and Odors: Some protozoa can produce metabolic byproducts that impart
off-flavors and odors to food.
• Texture Changes: Protozoa can break down food components, leading to undesirable
texture changes, such as softening or liquefaction.
• Spoilage of High-Moisture Foods: Foods with high water content, like dairy products,
are more susceptible to protozoal spoilage.
Names of Food-borne Protozoa

• The main protozoa of concern in developed countries are Toxoplasma,

Cryptosporidium and Giardia, and these can be a problem in immunocompromised
people.

• Other protozoa such as Entamoeba histolytica, Cyclospora cayetanensis and

Sarcocystis can be a food-borne problem in non-industrialised countries.

• C. cayetanensis has emerged as a food-borne pathogen in foods imported into North

America from South America.

• Plasmodium, Trypanosoma, Entamoeba, and other protozoans can be dangerous.

Case Study: Protozoal Spoilage of Yogurt
Protozoal spoilage of yogurt is a relatively rare but documented occurrence.
Incident
•Scenario: A dairy company producing yogurt experienced customer complaints about the
texture and taste of their yogurt products.
•Symptoms: Consumers reported that the yogurt had an unusual texture, appearing watery
and grainy, and had an off-flavor.
•Investigation: The company initiated an investigation to identify the source of the problem.
Findings
•Protozoal Contamination: Laboratory tests identified the presence of protozoa in the
spoiled yogurt samples.
•Species Identified: The specific protozoa responsible for the spoilage were identified as
species capable of feeding on the milk solids and producing metabolic byproducts that
affected taste and texture.
•Spoilage Mechanisms: The protozoa were found to break down the proteins and other
components in the yogurt, leading to the observed changes in texture and flavor
Summary
• Protozoal spoilage of foods, such as yogurt, can occur when specific protozoa
contaminate the product.
• The presence of protozoa in high-moisture foods can lead to textural changes and off-
flavors.
• Maintaining strict quality control and sanitation measures during dairy product processing
is essential to prevent protozoal spoilage.
In conclusion, while food spoilage by protozoa is less common than spoilage by bacteria or
fungi, it can still have a significant impact on food quality, particularly in high-moisture
products like yogurt. Proper quality control and sanitation measures in food processing
are crucial to prevent protozoal spoilage and ensure food safety and quality.
 Algae in Food Spoilage
Food spoilage by algae is relatively uncommon compared to other microorganisms like
bacteria, fungi, and molds. Algae are primarily aquatic microorganisms that are known for
photosynthesis, but they can still impact certain food products, especially those with high
water content.
Characteristics of Algae:
1. Algae are diverse, photosynthetic microorganisms that can be found in various
aquatic and damp environments.
2. They come in a variety of colors, including green, red, and brown.
3. Algae can grow in foods with high moisture content,
making seafood and salad greens susceptible targets.
How Algae Spoil Food:
Algae can spoil food through the following mechanisms:

• Off-flavors and Odors: Certain algae species produce compounds that impart off-
flavors and odors to food products. This can make the food unappetizing and
unpalatable.
• Textural Changes: Algae can alter the texture of food items, making them slimy or
rubbery in texture.
• Color Changes: Some algae can cause discoloration in foods, resulting in undesirable
appearances.
 What are poisonous algae?
•Marine algae (dinoflagellates), yellow-brown algae (diatoms), and blue-green algae (cyanobacteria)
produce phycotoxins like saxitoxin, gonyautoxin, okadaic toxin, brevetoxin, ciguatoxin, domoic
toxin, palytoxin, and tetrodotoxin.
•These toxins are toxic to humans causing gastrointestinal disorders causing algal pollution and killing
many aquatic animals.
•Algae such as Prymnesium parvum and dinoflagellates Gymnodinium veneficum, Gymnodinium
brevis and Gonyaulax monitala are poisonous to fishes.
•Human shellfish poisoning is caused by the dinoflagellates Gonyaulax tamarensis and Gonyaulax
catenella.
•Many fishes are non-poisonous but become poisonous when these toxic algae are consumed by them
and the chain goes by the human consumption of herbivorous fish and shellfish.
•Cyanobacteria are also known as blue-green algae and the species Nodularia spumigena is involved
in toxin production causing food poisoning in humans.
•Nodularia spumigena and Microcystis aeruginosa are found in brackish and freshwater and are
responsible for causing livestock poisoning.
•The major route of exposure is through the consumption of contaminated seafood products such as
contaminated clams, mussels, scallops, oysters, and other shellfish.
Case Study: Spoilage of Fresh Seafood by Algae
In coastal regions, seafood is a common target for algae-related food spoilage.
Incident
•Scenario: A seafood market in a coastal town consistently receives fresh catches of fish daily.
•Symptoms: Customers began complaining about the fish they purchased having an unusual
taste and odor.
•Investigation: Health authorities launched an investigation to determine the source of the
issue.
Findings
•Algae Contamination: The investigation revealed that the fish had been contaminated by
certain algae species when caught in waters with an algal bloom.
•Algal Toxins: Some of the algae present in the bloom produced toxins that were absorbed by
the fish.
•Spoilage Mechanisms: The algae toxins caused off-flavors and odors in the fish, making
them unsuitable for consumption.
Key Takeaways
• Algae-related food spoilage can occur in seafood when fish are exposed to
algal blooms.
• Certain algae species produce toxins that affect the taste and quality of the
food.
• Monitoring and quality control of seafood during harvesting and processing
are essential to prevent algae-related spoilage.

In conclusion, while food spoilage by algae is relatively uncommon, it can still

have a significant impact on food quality, particularly in seafood and salad
greens. Understanding the potential risks associated with algal
contamination and implementing appropriate monitoring and quality control
measures are crucial to prevent algae-related food spoilage and ensure food
safety and quality.
Food spoilage by viruses
Food spoilage caused by viruses is relatively rare compared to spoilage by other
microorganisms like bacteria, fungi, and molds. This is because viruses are obligate intracellular
parasites that require a host cell to replicate, and they do not grow on food in the same way that
bacteria and molds do. However, viruses can still play a role in foodborne illnesses.
Characteristics of Viruses:
• Viruses are tiny infectious agents made up of genetic material (DNA or RNA) enclosed in a
protein coat.
• They cannot grow or reproduce outside of a living host cell.
• Viruses are highly resistant to environmental conditions, including extreme temperatures and
disinfectants.
Transmission of Viruses in Food
Foodborne viruses can contaminate food through several means:
Infected food handlers: Viruses can be shed in the feces or respiratory secretions of
infected individuals. If food handlers do not practice good hygiene, they can transfer
viruses to food during preparation.
Contaminated water: Waterborne viruses can contaminate seafood and produce when
they come into contact with contaminated water sources.
Raw or undercooked animal products: Certain viruses, like hepatitis E, can be
transmitted through consumption of raw or undercooked animal products.
Impact on Food Safety:
1. Foodborne viruses can cause foodborne illnesses with symptoms such as nausea,
vomiting, diarrhea, and fever.
2. Common foodborne viruses include norovirus and hepatitis A virus.
Case Study: Norovirus Outbreak from Contaminated Salad
In 2016, a well-documented norovirus outbreak occurred in the United States, highlighting
the potential risks of foodborne viruses. The outbreak was traced back to a popular fast-food
chain and involved contaminated salad greens.
•Incident: Multiple customers reported symptoms of gastroenteritis (nausea, vomiting,
diarrhea) after consuming salads from the chain.
•Investigation: Health authorities launched an investigation to identify the source of the
outbreak.
•Identification: Norovirus was identified as the causative agent through stool samples from
affected individuals and food samples from the restaurant.
•Contaminated Greens: The source of the contamination was traced back to the salad
greens, which had been contaminated by a food handler with norovirus who did not follow
proper handwashing procedures.
 Food spoilage by Viruses
• Human Norovirus followed by hepatitis A virus are the most common foodborne
viruses, which are transmitted by food consumed raw, such as shellfish, fresh
vegetables, and berry fruit.

• In developed countries, hepatitis E virus is increasingly being recognized as an emerging

viral foodborne pathogen that includes zoonotic transmission via pork products.

• Other viruses such as Enterovirus (EV), human Rotavirus (HRV), hepatitis E virus
(HEV), astrovirus (AstV), Aichi virus (AiV), Sapovirus, Coronavirus, Parvovirus, and
human adenovirus (HAdV) can also be transmitted by food.
Summary
• This case study illustrates how food handlers who carry viruses can introduce them into the
food supply chain, leading to outbreaks.
• Norovirus is highly contagious and can survive on surfaces and in food for extended
periods.
• Proper food safety measures, including handwashing and hygiene, are critical in preventing
the transmission of foodborne viruses.
In conclusion, while viruses themselves do not typically cause food spoilage in the same way
as bacteria and molds, they can play a significant role in foodborne illnesses and outbreaks
when transmitted through contaminated food. Proper food handling and hygiene practices are
essential to mitigate the risk of foodborne virus contamination and ensure food safety.
 Fungi in Food Spoilage
Food spoilage by fungi is a common occurrence, with various fungal species capable of
causing deterioration in different food products. Fungi include molds and yeasts, which are
eukaryotic microorganisms that can thrive in a wide range of environmental conditions. They
play a significant role in food spoilage due to their ability to grow on various food items and
produce visible signs of spoilage.
Characteristics of Fungi:
1. Fungi are eukaryotic microorganisms that include molds and yeasts.
2. They can grow on foods with different moisture levels, pH values, and temperatures.
3. Fungi produce enzymes that break down food components, leading to changes in taste,
texture, and appearance.
 How Fungi Spoil Food

Molds: Molds are multicellular fungi that grow as hyphae (filaments). They are often
recognized by their visible mycelial growth, which appears as fuzzy spots or patches on
food.
Case Study: Mold growth on bread.

• Bread is a common target for mold growth due to its high moisture content and nutrient-
rich composition.
• Mold spores from the environment land on the bread's surface.
• When conditions are favorable (moisture, temperature), molds grow rapidly, forming
visible colonies.
• Mold hyphae penetrate the bread, releasing enzymes that break down starches and other
components, causing it to become stale, discolored, and develop an off-flavor.
Yeasts: Yeasts are single-celled fungi that can ferment sugars in food, leading to the production
of alcohol and carbon dioxide.

Case Study: Fermentation of fruit juice.

• Fruit juices contain natural sugars and are susceptible to yeast fermentation.
• When yeast cells are introduced to fruit juice, they consume sugars and produce alcohol
and CO2, leading to the formation of carbonation and an alcoholic taste.
• The change in taste and appearance makes the juice unpalatable and unsuitable for long-
term storage.
Mycotoxins:
1. Some molds produce mycotoxins, which are toxic compounds that can pose health risks
if ingested.
2. Mycotoxins are resistant to heat and cannot be destroyed by cooking.
3. Contaminated grains (e.g., aflatoxins in peanuts) and nuts are known sources of
mycotoxin-related food spoilage.

Prevention and Control:

1. Proper storage: Keep food items in a cool, dry place with proper ventilation to reduce
moisture and humidity.
2. Packaging: Use airtight containers to limit exposure to air, which can promote fungal
growth.
3. Temperature control: Refrigerate perishable foods to slow down fungal growth.
4. Inspection: Regularly check stored food items for signs of fungal growth and discard
affected products.
• In summary, fungi, including molds and yeasts, can cause food spoilage by growing on
various food products, altering their taste, texture, and appearance.

• Mold growth on bread and yeast fermentation in fruit juice are common examples of fungal
food spoilage.

• It's essential to prevent and control fungal growth through proper food storage, packaging,
and temperature control to maintain food quality and safety.

• Additionally, the presence of mycotoxins in some fungi highlights the importance of food
safety measures in avoiding health risks associated with fungal-contaminated foods.