Transcript Slide 1

MICROBIOLOGY
OF THERMALLY
PROCESSED FOODS
Chapter 2
1
Introduction
• Microbiology is the knowledge
of living forms so small that
they can be seen only with the
aid of a microscope
• The forms observed have
been referred to in a number
of ways
•
•
•
•
Germs
Microbes
Bacteria
Microorganisms
2
Introduction
• Leeuwenhoek – the inventor of the
microscope in the early 1700s
• First to observe these microscopic living
forms
• Called them "wee beasties“
• The science of microbiology is applied in a
number of fields, such as medicine,
agriculture, industry, and food
preservation
3
Introduction
• Food microbiology is the focus of this
chapter
• Food microbiology is the basis for
producing commercially sterile, shelf
stable foods
4
The Microbiology of Food
Processing
• The history of the canning of foods began in
1810 with Nicholas Appert, a French
confectioner
• Appert discovered that placing food in glass
containers, sealing them with corks, and
heating them in boiling water would usually
prevent spoilage
5
The Microbiology of Food
Processing
• Although he was a thorough and careful
worker, the science of microbiology was
unknown at the time, and he was unable to
explain why his method was successful
• He believed that the combination of heat
and the exclusion of air "averted the
tendency to decomposition"
6
The Microbiology of Food
Processing
• Some 50 years later, Louis Pasteur showed
that certain microorganisms are responsible
for fermentation and decay
• He conducted experiments on food
preservation, and the term "pasteurization"
bears his name
• Although Pasteur's findings could have
explained why Appert's method was
successful, they were not applied
7
immediately
The Microbiology of Food
Processing
• Thus, in the early days of food processing,
the causes of many spoilage incidents
remained unknown
• Numerous theories were advanced in a vain
effort to learn the cause of these mysterious
problems
• It was firmly believed by many processors
that without vacuum, canned foods
would not keep
8
The Microbiology of Food
Processing
• Research in food microbiology, started at
the Massachusetts Institute of Technology in
1895, ultimately concluded that the
seemingly mysterious spoilage of canned
foods resulted from failure to apply sufficient
heat to destroy microorganisms
9
Characteristics and Behavior of
Microorganisms
• All raw foods normally contain
microorganisms that will eventually
cause spoilage unless they are
controlled or destroyed
• Food preservation is a competition
between the human species and
microorganisms
• We attempt to preserve the food that
10
microorganisms attempt to utilize
Characteristics and Behavior of
Microorganisms
• Food preservation requires that
microorganisms be controlled or
destroyed
• Need to know what they are and how
they behave
11
Characteristics and Behavior of
Microorganisms
• The microorganisms of primary concern
to the food processor are molds, yeasts,
and bacteria
• They can grow in food or the processing
environment under suitable conditions
12
Characteristics and Behavior of
Microorganisms
• These microorganisms may be
divided into groups on the basis of
their microscopic characteristics or
visual appearance in mass
growths, called colonies
13
14
Characteristics and Behavior of
Microorganisms
• Other microorganisms of concern
• Viruses
• Parasites
• Do not multiply in foods but can get into
food through contaminated water or
other sources
15
Characteristics and Behavior of
Microorganisms
• The following factors are also important
in the classification of microorganisms:
• The materials they can use as foods
• The byproducts resulting from the
breakdown of these foods
• Their tolerance to oxygen
• Their growth temperatures
• Their resistance to such destructive
agents as heat and chemicals
16
Useful Functions of
Microorganisms
• Many of the thousands of microorganisms
that have been discovered and identified
perform some useful function
• Without microorganisms, we would not have
some of the tasty foods we enjoy, such as
breads, cheese, wine, beer, sauerkraut,
sausages, and other fermented foods
17
Useful Functions of
Microorganisms
• Microorganisms are needed to make
products useful to industry and medicine,
such as enzymes, antibiotics, glycerol and
other alcohols
• Microorganisms have the ability to break
down organic matter and return it to the
earth in the form of smaller molecules that
can be used by other organisms
• These smaller molecules become food
for plants, which in turn provide food for
18
animals
Some Microorganisms Cause
Disease
• About 1865, the "Germ Theory of Disease"
was accepted
• This theory teaches that most diseases of
humans, animals or plants are caused by
specific microorganisms
19
Some Microorganisms Cause
Disease
• The microorganism, or the substances it
produces, must invade the human, animal or
plant body to cause the disease
• Microorganisms that cause disease are
often referred to as pathogens
20
Some Microorganisms Cause
Disease
• Fortunately, very few of the known
microorganisms are harmful to humans
• While many diseases can be transmitted
from person to person or from animals to
humans, only a few can be transmitted
through foods
21
Some Microorganisms Cause
Disease
• Although it is becoming recognized that the
vast majority of foodborne illnesses are
caused by viruses such as noroviruses
• Bacterial pathogens are most frequently
identified as the cause of illness because of
better detection
22
Some Microorganisms Cause
Disease
• The majority of laboratory-diagnosed cases
of bacterial foodborne illnesses are caused
by just a few microorganisms
•
•
•
•
•
Salmonella spp.
Campylobacter
Shigella spp.
Clostridium perfringens
Staphylococcus aureus
23
Significant Microorganisms in
Food Processing
24
Molds
• Molds exhibit some of the characteristics of
the higher plants
• They are multicellular organisms forming
tubular filaments
• Molds demonstrate branching and reproduce
by means of fruiting bodies, called spores,
which are borne in or on aerial structures
• Their mycelia – intertwined filaments – may
resemble roots
• Molds are many times larger than
bacteria and somewhat longer than
yeasts
25
Molds
• Molds are widely distributed in nature, both
in the soil and in the dust carried by air
• Under suitable conditions of moisture, air
and temperature, molds will grow on almost
any food
• The black or green discoloration that
appears on moldy bread is a common
example of mold growth
26
Molds
• Molds are also able to survive on a wide
variety of substances not normally thought
suitable for supporting life
• These include concentrated solutions of some
acids
• Water containing minute quantities of certain
salts
• Certain pastes used in labeling
27
Molds
• Molds grow readily on the walls and ceilings
of buildings with high humidity and
considerable moisture condensation
• Mold growth can occur even in refrigerators,
because molds are much more tolerant to
cold than to heat
28
Molds
• Molds are capable of consuming acids,
thereby raising the pH of products
• On very rare occasions, their growth in foods
has removed the acid conditions that inhibit
growth of Clostridium botulinum, a food
poisoning organism discussed later in this
chapter
29
Molds
• Mold spoilage of food in closed, processed
containers is rare but not impossible
• Most molds have little heat resistance and
cannot survive the thermal processes for
low-acid canned foods
• Therefore, if present, it is the result of
serious underprocessing or post-processing
contamination
• Since molds need oxygen to grow, only
slight growth can occur unless the food
container has an opening to the outside
environment
30
Molds
• Molds such as Byssochlamys fulva,
Talaromyces macrosporus, Neosartorya
fischeri and others have been implicated in
the spoilage of some canned fruit drinks, fruit
and fruit-based products
• Because of their ability to form spores which
enables them to survive adverse conditions,
these molds are reportedly capable of
withstanding the mild heat treatment used to
preserve these high-acid products
31
Molds
• The heat-resistant sporeforms of these
molds may survive more than one minute at
198ºF in acid or acidified foods
• However, to achieve this degree of
resistance, the organisms need days to
mature and produce the heat-resistant
spores
• Therefore, daily sanitation of equipment and
raw product containers is extremely
important in controlling the growth of these
organisms and preventing the
development of the resistant spores
32
Molds
• Mold growth in thermally processed,
commercially sterile foods does not present
a significant public health problem
• In fact, mold is used in the ripening process
of some cheeses and sausages
33
Yeasts
• Another microorganism of importance to food
preservation is yeast
• Yeasts are single cell microscopic living
bodies, usually egg-shaped
• They are smaller than molds but larger than
bacteria
34
Yeasts
• Their greatest thickness is about 1/2,000 of
an inch
• Yeasts reproduce mainly by budding
• A small bud forms on the parent yeast cell
and gradually enlarges into another yeast
cell
• A few varieties of yeast may form spores
within a special cell
• Later these spores may form new yeast cells
35
Yeasts
• Yeasts are widely found in nature and are
particularly associated with liquid foods
containing sugars and acids
• They are quite adaptive to adverse
conditions such as acidity and dehydration
• Like molds, yeasts are more tolerant of cold
than of heat
36
Yeasts
• Most yeast forms are destroyed on heating to
170ºF
• Spoilage may result from the presence of
yeast in canned food, but if this happens,
severe underprocessing or leakage must be
suspected
• Usually the growth of yeasts results in the
production of alcohol and large amounts of
carbon dioxide gas
• The gas will swell the container
37
Yeasts
• Yeast growth in processed foods does not
present a significant public health problem
• Yeasts are used in the production of bread
and fermented beverages
38
Bacteria
• Bacteria are the most important and
troublesome microorganisms for the food
processor
• Bacteria are single-celled living bodies so
small that individually they can be seen only
with the aid of a powerful microscope
• They are among the smallest living creatures
known
39
Bacteria
• The cells of bacteria vary in length from
1/25,000 to 1/1,000 of an inch
• The number of these tiny microorganisms
that could be placed on the head of a pin
would equal the population of a large city!
• Viewed with a microscope, bacteria appear
in several shapes or forms
40
Bacteria
• Those most important in processed food
spoilage are either round in shape (called
“cocci”) or rod-shaped (called “rods”)
• Most bacteria in themselves are
comparatively harmless, but they excrete
enzymes that can produce undesirable
changes in food
41
Bacteria
• Some bacteria, however, are pathogenic
• In some cases, the microorganisms can
produce poisonous substances
42
Reproduction of Bacterial Cells
• Bacteria reproduce by division
• Microbiologists call this process “fission”
• When a bacterial cell is ready to divide, the
cell material gradually increases until its
volume is almost doubled
• The cocci shapes become oval while rod
shapes stretch to nearly twice their
length
43
Reproduction of Bacterial Cells
• The cell then constricts in the middle
• This constriction deepens until the cell
contents are held in two distinct
compartments separated by a wall
• These two compartments finally separate
to form two new cells which are exact
duplicates of the former cell and of each
other
44
Reproduction of Bacterial Cells
• The cell then constricts in the middle
• This constriction deepens until the cell
contents are held in two distinct
compartments separated by a wall
• These two compartments finally separate
to form two new cells which are exact
duplicates of the former cell and of each
other
45
46
Reproduction of Bacterial Cells
• Since the reproduction of bacteria
increases the numbers, it is often referred
to as “growth” or “doubling”
• Bacterial cells in the active stage of growth
and metabolism are sometimes referred to
as vegetative cells
47
Reproduction of Bacterial Cells
• Experiments conducted to determine the
growth rate of bacteria under favorable
conditions have found that each cell
divides on the average about once every
20 or 30 minutes
• At this rate of cell division, each single cell
will produce four cells at the end of the first
hour
48
Reproduction of Bacterial Cells
• At the end of two hours, each cell will have
produced 16 new cells
• After 15 hours, each parent cell will have
produced 1,000,000,000 (one billion) cells
identical to the original
49
Reproduction of Bacterial Cells
• For example, if there were 75,000 bacteria
per square inch on a conveyor belt, by the
end of one hour there could be 300,000
bacteria per square inch of that belt
• At the end of a three-hour shift, the
bacteria count per square inch of belt
surface could be 4,800,000
50
51
Reproduction of Bacterial Cells
• Fortunately, conditions for bacterial growth
never remain favorable long enough for
continuous unrestricted reproduction of the
organism
• Without a constant supply of available
fresh food or favorable environmental
conditions, bacterial growth becomes
limited or prohibited
52
Reproduction of Bacterial Cells
• Also, large numbers of bacteria result in an
accumulation of substances that are
byproducts of bacterial growth and that
also act to inhibit growth
• Cells may die when growth stops due to
pollution of their environment
53
Reproduction of Bacterial Cells
• However, if the microorganism is a type
that forms resistant but dormant spores
• These cells can remain alive under
conditions that kill other cells
54
Sporeforming and
Non-Sporeforming Bacteria
• Bacteria can be divided into two groups
based on their ability to form or not to
form spores
• One group of bacteria does not form
spores and only exists as vegetative cells
• Practically all of the round-shaped (cocci)
bacteria and many of the rod-shaped
bacteria cannot form spores
• Thus they are classified as nonsporeformers
55
Sporeforming and
Non-Sporeforming Bacteria
• Bacteria can be divided into two groups
based on their ability to form or not to
form spores
• A number of the rod-shaped bacteria have
the ability to produce spores
• They are classified as sporeformers
• Sporeformers have both a vegetative cell
and spore form
56
Spore size, shape and location
can help with identification
57
Sporeforming and
Non-Sporeforming Bacteria
• Spores are a dormant stage in the
normal growth cycle of these organisms
• The spores have the ability to survive a
wide range of unfavorable conditions
• Spores have been compared to plant
seeds in that they will germinate and
grow when conditions are suitable
58
Sporeforming and
Non-Sporeforming Bacteria
• Spores in molds and yeasts represent
reproductive bodies
• Bacterial spores are a resting stage in
the growth cycle of the organism
• When a bacterial spore germinates, it is
simply the same organism continuing its
growth process
59
Resistance of Spores to
Environment
• In general, bacterial spores are extremely
resistant to heat, cold and chemical agents
• Some bacterial spores can survive in boiling
water for more than 16 hours
• The same organisms in the vegetative state
and non-sporeforming bacteria will not
survive heating in boiling water
60
Resistance of Spores to
Environment
• As a general rule spores that successfully
resist heat are also highly resistant to
destruction by chemicals
• There are bacterial spores that can survive
more than three hours in sanitizing
solutions normally used in a food
processing plant
• Non-sporeforming bacteria are readily
destroyed by these sanitizing agents
61
Source of Foodborne Organisms
• Soil or water from which food is obtained is
the most common source of foodborne
organisms and spores
• Leafy vegetables, such as spinach and
other greens that grow close to the soil,
usually have high numbers of bacteria and
bacterial spores.
62
Source of Foodborne Organisms
• Asparagus and mushrooms, which grow
through the soil, are always contaminated
with spores
• Crops grown in soil from river bottomlands,
lake beds and alluvial plains also contain
high numbers of spores
63
Conditions
Affecting the Growth of Bacteria
• To control or eliminate bacteria one must
know the growth requirements for each
bacterial group
• The following factors will influence the
growth requirements of bacteria
•
•
•
•
•
Food
Moisture
Oxygen
Temperature
pH
64
Food Requirements
• A suitable food supply is the most important
condition affecting the growth of bacteria
• Every living cell requires certain nutrients to
multiply
• Nutrients for bacterial cells include solutions
of sugars or other carbohydrates, proteins,
and small amounts of other materials such
as phosphates, chlorides and calcium
• If the nutrient supply is removed,
bacteria will not multiply
65
Moisture Requirements
• The concentration of moisture and its
availability in a food are important factors in
preventing bacterial growth
• The bacterial cell has no mouth; therefore
its source of nutrients must be in a soluble
form to enter the cell through the cell wall
• Without sufficient available moisture, it
would be impossible for nutrients to transfer
into and waste products to transfer out of
the cell
66
Oxygen Requirements
• Some bacteria – called aerobes – require
free oxygen in order to survive
• For others the reverse is the case – the
smallest quantity of free oxygen prevents
their growth
• These bacteria are called anaerobes
• The majority of bacteria are neither strict
aerobes nor strict anaerobes but can
tolerate to some degree either the presence
or absence of oxygen
• These are known as facultative anaerobes
67
Temperature Requirements
• For each of the bacterial groups there is an
optimum (most favorable) temperature
range for growth
• Temperatures below and above the
optimum for each group adversely affect the
growth of the organism
• Bacterial groups bear names that indicate
their relationships to temperature
• These groups are classified as
• Psychrotrophic
• Mesophilic
• Thermophilic
68
Classification of Bacteria by
Temperature Range for Growth
Classification
Optimum Temperature
Psychrotrophs
58ºF to 68ºF (14ºC to 20ºC)
Mesophiles
86ºF to 98ºF (30ºC to 37ºC)
Thermophiles
122ºF to 150ºF (50ºC to 66ºC)
69
The Psychrotrophic Group
• Psychrotrophs (“psychro” for cold and
“trophs” for growing) are bacteria that grow
best at 58ºF to 68ºF
• But can grow slowly in or on food at
refrigerator temperatures 40ºF
• None of these bacteria – except Clostridium
botulinum Type E and non-proteolytic
strains of types B and F – is of concern to
low-acid or acidified canned foods
70
The Mesophilic Group
• Mesophiles (“meso” for middle and “phile”
for love) are bacteria that grow best at
temperatures of 86ºF to 98ºF
• This is the normal range of warehouse
temperatures
• All of the microorganisms that affect food
safety grow within this mesophilic
temperature range
• Although some may be considered
psychrotrophic as well.
71
The Mesophilic Group
• The sporeforming organism C. botulinum is
a member of this group, although, as noted
above, some strains are considered
psychrotrophs
72
The Thermophilic Group
• Thermophiles (“thermo” for heat, “phile” for
love) are bacteria that grow at high
temperatures
• Thermophilic bacteria are found in soil,
manure, compost piles, and even hot
springs
• Many are sporeforming bacteria and are
divided into two groups based on the
temperature at which the spores will
germinate and grow
73
The Thermophilic Group
• If the spores will not germinate and grow
below 122ºF, the bacteria are called
obligate thermophiles
• Meaning that the high growth temperature is an
absolute requirement
• If growth occurs at thermophilic
temperatures of 122ºF to 150ºF and at
lower temperatures – for example at 100ºF–
the bacteria are called facultative, meaning
they have the ability to grow at both
temperature ranges
74
The Thermophilic Group
• Some of the obligate thermophiles can grow
at temperatures up to 170ºF
• Laboratory tests have indicated that the
spores of these bacteria are so heatresistant that they can survive for more than
60 minutes at temperatures of 250ºF
• Thermophilic bacteria do not produce
poisons during spoilage of the foods and do
not affect food safety
75
pH Requirements
• In general pH refers to the degree of acidity
or alkalinity
• The pH of a food influences the types of
microorganisms that will grow in it
• In general, yeast and mold grow at a lower
pH compared with bacteria
• All bacteria have an optimum pH range for
growth - generally around neutral pH
76
pH Requirements
• All bacteria have a minimum below which
they will not grow and a maximum above
which they cannot grow
• The pH of foods can be adjusted to help
control microbial growth
• The pH of a food is extremely important
with respect to the control of Clostridium
botulinum
77
Clostridium botulinum and Botulism
• Clostridium botulinum is of great concern to
home and commercial canners for the
following reasons:
• When it grows it can produce a deadly toxin or
poison
• It can be isolated from soil or water practically
everywhere in the world
78
Clostridium botulinum and Botulism
• The term "Clostridium" indicates that the
organism is able to grow in the absence of
air or oxygen and is a sporeformer
• The ability to form spores enables it to
survive a wide range of unfavorable
conditions, such as heat and chemicals
79
Clostridium botulinum and Botulism
• The term "botulinum" comes from the Latin
word "botulus," a sausage, because the
organism was first isolated from a sausage
that had produced the illness now called
"botulism"
80
Clostridium botulinum and Botulism
• Because C. botulinum spores are found
everywhere, any raw food may be
contaminated with them
• It is only when the vegetative form of the
organism grows in a food that the toxin or
poison is produced.
81
Clostridium botulinum and Botulism
• Because certain types of C. botulinum
spores are very heat resistant and are able
to survive five to 10 hours in boiling water, it
is necessary to apply much higher
temperatures such as 250ºF to destroy the
spores
• The toxin is not heat resistant; it can be
inactivated by boiling temperatures –
212ºF
82
Clostridium botulinum and Botulism
• Certain strains of C. botulinum are called
putrefactive because this term describes
the odor produced during their growth
• These strains require proteins in order to
grow, and they grow best at temperatures
between 86ºF and 98ºF, although growth
can occur at any temperature between 50ºF
and 100ºF
83
Clostridium botulinum and Botulism
• Other strains require carbohydrates, such
as sugars and starch, and do not produce
putrefactive odors
• Some of these strains are associated with
marine environments; they tolerate lower
temperatures of 40ºF and more oxygen
than other types
• Their spores will not withstand heating
to 212ºF
84
Effect of pH on C. botulinum Growth
• The pH of the product may determine
whether or not C. botulinum has the ability
to grow and produce its toxin
• Scientific investigation has determined that
the spores of C. botulinum will not
germinate and grow in food below pH 4.8
• A pH of 4.6 has been selected as the
dividing line between acid and low-acid
foods
85
Effect of pH on C. botulinum Growth
• Spores of C. botulinum and other spoilage
types can be found in both acid and lowacid foods
• In acid or acidified products, the pH is a
critical factor for control, with a finished
equilibrium pH of 4.6 or less, so that growth
and toxin formation will not occur even if the
spores of C. botulinum are present
86
Effect of pH
on Required Heat Treatment
• The application of mild heat destroys all
bacteria that are non-sporeformers or all
vegetative cells in either low-acid or acid
foods, including the vegetative cells of C.
botulinum
• In low-acid foods, high heat must be applied
to kill the spores of C. botulinum or the
spores of other food spoilage
organisms
87
Effect of pH
on Required Heat Treatment
• Thus, these foods must be heat processed
under pressure
• In acid foods, there is no concern with the
spores of C. botulinum
• These spores are prevented from
germinating and growing because the pH is
4.6 or below
88
Effect of pH
on Required Heat Treatment
• Since only the vegetative cells must be
destroyed in acid foods, boiling water cooks
or hot-fill and hold procedures may be used
89
90
Approximate pH Range
for Selected Foods
Lemon Juice
Apples
Blueberries
Sauerkraut
Orange Juice
Pineapple, canned
Apricots
Tomatoes, canned
Peaches, canned
Pears, canned
Bananas
Beets, canned
Asparagus, canned
Beef
Carrots
Peppers, green
Papaya
2.0 - 2.6
3.1 - 4.0
3.1 - 3.3
3.3 - 3.6
3.3 - 4.2
3.4 - 4.1
3.3 - 4.0
3.5 - 4.7
3.7 - 4.2
4.0 - 4.1
4.5 - 5.2
4.9 - 5.8
5.0 - 6.0
5.1 - 7.0
4.9 - 5.2
5.2 - 5.9
5.2 - 6.0
Tuna
Sweet Potatoes
Onions
White Potatoes
Spinach
Beans
Peas, canned
Corn, canned
Soy Beans
Mushrooms
Clams
Salmon
Coconut milk
Milk
Garbanzo Beans
Chicken
Eggs, whole
5.2 - 6.1
5.3 - 5.6
5.3 - 5.8
5.4 - 5.9
5.5 - 6.8
5.6 - 6.5
5.7 - 6.0
5.9 - 6.5
6.0 - 6.6
6.0 - 6.7
6.0 - 7.1
6.1 - 6.3
6.1 - 7.0
6.4 - 6.8
6.4 - 6.8
6.5 - 6.7
7.1 - 7.9
91
Control of
Bacteria by Water Activity
• For thousands of years people have dried
fruits, meats and vegetables as a method of
food preservation
• It was also discovered that the addition of
sugar would allow preservation of foods
such as candies and jellies
• Salt preservation of meat and fish has been
practiced over the ages
92
Control of
Bacteria by Water Activity
• As late as 1940, food microbiologists
thought that the percentage of water in a
food product controlled microbial growth
• Gradually they learned that it is the
availability of the water that is the most
important factor influencing growth
93
Control of
Bacteria by Water Activity
• The measure of the availability of water in a
food is made by determining the water
activity
• Water activity is usually designated with the
symbol “aw.”
94
Control of
Bacteria by Water Activity
• When substances are dissolved, there is
substantial reaction between the substance
and the water
• A number of the molecules of the water are
bound by the molecules of the substances
dissolved
• All of the substances dissolved in the water
reduce the number of unattached water
molecules
95
Control of
Bacteria by Water Activity
• This reduces the amount of water available
for microbial growth
• The extent to which the water activity is
lowered depends primarily on the total
concentration of all dissolved substances
96
Control of
Bacteria by Water Activity
• Thus, if some ingredient – such as sugar,
salt, raisins, dried fruits, etc. – is added to
food, it competes with the microorganism
for available water
• The water-binding capacity of a particular
ingredient influences the amount of water
left for the growth of microorganism
97
Control of
Bacteria by Water Activity
• Most bacteria, yeasts and molds will grow
above a water activity of 0.95
• Spores of C. botulinum are generally
inhibited at a water activity of about 0.93 or
less
98
Minimum aw Requirements for
Microorganism Growth
Most molds (e.g., Aspergillus)
Most yeasts
C. botulinum
Staphylococcus aureus3
Salmonella3
0.751
0.882
0.93
0.85
0.93
some strains – 0.61
2 some strains – 0.62
3 Non-sporeforming food-poisoning bacteria readily destroyed by heat.
1
99
Control of
Bacteria by Water Activity
• Since most foods have a water activity
above 0.95
• We need to decrease the amount of water
available to spores
• To a point where they are inhibited
• Apply mild heat treatment to destroy the
vegetative cells
100
Control of
Bacteria by Water Activity
• Examples of foods preserved with this
method are
•
•
•
•
•
•
•
Some cheese spreads
Peanut butter
Honey
Syrups
Jams and jellies
Canned breads
Confectionery preparations - toppings
101
Water activity of some
common foods
•
•
•
•
•
•
•
•
•
Liverwurst
Cheese Spread
Caviar
Fudge Sauce
Semi-moist Pet Food
Salami
Soy Sauce
Peanut Butter – 15% total moisture
Dry Milk – 8% total moisture
0.96
0.95
0.92
0.83
0.83
0.82
0.80
0.70
0.70
102
Control of
Bacteria by Water Activity
• As far as C. botulinum is concerned, a
water activity of 0.85 provides a large
margin of safety
• Studies with this organism show that an
accurate water activity of 0.93 plus a mild
heat treatment will give commercial sterility
103
Control of
Bacteria by Water Activity
• Some questions exist about the precision or
accuracy of the instruments and methods
used to determine water activity and about
some factors that control water activity
• If water activity plus pasteurization is used
to control commercial sterility, data must be
obtained and records kept to show that the
process yields commercial sterility
104
Control of
Bacteria by Water Activity
• The critical factors in the control of water
activity
• The ingredients in the final product
• Their effect on water binding capacity that is
measured by the water activity
• The formulation of the product to give the
required water activity must be predetermined and very accurately
controlled at the time of packing
105
Control of
Bacteria by Water Activity
• The critical points for supervision are the
product preparation and the achievement of
the required center temperature in the final
product
• Measurement of water activity is generally
used as verification for correct formulation
• Samples of the final product should be
checked as frequently as necessary to
ensure that the water activity is being
achieved
106
Regulatory Requirements
Related to Water Activity
• Under the FDA regulation 21 CFR Part 113,
a canned food with a water activity greater
than 0.85 and a pH greater than 4.6 is
considered a low-acid food, and its
minimum heat process will have to be filed
by the individual packer
• If reduced water activity is used as an
adjunct to the process, the maximum water
activity must also be specified
107
Regulatory Requirements
Related to Water Activity
• If the pH of the product has been adjusted
to 4.6 or less and the water activity is
greater than 0.85, the product is covered by
the acidified food regulation (21 CFR Part
114) and requires only enough heat to
destroy vegetative bacterial cells
108
Regulatory Requirements
Related to Water Activity
• Both low-acid and acidified low-acid meat or
poultry containing products are subject to
the USDA canning regulations if the water
activity is greater than 0.85
109
Regulatory Requirements
Related to Water Activity
• Any non-meat containing food, regardless
of the pH, with an water activity of 0.85 or
less is not covered by the regulations for
either the low-acid food (21 CFR Part 113)
or the acidified food (21 CFR Part 114)
• However, these products are covered by
FDA’s Current Good Manufacturing
Practices (CGMPs) regulation (21 CFR
Part 110
110
Regulatory Requirements
Related to Water Activity
• Meat or poultry containing products with a
water activity of 0.85 or less are not
covered by the USDA/FSIS canning
regulations but are covered by other
regulations not discussed in this book, such
as the USDA/FSIS meat and poultry
HACCP regulation (9 CFR Part 417) and
Sanitation SOP regulation (9 CFR Part 416)
111
Methods for Determining aw
• One commonly used method is an electric
hygrometer with a sensor to measure
equilibrium relative humidity (ERH)
• The instrument was actually devised by
weathermen, and the sensors are the same
as those used to measure relative humidity
in air
112
Methods for Determining aw
• A variety of hygrometers are available by
different manufacturers
• A dew point instrument is also commonly
used to measure water activity
113
114
Methods for Determining aw
• The equilibrium relative humidity above the
food in a closed container divided by 100 is
a measure of the available moisture – the
water activity
• ERH (expressed as a percent) and water
activity (expressed as a decimal fraction)
are numerically the same
• For example, an ERH of 85% is equivalent
to a water activity of 0.85
115
Methods for Determining aw
• The measuring principle may be different
for different instruments
• In determining the water activity using a
hygrometer, 30-90 minutes may be required
for the water vapor (relative humidity) to
reach equilibrium
• New models of hygrometers may be a lot
faster (result is available in about 5 to10
minutes)
116
Methods for Determining aw
• A dew point instrument is usually much
faster – generally about 5 minutes
• This instrument measures the temperature at
which condensation occurs on a cooled mirror
in the headspace of the sample chamber
• The water activity is computed by converting
sample and mirror temperatures to vapor
pressures and calculating the ratio
117
Methods for Determining aw
• A single measurement of water activity on a
food provides information as to which types
of microorganisms are most likely to cause
spoilage and how close the water activity is
to the safety limits
118
Salt and Water Activity
• The use of salt is another method of
preservation
• This is particularly applicable to salt-cured
meats and fish
• In salt-cured meats, salt is usually
supplemented with other ingredients, such
as nitrite, that aid in spoilage prevention
119
Salt and Water Activity
• In all cases the salt is necessary to inhibit
the growth of sporeforming bacteria, such
as C. botulinum
• Only enough heat is applied to kill the nonheat resistant types
• Strains of C. botulinum that grow in a
suitable food containing 7 percent salt are
known
120
Salt and Water Activity
• Their growth, however, is inhibited at a
concentration of 10 percent, which is
equivalent to a water activity of
approximately 0.93
• Although growth can occur at 7 percent, no
toxin has yet been demonstrated in this
concentration of salt
121
Spoilage of Thermally Processed,
Commercially Sterile Foods
122
Spoilage of Thermally Processed,
Commercially Sterile Foods
• The most obvious indicator of spoilage in
processed food is a swollen container –
bulging at one or both ends
• This implies that the food has possibly
undergone spoilage, possibly by the action
of gas-forming bacteria
• Consumers are cautioned not to use any
container with a bulged end or ends,
even though the swelling may be of
123
non-microbial origin
Indications of Bacterial Spoilage
• Most bacteria produce gas when allowed to
grow in a canned food
• The production of this gas is what causes
the containers to swell
• Exceptions to this are the flat-sour
sporeforming organisms which produce
acid and sour the food without producing
gas, leaving the container ends flat
• These organisms are an economic but not
a public health problem
124
Indications of Bacterial Spoilage
• The appearance and odor of the container
contents may also indicate spoilage
• If the product is broken down and mushy, or
if a normally clear brine or syrup is cloudy,
spoilage may be suspected
• In jars, a white deposit may sometimes be
seen on the bottom or on pieces of food
• This may be a sign of spoilage, but could
also be starch precipitated from certain
foods
125
Indications of Bacterial Spoilage
• Microbial spoilage of thermally processed
product may result from one of five causes
(1) Incipient spoilage – growth of bacteria and/or
yeast and mold before processing
(2) Post-process contamination – growth of
microorganisms that have gotten into the
product after processing
(3) Under-processing – growth of bacteria that
have survived due to inadequate
processing
126
Indications of Bacterial Spoilage
• Microbial spoilage of thermally processed
product may result from one of five causes
(4) Thermophilic spoilage - growth of bacteria
that survive the thermal process, but will only
grow if environmental conditions result in
suitable elevated temperatures
(5) Spoilage by acid-tolerant sporeforming
bacteria
127
Incipient Spoilage Before Processing
• Processed food is sometimes held too long
between closing the containers and thermal
processing
• Such processing delays may result in
growth of bacteria and possibly yeasts and
molds normally present in the food resulting
in spoilage before thermal processing
128
Incipient Spoilage Before Processing
• This type of spoilage is referred to as
"incipient spoilage" and may result in an
adulterated product
• The degree of spoilage depends on the
time and temperature conditions during the
delay
129
Incipient Spoilage Before Processing
• The resulting loss of vacuum may lead to
extensive internal pressures in the
containers during retorting
• The build-up of internal pressure strains the
container seams or seals and increases the
potential for leaker spoilage
• Some containers may actually buckle or
rupture, rendering them unusable
• Steps should be taken to avoid such a
delay before retorting the containers130
Post-Process Contamination
• Post-process contamination is most often
suspected when microscopic examination
of spoiled product or cultures of spoiled
product reveals a variety of microorganisms
consisting of non-sporeforming bacteria of
various shapes and sizes and possibly
yeast and mold
• Rapid appearance of swollen containers is
also common
131
Post-Process Contamination
• Swollen containers begin to appear in the
warehouse shortly after processing and
may continue to appear for a considerable
time afterwards
• Generally with the mixture of microorganisms present there will be some that
produce gas which causes the swollen
containers
132
Post-Process Contamination
• However, if many swells are present, a
small percentage of normal appearing
containers may in fact be spoiled by flatsour microorganisms
• Therefore, normal appearing containers
should be examined with this potential in
mind
133
Post-Process Contamination
• The cause of post-process contamination
will depend on whether the thermal process
takes place after the product is filled and
sealed into the container or before the
product is filled and sealed into the
container
134
Post-Process Contamination
• Raw product being filled and sealed into a
non-sterile container and then processed to
produce a commercially sterile, hermetically
sealed container of product is often called
“conventional canning”
135
Post-Process Contamination
• Post-process contamination in conventional
canning is typically the result of
• Inadequately formed seams
• Container damage
• Contaminated cooling water leaking into the
container during cooling
136
Post-Process Contamination
• These conditions will allow microorganisms
to enter or leak into the container after
processing
• As a result, this type of spoilage came to be
know as “leaker” spoilage
137
Post-Process Contamination
• Product and containers being sterilized in
separate operations, brought together in a
sterilized environment for filling and sealing
to produce a commercially sterile,
hermetically sealed container of product is
the feature of aseptic processing
138
Post-Process Contamination
• For aseptic processing post-process
contamination represents a problem far
more complicated than simple leaker
spoilage
• Finding a mixture of microorganisms in
spoiled aseptically processed products can
result from more than just container defects
139
Post-Process Contamination
• For example, spoilage could also result
from the sterility of the processing system
or the filler being compromised by an
improperly actuated valve, a faulty seal, or
a barrier failure
• The sterility of the air used to maintain
sterility in the filler or surge tank may be
compromised by an improperly installed
filter or a filter failure
140
Post-Process Contamination
• In other words, the processing system, the
surge tank, or the filling machine may have
leaked, not the container
• The spoilage could also be the result of
inadequate pre-sterilization of the
processing system, the surge tank, and/or
the aseptic filling machine and have
nothing to do with leakage
141
Post-Process Contamination
• The term leaker spoilage is typically not
used when referring to post-process
contamination in aseptically produced
products
142
Inadequate Heat Processing
• Inadequate heat processing means that the
product did not receive the proper process
• To destroy all microorganisms of public health
significance
• To destroy all microorganisms of non-health
significance that could spoil the product under
normal conditions of storage and distribution
143
Inadequate Heat Processing
• The degree of inadequate heat processing
can range from
• Severe under-processing
• Were the product and/or container has seen little or
no process
• To just under-processing
• Where a process has been delivered, but not to the
extent to render commercial sterility
144
Inadequate Heat Processing
• Microscopic examination of severely underprocessed product will often reveal a
mixture of microorganisms similar to that
seen in post-process contaminated product
• Microscopic examination of underprocessed product will generally reveal pure
culture of more resistant species
145
Inadequate Heat Processing
• The reduced process will destroy less
resistant microorganisms leaving the more
resistant to survive and grow
• If the heat process is inadequate to destroy
C. botulinum, the situation is most
hazardous, since the health of the
consumer may be affected
146
Inadequate Heat Processing
• A heat process may be inadequate for a
variety of reasons, including but not limited
to the following list:
(1) If the time and/or temperature specified in
the scheduled heat process for the particular
product in the particular size of container or its
equivalent is not used
(2) If the scheduled heat process was not
established properly
147
Inadequate Heat Processing
• A heat process may be inadequate for a
variety of reasons, including but not limited
to the following list:
(3) If the scheduled heat process is not properly
applied because of some mechanical or
personnel failure
148
Thermophilic Spoilage
• Generally, the higher the temperature at
which a sporeforming organism can grow,
the greater the heat resistance of its spores
will be
• Thus, the spores of thermophilic bacteria
usually have a greater heat resistance than
the spores of mesophilic bacteria
149
Thermophilic Spoilage
• The spores of thermophilic bacteria are so
resistant to heat that thermal processes
designed to kill the mesophilic bacteria
spores may not be adequate to destroy
thermophilic bacteria
• In order to prevent thermophilic spoilage,
the product must be properly cooled and
held below thermophilic temperatures
150
Thermophilic Spoilage
• For products such as peas, corn, certain
baby foods and meat, where thermophilic
spoilage may be a problem, processors
should exercise great care in preventing
product contamination by thermophilic
bacteria
151
Thermophilic Spoilage
• Processors should use ingredients – such
as sugar, starch and spices – that the
supplier guarantees are free of thermophilic
bacteria or that meet specifications for
thermophiles for canning processes
152
Thermophilic Spoilage
• Thermophiles may grow in equipment that
contacts food if the temperature is within
their growth range
• Consequently, product should always be held
at 170ºF or higher or at room temperature to
prevent the growth of thermophiles
• Extreme care should be taken to cool the
product promptly below 105ºF after thermal
processing and to store these products
below 95ºF
153
Spoilage by Acid-Tolerant
Sporeformers
• Acidified foods do not require a severe
thermal process to assure product safety
• Therefore, a variety of spoilage-causing,
acid-tolerant sporeformers may survive the
process
• A thermal process for acidified foods is
designed to inactivate a certain level of
these sporeformers
154
Spoilage by Acid-Tolerant
Sporeformers
• Their survival is typically a result of
excessive pre-processing contamination
• Sometimes underprocessing, either due to
an inadequate process or to process
deviations, may also result in the survival of
these acid-tolerant sporeformers
• Butyric acid producing anaerobes
• Aciduric flat-sour sporeformers
155
Spoilage by Acid-Tolerant
Sporeformers
• The butyric acid producing anaerobes, such
as Clostridium butyricum and Clostridium
pasteurianum, are mesophilic sporeformers
• The spores are capable of germination and
growth at pH values as low as 4.2-4.4 and
consequently are of spoilage significance in
acidified foods, particularly if the pH is
above 4.2
156
Spoilage by Acid-Tolerant
Sporeformers
• Spoilage by butyric acid anaerobes may be
controlled either by lowering the pH of the
product to below 4.2 or by increasing the
thermal process
• Growth of these organisms in foods is
characterized by a butyric odor and the
production of large quantities of carbon
dioxide and hydrogen gas
157
Spoilage by Acid-Tolerant
Sporeformers
• Occasionally strains will be encountered
that can grow at a pH lower than 4.2.
• If these strains are present in high numbers,
the heat process may be inadequate and
spoilage may occur
158
Spoilage by Acid-Tolerant
Sporeformers
• An unusual spoilage incident occurred in
canned, acidified mung bean sprouts in
which several acid-tolerant sporeformers
were isolated
• The organisms, which grew and sporulated
to high numbers during the sprouting
process, were able to grow at a pH as low
as 3.7
159
Spoilage by Acid-Tolerant
Sporeformers
• Aciduric flat-sours bacteria are facultative
anaerobic sporeformers that seldom
produce gas in spoiled products
• The ends of spoiled cans remain flat; hence
the term “flat sour”
• Spoiled products have an off-flavor that has
been described as “medicinal” or “phenolic”
160
Spoilage by Acid-Tolerant
Sporeformers
• These organisms (Bacillus coagulans) have
caused spoilage in acid foods such as
tomato products
• It is necessary to ensure that the thermal
process is adequate to inactivate an
expected number of spores
• The load of flat-sour spores is determined
through bacteriological surveys
161
Spoilage by Acid-Tolerant
Sporeformers
• Pinpointing the ingredient that is
contributing the most to the total spore load
may prove beneficial in process control
• For example, proper handling of fruits and
vegetables prior to use, such as washing
and culling, may also help to reduce spore
loads
162
Spoilage by Acid-Tolerant
Sporeformers
• Alicyclobacillus spp., such as
A. acidoterrestris and A. acidocaldarius, are
flat-sour sporeformers that can grow at a
pH as low as 3 in shelf stable juice and
other beverage products
• Spoilage caused by Alicyclobacillus spores
has been reported in a variety of juices and
beverages (in particular apple juice
products), especially when the product
163
packaging allows oxygen transmission
Spoilage by Acid-Tolerant
Sporeformers
• The spoilage can be minimized by multiple
approaches
• Treating a selected ingredient with an
intensified thermal process (at temperatures
above 212°F)
• Product formulation
• Limiting oxygen availability
• Rapid cooling of finished products
164
Spoilage by Acid-Tolerant
Sporeformers
• Some flat-sour facultative aerobes, such as
Bacillus stearothermophilus, are
thermophiles
• Proper cooling after thermal processing and
avoiding high temperatures during storage
are essential since the thermal process for
acid food is not sufficient to destroy their
spores
165
Spoilage by Acid-Tolerant
Sporeformers
• Fortunately, most food processing
operations do not provide anaerobic
conditions; therefore, heavy build-up of
acid-tolerant anaerobic sporeformers
seldom occurs
• However, if dead ends exist in processing
lines, a build-up of anaerobic sporeformers
in the dead ends can occur
166
Spoilage by Acid-Tolerant
Sporeformers
• As a result, under-processing spoilage may
occur because of the heavy load of spores
contaminating the product from the dead
ends
• This is why dead ends must be avoided in
processing lines
167
Spoilage by Acid-Tolerant
Sporeformers
• The spoilage pattern within the affected lots
is often spotty and scattered
• This is more typical of post-processing
spoilage that is due to container leakage,
than the pattern expected from
sporeformers that survive a thermal process
168
Spoilage by Acid-Tolerant
Sporeformers
• Thermal processing records and other
processing parameters usually give no
indication of any irregularities
• In most cases, the problem can be
identified only by investigation at the
factory, which includes a bacteriological
survey, plus the absence of demonstrable
leakage and package defects in the
spoiled containers
169
Spoilage by Acid-Tolerant
Sporeformers
• Spoilage by the thermophilic anaerobe
Clostridium thermosaccharolyticum has
been seen in canned tomato products in the
pH range 4.1 to 4.5
• The thermal process for acidified foods is
not adequate to destroy the spores of the
organism; however, the problem will not
occur if the product is properly cooled
and stored at temperatures below 95°F
170
Non-Microbial Food Spoilage
• Bacterial spoilage is recognized as the most
frequent cause of abnormal conditions in
canned foods
• However, non-bacterial causes of spoilage
are also important to recognize
• The following are causes of non-microbial
food spoilage
171
Non-Microbial Food Spoilage
(1) Chemical reaction of food components
with the metal inner surfaces of the
container
• May produce hydrogen gas
• The accumulation of gas can dissipate the
vacuum in the container and cause the
container to swell
• These hydrogen swells do not cause concern
from a public health standpoint
172
Non-Microbial Food Spoilage
(1) Chemical reaction of food components
with the metal inner surfaces of the
container
• However, consumers are advised to reject
containers with bulged ends, since they cannot
distinguish between a hydrogen swell and
swelling caused by microbial growth
173
Non-Microbial Food Spoilage
(2) The chemical reaction of food acids on the
surface of metal cans
• May advance to the point of causing pinhole
perforations
• Bacteria could enter the can through these
pinholes, causing secondary spoilage
174
Non-Microbial Food Spoilage
(3) Overfilling of containers
• May cause bulged ends or appearance of
spoilage, particularly in smaller can sizes and in
those with large lid area in proportion to height
(4) Closing cans with zero or low vacuum
• May give the appearance of spoilage
• Such cans – referred to as flippers – often
become slightly swollen when transported
to locations with a high altitude
175