1 MICROBIOLOGY OF THERMALLY PROCESSED FOODS Chapter 2

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MICROBIOLOGY MICROBIOLOGY OF THERMALLY OF THERMALLY

PROCESSED FOODSPROCESSED FOODSChapter 2Chapter 2

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IntroductionIntroduction• 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

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IntroductionIntroduction• 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

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IntroductionIntroduction

• Food microbiology is the focus of this chapter

• Food microbiology is the basis for producing commercially sterile, shelf stable foods

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The Microbiology of The Microbiology of Food ProcessingFood 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

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• 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"

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• 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 immediately

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• 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

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• 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

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Characteristics and Characteristics and Behavior of Behavior of

MicroorganismsMicroorganisms• 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

microorganisms attempt to utilize

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MicroorganismsMicroorganisms• Food preservation requires that

microorganisms be controlled or destroyed

• Need to know what they are and how they behave

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MicroorganismsMicroorganisms• 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

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MicroorganismsMicroorganisms• These microorganisms may be

divided into groups on the basis of their microscopic characteristics or visual appearance in mass growths, called colonies

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MicroorganismsMicroorganisms• Other microorganisms of concern• Viruses• Parasites

• Do not multiply in foods but can get into food through contaminated water or other sources

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MicroorganismsMicroorganisms• 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

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Useful Functions of Useful Functions of MicroorganismsMicroorganisms

• 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

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Useful Functions of Useful Functions of MicroorganismsMicroorganisms

• 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 animals

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Some Microorganisms Some Microorganisms Cause DiseaseCause 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

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• 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

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• 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

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• 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

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• 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

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Significant Significant Microorganisms in Food Microorganisms in Food

ProcessingProcessing

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MoldsMolds• 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

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MoldsMolds

• 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

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MoldsMolds

• 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

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MoldsMolds

• 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

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MoldsMolds

• 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

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MoldsMolds• 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

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MoldsMolds

• 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

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MoldsMolds• 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

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MoldsMolds

• 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

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YeastsYeasts

• 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

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YeastsYeasts• 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

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YeastsYeasts

• 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

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YeastsYeasts• 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

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YeastsYeasts

• Yeast growth in processed foods does not present a significant public health problem

• Yeasts are used in the production of bread and fermented beverages

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BacteriaBacteria

• 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

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BacteriaBacteria

• 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

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BacteriaBacteria

• 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

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BacteriaBacteria

• Some bacteria, however, are pathogenic

• In some cases, the microorganisms can produce poisonous substances

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Reproduction of Bacterial Reproduction of Bacterial CellsCells

• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• However, if the microorganism is a type that forms resistant but dormant spores

• These cells can remain alive under conditions that kill other cells

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Sporeforming and Sporeforming and Non-Sporeforming Non-Sporeforming

BacteriaBacteria• 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 non-sporeformers

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BacteriaBacteria• 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

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Spore size, shape and location can help with identification

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BacteriaBacteria• 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

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BacteriaBacteria• 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

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Resistance of Spores to Resistance of Spores to EnvironmentEnvironment

• 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

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Resistance of Spores to Resistance of Spores to EnvironmentEnvironment

• 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

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Source of Foodborne Source of Foodborne OrganismsOrganisms

• 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.

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Source of Foodborne Source of Foodborne OrganismsOrganisms

• 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

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ConditionsConditions Affecting the Growth of Affecting the Growth of

BacteriaBacteria• 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

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Food RequirementsFood 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

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Moisture RequirementsMoisture 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

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Oxygen RequirementsOxygen 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

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Temperature RequirementsTemperature 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

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Classification of Bacteria by Temperature Range for Growth

Classification Optimum Temperature

Psychrotrophs

Mesophiles

Thermophiles

58ºF to 68ºF (14ºC to 20ºC)



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The Psychrotrophic GroupThe 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

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The Mesophilic GroupThe 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.

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The Mesophilic GroupThe Mesophilic Group

• The sporeforming organism C. botulinum is a member of this group, although, as noted above, some strains are considered psychrotrophs

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The Thermophilic GroupThe 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

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• 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

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• Some of the obligate thermophiles can grow at temperatures up to 170ºF

• Laboratory tests have indicated that the spores of these bacteria are so heat-resistant 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

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pH RequirementspH 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

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pH RequirementspH 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

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Clostridium botulinumClostridium botulinum and and BotulismBotulism

• 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

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• 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

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• 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"

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• 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.

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• 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

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• 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

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• 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

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Effect of pH on Effect of pH on C. botulinum C. botulinum GrowthGrowth

• 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

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Effect of pH on Effect of pH on C. botulinum C. botulinum GrowthGrowth

• Spores of C. botulinum and other spoilage types can be found in both acid and low-acid 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

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Effect of pHEffect of pH on Required Heat Treatment 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

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Effect of pHEffect of pH on Required Heat Treatment 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

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Effect of pHEffect of pH on Required Heat Treatment 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

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Approximate pH Rangefor Selected Foods

Lemon JuiceApplesBlueberriesSauerkrautOrange JuicePineapple, cannedApricotsTomatoes, cannedPeaches, cannedPears, cannedBananasBeets, cannedAsparagus, cannedBeefCarrotsPeppers, greenPapaya

2.0 - 2.63.1 - 4.03.1 - 3.33.3 - 3.63.3 - 4.23.4 - 4.13.3 - 4.03.5 - 4.73.7 - 4.24.0 - 4.14.5 - 5.24.9 - 5.85.0 - 6.05.1 - 7.04.9 - 5.25.2 - 5.95.2 - 6.0

TunaSweet PotatoesOnionsWhite PotatoesSpinachBeansPeas, cannedCorn, cannedSoy BeansMushroomsClamsSalmonCoconut milkMilkGarbanzo BeansChickenEggs, whole

5.2 - 6.15.3 - 5.65.3 - 5.85.4 - 5.95.5 - 6.85.6 - 6.55.7 - 6.05.9 - 6.56.0 - 6.66.0 - 6.76.0 - 7.16.1 - 6.36.1 - 7.06.4 - 6.86.4 - 6.86.5 - 6.77.1 - 7.9

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Control of Control of Bacteria by Water ActivityBacteria 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

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• 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

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• 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.”

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• 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

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• 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

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• 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

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• 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

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Minimum aw Requirements for Microorganism Growth

Most molds (e.g., Aspergillus) 0.751

Most yeasts 0.882

C. botulinum 0.93Staphylococcus aureus3 0.85Salmonella3 0.93

1 some strains – 0.612 some strains – 0.623 Non-sporeforming food-poisoning bacteria readily destroyed by heat.

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• 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

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Control of Control of Bacteria by Water ActivityBacteria 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

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Water activity of some common foods

• Liverwurst 0.96• Cheese Spread 0.95• Caviar 0.92• Fudge Sauce 0.83• Semi-moist Pet Food 0.83• Salami 0.82• Soy Sauce 0.80• Peanut Butter – 15% total moisture 0.70• Dry Milk – 8% total moisture 0.70

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• 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

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Control of Control of Bacteria by Water ActivityBacteria 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

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Control of Control of Bacteria by Water ActivityBacteria 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 pre-determined and very accurately controlled at the time of packing

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Control of Control of Bacteria by Water ActivityBacteria 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

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Regulatory RequirementsRegulatory Requirements Related to Water Activity 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

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• 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

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• 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

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• 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

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• 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)

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Methods for Determining aMethods for Determining aww

• 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

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• A variety of hygrometers are available by different manufacturers

• A dew point instrument is also commonly used to measure water activity

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• 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

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• 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)

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• 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

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• 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

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Salt and Water ActivitySalt 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

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• 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 non- heat resistant types

• Strains of C. botulinum that grow in a suitable food containing 7 percent salt are known

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• 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

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Spoilage of Thermally Spoilage of Thermally Processed, Commercially Processed, Commercially

Sterile FoodsSterile Foods

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Spoilage of Thermally Spoilage of Thermally Processed, Commercially Processed, Commercially

Sterile FoodsSterile 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 non-microbial origin

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Indications of Bacterial Indications of Bacterial SpoilageSpoilage

• 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

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• 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

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• 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

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• 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

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Incipient Spoilage Before Incipient Spoilage Before ProcessingProcessing

• 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

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• 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

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• 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 containers

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Post-Process ContaminationPost-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

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• 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 micro-organisms present there will be some that produce gas which causes the swollen containers

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• However, if many swells are present, a small percentage of normal appearing containers may in fact be spoiled by flat-sour microorganisms

• Therefore, normal appearing containers should be examined with this potential in mind

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• 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

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• 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”

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• The term leaker spoilage is typically not used when referring to post-process contamination in aseptically produced products

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Inadequate Heat ProcessingInadequate 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

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• 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

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• Microscopic examination of severely under-processed product will often reveal a mixture of microorganisms similar to that seen in post-process contaminated product

• Microscopic examination of under-processed product will generally reveal pure culture of more resistant species

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• 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

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• 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

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• 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

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Thermophilic SpoilageThermophilic 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

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• 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

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• 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

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• 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

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• 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

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Spoilage by Acid-Tolerant Spoilage by Acid-Tolerant SporeformersSporeformers

• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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”

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• 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

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• 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

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• 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 packaging allows oxygen transmission

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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• 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

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Non-Microbial Food SpoilageNon-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

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(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

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(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

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(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

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(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

Documents

1 MICROBIOLOGY OF THERMALLY PROCESSED FOODS Chapter 2