FOOD MICROBIOLOGY
Food microbiology:
Food microbiology is the study of the microorganisms that inhabit, create, or contaminate
food. Of major importance is the study of microorganisms causing food spoilage
Importance of microbiology
The importance of microbiology includes:
Used in biomedical research, creation of medicines, environmental applications and
new research tools.
Disease causing organisms include: protists, bacteria, viruses and other
microorganisms.
Bacteria are important for fixing N2 in a usable form for plants.$
Bacteria and some fungi are important in decomposition and recycling of raw
materials.
Industry applications of microbiology: waste management, food industry, mining,
medicine, research and biotechnology.
Primary sources of micro-organisms found in food
1. Soil and water (Air and dust)
Grouped together because of atmospheric cycling.$ Soil and water are common sources of
important pathogenic and spoilage microorganisms, which is why it is important to
thoroughly wash raw foods with good quality water.$ Air and dust are important sources of
microorganisms during food processing and can influence food quality in the home as well.
2. Plants and plant products
Although most soil and water borne microbes will contaminate plants, very few types
actually persist on them.$ Those that persist, such as lactic acid bacteria and some yeasts,
must be able to adhere to the plant material and to utilize it for growth.
3. Food Utensils
Another important source for cross contamination of raw and cooked foods e.g. cutting
blocks, food trays where raw food was held (BBQ plate).
4. Intestinal tracts of humans and animals
Poor sanitation practices (use of polluted water, poor personal hygiene) lead to
contamination from these sources. And many pathogens are transmitted by this route.
5. Food handlers
Microbiota on hands, garments, etc. reflects the habits of the individual.$ This can include
microorganisms from virtually any environmental source.
6. Animal feeds
Very important source of Salmonella in poultry and of Listeria monocytogenes (from silage)
in dairy and meat animals.
7. Animal hides
E.g. Microbiota of raw milk influenced by that of the udder.
Synopsis/Symptoms of common food borne moulds
Yeasts Involved in Food Spoilage
Yeast genera Product spoilage
Saccharomyces Low sugar products
Zygosaccharomyces(osmophilc) Honey, Syrups, Molasses, Wine,Soysauce
Pichia Wines
Hansenula Beer
Debaryomyces Meat brine, Cheese, Sausages etc.
Hanseniospora Fruit juices
Torulopsis Milk products, Fruit juices, acid foods
Rhodotorula Meat, Saurkraut
Candida High acid foods, Salt foods, butter
Brettanomyces Beer, Wines
Trichosporon Chilled beer
INTRINSIC AND EXTRINSIC PARAMETERS OF FOODS THAT AFFECT
MICROBIAL GROWTH
Intrinsic Parameters
These are inherent properties of foods which influence how well microbes can grow in those
foods. Some foods can support the growth of a wide array of microbes while growth in
others is limited to a few genera or species (e.g. milk-almost anything grows vs. beef jerky
where growth is typically restricted to xerophilic molds).
1. PH
Most microbes grow best around 7.0, and very few grow below 4.0 (lactic acid bacteria,
yeasts and molds are some exceptions. Fortunately, most pathogenic bacteria are very
susceptible to low pH, so the heat treatment required to destroy C. botulinum in high acid
foods like tomatoes is lower than that given to low acid products like green beans.$ Type of
acid present in the food may also influence growth.$ Lactic, sorbic & propionic acid are
generally inhibitorier than HCl.$ Effects of PH;
1.$$$ Retards the function of microbial enzymes
2.$$$ Retards the cells' ability to transport nutrients across its' membrane.
2. Moisture Content
One of the oldest methods to preserve foods is drying, which restricts the water available to
microbes for chemical reactions.$ The water requirements of a microbial species is defined
by water activity (aw):
$$$$$$$$ Aw$= p =$$$ vapor pressure of the food (p)
$$$$$$$$$$$$$ $$ po$vapor pressure of pure water at the same temperature (po)
The aw$of pure water = 1.00, and that of most fresh foods is >0.99.
$Ways to reduce aw:
1. Drying (xerophiles may be a problem).$
2. High solute concentration (sugar or salting); limits the availability of "free" water
(osmophiles or halophiles may still be a concern).$
3. Freezing (energy intensive)$
$3.9 Oxidation-Reduction Potential- (O/R or Eh)
Defined as the relative ease with which a medium gains (reduction) or loses (oxidation)
electrons.$
O/R potential of food determined by:
1. Eh of original food.
2. Poising capacity (resistance to change in Eh).
3. Oxygen$tension of atmosphere around the food and its access to the food
Examples - packaged food, ground or whole food.
4.$ Antimicrobial Compounds
Some foods resist microbial spoilage because they contain natural substances that inhibit
microbial activity. Examples include:$
1. Essential oils, often in spices$
2. Milk also contains antimicrobial proteins such as lactoferrin, which tightly binds Fe3.
3. Eggs contain significant amounts of lysozyme as well as conalbumen, a protein that
chelates iron, copper and zinc to limit their availability to infecting MOs.
4. Many plants, fruits and vegetables produce hydroxycinnamic acid derivatives (e.g.
ferulic, caffeic acids) which inhibit bacteria and some fungi.
5. Lactic fermentation of foods can result in the synthesis of several natural inhibitors
including organic acids, H2O2, and bacteriocins.
6.9 Biological Structures
Natural covering of some foods excludes or resists microbial invasion and proliferation: $
1. Shells of egg and nuts
2. Skin of fruits, vegetables and even fish and animals all retard the rate of spoilage.
EXTRINSIC PRAMETERS9
Properties of food storage environment.$
The most important parameters are:
1. Temperature
Storage temperature is the single most important parameter that affects the spoilage of highly
perishable foods.
a. Psycrotrophic and mesophilic species may grow at refrigeration temps and thermophiles
may grow in warmed foods.
b. Storage temperature also affects the quality o foods.
- High temperatures will prevent microbial grwth but may lead to thermal breakdown
of food constituents.
- Low temperature storage (refrigeration or freezing) also decreases the quality of
some foods.$ Ice crystal formation….sublimation
2. Relative Humidity
$Although R.H. of the storage environment primarily affects the amount of water available to
support microbial growth on a food surface, it can also influence the aw$of a food by
allowing the food to gain or lose moisture.
3. Atmosphere (presence and concentration of gases)
Earth's' atmosphere is composed of approximately: 78% N2, 21% O2, 0.03% CO2 and 0.07%
other gases
1. Growth of various Microbes will be heavily influenced by their ability to grow in the
presence or absence of O2.
a. Clearly, molds will not be a problem in vacuum-pkgd meats.
b.$C. botulinum will not grow on the surface of raw chicken.
2. In addition to the influence of 02, food microbiologists have also discovered that
manipulation of the atmospheric content can also influence microbial composition and
growth:
a. Storage of fruits in a modified atmosphere (MA) of up to 10% CO2, for example, retarded
spoilage by many fungi.
b. High levels of CO2 are also used during storage and packaging of meats to increase
storage life.
Analysis of this phenomenon have indicated that G- were more sensitive to increased CO2
concentrations than G+, while lactic acid bacteria and anaerobes were among the most
resistant.
Although the inhibitory effect of CO2$has been recognized for over 100 years, the precise
mechanism for CO2$inhibition of MO is still unclear.
c. Use of N2 instead of CO2 has a similar effect.
FOOD PRESERVATION
Introduction
Food preservation is the process of treating and handling food to stop or slow down spoilage
(loss of quality, edibility or nutritional value).Preservation usually involves preventing the
growth of bacteria, yeasts, fungi, and other micro-organisms (although some methods work
by introducing benign bacteria, or fungi to the food), as well as retarding the oxidation of fats
which cause rancidity. Food preservation also includes processes which inhibit natural
discoloration that can occur during food preparation, such as the enzymatic browning
reaction in apples after they are cut. Many processes designed to preserve food will involve a
number of food preservation methods. Preserving fruit, by turning into jam, for example,
involves boiling (to reduce the fruit’s moisture content and to kill bacteria, yeasts, etc),
sugaring (to prevent their re-growth) and sealing within an airtight jar (to prevent
recontamination).Maintaining or creating nutritional value, texture and flavor is an important
aspect of food preservation, although, historically, some methods drastically altered the
character of the food being preserved. In many cases these changes have now come to be
seen as desirable qualities – cheese, yoghurt and pickled onions being common examples.
Role of food preservation:
1. Eliminate any potential microbiological harm to the consumer
2. Maintain quality of food (sensory perceptions)
3. Maintain nutritional value within limits dictated by the production of a safe food
product
- No preservation method will completely eliminate spoilage phenomena.
Preservation methods
1. Thermal processing
Application of heat
- Inactivate enzymes
- Kill microorganisms. Most bacteria are killed in the range 82-93c. spores are not
destroyed even by boiling water at 100c for 30 min to ensure sterility (total microbial
destruction, Including spores) a temp. of 121c must be maintained for 15 min or
longer.
- Blanching
- Pasteurization
- Commercial sterilization
- Removal of heat (cold processing) - lowering temperature of food decreases the rate
of enzymatic, chemical and microbial reactions in food, storage life is extended
- Refrigeration
- Freezing
2. Control of water content (drying)
microorganisms require free water
free water is removed from the food and therefore, from microbial cells
multiplication will stop
Water unavailable for chemical/biochem. reactions
storage life extended
freezing water
physical removal of water from food (dehydration)
removal of some of the water from food (concentration)
addition of substances that bind water in food, making it unavailable (sugar, salts)
3. Radiation
ionizing radiation inactivate microorganisms in food, destroy storage pests
microwave treatment to inactivate enzymes
infrared radiation to cook food and keep food hot
ultraviolet energy to sterilize air and water used in food processing
4. Atmosphere composition
removal of oxygen
inhibits O2-dependant enzymatic and chemical reactions
inhibits growth of aerobic microorganisms
e.g. use of paraffin wax, nitrogen back flushed bags (potato chips), controlled atmosphere
storage, vacuum packaging of fresh food (cured meats)
5. Fermentation
specific microorganisms are used (starter cultures)
facilitate desirable chemical changes
enhance longer storage life
produce acids, alcohol that will prevent growth of undesirable microorganisms and
antimicrobial agents
Food preservation methods
Method Advantages Disadvantages
Drying (e.g. freeze-
drying, spray-drying,
sun-drying)
Produces concentrated
form of food.
Inhibits microbial
growth & autolytic
enzymes.
Retains most nutrients.
Can cause loss of some
nutrients, particularly
thiamin & vitamin C.
Sulphur dioxide is
sometimes added to dried
fruits to retain vitamin C,
but some individuals are
sensitive to this
substance.
Smoking Preserve partly by
drying, partly by
incorporation of
substances from smoke.
Eating a lot of smoked
foods has been linked
with some cancers in
some parts of the world.
Refrigeration Slows microbial
multiplication.
Slows autolysis by
enzymes
Slow loss of some
nutrients with time
Freezing Prevents microbial
growth by low
temperature &
unavailability of water.
Generally good retention
of nutrients.
Blanching of vegetables
prior to freezing causes
loss of some B-Group
vitamins and vitamin C.
Unintended thawing can
reduce product quality.
Adding salt or sugar Makes water unavailable
for microbial growth.
Process does not destroy
nutrients.
Increases salt and sugar
content of food.
High heat processing
(e.g. pasteurization)
Inactivates autolytic
enzymes
Destroys
microorganisms.
Loss of heat-sensitive
nutrients.
Canning (involves high
heat processing)
Destroys
microorganisms &
autolytic enzymes.
Water-soluble nutrients
can be lost into liquid in
can.
Chemical preservatives Prevent microbial
growth
No loss of nutrient.
Some people are sensitive
to some chemical
preservatives.
Ionizing radiation Sterilizes foods (such as
spices) whose flavour
would change with
heating.
Inhibits sprouting
potatoes
Extends shelf life of
strawberries and
mushrooms
Longer shelf life of fresh
foods can lead to greater
nutrient losses than if
eaten sooner after
harvesting.
