Factors Affecting Growth of Microorganisms Flashcards
Raw Food Microflora comes from:
- Animals, plants, and people
- Irrigation and wastewater
- manure
- Infected food animal
- Human and animal excrete
- Food handler
- Raw materials
- Flies and pests
- Polluted environment
- Air and water
- Equipment
- Ingredients
Death/Survival contamination depends on:
- Processing conditions (heat, exposure, etc.)
- Sanitation conditions
- Storage conditions
- Composition of foods
- Types of microorganisms
Raw food microflora -> Death/Survival contamination -> Food product microflora
when applied to microbiology, ecology can be defined as “the study of the interactions between the chemical, physical, and structural aspects of a niche and the composition of its specific microbial population”.
- the term “interactions” emphasizes the dynamic complexity of food ecosystems.
Factors that affect how microbes grow in food:
- What’s in the food (intrinsic factors)?
- What’s outside of the food (extrinsic factors)?
- What’s being done to the food (processing factors)?
- What other microbes are doing to the food (microbe factors)?
Intrinsic factors:
- Nutrients in the food substrate
- pH
- water activity or aw
- anti-microbials naturally occurring in the food substrate
- redox potential or Eh
- biological protective structure
Extrinsic factors:
- Temperature during preparation and storage
- Relative humidity of the environment
- Presence and concentration of gases in the environment
- Presence and activities of other microorganisms
Food processing:
- Slicing
- Mixing
- Washing
- Packing
- Pasteurization
- Smoking
- Any type of handling, including addition of additives and preservatives
Other microbes in the food as factor:
- Competition
- Growth rate
- Mutualism
- Antagonism
- Commensalism
- Changes in pH or nutrients or any other intrinsic factor in the food matrix
Which intrinsic factor:
- sources of carbon, oxygen, hydrogen, nitrogen, phosphorus, sulfur, and other elements.
Carbs = CHO
Available nutrients
pH range of most edible foods
pH 4 to pH 8
pH of sodas and most fruit
pH 4
pH of coffee and tomato
pH 5
pH of banana and broccoli
pH 6
pH of distilled water
pH 7
pH of seaweed
pH 8
Foods can be grouped as:
based on pH
- low-acid foods (pH above 5.2)
- medium-acid foods (pH 5.2 - 4.6)
- acid foods (pH 4.6 – 4.0)
- high acid foods (pH below 4.0)
The acid in the foods can be either:
- present naturally (as in fruits)
- produced during fermentation
- added during processing (as in salad dressings)
pH of Egg albumen
> 7.0
pH of milk, ham, bacon, poultry, and fish
7.0 - 6.5
pH of raw beef, vegetables, vacuum-packed meat, and melons
6.5 - 5.3
pH of cottage cheese, fermented vegetables, and fermented meats (summer sausage), and many sauces and soups
5.3 - 4.5
pH of tomatoes, fruits and fruit juices, yogurt, pickles, and sauerkraut
<4.5
- each unit on the pH scale represents a 10-fold difference: a food with a pH of 6 is 10 times more acidic than one with a pH of 7; pH 5.0 is 100 times more acidic.
most foodborne bacteria grow in acidic conditions
Which intrinsic factor:
- water availability
- nutrients in soluble form
- water for hydrolysis by enzyme
Water activity (aw)
is a measure of the availability of water for biological functions and relates to water present in a food in “free” form
water activity (aw)
is necessary to hydrate the hydrophilic molecules and dissolve the solutes, and is not available for a biological function; does not contribute to aw
bound water
Water activity of a food can be expressed by the ratio of water vapor pressure (p) of the food to that of pure water vapor pressure (p0 is 1) at the same temperature
aw =(vapor pressure of food/vapor pressure of pure water)
aw = P/P0
water activity of milk powder and instant coffee
0.2
water activity of biscuits
0.3
water activity of pasta
0.5
water activity of dried fruit
Xerophilic moulds osmophilic
0.6
water activity of honey
0.7
water activity of jams and jellies
Most spoilage molds
0.8
water activity of spoilage yeasts and the minimum water activity for bacterial growth
0.85
water activity of cheese
0.97
water activity of bread
0.95
water activity of potentially hazardous foods (PHF)
0.85 - 1
2 types of foods according to moisture
- High-moisture foods such as fruits, vegetables, meats, and fish have an aw of >0.98
- Intermediate moisture foods (jams, sausages, etc.) have aw levels of 0.7 to 0.85
decreasing aw increases the lag phase of growth and decreases the growth rate
in general, (Gram-negative, Gram-positive) bacteria have the highest aw requirement (0.93) than (Gram-negative, Gram-positive) bacteria (0.90)
Gram-negative; Gram-positive
most spoilage bacteria do not grow below 0.90
(fungi, bacteria) require higher values of aw for growth than (fungi, bacteria)
bacteria; fungi
most bacteria grow well with aw 0.995–0.998
Pseudomonas spp. = above aw of 0.96
most members of the family Enterobacteriaceae = grow
only above aw of 0.93
Microorganisms according to the water activity environments they can grow in:
- Halotolerant microbes grow in high concentrations of salt
- Osmotolerant microbes grow in high concentrations of organics (e.g., sugars)
*Xerotolerant microbes grow on dry foods
Shelf life of most fresh foods, meat, fish, poultry, fruit, and vegetables
Days
Shelf life of cured meat products
Weeks
Shelf life of syrups and salted foods
Months
Shelf life of dried foods
Years
mycotoxins:
Aflatoxins
Ochratoxins
Patulin
molds that produce the mycotoxin aflatoxins
Aspergillus flavus
Aspergillus parasiticus
molds that produce the mycotoxin ochratoxins
Aspergillus ochraceus
Penicillium cyclopium
molds that produce the mycotoxin patulin
Penicillium expansum
Penicillium patulum
a food substrate in which microbes are able to survive, replicate, and colonize; food in any form or state that is capable of supporting the growth of disease-causing microorganisms or the production of toxins
Potentially hazardous food (PHF)
pH and water activity levels that make foods potentially hazardous
pH >4.6
aw >0.85
Antimicrobials and the foods where they can be found
- cloves, cinnamon, and sage (eugenol)
- cinnamon (cinnamic aldehyde)
- oregano (carvacrol and thymol)
- thyme (thymol)
- garlic (allicin)
- ginger (olioresin)
- egg whites (lysozyme)
- mustard (allyl isothiocyanate)
- cow’s milk (lactoferrin, immunoglobulin, conglutinin, lysozyme.)
- egg albumen (Ovotransferrin)
- cranberries (benzoic acid and sorbic acid)
- garlic, onions, and leeks (isothiocyanates and thiosulfate)
- milk (lactoperoxidase system in milk with the natural content
of lactoperoxidase and thiocyanate)
Antimicrobials can be produced by microorganisms in foods:
- propionic acid-producing by __________in cheese inhibits molds
- H2O2, organic acids, and ethanol-producing by __________ in fermented foods inhibit other microorganisms
- alcohols producing by _________ inhibit other microorganisms
- bacteriocins (nisin, plantaricin, pediocins) by ____ in fermented foods
propionic bacteria; lactic acid bacteria (LAB); wine yeasts; LAB
when a substrate loses electron, the substrate is _______
oxidized
a substrate that gains electrons becomes _______
reduced
- when electrons are transferred from one compound to another, the redox potential (Eh) is created between two compounds
- the redox potential is measured in __________
electrical units of millivolts (mV)
- oxidized substrate carries out (positive redox potential (+mV), negative redox potential (- mV)) and
reduced substrate, (positive redox potential (+mV), negative redox potential (- mV))
positive redox potential (+mV); negative redox potential (- mV)
Which intrinsic factor tells how much natural oxygen is present in the food
Redox potential (O/R; Eh)
- increasing air (oxygen) will increase the redox potential
- transfer of electrons drives many chemical activities in microbes (actions of enzymes, metabolic activities)
- microbial growth in a food will reduce its redox potential
- increasing acidity will increase the redox potential
- Oxygen content in the food mostly increases during mincing, cutting, chopping, and grinding of foods; this leads to maximum positive Eh and supports the microbial growth.
- natural covering of foods provides excellent protection against the entry and damage by spoilage microorganisms
Biological protective structure
hide, scales, egg shells
microbes that grow in the temperature range of -7 to 17 degrees celsius
Psychrophiles
microbes that grow in the temperature range of -7 to 35 degrees celsius
Psychrotrophs
microbes that grow in the temperature range of 10 to 50 degrees celsius
Mesophiles
microbes that grow in the temperature range of 42 to 85 degrees celsius
Thermophiles
microbes that grow in the temperature range of 65 to 110 degrees celsius
Hyperthermophiles
Minimum temperature
Optimal temperature
maximum temperature
freeze food to ________ for storage
below -18°C
Store and thaw food at __________
4°C or below
Avoid storing potentially hazardous food in this temperature range; bacteria multiply rapidly
4°C to 60°C = Danger zone
Hold foods hot at ________
60°C or above
Reheat food to _______
74°C or above
Scotland’s recommended cooking temperature
82°C
England’s recommended cooking temperature
70°C
Classes of food based on perishability
- Perishable
- foods that spoil quickly within 1 or 10 days
- high water content
- short shelf life - Semi-perishable
- foods that can last for 1-2 weeks
- less water content
- medium shelf life - Non-perishable
- foods that last for 1 year
- no water content
- long shelf life
_________ is the most important parameter that affects the spoilage of highly perishable foods
Temperature of storage
- When the aw of a food is set at 0.60, it is important that this food be stored under conditions of RH that do not allow the food to pick up moisture from the air and thereby increase its own surface and subsurface aw to a point where microbial growth can occur
Relative humidity (RH) (expressed as a percent) measures water vapor, but RELATIVE to the temperature of the air
it is a measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature.
Relative humidity
*Moisture equilibrium is established when:
- foods with low aw values are placed in environments of high RH
- foods with a high aw lose moisture when placed in an environment of low RH
- the higher the temperature, the lower the RH, and vice versa
__________ require oxygen to live; they are growing at the top of the tube of nutrient broth.
provide example:
Obligate aerobes
Mycobacterium tuberculosis
__________ are bacteria that grow with or without oxygen but grow best in oxygen, with most growth near the top of the tube. These bacteria are growing throughout the tube of nutrient broth.
Provide example:
Facultative anaerobes
E. coli
______ do not grow in the presence of oxygen, shown growing at the bottom of the tube.
Provide example:
Obligate anaerobes
Clostridium botulinum
________ bacteria are not affected by oxygen levels and can grow equally well in aerobic or anaerobic conditions (and anything in between).
Provide example:
Aerotolerant anaerobes
Lactobacillus
_______ bacteria grow best in low concentrations of oxygen, just below the surface of the nutrient broth in the tube
Provide example:
Microaerophilic
Campylobacter jejuni
One microbe community harms another
Example: The growth of LAB, leuconostoc, produces acids that inhibit the growth of E. coli.
Penicillium, the black bread mold, secretes chemicals that kill bacteria
Competition and antagonism
Both microbe communities benefit each other
Example: The growth of one strain of LAB, Leuconostoc, promotes the growth of another type of LAB, lactobacillus
Mutualism
One microbe community benefits from growth but doesn’t affect the other community
Example: During fermentation of apples into vinegar, yeast will break down sugars into alcohol. Acetobacter bacteria benefit from this and break down the alcohol into acetic acid. This end product is not harmful to yeasts.
Commensalism
