Food Microbiology

Question 1

MacConkey Agar

Answer 1

◦ Selective and differential media
◦ Used for the isolation of coliforms and intestinal pathogens in water, dairy
products and biological specimens
◦ Used in the differentiation of lactose fermenting from lactose non-
fermenting gram-negative bacteria
◦ Fermentation of carbohydrates by bacteria results in production of acid and
gas
◦ Acid production lowers pH of media – colour change

Question 2

Intrinsic factors affecting microorganisms in food

Answer 2

Factors related to the food itself;
inherent part of a food, so hard
to manipulate/change:
* Food composition/nutrient
content
* pH
* Water activity
* Oxidation-reduction potential
* Antimicrobial constituents
* Biological structures

Question 3

Extrinsic Factors affecting microorganisms in food

Answer 3

Factors related to the food environment:
* Temperature of storage
* Relative humidity
* Gases in the environment

Question 4

Mycotoxins

Answer 4

Mycotoxins are secondary metabolites produced by certain molds e.g.
Aspergillus spp, Penicillium spp and Fusarium spp
*The molds grow on a variety of different crops and foodstuffs including
cereals, nuts, spices, dried fruits, apples and coffee beans
*Often grow under warm and humid conditions

Question 5

high risk foods and associated bacteria

Answer 5

*Meat and Poultry:
Salmonella, Campylobacter, Listeria, E. coli O157:H7, Clostridium perfringens
*Milk and dairy products:
Listeria, Campylobacter, Staphylococcus
*Seafood (fish and shellfish):
Vibrio, Salmonella,Shigella
*Vegetables (esp. salad vegetables):
Listeria, Salmonella, E. coli O157:H7,
other types of E. coli
*Eggs:
Salmonella enteritidis
*Chocolate:
Salmonella montevideo

Question 6

Food preservation methods

Answer 6

• low temperature
• reduction in water activity
• reduction in pH
• removal of oxygen
• chemical preservatives
• high temperature
• high hydrostatic pressure
• radiation - UV and gamma
• biopreservation

Question 7

Date labelling of food - Use-by

Answer 7

SAFETY

◦Foods can be eaten until the use-
by date but not after
◦Used on food that goes off quickly
e.g. meat products or ready-to-eat
salads.
◦Storage instructions must be
adhered to
◦After the use-by date, don’t eat it,
cook it or freeze it

Question 8

Date labelling of food - best before

Answer 8

QUALITY

◦Food will be safe to eat after this date
but may not be at its best
◦E.g. flavour and texture might not be as
good
◦Best before dates appear on a wide
range of foods including: frozen foods,
dried foods, tinned foods
◦Storage instructions must be adhered
to

Question 9

Low temperature preservation

Answer 9

*Slows/prevents microbial growth
*When temperatures are reduced below the
optimum for growth of a particular
microorganism:
◦ Metabolic reactions that are enzyme
catalyzed slow down as temperature
decreases
◦ Lag time and generation time increase
◦ Growth rate decreases
◦ Cell division ceases as the temperature
approaches the minimum for growth
◦ Refrigeration (temp 1- 40C) retards
microbial growth but only for a limited time
◦ During freezing, temp is much lower (-200C)
and water is unavailable for microbes

Question 10

Reduction in water acitivity

Answer 10

• Drying/dehydration – water is removed e.g. dried fruits, dried pasta
• Addition of solutes like sugar or salt (re notes on aw):
• Salt (NaCl)
  ◦ Oldest preservative known
  ◦ Sodium chloride exerts a drying effect on both food and
  microorganisms
  ◦ Causes plasmolysis of cells because water passes out of cell to try
  and dilute high salt concentrations
  ◦ Used for fish and meat traditionally
• Sugar – high sugar content also causes loss of water from microbial
  cells

Question 11

High temperature preservation

Answer 11

Low temperature holding (LTH) :
- 68.2 0C, 30 minutes

High-temperature short-time (HTST):
- 72 0C, 15 seconds

High-temperature Long-Time (HTLT):
- 80 0C, 30 seconds

Ultra-high Temperature (UHT):
- 138 0C, 3 seconds

Question 12

Pasteurisation

Answer 12

heating food to a temperature that kills disease
causing microorganism and that substantially reduces numbers of
spoilage organisms
*Duration and temperature of pasteurisation process (and other heat
treatment processes) is based on knowledge of food composition and
microbiota:
◦ Water, fat, salts, carbohydrates
◦ pH
◦ Proteins and other substances
◦ Numbers of microorganisms
◦ Age/growth phase of microorganisms

Question 13

Sterilisation

Answer 13

Implies the destruction of all viable organisms as may be measured by
an appropriate plating or enumeration method:
*UHT pasteurisation
*Canning :
*Canned foods are heated in special containers called retorts
*Heated around 1150C for 25- >100 mins
*Cans are then rapidly cooled with cold water
*Canned foods are sometimes called ‘commercially sterile’ to indicate
that the number of survivors is so low as to be of no significance
during subsequent storage

Question 14

Packaging

Answer 14

*Vacuum packaging:
◦ Air is removed
◦ It’s return is prevented by an airtight seal surrounding the food
within the packaging material
*Modified atmosphere packaging (MAP):
◦ Atmosphere food is packaged in is modified with a mixture of natural
gases in carefully controlled proportions
◦ Gases used based on type of food in the package and the sort of
decay or change that the food undergoes

Question 15

Radiation

Answer 15

Radappertization
Three sources of radiation used:
◦ X rays beams—send stream of high-energy electrons reflected from
heavy metal
◦ Electron beams—send stream of high-energy electrons propelled
from accelerator
◦ Gamma irradiation—cobalt-60 or cesium-137 used with moist foods
because only effective if it generates reactive oxygen species from
water
Radappertization extends shelf life of meats, seafood, fruits, and
vegetables.