Food Microbiology

Question 0

Why is food composition an important intrinsic factor?

Answer 0

Microbes require water, energy source, vitamins and minerals etc. and have different requirement levels for growth. Molds have the least requirements, followed by yeasts, gram neg and gram positive bacteria.

Question 1

Name 6 intrinsic factors affecting microbial growth

Answer 1

Food composition
pH levels
Moisture content
Oxidation-reduction potential
Antimicrobial constituents
Physical structure

Question 2

Describe four types of food spoilage that occurs based on the COMPOSITION of the food

Answer 2

Pectin in fruits undergoes pectinolysis which produces methanol and uronic acids- soft rot, loss of fruit structure etc.
Proteins in meat undergo proteolysis to produce amino acids, peptides, ammonia, H2S etc. which causes bitterness, souring, bad odour, greening.
Carbohydrates in starchy foods undergo hydrolysis or fermentation which produces organic acids, CO2, alcohols- leads to acidification and souring.
Lipids found in butter undergo fatty acid degradation to produce glycerol, mixed fatty acids which causes rancidity and bitterness

Question 3

Why are pH levels important in affecting microbial growth and what are the ideal levels for microbes to grow at?

Answer 3

Adverse pH affects enzyme function and transportation of nutrients. Molds and yeasts thrive in levels of 3.5 or lower while bacterias prefer 5-5.6.

Question 4

How do microbes protect themselves from adverse pH levels?

Answer 4

They use amino acid decarboxylases and amino acid deaminases to help buffer acid and alkaline conditions respectively.

Question 5

How can the substrate pH be increased?

Answer 5

By metabolizing the acids present (Ie. Clostridium acetobutylicum)

Question 6

Why is the moisture content important?

Answer 6

Aw reflects the amount of water available to microorganisms. By decreasing aw, less water is available for use by microbes.

Question 7

How does one decrease aw?

Answer 7

By increasing the salt concentration

Question 8

What are the exceptions to stopping microbe growth by decreasing aw?

Answer 8

Osmophilic microbes prefer high osmotic pressure.
Halophilic microbes can grow in high NaCl concentrations
Xerophilic microbes prefer low aw

Question 9

What is oxidation-reduction potential?

Answer 9

Eh is the ease at which the food substrate loses or gains electrons. Anaerobic bacteria require a negative Eh (reuced)while aerobic bacteria require a positive Eh (oxidised).

Question 10

How can Eh be altered?

Answer 10

It can be altered by microbial growth and cooking. Aerobic growth depletes oxygen, lowering Eh.
Metabolic byproducts of anaerobic growth (H2S) will lower Eh

Question 11

With reference to antimicrobial constituents, why do people spice their food?

Answer 11

Original hypotheses were to make food taste better, eat-to-sweat theory, disguise the taste of spoilt food, to add nutritional value to food.
It was found that hotter countries use spices more frequently and veg based meals use less spice than meat based ones. Spices like garlic, onion, oregano were antibacterial so the ultimate reason is to kill food-borne bacteria and fungi

Question 12

How can the physical structure affect spoilage?

Answer 12

It can retard, prevent or promote spoilage. Outer coverings of seeds, fruits, nuts provide protection.
Grinding food increases the surface area and alters the cellular structure. This distributes contaminating microbes throughout the food.

Question 13

Describe how avian eggs are an example of the effects of intrinsic factors on microbial growth

Answer 13

Food composition: conalbumin scavenges metal ions and makes them unavailable to bacteria and avidin forms a complex with biotin.
PH: a very alkaline egg white (9.1-9.6)
Antimicrobial constituents: lysozyme targets the cell walls of gram positive bacteria
Physical structure: outer shell and membranes act as a barrier to entry by microbes.

Question 14

Explain how honey is an example of the effects of intrinsic factors on microbial growth

Answer 14

Moisture content: low aw of honey (0.6) means little moisture is available to support microbial life
PH: low pH (3.2-4.5) discourages growth
Antimicrobial constituents: methylglyoxal, hydrogen peroxide forms in certain honeys when diluted

Question 15

Name four extrinsic factors that affect microbial growth

Answer 15

Temperature
Relative humidity
Presence and concentration of gases
Presence of other microbes

Question 16

Why is temperature so important in microbial growth?

Answer 16

Affects the spoilage of highly perishable foods.

Question 17

Name three ranges of temperatures that some microbes grow at

Answer 17

Pyschrotrophs: 7-30 degrees (Pseudomonas and Entercoccus)
Mesophiles: 20-45 degrees (Staphylococcus)
Thermophiles: 55-65 degrees (Bacillus and Clostridium)

Question 18

How does low term pasture affect microbial growth and why can’t it be used for all food?

Answer 18

Slows growth but low temps compromise the maintenance and quality of some foods

Question 19

Why is relative humidity important for microbial growth?

Answer 19

Microbial growth is initiated at a higher Rh even at lower temperatures (dried foods placed in moist environments can absorb moisture- thereby allowing microbial growth). Surface spoilage can be reduced by storage under low Rh conditions.

Question 20

Describe two methods of packaging that control the atmosphere surrounding the food.

Answer 20

High CO2 modified atmosphere packaging: high CO2 interferes with metabolism and membrane transport, and also retards the rotting of fruit and inhibits gram negative bacteria.
Ozone modified atmosphere packaging: superoxide anions inside the cells are transformed into highly toxic peroxide and oxide (resulting in antimicrobial activity)

Question 21

Hw can the presence of other microbes affect the microbial growth of certain bacteria?

Answer 21

Food microbiota can produce substances that are inhibitory or lethal to other microorganisms.

Question 22

Give two examples of foods that show how extrinsic factors affect microbial growth.

Answer 22

Yoghurt:
- temperature: storage at 4 degrees discourages microbial growth of mesophiles and thermophiles
- presence of other microbes: lactic antagonism (lactic acid bacteria discourage contamination by other microorganisms)
Biltong:
- relative humidity: low Rh of sealed product prevents growth of microbes
- modified atmosphere packaging: high CO2 concentration in the internal atmosphere prevents microbial growth.

Question 23

Describe the chemistry of some food spoilage (degradation of substances)

Answer 23

Carbohydrates: do not yield major odours and the conversion to ethyl alcohol gives alcoholic taste
Amino acids: results in foul smelling compounds, cysteine breakdown yields H2S (rotten egg smell), tryptophan breakdown into indole and skatole (faecal smell) and putrefaction- anaerobic breakdown of proteins.
Fats: broken down into fatty acids (rancid smell)
Sliminess: from bacterial capsule production
Swelling and souring: microbial metabolism produces acid and gas

Question 24

Why are meat and fish easily spoiled?

Answer 24

Protein rich foods and can become contaminated during slaughter, storage or handling

Question 25

Describe the type of spoilage that can occur in meat

Answer 25

Organ meats (liver, kidney etc) are less compact than muscle tissue and thus spoil easily. Lactobacillus (gram positive rod) and Leuconostoc (gram positive coccus) leave a greening on meat surfaces. Streptococcus is responsible for the formation of slime and souring of processed meats.

Question 26

Why are fish easily spoilt?

Answer 26

Shellfish generally obtain their food by filtering particles from after (this can concentrate pathogens like hepatitis A or Vibrio cholera). Marine algal toxins produced by phytoplankton species are ingested by shellfish and fish. Consumption of contaminated seafood results in seafood poising syndromes (paralytic, neurotoxic, amnesic, diarrhetic etc)

Question 27

Describe how some bakery products can be favourable for microbial growth

Answer 27

Cream fillings and toppings provide favourable conditions for growth.
Custards made with whole eggs may be contaminated with salmonella
Whipped cream may contain Lactobacillus and Streptococcus

Question 28

Describe the effects of Aspergillus flavus contamination in grains

Answer 28

The ascomycete produces aflatoxins which accumulate in stored grain such as wheat, peanuts, soybeans and maize. These toxins have been implicate din liver and colon cancers

Question 29

Describe the grain spoilage caused by Claviceps purpurea

Answer 29

Causes ergot poisoning. This causes convulsions, hallucinations (LSD is derived from this toxin).
The symptoms reported at the Salem Witch trials were linked to ergot poisoning.
Rye, wheat and barley are susceptible to this type of spoigad

Question 30

Describe the spoilage that occurs in fruits and veggies

Answer 30

Spoilage is initiated by mood that weakens the outer skin. Bacteria then degrade the carbs and the high Eh favours aerobes and facultative anaerobes.
Vegetables: Pseudomonas, Bacillus, Penicillium, Aspergillus
Fruits: Actobacter, Penicillium, Aspergillus and Rhizopus

Question 31

Describe how vinegar is made

Answer 31

Yeasts ferments the fruit juice and the alcohol is converted to vinegar by Actobacter aceti.
A vat is filled with vinegar-soaked wood shavings, on which grows Actobacter aceti. Air enters via bungholes or by an air compressor at the bottom of the vat.
Alcohol drips through the wood shavings and the bacteria convert the alcohol into acetaldehyde and then into acetic acid.
This is then diluted from 14% to 5%.

Question 32

How is yoghurt made?

Answer 32

A fermentation product of milk brought about by the action of lactic acid bacteria. The production of lactic acid causes casein to coagulate and precipitate.
Lactic acid and carbonyl compounds contribute to the flavour.
S. thermophilus (produces pyruvic acid, formic acid and CO2) and L. bulgaricus (produces peptides and amino acids) are typically used as a starter culture- each enhances the growth and acid production of the other.

Question 33

What do the starter culture and secondary culture in cheese do?

Answer 33

Starter culture brings about initial coagulation of casein and the secondary culture is for flavour development during ripening.

Question 34

Describe how cheese is made

Answer 34

Lactic acid starter cultures (usually S. thermophilus and L. lactis) are added to warm milk and change the small amounts of lactose to lactic acid.
This acidified the milk at a faster rate and prepares it for the next stage.
Rennet (chymosin) is added to the milk and a soft curd is produced.
The curds are cut to form small funds and separated from the whey and allowed to mat together to form large slabs of hard curd.
Salt is then added and the curd is moulded and pressed into moulds for maturing

Question 35

What is whey?

Answer 35

Water, unfermented lactose and albumen)

Question 36

Why is salt added?

Answer 36

Flavour
aids in the releases of more whey
Controls further acid production
Discourages growth of harmful bacteria

Question 37

Why was a firm coating needed to prevent damage or spoilage and what was it usually made of?

Answer 37

Cheese was often matured is less than ideal conditions.

Coating was made of bandaging, waxing, larding or growing a molded surface.

Question 38

Describe some secondary microbiota added to the cheese during maturation

Answer 38

Swiss: starter culture of L. bulgaricus and S. thermophilus. Propionibacterium shermanii is added during ripening.
Blue: penicillium roquefortii is added to curd
Camembert: penicillium camemberti is added to the surface of the cheese so it ripens inwards

Question 39

Describe the process of fermenting cacao beans

Answer 39

Pulp pH of the cacao beans is low (citric acid content) and the pulp sugar concentration is high.
Yeasts (Candida rugosa, Kluyveromyces marxianus) hydrolyse the pectin that covers the seeds and ferments the sugars to release ethyl alcohol and CO2.
As the temperature and alcohol increase, the yeasts are inhibited and lactic acid be atria increase in number.
Lactic acid production drives the pH down and encourages acetic acid bacteria growth.
The mixture is stirred to aerate it and the oxygen and low pH allows the bacteria to convert the ethanol to acetic acid.
Fermentation lasts 5-7 days and spoilage occurs after this because microbes start growing on the seeds.
The beans are then dried and roasted

Question 40

Why is acetic acid important in the cacao bean fermentation process?

Answer 40

It kills the sprout inside the seed which releases enzymes that cause further degradation of proteins and carbohydrates (increases taste)

Question 41

Name 6 types of food preservation

Answer 41

Heat
Low temperatures
Drying
Osmotic pressure
Chemical preservation
Radiation

Question 42

Why is heat effective in preserving food and how does moisture and pH affect the effectiveness of temperature?

Answer 42

Heat kills microorganisms by changing the physics and chemical properties of their proteins.
Moisture and pH affects the heat sensitivity of microbes so less heat is required.

Question 43

Describe two methods of heat preservation

Answer 43

Sterilisation:
Destruction of all viable microorganisms
Can be done pre or post packaging
Food items last longer
Pasteurisation:
Reduction in the abundance of pathogenic and spoilage microbes
Three main types
1) low temp, long time (63 degrees for 30m)
2) high temp, short time (72 degrees for 15s)
3) ultra high temp (140-150 degrees for 1-2s)

Question 44

How does refrigeration help slow microbial growth?

Answer 44

Their growth is slowed and their numbers are kept low and spoilage is minimised. Food is preserved without destroying the appearance, taste or cellular integrity.
But psychrotrophic bacteria can survive and cause green meat surfaces, rotten eggs and sour milk

Question 45

How does drying preserve food?

Answer 45

It decreases the available moisture of the food item to the point where food spoilage microbes cannot proliferate (aw is less than 0.6)
Eg fruits (raisins), meat (biltong)

Question 46

How is osmotic pressure affected to preserve food?

Answer 46

Addition of salts or sugars to food items creates a hypertonic environments (causes plasmolysis of microbial cells)
Eg jam preserves and sweetened condensed milk

Question 47

How is radiation used to preserve foods.

Answer 47

UV light and gamma rays are used for food preservation.
Meat storage facilities use UV lights to reduce surface contamination of meat products.
Gamma rays are used to extend the shelf life of veggies, fish and poultry.

Question 48

How do chemical preservatives preserve foods.

Answer 48

They damage microbial membranes and affect the uptake of essential organic substances (eg amino acids).
They are used primarily against molds and yeasts.