(4) Food Microbiology

Question 1

What are the two types of lactic acid bacteria used in food production and how do they differ?

Answer 1

Homofermentative: yields primarily lactic acid
Heterofermentative: yields lactic acid AND other flavorful compounds

Question 2

What are the two most used microorganisms in food microbiology?

Answer 2

Lactic acid bacteria

Saccharomyces strands

Question 3

How is yogurt produced, microbiologically speaking?

Answer 3

Lactic acid bacteria are incubated at 42°C and produce acid -> denatures casein -> thickens

Question 4

What are the two basic roles of microorganisms when producing cheese?

Answer 4

Curdling: lactic acid bacteria and/or rennin precipitate proteins to make curd
Ripening: lactic acid bacteria w/ or w/o other organisms make flavor and texture change

Question 5

How are the 5 different cheese ripened?

Answer 5

(lactic acid bacteria = LAB)
Cheddar = LAB
Swiss = LAB + propionibacteria
Oka = LAB + Geotrichum (surface)
Blue & Roquefort = Penicillium Roqueforti
Camambert & Brie = Penicilum + Brevibacterium (surface)

Question 6

How is vinegar produced?

Answer 6

Not trough fermentation:

Acetobactor (strict aerobe) converts ethanol into acetic acid

Question 7

In wine making, what is first added?

Answer 7

Metabisulfite -> to kill wild yeast present on grapes

Question 8

How is red wine and white wine production different?

Answer 8

White wine: skin is removed

Red wine: skin is kept

Question 9

How is dry wine and sweet wine production different?

Answer 9

Dry = no more sugar => all fermented
Sweet = not all fermented or added after full fermentation

Question 10

What are the 6 steps in beermaking?

Answer 10

Malting: germinate grains -> produce amylase; obtain MALT
Mashing: soak malt in warm water -> amylases transform starch in maltose + glucose; Mash is filtered to obtain WORT
Hops (antimicrobial activity + flavor) added to wort
Boiling of wort (get virtually sterile)
Fermentation:
- S. carlsbergensis {carlsB for Bottom} = bottom yeast = Lager {bottom is Lower -> Lager}
- S. cervisiae = top yeast = Ale
Polishing (filter, add flavorings…)

Question 11

How are distilled beverages produced?

Answer 11

Malting, Mashing, Fermenting, Distillation, Aging and/or Add flavors

Question 12

How is Scotch whiskies production different than other distilles beverages?

Answer 12

The wort is not boiled; mixed fermentation done by LAB + yeasts

Question 13

How can Food Spoilage be defined?

Answer 13

Any change making product unacceptable to the consumer

Question 14

What microorganism (3) spoil foods?

Answer 14

Bacteria, molds & yeasts

Question 15

What 6 factors affect food spoilage and their characteristics?

Answer 15

• Moisture content (water activity; if a-w <0.9, most are -inhibited; if a-w <0.8, only specialized organisms survive)
• Temperature (hot > 60°C, cold <4.5°C; prevents further growth)
• pH (molds and yeast can survive below 4.5, bacteria cannot)
• Oxygen availability (required for molds)
• Chemical composition (proteins & fat not accessed by yeasts)
• Surface Vs. Inside

Question 16

What are the 3 specialized organisms that can survive in low water activity?

Answer 16

Xerophiles (dry env.)
Osmophiles (high osmolarity {sugar})
Halophule (high salt)

Question 17

How does food poisoning/intoxication differs from food infection?

Answer 17

Poisoning/intoxication = caused by toxins => symptoms appear quickly
Infection = organisms ingested infect host => delayed symptoms

Question 18

Describe two examples of food poisoning/intoxication

Answer 18

Botulism: botulinum toxin is heat-labile (80°C for 10 min), spores are not (dangerous only to infants)
Staphylococcus aureus: carried by humans -> food contamination -> exotoxin (Vs. endo) affecting intestinal cells = enterotoxin

Question 19

Describe two examples of food infection

Answer 19

Salmonella enterica: infects and grows in intestinal cells. Killed by cooking
E. Coli O157:H7 produces AB-type (shiga-like toxin) once in intestinal cells. If toxin enters bloodstream, may damage kidney cells (hemolytic uremic syndrome). Killed by cooking

Question 20

What are the 3 main goals of food preservation?

Answer 20

Prevent/delay decomposition by microorganisms
Destroy pathogens or inhibit their growth
Prevent/delay self decomposition by enzymes in food

Question 21

What are 6 food preservation techniques?

Answer 21

• Refrigerating & Freezing: slows growth, freezing reduces water activity
• Pasteurization: reduce number of microorganisms
• Canning: 100°C for acidic; 121°C for low-acid
• Water availability:
  a) drying
  c) lyophilisation
  c) addition of salt/sugar
• Irradiation: protects food’s surface
• Chemicals: control growth microorganisms with Generally Recognized as Safe (GRAS)

Question 22

What are 3 pasteurization methods for milk?

Answer 22

Long Time Low Temp: 30min, 62.8°C
High Temp Short Time: 15 sec, 71.2°C
Ultrahigh temperature: 2 sec, 141°C (virtually sterile)

Question 23

In canning, what is the 12D process?

Answer 23

Norm by which if kill 10^12 botulinum spores, consider product sterile

Question 24

What are 3 GRAS chemicals?

Answer 24

Nitrite -> C. botulinum
Sulfites -> wild yeast
Nisin (produced by lactococcus lactis {cheese})

Question 25

What are primary metabolites?

Answer 25

Metabolites produced during exponential growth (alcohol)

Question 26

What are secondary metabolites?

Answer 26

Metabolites produced near the stationary phase (end of growth) -> antibiotics

Question 27

In industrial microbiology, what is considered fermentation?

Answer 27

Any microbiological process (aerobic or anaerobic)

Question 28

What two types of organism variation can be used in industrial microbiology?

Answer 28

Spontaneous mutants: overproduce desired metabolites

Genetically modified organisms: cloned genes expressed in bacteria/yeasts