Food Microbiology I

Question 1

Lactic acid bacteria

Molds/yeasts used in fermentation

Answer 1

Both use lactic acid bacteria: gram-positive, aerotolerant anaerobes, rods or cocci

Saccharomyces yeast and Aspergillus mold used mainly

Question 2

Homofermentative vs. heterofermentative

Answer 2

Homofermentative: fermentation yields primarily lactic acid products like pickles and sauerkraut

Heterofermentative: lactic acid + variety of flavorful compounds like in fermented milk products, fermented sausage

Question 3

Microbes used in milk products

Answer 3

Pasteurization of milk removes lactic acid bacteria

Yogurt: Streptococcus thermophilus and Lactobacillus bulgaris denature casein to thicken yogurt
- temp 42°C

Cheese:
1) Curdling by lactic acid bacteria or rennet
2) Ripenings by lactic acid bacter + other organisms such as Aspergillus mold

Question 4

Curd

Supernatant

Rennin

Answer 4

Curd = precipitated proteins, fat and other suspended material

Supernatant = whey fraction, high BOD and requires treatment

Rennin = enzyme added which cleaves casein and forms curds
- adding head further solidifies curd

Question 5

Cheddar, swiss, gouda

Answer 5

Wrapped and aged 3-12+ months:
Cheddar: lactic acid only
Swiss: Propionibacteria –> propionic acid and acetic acid (flavor) and CO2
Gouda

Question 6

Oka

Blue, roquefort

Camembert, brie

Answer 6

Oka: brined and washed with Geotrichum

Blue, roquefort: ripening with Penicillum roquefortii and blue spores mix with curds
- 3-6 months ripening

Camembert, brie: unwrapped, surface Penicillum + Brevibacterium
- ripening in curing room 1-5 months, requires good sanitary practices
- proteases soften cheese

Question 7

White wine

Red wine

Pink wine

Dry wine

Sweet wine

Answer 7

White wine: Must sits for 16-24 hours before being pressed and pomace discarded and then yeast is added

Red wine: Yeast added to must and then pomace removed after fermentation

Pink wine: partial fermentation with skins

Dry wine: all sugar is fermented out

Sweet wine: some sugar is left or added after fermentation

Question 8

Microbes used in wine

Answer 8

Fermentation with S. cerevisiae and S. ellipsodeus which are Metabisulfite tolerant and 12-14% alcohol tolerant

Wild yeasts are less tolerant and would cease fermentation sooner

Question 9

Beer making first phases until wort

Answer 9

Malting: Grains (barley) germinate

Grains are then dried and crushed –> malt

Malt is soaked in warm water and then mashed

Amylase is released and degrades starch –> fermentable sugars

Mash is filtered –> wort

Question 10

Beer making second phases from wort

Answer 10

Hops added to wort and boiled to near sterilization
- hops for flavor/bitterness and antimicrobial action

Wort is filtered (commercial) and then cooled

Yeast is added specific to type of beer desired

Polishing phase: carbonation, filtration, pasteurization

Question 11

Ale vs. lager vs. light beer yeasts

Answer 11

Ale: made with Saccharomyces cerevisiae

Lager: made with Saccharomyces carlsbergensis

Light beer: yeasts genetically manipulated to use up all the sugars in the wort

Question 12

Starches used in distilled alcohol

Answer 12

Vodka: wheat, rye, potatoes

Gin: grains or starchy product + juniper

Rum: sugar cane

Brandy/cognac: wine

Whiskey: malt brews, wort not boiled
- mix of resident lactic acid bacteria and yeasts
- aged in wood casks

No hops added

Question 13

Vinegar

Answer 13

Not fermentation - strict aerobes

Acetic acid bacteria oxidize ethanol –> acetaldehyde –> acetic acid
- requires constant supply of ethanol to not convert acetic acid to CO2 + H2O

Bacteria: Acetobacter

Question 14

Poor growth circumstances for mold, yeast and bacteria

Answer 14

Poor conditions for:

Mold: low temperature <10°C or high >40°C or absent oxygen

Yeast: high temp > 40°C or protein or fat foods

Bacteria: high acidity <4.5 pH and low moisture