Microbiology Week 15- Food Microbiology

Fermented foods

Produced by controlled activity of bacteria, yeasts, & molds

Fermentation

• Means that an organic compound serves as electron acceptor

• Generally used by food scientists to include any desirable change in food by microbes

Intrinsic factors

Inherent conditions of foods

• Affect which microbes predominate

Water availability

moisture

water activity (a_w)

used to indicate amount of water available

Water a_w

1.0

Fresh foods a_w

typically above 0.98

Jam a_w

0.85

Bacteria a_w

above 0.90 for growth

Fungi a_w

As low as 0.80

pH

• Many bacterial species, most pathogens cannot grow below pH 4.5

• can determine whether toxins can be produced

Lactic acid bacteria

Obligate fermenters

• Can grow at pH 3.5

  • Produce lactic acid as end product

  • Used to make foods from meat, dairy, vegetables

Fungi pH

Can grow at lower pH than most spoilage bacteria

Clostridium botulinum

<pH 4.5, does not grow or produce toxin

Lactic acid bacteria species

• Lactobacillus

• Lactococcus

• Streptococcus

• Leuconostoc

• Pediococcus

• Some produce other flavorful and aromatic compounds

Lactic acid Dairy

yogurt, sharp cheeses

Lactic acid Vegetable

pickles, soy sauce, sauerkraut, kimchi

Saccharomyces cerevisiae

converts sugars into Ethanol and CO₂

Bread

wheat sugars

Beer

malted barley

Wine

fruit, mostly grapes

Spirits

distilled variety of alcoholic fermentation

Lager yeast

Saccharomyces pastorianus (a hybrid of S. cerevisiae) ferments at 8-14°C and takes 10-21 days

Ale yeast

Saccharomyces cerrevisiae ferments at 14-23°C and takes 5-7 days

• Excess yeast is removed

• Beer is aged, then filtered, kegged, bottled or canned

Cocoa

• Spontaneous fermentation for about 6 days

• Pink beans turn orange, and the flavor improves

• Afterwards, they are sun- or artificially-dried

Cocoa microbes

• Yeast

• Lactic acid bacteria

• Acetic acid bacteria

• Bacillus species