Week 7: Microbes and Food

Question 1

Major advances in microbiology in the 1800s led to the understanding of
the relationship between microbe(s) and product

Answer 1

• Optimize fermentation process to yield best product (flavour, yield)
• Reduced contamination by other species

Question 2

Saccharomyces cerevisiae

Answer 2

yeast

Question 3

bread fermentation

Answer 3

uses sugars from the grains for their metabolism
Because the dough is an anaerobic
environment, the yeast undergoes
fermentation converting the pyruvate
made during glycolysis into ethanol and
CO2

Question 4

wine fermentation

Answer 4

• Grapes are crushed to acquire the must
(juice)

• Treated with sulfur dioxide; prevents
contamination from other naturally occurring
microbes to obtain a more predictable product/
flavour

• Appropriate strain of S. cerevisiae added;
fermentation proceeds

• Fermentation terminates as the yeast is killed
by its own waste product (ethanol) ~7-18%
• Fermented musk placed in vats for settling,
filtration and flash pasteurization

Question 5

beer fermentation

Answer 5

• Saccharomyces sp. are also used in beer
production

• Fermentation typically yields and alcohol content
between 3.5-6%

• After fermentation, beer is typically filtered and
pasteurized to remove sediment and
contaminating microbes

• Carbonation is required since CO2 is released during the fermentation process

Question 6

pickling

Answer 6

• Process usually relies on the naturally
occurring microbes on the surface of the
foods
• Salt is added to eliminate Gram-negative
species and allow the lactic acid
producing species to undergo
fermentation

Question 7

vinegar

Answer 7

• Made by introducing an ethanol-
based product of fermentation to an

aerobic bacteria in the Acetobacter
genus.
• Acetobacter sp. convert ethanol to
acetic acid (vinegar)
• Different fermented liquids with
ethanol are used to make different
vinegars

Question 8

soy sauce

Answer 8

• Made by introducing the mold
Aspergillus sp. to a mixture of cooked
soy beans and wheat
• Fungus digests the complex
carbohydrates in the beans and
wheat to simple sugars and amino
acids
• Solution is mixed with salty brine and
halo-tolerant yeast and bacteria
• Fermentation is allowed to proceed
for up to one year

Question 9

yogurt

Answer 9

Milk is thickened and then two types of lactic acid producers are added for fermentation;
• Streptococcus thermophilus - Tart taste and low pH denatures
proteins to provide the semisolid
texture
• Lactobacillus bulgaricus - Releases aromatic compounds
providing the distinct yogurt smell

Question 10

cheese

Answer 10

• Traditionally, curd (solids) were made using lactic
acid producers; however, today the clotting
enzyme renin is added
• Curd is separated from the whey (liquid) and the
pH, salt and moisture content are adjusted
before it is shaped
• From here, some cheeses are ready to eat,
others undergo an ageing process
• Appropriate microbes colonize the surface of the
curd and their enzymes seep into the curd

Question 11

direct consumption of microbes

Answer 11

• Mushrooms

* Vegemite (S. cerevisiae) with added flavouring

Question 12

processing of microbes

Answer 12

The bacterium Erwinia dissolvens easily digests the pulp around coffee beans before roasting

Question 13

vitamin production and microbes

Answer 13

• Selective growth of microbes to promote the production of specific vitamins often used as
additives in commercial food products such as breads, cereals:
• B12 - Pseudomonas sp. & Propionibacterium sp. (bacteria)
• B2 (Riboflavin) – Ashbya gossypii (yeast)`

Question 14

amino acid production and microbes

Answer 14

• Used as food additives (essential amino acids) and flavour enhancers