Experiment 3 - Microbial Analysis of Cheese

What is cheese the result of?

Metabolic activity if microorganism

What are lactic acid-producing bacteria members of?

Streptococcus and/or Lactobacillus

What does these genera do?

Initiate cheese-making processes.

What are the organic acids produced by these organisms responsible for?

The acidification (souring) of the milk and the characteristic cheese flavours.

Why are other microorganisms added to the sour milk?

The enzymes they produce cause protein molecules (curd) to separate from the much of the mixture

What happens to the liquid portion (the whey) of the mixture?

Drawn off to and leaves the curd to further process and become the actual cheese product

What are the three groups of cheese?

1. Soft acid-curd cheeses

2. Hard rennet-curd cheeses

3. Semisoft rennet-curd cheeses

What is one way to defect that can occur in cheddar cheese?

The formation of gas.

How does the formation of gas occur in cheddar cheese?

Due to the presence of gas-producing organisms

What are examples of gas-producing organisms?

Clostridium and/or coliform bacteria like Escherichia coli and Klebsiella aerogenes.

How else might spoilage of cured cheese occur?

Due to the presence of “dairy mold”

What species may grow on the surfaces or in the crevices of cheese to give it an off flavour?

Geotrichum, Cladosporium, and Penicillium

What are internal factors?

Nutrient content and pH

What are external factors?

Conditions under which food is stored

What is the most common source of coliform or molds that are not a part of dairy’s normal flora?

Improper storage or handling

As a milk product, what is cheese high in?

Lactose and casein (protein)

How can lactose and casein affect cheese?

Makes it prone to spoilage by lactic acid bacteria and fungi

What does the contamination of Clostridium sp. or Esherichia coli look like?

Produces gas

What does the growth of Alcaligenes sp. or Klebsiella sp. look like?

Causes the product to look slimy

When does most spoilage occur

Late in the exponential growth cycle and may not be noticeable in taste or appearance until its peak

What can the consumption of spoiled food lead to?

Illness

What is the purpose of using different agar?

Different microorganisms have different metabolic requirements in order to grow, some are limited and some take advantage of their sources (generalists)

What is All Purpose Tween 80 (APT) agar?

It is an enrichment media meant to facilitate growth and will not actively inhibit the growth of a particular organism.

What does APT agar permit the growth of?

"lactic acid" bacteria such as Streptococcus and Lactobacillus species, that are necessary for cheese production and produce no defects

What properties does APT agar?

It is non-selective and non-differential.

How does growth look like on APT agar?

If an organism is able to process the caesin peptone, dextrose, and metallic salts within the agar it can grow fairly well.

What is Eosin Methylene Blue (EMB) agar?

Identifies Gram-negative enteric bacteria

What types of bacteria is identified on EMB agar?

Enterobacteriaceae

What bacteria is permitted to grow on EMB

Gram positive bacteria are inhibited by the crystal violet dye in the media and will not
grow.

What properties does EMB agar have?

It is selective and differential

How is EMB agar selective?

Medium partially inhibits Gram-positive bacteria.

How is EMB agar differential?

Eosin and methylene blue differentiate among organisms

What does Esherichia coli look like on EMB agar?

Colonies are purple and typically have a metallic green sheen because they are strong
fermenters

What does Enterobacter aerogenes look like on EMB agar?

Colonies are pink, indicating that they are weak fermenters

What do other organisms look like on EMB agar?

Colonies are clear or translucent, indicating they do not ferment lactose

What is Malt Extract Agar (MEA) used for?

A general purpose growth media to isolate and cultivate yeasts and molds from clinical samples, as well as a wide range of environmental sources

What properties does MEA have?

It is an enriched media that is nonselective and non-differential

What does MEA contain?

Peptones as a source of amino acids and nitrogenous compounds

Why is the pH adjusted for MEA? What is the pH adjusted to?

To enhance the growth of fungi and to slightly inhibit bacterial growth commonly found as environmental contaminants. It is adjusted to 5.5.

What else is added to MEA?

Dextrose is added to the medium to provide a carbon and energy source for the fungi

What does EMB agar detect?

Coliforms that may be responsible for gas defects

What does MEA detect?

Molds that may cause cheese defects.