14- Food microbiology

Question 1

What includes food microbio.?

Answer 1

Microorganisms used to produce a food or a beverage.

Microorganisms that can spoil food or make a food harmful.

Question 2

Tell me about fermented food products. Most often organisms used? Homofermentative, Heterofermentative? Use of lactic acids?

Answer 2

• Many of the food and beverages that we consume are products of microbial action on raw material: sauerkraut, pickles, salami, cheeses, yogurt, alcoholic beverages.
• Microorganisms that are the most often involved are lactic acid bacteria and a variety of yeasts.
• Lactic acid bacteria: Gram-positive, aerotolerant, anaerobes, rods or cocci.

– Homofermentative: fermentation yields primarily lactic acid (pickles, sauerkraut).

– Heterofermentative: fermentation yields lactic acid AND a variety of flavorful
compounds (fermented sausages such as salami, fermented milk products).

– Lactic acid reduces food spoilage and improves storage qualities (brings the pH down).
– Flavor compounds produced by different organisms give each product its distinctive
taste.

• Yeasts (alcoholic beverages): variety of Saccharomyces strains.

Question 3

Tell me about Sauerkraut and pickles: Leuconostoc mesenteroides? Lactobacillus plantarum?

Answer 3

• Cabbage is simply shredded and salted (final concentration of 2.2-2.8% NaCl, restricts growth of spoilage microorganisms).

Microorganisms are naturally
present on cabbage:
– Leuconostoc mesenteroides: lactic acid, flavor compounds, killed when acidity reaches
0.7-1.0%
– Lactobacillus plantarum: lactic acid only, increases acidity to 1.6-1.8%.

• Pickles:
– Use natural flora
– Kill natural flora and use pure culture of Pediococcus cerevisiae and Lactobacillus
plantarum (also used to produce fermented sausages, salami, bologna).

Question 4

Tell me about milk product: milk, yogurt, cheese (curdling, ripening)

Answer 4

• Milk naturally contains lactic acid bacteria, but is usually pasteurized to remove potential pathogens. Desired microorganisms are then added back to give the final product.

• Yogurt: cultures of Streptococcus thermophilus and Lactobacillus bulgaris are added and incubated at 42°C.
– Streptococci produce some acids and favor the growth of Lactobacillus which produce most of the acid.
– The acid denatures casein (milk protein) which results in thickening.
– Other microorganisms are sometimes added (BL, Bifidobacterium animalis)

• Cheeses: hundreds of microorganisms may be involved. Raw milk cheese may
contain pathogens. Two basic roles:
– Curdling: lactic acid bacteria precipitate the proteins -> curd
– Ripening: lactic acid bacteria ± other organisms -> flavor

Question 5

Explain Cheese – curdling (rennin)

Answer 5

• Starter cultures of lactic acid bacteria are added. The microorganisms ferment lactose
and produce lactic acid which promotes the development of the curd (precipitated proteins, fat and other suspended material) and whey (supernatant).

• Usually, an enzyme, rennin, is added to the culture. Rennin cleaves casein which results
in curd formation. (Rennin or lactic acid bacteria work).

• Whey is drained off: starting material for other processes, or discarded (high BOD, must be treated like sewage).

• Curd is then heated (T° depending on final product): more whey is expelled and the
curd becomes more solid.

• Salt may be added: flavor, promotes extraction of water, inhibits growth of spoilage
organisms.

• Packaging (fresh cheese), or other processes: pressing, brining, ripening.

Question 6

Explain Cheese - ripening (cheddar, swiss gouda, oka)

Answer 6

• Ripening leads to changes in texture, consistency, and flavor. Due to the presence of specific microorganisms added with the starter culture or spread on the
surface of the pressed curd.

Some of the ripening is due to the lactic acid bacteria, rennin or other enzymes present.

• Cheddar, Swiss, Gouda:
– Wrapped
– Cheddar: ripening involves the lactic acid bacteria only.
– The longer the ripening the higher the acidity and sharpness.
– In Swiss cheese: propionibacteria produce
propionic acid and acetic acid (flavor), and CO2 (bubbles).
– 3-12 months (or longer)

• Oka: pressed curd is brined, surface is washed frequently. Growth of Geotrichum.

Question 7

Explain Cheese - ripening (blue, roquefort, camembert, brie)

Answer 7

• Blue and Roquefort:
– Ripening is mainly due to the mold Penicillium roquefortii.
– Spores from the mold are mixed with the milk (or the curd)
– Small holes are drilled in the curd before wrapping.
– 3-6 months.
• The mold grows in the holes and produces spores (blue).

• Camembert, Brie:
– Unwrapped
– Mixture of mold (Penicillium) and bacteria (Brevibacterium) that are spread on the surface of the curd.
– Ripened in curing room (high
humidity)
– 1-5 months.
– Proteases secreted by the
microorganisms is responsible for softening of the cheese.
– Good sanitary practices are required so that the cheese does not get contaminated.

Question 8

Wine, explain (dry n sweet)

Answer 8

• Must = crushed grapes
• Metabisulfite (Na2S2O5) is added to kill wild yeasts.
• Sugar and organic acids may be added depending on the final product.
• S. cerevisiae and S. ellipsoideus: resistant to
Na2S2O5 and tolerate 12-14% alcohol.

• Dry wine: all the sugar is fermented.
• Sweet wine: some sugar is leB or added after fermentation.

Question 9

Vinegar, explain

Answer 9

• Acetic acid bacteria oxidize ethanol to acetic acid – strict aerobes, this is not fermentation.
• ethanol -> acetaldehyde -> acetic acid

– Acetobacter: used in all commercial applications, very efficient but in the absence of
ethanol, it will oxidize acetic acid to CO2 and H2O – some ethanol must remain to prevent this.

Question 10

Brewing, key factors?

Answer 10

Key factors: water, malt, hops, yeasts and condition of fermentation (T°, time).

Question 11

Brewing- Malting and mashing

Answer 11

• Grains contain starch that cannot be digested by yeasts.
• Grains (barley) are allowed to germinate, producing amylase –> MALTING
• The grains are then dried and crushed -> MALT.
• Malt is then soaked in warm water (65°C) for 1-2h à MASHING. The amylase degrades starch to fermentable sugars (glucose and maltose).
• The mash is then filtered -> WORT

Question 12

Brewing- boiling

Answer 12

• Hops is added to the wort. Wort is boiled for 1-2 hours (near sterilization).

• Hops gives flavor and bilerness. It also has antimicrobial properties and prevents lactic acid bacteria from growing in the final product.
• Commercial beer: filtered.
• Cooled to 20°C.

Question 13

Brewing- fermentation

Answer 13

• Yeast is added:
– Lager: Saccharomyces carlsbergensis (bolom yeasts, 6-12 °C) .
– Ale: Saccharomyces cerevisisae (top yeasts,
carried up by CO2 during fermentation, 14-23 °C).

• Polishing: filtration, carbonation,
pasteurization, etc.

• Light beers: yeasts manipulated genetically so they can use all the sugars in the wort.

Question 14

Distilled alcoholic beverage, explain

Answer 14

• Similar to beer making process but hops is not added.

Different grains can be used:
– Vodka: wheat, rye or potatoes
– Gin: grains or starchy product + juniper oil and other flavor ingredients
– Rum: sugar cane (molasses)
– Brandy/Cognac: wine
– Whiskies: malt brews

• Alcohol is distilled.
• Aged, diluted with water.
• Flavor ingredients are added

• Whiskies: malt brew (Scotch), other cereals (corn in the US). Wort is not boiled, mixed fermentation of added yeasts and resident lactic acid bacteria.

• Factors:
– Water
– Resident lactic acid bacteria
– Source and cultivar of the barley
– Shape of the still
– Ageing in wood cask.

Question 15

Food spoilage, explain

Answer 15

• Food spoilage: any change in the appearance, smell, taste that makes it unacceptable to the consumer. MAY still be safe to eat, not necessarily a health hazard.

• In North America, 10-20% of the foods are not suitable for consumption because of spoilage, up to 50% in developing countries.
• Wide variety of microorganisms can cause spoilage: molds, yeasts, bacteria.

• Factors affecting spoilage:
– Types of microorganisms involved.
– Extent of growth of the microorganisms.
– Moisture content of the food.
– Temperature.
– pH.
– Availability of oxygen.
– Chemical composition and physical state.
– Surfaces VS insides, grinding distributes the surface contaminant

Question 16

Food spoilage: Moisture content, explain

Answer 16

• All microorganisms need water.
• Availability of water to microorganisms, water activity: aw.

• Solutes dissolved in water reduce availability of water to microorganisms (vapor pressure decreases).

• Fresh food: aw > 0.95
• Most spoilage microorganisms can grow to an aw as low as 0.9.
• aw < 0.9 : most bacteria and yeasts are unable to grow. Many molds can still grow.

• aw < 0.8 : only specialized organisms can grow:
– Xerophiles (dry environment, low osmolarity)
– Osmophiles (high osmolarity: high sugar)
– Halophiles (high salinity)
• Staphyloccocus aureus grows at aw > 0.83

Question 17

Food spoilage: Temperature, explain

Answer 17

• Crucial factor influencing spoilage.
• Hot foods should be kept above 60°C (steam table).
• Cold food should be kept below 4.5°C.
• Psychrophiles (grow best at low T°)
• Psychrotrophs (able to grow at low T°)
• Most pathogens do not grow at 4.5°C, except: Yersinia enterocolitica, Listeria
monocytogenes, Clostridium botulinum.

• Microorganisms can survive for extended periods of time at 4°C, if the temperature is then allowed to increase, microorganisms can resume growth.

Question 18

Food spoilage: pH, explain

Answer 18

• Most foods are neutral or acidic
• Bacteria:
– Many grow at a pH as low as 5.
– Lactic acid bacteria at pH 4
– Very few bacteria able to grow below pH 4
• Molds and yeasts:
– Many can grow at pH 4 and lower.
– Main spoilage organisms in acidic food (below pH of 4.5).

Question 19

Food spoilage: Oxygen, explain

Answer 19

• Required for molds and many aerobic bacteria and yeasts.
• Excluding oxygen is a good strategy to prevent spoilage of food (vacuum packaging, canning, modified atmosphere).

Also reduces oxidation
of the product.
• Oxygen diffusion
• Some yeasts and many bacteria can grow under anaerobic conditions; can
cause swelling of the package (CO2 production).

Question 20

Food spoilage: chemical component (meat, vegetable), explain

Answer 20

• Nutrients available determine the types of organisms that grow.
• Enzymes produced and secreted will determine the type of foods that the
microorganisms can use.

• Meat: high in proteins and fat. Requires proteases and lipases – proteolytic and lipolytic microorganisms: bacteria and molds.

• Vegetables: high in starch, cellulose, pectin;
Fruits: high in sugars and pectin.
Require saccharase, cellulase, pectinase - saccharolytic, cellulolytic and pectinolytic
microorganisms (bacteria, yeasts, molds).