Food Microbiology Flashcards
What microorganisms are most invovled in fermented food products
- Lactic acid bacteria
- Variety of yeasts
Homofermentative
Fermentation yields:
* Primarily lactic acid
Heterofermentative
Fermentation yields
* Lactic acid
* Variety of flavorful
compounds
Function of lactic acid
Reduces food spoilage and improves storage qualities
Function of flavorful compounds
Give each product its distinctive
taste
Lactic acid bacteria: Description
Gram-positive, aerotolerant anaerobes, rods or cocci
Milk products
- 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 & incubated at 42°C
* Streptococci produce some acids & favor the growth of Lactobacillus
* Lactobacillus produce most of the acid
* The acid denatures casein (milk protein) which results in thickening
Cheeses
2 basic roles:
* Curdling
* Ripening
Cheese Curdling: Process
- LAB ferment lactose and produce lactic acid
- Rennin cleaves casein
- Whey is drained off (disposed of)
- Curd is heated
- Salt may be added to add flavor and promotes extraction of water which inhibits growth of spoilage organisms
Cheese ripening: Result
Ripening leads to changes in texture, consistency, and flavor
Ripening: Cheddar, Swiss, Gouda
Cheddar:
- Involves the LAB only
Swiss:
- propionibacteria produce propionic acid & acetic acid (flavor), & CO2
Ripening: Blue and roquefort
Penicillium roquefortii
* Spores are mixed with the milk
* Small holes are drilled in the curd
* The mold grows in the holes & produces spores (blue)
Ripening: Camember, Brie
Mixture of mold (Penicillium) and bacteria (Brevibacterium)
* Proteases secreted by the microorganisms is responsible for softening of the cheese.
Wine
Consists of:
* crushed grapes
* S. cerevisiae
* S. ellipsoideus
Metabisulfite is added to kill wild yeasts
* S. cerevisiae and S. ellipsoideus are resistant to Metabisulfite and tolerate 12-14% alcohol
Sugar and organic acids may be added
depending on the final product
Dry wine
All the sugar is fermented
Sweet wine
Some sugar is left or added
after fermentation.
Brewing: Process
- Malting and mashing
- Boiling
- Fermentation
Malting and mashing
Malting:
* Grains (barley) are allowed to germinate, producing amylase
Malt:
* The grains are then dried and crushed
Mashing:
* Malt is then soaked in warm water (65°C) for 1-2h
* The amylase degrades starch to fermentable sugars
Wort:
* The mash is then filtered
Boiling
- Hops is added to the wort
- Wort is boiled for 1-2 hours
Hops
- Gives flavor and bitterness
- Has antimicrobial properties and prevents LAB from growing in the final product
Fermentation
Yeast is added
* Lager: Saccharomyces carlsbergensis
* Ale: Saccharomyces cerevisisae
Polishing: filtration, carbonation, pasteurization…
Distilled alcoholic beverages
Similar to the brewing making process but hops is not added
Vinegar
Acetic acid bacteria oxidize ethanol to acetic acid
* strict aerobes, this is not fermentation.
Acetobacter
- Used in all commercial applications
- Very efficient but in the absence of ethanol, it will oxidize acetic acid to CO2 & H2O
- some ethanol must remain to prevent this
Food spoilage
Any change in the appearance, smell, taste that makes it unacceptable to the consumer.
Factors affecting spoilage
- Moisture content of the food
- Temperature.
- pH.
- Availability of oxygen.
- Chemical composition
Moisture content
- All microorganisms need water
- Solutes dissolved in water reduce availability of water to microorganisms
- a w < 0.9 : most bacteria and yeasts are unable to grow
- a w < 0.8 : only specialized organisms can grow
Temperature
Hot foods should be kept above 60°C
Cold food should be kept below 4.5°C
pH
Most foods are neutral or acidic
Bacteria:
* Many grow at a pH as low as 5.
* LAB 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
Oxygen
- Required for molds and many aerobic bacteria and yeasts
Chemical composition
- Nutrients available determine the types of organisms that grow
Meat:
* high in proteins and fat
* Requires proteolytic and lipolytic microorganisms
Vegetables and fruits
* high in starch, cellulose, pectin
* Require saccharolytic, cellulolytic and pectinolytic microorganisms
