Microbiology of food Flashcards
What are the organisms mostly involved in fermentation of food?
Yeast and lactic acid bacteria
What are the characteristics of lactic acid bacteria ?
Gram-positive
Aerotolerant anaerobes
Rods and cocci
What are the 2 kinds of lactic acid bacteria (categories…)?
Homofermentative: Fermentation yields lactic acid (pickles, sauerkraut)
Heterofermentative: fermentation yields lactic acid AND a variety of flavourful compounds (fermented sausages such as salami, fermented milk products)
What strain of yeast is used in fermentation of alcoholic beverages?
Saccharomyces
How is Sauerkraut made?
Cabbage shredded and salted. Microorgansims are NATURALLY present on cabbage.
What is the final concentration of salt in sauekraut?
2.2-2.8 % of NaCl
Which restricts growth of microorganisms
What are the microorganisms that participate in fermentation of sauerkraut?
Leuconostoc mesnteroides, is killed when acidity reaches 0,7-1%
Lactobacillus plantarum: lactic acid only increases acidity to 1.6-1.8%
What are the microorganisms used in pickles?
Pediococcus cerevisiae and Lactobacillus platarum
Are all the microorganisms killed when pasteurization of milk?
No. Only removes potential pathogens. Desired microorganisms are then added back to give the final product.
What are the organisms used for yogurt?
Streptococcus thermophilus and lactobacillus bulgaris are added and incubated at 42 degrees.
how is the texture of yogurt developed?
Acid denatures casein (milk prots) which results in thickening
What are the organisms involved in cheese making?
hundreds of organisms may be involved
What are the 2 basic roles of cheese making?
CURDLING: lactic acid bacteria precipitate the proteins => curd
RIPENING: lactic acid bacteria +/- other organims => flavor
What are the steps in curdling?
- Starter culture of lactic acid bacteria is added. Microorganisms ferment LACTOSE and produce LACTIC ACID => promote development of curd (precipitated prots, fat and other suspended material) and whey (supernatant).
- Usually, an enzyme is added to culture. RENNIN cleaves casein, which results in CURD FORMATION
- Whey is drained off: staring material or discarded (HIgh BOD=> must be traeted like sewage)
- Curd is heated (temperature 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 or other processes: PRESSING, BRINING, RIPENING
What is ripening of cheese?
Leads to CHANGE IN TEXTURE, CONSISTENCY, and FLAVOR due to 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 LACTIC ACID BACT. , RENNIN or OTHER ENZYMES
More ripening of cheese means…
higher acidity and sharpness
What is special about ripening of cheddar, swiss cheese and oka cheese?
Cheddar: ripening invovles lactic bacteria only
Swiss: propinibacteria produced propionic acid and acetic acid and CO2
Oka: pressed curd is brined, surface wahed frequently. Growth of Geotrichum.
Explain blue requefort
Ripening is maunly due to mold (penicillium roquefortii)
Spores form mold mixed with the milk
Mall holes are drilled in the curd before wrapping -> MOLD GROWS IN THE HOLES and produces spores (blue)
What is special about ripening of camembert, brie?
Unwrapped
Mixture of mold and bacteria that spreads on the surface of the curd
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
What is the difference between red and white wine?
In white wine, the yeast is added after the press , in red, the yeast is added before the press
What is the “must” in wine making?
crushed grapes
What is added to the must (in wine making) to kill the wild yeast?
Metabisulfite
What are the 2 microorganisms resistant to metabisulfite and that tolerate 12-14% of alcohol? (in wine making)
S.cerevisae and S ellipsoideus
What is the difference between dry and sweet wine?
Dry: all the sugar is fermented
Sweet: some sugar is left or added after fermentation.
What is the process of vinegar making? what is microorganism involved?
Acetic acid bacteria oxidize ethanol to acetic acid- strict aerobes, this is not fermentation.
Ethanol=> acetaldehyde=> acetic acid
Acetobacter, very efficient but in absence of ethanol, it will oxidize acetic acid to CO2 and H2O
What are the steps in brewing beer?
Malting:
germination of grain, producing amylase=>
The grains are then dried and crushed=> MALT
Mashing:
Malt is soaked in warm water for 1-2h => ready for mashing
The amylase degrades starch to fermentable sugars
The mash is filtered=> WORT
Boiling
Hops is added to wort, wort is boiled.
hops= flavour, but also antimicrobial properties
Fermentation
Yeast is added to wort
- Lager: saccharomyce CARLSBERGensis
- ALE: Saccharomces cerevisae
What are the steps in distilled alcoholic beverages?
Process similar to beer making process but hops is not added. Different grains can be used.
- Alcohol is distilled
- Aged, diluted with water
- Flavor ingredients added
What are factors affecting food spoilage?
- Types of microorganisms involved
- Extent of growth of the microorganisms
- Moisture content of the food
- Temperature
- pH
- Availability of oxygne
- Chemical composition and physical state
- Surfaces vs insides, grinding distributes the surface contaminant
What is aw?
Indicates water ability… water ability factor…
What is the availability of water in fresh food?
aw > 0,95
At what lowest are most microorganisms able to grow ?
aw < 0,9
At what aw are only specialyzed organims able to grow? what are these organisms?
aw < 0,8
- Xerophiles (dry environment, low osmolarity)
- Osmophiles (high osmolarity: high sugar)
- Halophiles (high salinity)
At what aw does staphylococcus grow?
aw> 0.83
What is the difference between psychrophiles and psychotrophs?
Psychrophiles: grow best at low temp
Psychrotrophs: able to grow at low temp
What pathogens can grow under 4,5 degrees?
Yerisinia enterocolitica
Listeria monocytogenes
Clostridium botulinum
What pathogens can grow under 4,5 degrees?
Yerisinia enterocolitica
Listeria monocytogenes
Clostridium botulinum
At what lower pH can bacteria, yeast and mold grow?
Bacteria:
- Many row at oH 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 oH of 4.5)
What is a sign that there is yeast or bacteria growth in an airtight packaging?
Swelling of the package because prod of CO2
What enzymes are necessary for organism to grow on meat? what organism will grow?
proteases and lipases
bacteria and molds
What enzymes are necessary for organsims to grow on fruits and vegetables? What can grow?
Saccharase, cellulase, pectinase
Bacteria , yeasts, molds
What is the difference between food poisening and food infection?
Food poisening: caused by microbial TOXINS in food. Symptoms appear quickly.
Food infection: organisms are ingested with the food and multiply in the host. SYMPTOMS TAKE LONGER to develop. Illness may be due to tissue invasion , production of toxins, or both.
What is botulism?
Food poisoning: ingestion of the neurotoxin produced by clostridium botulinum
What is a characteristic of clostridium botulinum? What are the risk factors?
Anaerobe that produces spores
Risks:
- Home canning, insufficient heat to kill the sporte
- Processed foods that are not reheated (non-acid canned vegetables, sliced meats)
- Sportes germinate, growth, tixin is produced
How is the clostridium botulinum toxin destroyed?
Toxin is detroyed by heating (80 degrees, 10 minutes). Properly cooked foods are safe to eat. It destroyes the toxin, not the organisms.
What is the most common form of food poisening?
Staphylococcal food poisoning
What is staphylococcal food poisoning due to?
Due to enterotoxin (SE) produced by some strains of staphylococcus aureus (enterotoxigenic strains). The toxin is relatively heat-stable (30 min at 100 degree, 16h at 60 degrees)
What are the symptos of Staph food pois.? when do they occur?
nausea, vomiting, diarrhea. 1-6h.
What is an important characteracteristic of staphylococcus aureus?
It is halotolerant, can grow on salted sausage etc.
This food pathogen has 2000 servorars, can infect mammals, birds and reptiles
Salmonella
What are the 2 most frequent serovars of salmonella?
Enteridis and typhimurium
Where does Salmonella grow?
In intestinal tract
What is the concentration of organism of salmonella required to cause a disease?
10^5- 10^8
What are the symptoms of salmonella enterica, and with what is it associated?
Invasion and destruction of intestinal epithelium
Associated with uncooked or slightly cooked foods. or cooked foods that are contaminated (cross contamination)
What kills salmonella enterica?
Cooking.
What is E.Coli 0157:H7? characteristics..
- Fecal coliform
- DOES NOT GROW AT 44,5 degrees
- Causes enterohemorrhagie E.Coli (EHEC).
- COOKING KILLS microorganism
What does E.coli causes?
Food infection:
attaches to the intestinal mucosa and produces an AB-type exotoxin: shiga-like toxin that inhibits protein synthesis
The toxin also damages the underlying tissues=> causes bloody diarrhea (hemorrhagic colitis)
oxin travels through blood strem and DESTROY CELLS IN KIDNEY => causes hemolytic uremic syndrome (HUS), can result in acute renal failure and death.
What does food preocessing/preserving aims? (3)
- Prevent or delay decomposition by microorganisms (spoilage)
- Destroy pathogens or inhibit their growth
- Prevent or delay self-decomposition by enzymes present in the food.
What are the 3 types of pasteurization?
Long temperature long time (LTLT) : 30 min 62,8 degrees (bad taste, used in ice cream and cheese)
High temperature short time (HTST) : 15 sec 71,7 (milk)
Ultrahigh temperature (UHT) : 2 sec, 141 degrees (long shelf life at room temp)
Are LTLT ( long time long temperature) and HTST (high temperature short time) equally safe?
Yes
What does canning aim?
Seal food from the outside wolrd and heat the container to kill most, if not all, of the microorganisms. It creates an anaerobic enfironment.
What is the temp required when canning acidic vs alkaline foods?
100 degrees for acidic
up to 121 for low acid foods
What is the 12D process in canning?
Temperature and time of exposure must be sufficient to kill 10^12 spores of clostridium botulinum
Does drying necessarily kill microorganisms?
no, but inhibits their growth
What is the aim of irradiation?
reduce contamination of FRESH PRODUCT by pathogen and spoilage organisms
What are 3 dosages of irradiation in food preservation?
RADAPPERTIZATION=> kill all microorganisms
RADICIDATION=> kill specific microorganisms
RADURIZATION=> reduce overall contamination
What are chemicals used in food preservation, and what do they aim?
Nitrite: prevents outgrowth of C. botulinum
Sulfite: wine industry, inhibits wild yeast
Nisin: bacteriocin produced by lactococcus lactis
What chemical is used against listeria monocytogenes bacteria?
A bacteriophage preparation
