Microbiology Exam 4

Historically Important Preservation Methods

Fermentation, salting, drying, herbs/spices, and canning.

Fermentation

Accomplished through the metabolism of lactic acid bacteria, primarily species of Lactobacillus. Works through the microbial production of lactic acid to ensure the maintenance of a low pH.

Lactic Acid Fermentation Process

Lactic acid bacteria (LAB), such as Lactobacillus, consume carbohydrates (sugars) and convert them into lactic acid and carbon dioxide, which preserves food and enhances flavor.

Lactic acid fermentation is carried out by

lactic acid and acetic acid producing bacteria, primarily Lactobacillus

Fermented milk is produced by

adding starter cultures to the milk - a culture of living microorganisms which produce specific changes in the chemical composition and properties of the substrate

Cheese production

1. Enzyme rennin (chymosin), a protease, is added to fermenting milk to hasten protein coagulation

2. Whey is separated, pressed, and salted

3. Curds are then ripened to encourage changes in texture and flavor

4. Ripening can take weeks to years (longer ripening produces more acidic/sharper cheese)

Yogurt production

1. Milk is inoculated with a starting culture (typically includes streptococcus thermophilus & lactobacillus bulgaricus) at high temperature heating (180F)

2. Mixture is incubated at 40-45C/104-113F for several hours (optimal growth temps for bacteria in starting culture

3. Controlled incubation ensures proper levels of acid production, proteolytic activity, and flavor compound generation

Bacteria commonly used as probiotics:

• Bifidobacterium Bifidum

• Lactobacillus casei

• Bacillus subtilis

• Saccharomyces boulardii

• Bacillus coagulans

Pickled vegetables

Uses naturally occurring lactic acid bacteria already residing on vegetables (rather than using a starter culture)

Majority of store bought pickled vegetables are packed in acetic acid to pickle them

Kimchi

Fermented cabbage with other vegetables and seasoning

Uses naturally occurring lactic acid bacteria residing on vegetables

Traditionally fermented in a semi-permeable vessel called onggi

Very long shelf life, length of fermentation changes the flavor

Canning

• Heat processing at high temperature for a fixed period of time

• Destroys microbes and inactivates endogenous 'spoilage' enzymes.

• Oxygen is driven out, which serves to reduce microbial and oxygen-driven spoilage chemistry.

• A vacuum is formed upon cooling that prevents recontamination.

• The process is specifically designed to destroy endospores such as those from Clostridium botulinum; however, improper canning can actually promote germination if the temperature is not high enough or held long enough

Drying

Used to prevent growth and limit metabolic activities of microbes that cause food spoilage by creating an extremely low moisture content

Salting

Creates high osmotic pressure which is a safety consideration that limits microbial growth in preserved foods

Herbs & spices

Preserve food by providing naturally occurring anti-microbial properties that influence the safety and stability of the food product

Intrinsic factors that impact microbial growth in food.

• Water availability

• pH

• Nutrients

• Biological barriers

• Antimicrobial chemicals

Microbes multiply most rapidly in moist, nutritionally rich, pH neutral foods

Water availability

supports microbial growth. Foods like fresh meats and milk have high water content and therefore are more susceptible to microbial growth than dry foods like bread and nuts with low water content.

pH

lowers microbial growth - many harmful microorganisms are inhibited by acidic conditions

Lactic acid bacteria are _____ by a low pH

not inhibited and are used in fermentation for food production

Yeast and other fungi are able to serve at a relatively

low pH

Salt/salinity level

Most bacteria, fungi, and other pathogens cannot survive high amounts of salt because salt pulls moisture out of the cells, dehydrating the microbial cells and killing/inactivating them

How much salt concentration is needed to kill most harmful bacteria?

20%

Nutrient availability

Intrinsic factor that helps determine what organisms can grow in food

Biological barriers

Intrinsic factor such as rinds, shells, and outer coverings that can help protect foods from microbial invasions

Antimicrobial chemicals

Intrinsic factor - some foods and spices naturally contain antimicrobial chemicals that inhibit growth of organisms responsible for spoilage.

Extrinsic factors that impact microbial growth in food.

• Storage temperature

• Oxygen levels

• Humidity

Storage temperature

• At below freezing temps, water availability is significantly decreased —> halting microbial growth

• At low temps (above freezing) enzymatic action is very slow or nonexistent —> inability of microbe to grow

Atmosphere

• Presence or absence of oxygen affects type of microbial population

• Humidity - moisture promotes growth of spoilage microorganisms

Cooking

can destroy non-spore forming organisms and alters the characteristics of food

Refrigeration

Preserves food by slowing the growth rate of spoiling organisms (many organisms are unable to multiply in low temps)

Freezing

Stops microbial growth (water unavailable due to formation of ice)

Portion of organisms remaining can grow when food is thawed

Drying

Inhibits microbial growth by decreasing available moisture but molds may grow eventually

Irradiation

damages microbial DNA

Pasteurization

The process of heating a liquid for a specific period of time to destroy pathogenic microorganisms.

Ultra-pasteurization (UP)

Holds the milk at a temperature of 140C (284F) for 2 seconds

Refrigerated, extended storage

Ultra High Temperature (UHT) Processing

Process holds milk at a temperature of 140C (284F) for 4-15 seconds

Milk is STERILIZED not pasteurized

Lets milk remain on the shelf without refrigeration

High temperature short time (HTST) pasteurization

72C (161F) for 15 seconds

Higher-heat shorter time (HHST) pasteurization

89C (191F) 1 second

90C (194F) 0.5 seconds

94C (201F) 0.1 seconds

96C (204F) 0.05 seconds

100C (212F) 0.01 seconds

Risks of ingesting raw milk

Germs from animals feces, skin, environment can get in the milk

The animal/animal’s udder can be infected

Insect, rodents, other small animals can get in milk

Milk processing plant can be unsanitary

Cross-contamination from dairy workers

Sterilization

The process of eliminating, removing, killing, or deactivating all forms of life and other biological agents

Psychrophile

Microorganism with a cold temperature optima, inhabit permanently cold environments

Around 4C

Mesophile

Midrange temp optima, humans

Average: 39C

Thermophile

High temp optima

Average: 60C

Hyperthermophile

VERY high temp optima

Around 88 - 106 C

Psychotolerant

Microorganisms that can grow at 0C but has an optimum of 20-40C

Foodborne Intoxication

Illness that results from ingestion of foods containing preformed microbial toxins. The microorganisms that produced the toxins do not have to infect the host, and onset is typically 4-12 hours.

Foodborne Infection

Microbial infection resulting from the ingestion of pathogen-contaminated food followed by growth of pathogen in the host; typical time of onset is 24-48 hours.

Factors that determine risk for developing foodborne illness:

• Type of microbe or toxin ingested

• Inoculum size - number microbes ingested, amount of toxin ingested

• Inadequate stomach acidity - organisms must be able to survive gastric acids in order to reach the small intestine and cause disease

• Reduction or change in normal microbiota of host - use of antibiotics

• Susceptible populations include - old, very young, hospitalized

Heat stable enterotoxins

Produced by strains of Staphylococcus aureus

Stable upon heating at 100C for 30 minutes

Protected from digestion in the GI tract by enzymes

Induce secretion of ions and water, resulting in watery diarrhea and abdominal cramping

Staphylococcus aureus food poisoning

Heat stable toxin NOT inactivated by cooking

Introduced into food during preparation

Food left at room temperature allows organism to grow and produce toxin

Clostridium botulinum

Gram +, spore-forming anaerobic rod

Paralytic disease caused by ingestion of a neurotoxin

Causes nausea, vomiting, visual impairment, loss of mouth/throat function, fatigue, loss of coordination, respiratory impairment, abdominal pain

Clostridium botulinum spores are found widely in

soil, dust, and honey

Bacillus cereus