Lecture 18 Food Microbiology

8/2/2019 Lecture 18 Food Microbiology

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Food Microbiology

Lecture #9


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Conditions for Spoilage

Water

pH

Physical structureOxygen

temperature


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Microorganism Growth in Foods


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Intrinsic Factors

composition

pH

presence and availability of water

oxidation-reduction potential

altered by cooking

physical structure

presence of antimicrobial substances


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Composition and pH

putrefaction proteolysis and anaerobic breakdown of proteins,

yielding foul-smelling amine compounds

pH impacts make up of microbial community and

therefore types of chemical reactions that occurwhen microbes grow in food


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Water availability

in general, lower water activity inhibits microbialgrowth

water activity lowered by: drying addition of salt or sugar

osmophilic microorganisms

prefer high osmotic pressure xerophilic microorganisms

prefer low water activity


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Physical structure

grinding and mixing increase surface area

and distribute microbes

promotes microbial growth

outer skin of vegetables and fruits slows

microbial growth


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Antimicrobial substances

coumarinsfruits and vegetables

lysozymecows milk and eggs

aldehydic and phenolic compoundsherbs

and spices

allicingarlic

polyphenolsgreen and black teas


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Extrinsic Factors

temperature

lower temperatures retard microbial growth

relative humidityhigher levels promote microbial growth

atmosphere

oxygen promotes growthmodified atmosphere packaging (MAP)

use of shrink wrap and vacuum technologies to

package food in controlled atmospheres


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Microbial Growth and Food

Spoilage food spoilage

results from growth of microbes in food

alters food visibly and in other ways, rendering itunsuitable for consumption

involves predictable succession of microbes

different foods undergo different types of

spoilage processestoxins are sometimes produced

algal toxins may contaminate shellfish and finfish


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Food Spoilage

Approximately 1/3rd of all food manufactured inworld is lost to spoilage

Microbial content of foods (microbial load):

qualitative (which bugs) and quantitative (how many

bugs)

Shelf life

Non-perishable foods (pasta)

Semiperishable foods (bread)Perishable foods (eggs)


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General Principles

Minimize contamination by:Good management processes

Acceptable sanitary practices

Rapid movement of food through processing

plant

Well-tested preservation procedures


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SpoilageMeat

Cutting board contamination

Conveyor belts

Temperature

Failure to distribute quickly

Fecal bacteria from intestines

Fish

Polluted watersTransportation boxes


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SpoilagePoultry and Eggs

Human contact

Penetration by bacteria

Milk and Dairy Products

Lactobacillus and Streptococcus species that survive

pasturization (sour milk)

Breads

Spores and fungi that survive bakingGrains

Fungi produce toxins


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Food-Borne Diseases

two primary types

food-borne infections

food intoxications


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Preventing Foodborne Disease

Food infections (microbes are transferred to

consumer)

Food poisoning (results from the toxinconsumption)


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Food-Borne Intoxications

ingestion of toxins in foods in which

microbes have grown

include staphylococcal food poisoning,botulism, Clostridium perfringens food

poisoning, andBacillus cereus food

poisoning


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Toxins

ergotismtoxic condition caused by growth of a fungus in

grains

aflatoxinscarcinogens produced in fungus-infected grains

and nut products

fumonisinscarcinogens produced in fungus-infected corn


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Controlling Food Spoilage


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Removal of Microorganisms

usually achieved by filtration

commonly used for water, beer, wine,juices, soft drinks, and other liquids


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Low Temperature

refrigeration at 5C retards but does not stop

microbial growth

psychrophiles and psychrotrophs can still causespoilage

growth at temperatures below -10C has been

observed


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High Temperature

canning

pasteurization


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Canning

food heated in

special containers

(retorts) to 115 C

for 25 to 100minutes

kills spoilage

microbes, but not

necessarily all

microbes in food


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Spoilage of canned goods

spoilage prior to

canning

underprocessing leakage of

contaminated water

into cans during

cooling process


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Pasteurization

kills pathogens and substantially reduces

number of spoilage organisms

different pasteurization procedures heat fordifferent lengths of time

shorter heating times result in improved flavor


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Water Availability


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Chemical-Based Preservation

GRAS

chemical agents generally recognized as safe

pH of food impacts effectiveness of

chemical preservative


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Radiation

ultraviolet (UV) radiationused for surfaces of food-handling equipment

does not penetrate foods

Gamma radiationuse of ionizing radiation (gamma radiation) to

extend shelf life or sterilize meat, seafoods,

fruits, and vegetables


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Detection of Food-Borne

Pathogens must be rapid and sensitive

methods include:

culture techniquesmay be too slowimmunological techniques - very sensitive

molecular techniques

probes used to detect specific DNA or RNA sensitive and specific


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comparison of PCR and

growth for detection of

Salmonella


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nucleic acid can be detected

even when plaque-forming

ability is lost


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Surveillance for food-borne

disease PulseNet

established by Centers for Disease Control

uses pulsed-field gel electrophoresis undercarefully controlled and duplicated conditionsto determine distinctive DNA pattern of eachbacterial pathogen

enables public health officials to link pathogensassociated with disease outbreaks in differentparts of the world to a specific food source


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Surveillance

FoodNet

active surveillance network used to follow nine

major food-borne diseasesenables public health officials to rapidly trace

the course and cause of infection in days rather

than weeks


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Helpful Suggestions

Refrigerate quickly

Wash hands

Clean cutting boardsLeftovers

Avoid home-canned foods


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Microbiology of Fermented

Foods

major fermentations used are lactic,

propionic, and ethanolic fermentations


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Fermentation

Any partial breakdown of carbohydrates taking

place in the absence of oxygen.


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Meat and Fish

sausages

hams

bologna

salami

izushifish, rice and vegetables

katsuobushituna


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Wine

White vs. Red: juice or juice and skin

Yeasts: Ferment when no oxygen around.

Saccharomyces species

Dry

SweetSparkling

Fortified


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Production of Breads

involves growth ofSaccharomycescerevisiae

(bakers yeast) under aerobic conditions

maximizes CO2 production, which leavens bread other microbes used to make special breads (e.g.,

sourdough bread)

can be spoiled byBacillus species that produce

ropiness


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Other Fermented Foods

silages fermented grass, corn, and other fresh animal feeds


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Microorganisms as Foods and

Food Amendments variety of bacteria, yeasts, and other fungi

are used as animal and human food sources

probioticsmicrobial dietary adjuvants

microbes added to diet in order to provide

health benefits beyond basic nutritive value

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Lecture 18 Food Microbiology