Lecture 18 Food Microbiology

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Food Microbiology


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Lecture #9:

Food Microbiology Lecture #9

Conditions for Spoilage:

Conditions for Spoilage Water pH Physical structure Oxygen temperature

Microorganism Growth in Foods:

Microorganism Growth in Foods

Intrinsic Factors:

Intrinsic Factors composition pH presence and availability of water oxidation-reduction potential altered by cooking physical structure presence of antimicrobial substances

Composition and pH:

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 occur when microbes grow in food

Water availability:

Water availability in general, lower water activity inhibits microbial growth water activity lowered by: drying addition of salt or sugar osmophilic microorganisms prefer high osmotic pressure xerophilic microorganisms prefer low water activity

Physical structure:

Physical structure grinding and mixing increase surface area and distribute microbes promotes microbial growth outer skin of vegetables and fruits slows microbial growth

Antimicrobial substances:

Antimicrobial substances coumarins – fruits and vegetables lysozyme – cow’s milk and eggs aldehydic and phenolic compounds – herbs and spices allicin – garlic polyphenols – green and black teas

Extrinsic Factors:

Extrinsic Factors temperature lower temperatures retard microbial growth relative humidity higher levels promote microbial growth atmosphere oxygen promotes growth modified atmosphere packaging (MAP) use of shrink wrap and vacuum technologies to package food in controlled atmospheres

Microbial Growth and Food Spoilage:

Microbial Growth and Food Spoilage food spoilage results from growth of microbes in food alters food visibly and in other ways, rendering it unsuitable for consumption involves predictable succession of microbes different foods undergo different types of spoilage processes toxins are sometimes produced algal toxins may contaminate shellfish and finfish

Food Spoilage:

Food Spoilage Approximately 1/3 rd of all food manufactured in world 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)

General Principles:

General Principles Minimize contamination by: Good management processes Acceptable sanitary practices Rapid movement of food through processing plant Well-tested preservation procedures


Spoilage Meat Cutting board contamination Conveyor belts Temperature Failure to distribute quickly Fecal bacteria from intestines Fish Polluted waters Transportation boxes


Spoilage Poultry 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 baking Grains Fungi produce toxins

Food-Borne Diseases:

Food-Borne Diseases two primary types food-borne infections food intoxications

Preventing Foodborne Disease:

Preventing Foodborne Disease Food infections (microbes are transferred to consumer) Food poisoning (results from the toxin consumption)

Food-Borne Intoxications:

Food-Borne Intoxications ingestion of toxins in foods in which microbes have grown include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning


Toxins ergotism toxic condition caused by growth of a fungus in grains aflatoxins carcinogens produced in fungus-infected grains and nut products fumonisins carcinogens produced in fungus-infected corn

Controlling Food Spoilage:

Controlling Food Spoilage

Removal of Microorganisms:

Removal of Microorganisms usually achieved by filtration commonly used for water, beer, wine, juices, soft drinks, and other liquids

Low Temperature:

Low Temperature refrigeration at 5 °C retards but does not stop microbial growth psychrophiles and psychrotrophs can still cause spoilage growth at temperatures below -10 °C has been observed

High Temperature:

High Temperature canning pasteurization


Canning food heated in special containers (retorts) to 115 °C for 25 to 100 minutes kills spoilage microbes, but not necessarily all microbes in food

Spoilage of canned goods:

Spoilage of canned goods spoilage prior to canning underprocessing leakage of contaminated water into cans during cooling process


Pasteurization kills pathogens and substantially reduces number of spoilage organisms different pasteurization procedures heat for different lengths of time shorter heating times result in improved flavor

Water Availability:

Water Availability

Chemical-Based Preservation:

Chemical-Based Preservation GRAS chemical agents “generally recognized as safe” pH of food impacts effectiveness of chemical preservative


Radiation ultraviolet (UV) radiation used for surfaces of food-handling equipment does not penetrate foods Gamma radiation use of ionizing radiation (gamma radiation) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables

Detection of Food-Borne Pathogens:

Detection of Food-Borne Pathogens must be rapid and sensitive methods include: culture techniques – may be too slow immunological techniques - very sensitive molecular techniques probes used to detect specific DNA or RNA sensitive and specific

PowerPoint Presentation:

comparison of PCR and growth for detection of Salmonella

PowerPoint Presentation:

nucleic acid can be detected even when plaque-forming ability is lost

Surveillance for food-borne disease:

Surveillance for food-borne disease PulseNet established by Centers for Disease Control uses pulsed-field gel electrophoresis under carefully controlled and duplicated conditions to determine distinctive DNA pattern of each bacterial pathogen enables public health officials to link pathogens associated with disease outbreaks in different parts of the world to a specific food source


Surveillance… FoodNet active surveillance network used to follow nine major food-borne diseases enables public health officials to rapidly trace the course and cause of infection in days rather than weeks

Helpful Suggestions:

Helpful Suggestions Refrigerate quickly Wash hands Clean cutting boards Leftovers Avoid home-canned foods

Microbiology of Fermented Foods:

Microbiology of Fermented Foods major fermentations used are lactic, propionic, and ethanolic fermentations


Fermentation Any partial breakdown of carbohydrates taking place in the absence of oxygen.

Meat and Fish:

Meat and Fish sausages hams bologna salami izushi – fish, rice and vegetables katsuobushi – tuna


Wine White vs. Red: juice or juice and skin Yeasts: Ferment when no oxygen around. Saccharomyces species Dry Sweet Sparkling Fortified

Production of Breads:

Production of Breads involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions maximizes CO 2 production, which leavens bread other microbes used to make special breads (e.g., sourdough bread) can be spoiled by Bacillus species that produce ropiness

Other Fermented Foods:

Other Fermented Foods silages fermented grass, corn, and other fresh animal feeds

Microorganisms as Foods and Food Amendments:

Microorganisms as Foods and Food Amendments variety of bacteria, yeasts, and other fungi are used as animal and human food sources probiotics microbial dietary adjuvants microbes added to diet in order to provide health benefits beyond basic nutritive value

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