41 Microbiology of Food Copyright © McGraw-Hill Global Education Holdings, LLC. Permission required for reproduction or display.

41.1 Microbial Growth and Food Spoilage Differentiate between intrinsic and extrinsic factors that influence food spoilage Explain how the composition of a food item helps determine the kind of microbial growth that might occur List two physical and two biological features of a food that influence spoilage

Microbial Growth and 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

Microbial Growth and Food Spoilage Microbial growth is controlled by intrinsic factors factors related to the food itself extrinsic factors environment where food stored

Intrinsic Factors Food composition - carbohydrates mold predominates degrades food by hydrolysis little odor ergotism hallucinogenic alkaloids released by Claviceps purpurea may cause death

Intrinsic Factors Food composition – proteins or fats Putrefaction bacterial growth predominates Putrefaction proteolysis and anaerobic breakdown of proteins; foul smelling amine compounds Unpasteurized milk spoilage acid production followed by putrefication Butter short-chained fatty acid production; rancid butter

Intrinsic Factors pH Presence and availability of water impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food e.g., low pH favors yeast and mold Presence and availability of water in general, lower water activity inhibits microbial growth

Intrinsic Factors Oxidation-reduction potential Physical structure altered by cooking lower redox – more bacteria and anaerobes Physical structure grinding and mixing distribute microbes; promotes microbial growth outer skin of vegetables and fruits slows microbial growth

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 Temperature Relative humidity Atmosphere 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

41.2 Controlling Food Spoilage Review the limitations of refrigeration Explain why canning is not always a fail-safe means of preserving food Compare and contrast two pasteurization methods Describe the principle of water availability and how it can be used to preserve foods Explain how commonly used chemicals preserve food

41.2 Controlling Food Spoilage Express an informed opinion on the safety and efficacy of food irradiation Describe two microbe-based food preservatives Examine the packaging of food you frequently consume and assess its capacity to prevent spoilage

Controlling Food Spoilage Methods of preservation goal is to eliminate or reduce the populations of spoilage and disease causing microbes while maintaining food quality

Filtration Water, wine, beer, juices, soft drinks, and other liquids usually by filtration May better preserve flavor and aroma

Low Temperature Refrigeration at 5°C retards but does not stop microbial growth microorganisms can still cause spoilage with extended storage growth at temperatures below 10°C has been observed fruit juice concentrates ice cream some fruits

High Temperature 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 foods 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 Dehydration e.g., lyophilization to produce freeze-dried foods is commonly used to eliminate bacterial growth food preservation occurs as a result of free- water loss and an increase in solute concentration

Chemical-Based Preservation GRAS chemical agents “generally recognized as safe” agents include organic acids, sulfite, ethylene oxide gas, ethyl formate sodium nitrite – inhibits spore formation in meats, forms nitrosamines pH of food impacts effectiveness of chemical preservative

High Hydrostatic Pressure (HHP) Alternate way to preserve food Applies pressures from 100-800 milliPascals (MPs) without significant changes in temperature highly detrimental to cell membranes effective at eliminating eukaryotic microbes not as effective at elimination of Gram-positive microbes No industry standards for HHP conditions (yet)

Radiation Radappertization use of ionizing radiation (gamma radiation) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables excellent penetrating power – food not rendered radioactive kills microbes in moist foods by producing peroxides from water peroxides oxidize cellular constituents

Microbial Product-Based Inhibition Bacteriocins bactericidal proteins active against related species some dissipate proton motive force of susceptible bacteria some form pores in plasma membranes some inhibit protein or RNA synthesis

Microbial Product-Based Inhibition e.g., nisin from Lactococcus lactis used in low-acid foods to inactivate Clostridium botulinum during canning process e.g., bacteriophages that kill Listeria monocytogenes sprayed onto ready-to-eat meats prior to packaging

Packaging Modified atmosphere packaging (MAP) gases in stored food affect microbial growth shrink wrap materials and vacuum technology control atmosphere impermeable to gasses high CO2 content packaging can be used to prevent fungal growth high O2 content packaging produces superoxide radicals that inhibit microbial growth

41.3 Food-Borne Disease Outbreaks Summarize a major food-borne infection that had substantial health and economic consequences Describe the origin and consequences of a major food intoxication event in the United States Explain why these cases of food-borne disease outbreaks serve as cautionary tales for government regulatory agencies, food producers, and consumers Contrast and compare the aflatoxins with the fumonisins

Types of Food-Borne Disease About 48 million cases/yr in U.S. approximately 128,000 hospitalizations at least 3,000 deaths/yr in U.S. only 14 million attributed to known pathogens Pathogens Noroviruses, Campylobacter jejuni, Salmonella are major causes E. coli and Listeria are also important pathogens

Types of Food-Borne Disease Two primary types food-borne infections food intoxications Transmission breakdown in hygiene fecal-oral route key fomites also important

Food-Borne Infection Ingestion of pathogen, followed by growth, tissue invasion, and/or release of toxins Raw foods (e.g., sprouts, raspberries, and seafood) are important sources

Food-Borne Infections Listeriosis pregnant women, the young and old, and immunocompromised individuals most vulnerable responsible for the largest meat recall in U.S. at-risk people should not eat soft cheeses, refrigerated smoked meats, deli meats, and undercooked hot dogs

Food-Borne Intoxications Ingestion of toxins in foods in which microbes have grown Produce symptoms shortly after the food is consumed because growth of the disease- causing microorganism is not required Include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning

Other Food Disease Enterohemorrhagic E. coli (O157:H7) normal inhabitant of intestines of mammals including cattle, swine contamination of meat during slaughter in salad vegetables contaminated in field shiga toxin from horizontal gene transfer from Shigella may result in life-threatening hemolytic uremic syndrome, most common in children and elderly

Other Food Disease Fungus-derived toxins Algal toxins aflatoxins carcinogens produced in fungus- infected grains and nut products fumonisins carcinogens produced in fungus- infected corn Algal toxins contaminate fish and shellfish

41.4 Detection of Food-Borne Pathogens List the principles that guide the development of food- testing strategies Describe how a food-borne pathogen consumed by an individual might be traced back to the food source

Key U.S. Food Safety Legislation Driven in part by Upton Sinclair’s 1905 novel The Jungle, the Federal Meat Inspection Act was passed Other food safety legislation followed

Detection of Food-Borne Pathogens Must be rapid, sensitive and simple and for prevention Methods include: culture techniques – may be too slow but are most likely to detect food pathogens immunological techniques - very sensitive molecular techniques probes used to detect specific DNA or RNA sensitive and specific

Detection of Food-Borne Pathogens PulseNet established by Centers for Disease Control uses pulsed-field gel electrophoresis 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

Detection of Food-Borne Pathogens In addition to PFGE, PCR and RFLP are increasingly developed and used amplify small quantities species specific microarray techniques directly from food or water

41.5 Microbiology of Fermented Foods Compare and contrast mesophilic and thermophilic fermentations Differentiate between a yeast-lactic and a mold-lactic fermentation Outline the process by which cheese is made List some fermented meats Provide an overview of the production of wine, champagne, beer, and distilled spirits Describe a fermented food item not commonly eaten in the United States

Microbiology of Fermented Foods Chemical changes in food brought about microbial action Major fermentations used are lactic, propionic, and alcoholic fermentations

Fermented Milks Majority of fermented milk products rely on lactic acid bacteria (LAB) in the genera Lactobacillus, Lactococcus, Leuconostoc, and Streptococcus Gram-positives that tolerate acidic conditions, non- spore forming, aerotolerant, strictly fermentative

Fermented Milks Mesophilic (app. 20-30° C): Buttermilk/sour cream Lactobacillus spp. and Lactococcus lactis Thermophilic (app. 45° C): Yogurt Lactobacillus spp. and Streptococcus thermophilus Yeast lactic: Kefir (ferm. milk with 2% ethanol) yeasts, lactic acid bacteria, and acetic acid bacteria Mold lactic: Viili (Finnish fermented milk) filamentous fungi and lactic acid bacteria

Cheese Production Approximately 2,000 distinct varieties representing 20 general types Classified based on texture, hardness (soft, semi-soft, hard, very hard) All from lactic acid fermentation molds may further enhance flavor

Cheese Production milk curd cheese lactic acid bacteria and rennin removal of whey  ripening by microbial action  cheese

Meat and Fish Sausages Hams Bologna Salami Izushi – fish, rice, and vegetables Katsuobushi – tuna

Wines and Champagnes Enology (wine production) crushed grapes separation and storage of liquid (must) before fermentation fresh must treated with sulfur dioxide fumigant Saccharomyces cerevisiae or S. liposideus added for consistent results fermented for 3–5 days at 20–28oC

Wines and Champagnes Dry or sweet wines Racking controlled by regulating initial must sugar content Racking removes sediment produced during fermentation

Beers and Ales Cereal grains used for fermentation malt mash germinated barley grains having activated enzymes mash the malt after being mixed with water in order to hydrolyze starch to usable carbohydrates mash heated with hops hops provide flavor and assist in clarification of wort heating inactivates hydrolytic enzymes

Beers and Ales Wort inoculated (pitched) with desired yeast bottom yeasts used in production of beers top yeasts used in production of ales Freshly fermented beers are aged or lagered CO2 usually added at bottling Beer can be pasteurized or sterilized by filtration

Distilled Spirits Similar to beer-making process begins with sour mash mash inoculated with homolactic bacterium following fermentation, is distilled to concentrate alcohol

Production of Breads Involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions maximizes CO2 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 Sufu – from fermentation of tofu Sauerkraut (sour cabbage) – from wilted, shredded cabbage Pickles – from cucumbers Silage – from grass, chopped corn, and other fresh animal feeds

41.6 Probiotics Describe how a food is classified as a probiotic and identify two microorganisms used in probiotic foods

Probiotics and Standardization live microorganisms, which when administered in adequate amounts, confer a health benefit to the host specific requirements should be met Microorganisms Lactobacillus, Bifidobacterium

Possible Benefits of Probiotics Immunomodulation Control of diarrhea Possible modulation of Crohn’s Disease Lactobacillus acidophilus and Bifidobacterium help minimize lactose intolerance improve general intestinal health and balance may lower serum cholesterol may have anti-tumor activity

Probiotic Microbes in Farm Animals Probiotics Lactobacillus acidophilus in beef cattle decrease E. coli O157:H7 Bacillus strain in poultry limit colonization of gut by the process of competitive exclusion reduces Salmonella and Campylobacter