Transcript Chapter 41
Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Chapter 40 Microbiology of Food 1 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Intrinsic and Extrinsic Factors 2 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Intrinsic Factors • • • • composition pH presence and availability of water oxidation-reduction potential – altered by cooking • physical structure • presence of antimicrobial substances 3 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. • 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 4 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Water availability • measured in terms of water activity (aw) • 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 aw 5 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 6 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Physical structure • grinding and mixing increase surface area and distribute microbes – promotes microbial growth • outer skin of vegetables and fruits slows microbial growth 7 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 8 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 9 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 10 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 11 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 12 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 • algal toxins – contaminate fish and shellfish 13 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Aflatoxins Intercalate into DNA, causing frameshift mutations Four basic structures - B1, B2, G1, and G2 M aflatoxins in the milk of lactating animals that have ingested type B aflatoxins 14 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Basic structure of fumonisins Disrupt synthesis and metabolism of sphingolipids FB1, R = OH; FB2, R = H 15 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 16 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Controlling Food Spoilage 17 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Removal of Microorganisms • usually achieved by filtration • commonly used for water, beer, wine, juices, soft drinks, and other liquids 18 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Low Temperature • refrigeration at 5°C retards but does not stop microbial growth – microorganisms can still cause spoilage with extended spoilage – growth at temperatures below -10°C has been observed 19 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. High Temperature • canning • Pasteurization 20 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 21 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Spoilage of canned goods • spoilage prior to canning • underprocessing • leakage of contaminated water into cans during cooling process 22 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 23 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 freewater loss and an increase in solute concentration 24 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Chemical-Based Preservation • GRAS – chemical agents “generally recognized as safe” • pH of food impacts effectiveness of chemical preservative 25 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 26 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Radiation • ultraviolet (UV) radiation – used for surfaces of food-handling equipment – does not penetrate foods • radappertization – use of ionizing radiation (gamma radiation) to extend shelf life or sterilize meat, seafoods, fruits, and vegetables – kills microbes in moist foods by producing peroxides from water • peroxides oxidize cellular constituents • electron beams – electrons are electrically generated, so can be turned on only when needed – does not generate radioactive waste, but also does not penetrate foods as deeply as gamma radiation 27 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 • e.g., nisin – used in low-acid foods to inactivate Clostridium botulinum during canning process 28 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Food-Borne Diseases • two primary types – food-borne infections – food intoxications 29 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Food-Borne Infection • ingestion of microbes, followed by growth, tissue invasion, and/or release of toxins • raw foods (e.g., sprouts, raspberries, and seafood) are important sources 30 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 31 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Food Borne Infections • Salmonellosis – gastroenteritis from ingestion of contaminated meats, poultry or eggs • Campylobacter jejuni – transmitted by uncooked or poorly cooked poultry products, or raw milk – gastroenteritis • 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 32 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Food Borne Infections… • Escherichia coli – diarrhea caused by enteropathogenic, enteroinvasive and enterotoxigenic types – E. coli 0157:H7 is thought to have acquired enterohemorrhagic genes from Shigella, including genes for the shigalike toxin 33 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Food Borne Infections… • other organisms – viruses – protozoan pathogens – prions • other concerns – foods that are transported and consumed in uncooked state • sprouts • shellfish and finfish • raspberries 34 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Cultural versus Molecular Based Virus Detection Poliovirus type 2 35 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. sporocysts Cyclospora cayetanensis from raspberries. Bar = 5 μm. 36 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 37 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 38 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Molecular Probes and Food Microbiology Autoradiogram of a radioactively-labeled Listeria monocytogenes DNA probe against 100 Listeria spp. cultures. 39 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. PCR-based Pathogen Detedtion Comparison of PCR and growth for detection of Salmonella 40 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Surveillance for food-borne disease… • PulseNet – established by Centers for Disease Control and Prevention, USA – 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 41 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 42 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Microbiology of Fermented Foods • major fermentations - lactic, propionic, and ethanolic fermentations 43 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 44 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Fermented Milks • majority of fermented milk products rely on lactic acid bacteria belonging to the genera Lactobacillus, Lactococcus, Leuconostoc, and Streptococcus – gram-positives that tolerate acidic conditions, are non-spore forming, and are aerotolerant with a strictly fermentative metabolism 45 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Lactobacillus helveticus 46 Lactobacillus delbrueckii sub. bulgaricus Lactobacillus lactis Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Fermented Milks… • mesophilic – Lactobacillus and Lactococcus – buttermilk and sour cream • thermophilic – Lactobacillus and Streptococcus – yogurt • probiotics – Lactobacillus and Bifidobacterium – addition of microbes to the diet to improve health beyond basic nutritive value 47 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. A light microscope of Bifidobacterium sp. 48 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Examples of Bifid-amended dairy products. 49 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Fermented Milks… • Yeast-lactic fermentation – yeasts, lactic acid bacteria, and acetic acid bacteria – kefir • Mold-lactic fermentation – filamentous fungi and lactic acid bacteria – viili 50 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Cheese Production milk lactic acid bacteria and rennin curd removal of whey ripening by microbial action cheese 51 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 52 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Brie cheese (soft/ripened) Gouda cheese Limburger cheese (soft/ripened) cheddar cheese cream cheese (soft/unripened) Roquefort cheese Cottage cheese (soft/unripened) Swiss cheese 53 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Cheddar cheese production 54 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Fermented Meat and Fish • • • • • sausages hams bologna salami izushi – fish, rice, and vegetables (Lactobacillus spp.) • katsuobushi – tuna (Aspergillus glaucus) 55 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Production of Alcoholic Beverages • begins with formation of liquid containing carbohydrates in readily fermentable form – must • juice from crushed grapes – mashing • hydrolysis of complex carbohydrates in cereals by addition of water and incubation • yields wort – clear liquid containing fermentable carbohydrates 56 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. or racking – removes sediments Microbial oxidation of ethanol to acetic acid yields wine vinegar 57 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Beers and ales • malt – 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 58 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Beers and ales… • wort is then 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 59 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Beer Production (pitched) 60 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Distilled spirits • similar to beer-making process - following fermentation, is distilled to concentrate alcohol • Scotch whiskey – begins with sour mash • mash inoculated with homolactic bacterium 61 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 62 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 63 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 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 64 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Making sauerkraut 65 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Microorganisms as Foods and Food Amendments • variety of bacteria, yeasts, and other fungi (mushrooms) are used as animal and human food sources • probiotics – microbial dietary adjuvants : e.g. Spirulina, lactic acid bacteria, etc. – microbes added to diet in order to provide health benefits beyond basic nutritive value 66 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Benefits of probiotics • • • • • immunodilation control of diarrhea anticancer effects possible modulation of Crohn’s disease in beef cattle, – decrease E. coli (O157:H7) • in poultry, – limit colonization of gut by the process of competitive exclusion 67 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. Prebiotics • oligosaccharide polymers that are not processed until they enter large intestine • synbiotic system – combination of prebiotics and probiotics – results in production of certain acids that may be responsible for possible beneficial effects of probiotics 68