Lecture 7 - Microbiology of Food PDF

Lecture 7 - Microbiology of Food Intrinsic Factors: pH; presence and availability of water; physical structure; presence of antimicrobial substances Copyright © The McGraw-Hill Companies. Permission required for reproduction or display....

Lecture 7 - Microbiology of Food Intrinsic Factors: pH; presence and availability of water; physical structure; presence of antimicrobial substances Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 1 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 in general, lower water activity inhibits microbial growth water activity lowered by: Drying and the addition of salt or sugar osmophilic microorganisms - prefer high osmotic pressure xerophilic microorganisms - prefer low water activity Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 2 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 coumarins – fruits and vegetables lysozyme – cow’s milk and eggs aldehydic and phenolic compounds – herbs and spices polyphenols – green and black teas Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 3 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 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 4 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 different foods undergo different types of spoilage processes toxins are sometimes produced Toxins Ergotism - toxic condition caused by growth of a fungus in grains Aflatoxins - carcinogens produced in fungus-infected grains and nut products Copyright © The McGraw-Hill Fumonisins - carcinogens produced in fungus-infected corn Companies. Permission required for reproduction or display. 5 Controlling Food Spoilage Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 6 Removal of Microorganisms usually achieved by filtration commonly used for water, beer, wine, juices, soft drinks, and other liquids 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 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 7 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 goods spoilage prior to canning underprocessing leakage of contaminated water into cans during cooling process Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 8 Pasteurisation kills pathogens and substantially reduces number of spoilage organisms different pasteurization procedures heat for different lengths of time shorter heating times result in improved flavour Water Availability Reduced water availability Water removal, as with lyophilisation or freeze drying; use of spray dryers or heating drums; decreasing water availability by addition of solutes such as salt or sugar Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 9 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 Radappertisation - gamma radiation to extend shelf life or sterilize meat, seafoods, fruits, and vegetables Microbial Product-Based Inhibition Bacteriocins - bactericidal proteins active against related species, e.g., nisin; Used in low-acid foods to inactivate Clostridium botulinum during canning process Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 10 Food-Borne Diseases - two primary types: food-borne infections and food intoxications 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 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 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 11 Microbiology of Fermented Foods - Fermented Milks mesophilic – Lactobacillus and Lactococcus thermophilic – Lactobacillus and Streptococcus therapeutic – Lactobacillus and Bifidobacterium yeast lactic – yeasts, lactic acid bacteria, and acetic acid bacteria mold lactic – filamentous fungi and lactic acid bacteria Cheese Production Milk lactic acid bacteria and rennin curd removal of whey ripening by microbial action cheese Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 12 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 Benefits of probiotics control of diarrhea anticancer effects possible modulation of Crohn’s Disease Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 13 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 Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 14