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Food as a substrate for microbial growth
- 1. History Early Food Preservation900 AD – “Food Poisoning” Recognized 1795-Appert Developed Canning 1854-1864- Food Microbiology Becomes A Science- Louis Pasteur Why Study Food Microbiology? • Provide Clean, Safe, Healthful Food to Consumer •Food Permits and Control Microbial Growth •Prevent Food Spoilage and Food borne illness
- 2. Factors Influencing Growthof Microorganisms in Food The factors influence microbial growth and maintain the food quality in production as well as preservation Conditions naturally present in food termed intrinsic factors Environmental conditions are termed extrinsic factors
- 3. Intrinsic Factors pH Moisture Content Oxidation-reduction Potential Nutrient Content Antimicrobial Constituents • Multiplication of organisms in food greatly influenced by inherent characteristics of food (moist, nutritionally rich, pH neutral)
- 4. Water availability In general,lower water activity inhibits microbial growth Foods have different water availability Fresh meats & milk have high water content-Supports microbial growth Breads, nuts and dried foods have low water availability Defined populations can grow in these specific environments Water activity (aw) used to designate the amount of water available in foods Pure water has aw of 1.0 Most of the bacteria require aw of above 0.90 Most of the fungi require aw of above 0.80
- 5. Remove and/orBind Moisture Humectants and Dehydration Most Fresh Foods - aW > 0.99, Lowest aW for Pathogen 0.86 (Staphylococcus aureus) Halophilic 0.75 Xerophilic molds 0.61 Osmophilic yeasts 0.61 0 1 Pure waterNo water Water activity lowered by: 1. Drying 2. Addition of salt or sugar Osmophilic microorganisms- prefer high osmotic pressure Xerophilic microorganisms - prefer low water activity
- 6. pH Importantin determining which organisms can survive and thrive on specific foods Many microorganisms inhibited by acid conditions- Exception Lactic acid bacteria Lactic acid bacteria are used in fermentation process and other foods (unpasteurized milk) Fungi are able to survive at relatively low pH Most acid foods spoil by fungal contamination pH can determine the bacteria’s ability to produce toxin Toxin production of many organisms is inhibited by acid pH
- 7. EFFECTS OF pH Enzymes Nutrients Other Environmental Factors Temperature Salt No known pathogen grows below pH of 4.6 Clostridium botulinum
- 8. Oxidation- Reduction Potential O/R Potential – Eh Value “…ease with which the substrate loses or gains electrons.” Loss of electrons – oxidized Gain of electrons – reduced Aerobic, Anaerobic, Microaerobic and Facultative anaerobes Physical structure grinding and mixing increase surface area and distribute microbes promotes microbial growth outer skin of vegetables and fruits slows microbial growth
- 10. Nutrients Nutrients presentin food - determine which organisms can grow in foods Biological barriers outer coverings help to protect foods from microbial invasion Microorganisms will eventually breakdown the coverings and cause spoilage in some times Antimicrobial chemicals Some foods contain natural antimicrobial chemicals that inhibit growth of organisms responsible for spoilage
- 11. Extrinsic factors Extentof microbial growth is depend on storage of food Microbes multiply rapidly in warm, oxygen-rich environments Preventing growth and metabolic activities of organisms that preserves food quality Temperature : lower temperatures retard microbial Growth Relative humidity : higher levels promote microbial growth Atmosphere : • oxygen promotes growth • modified atmosphere packaging (MAP) • use of vacuum technologies to pack the food
- 13. Removal of Microorganisms •Usuallyachieved by filtration •Commonly used in water, beer, juices, wine, soft drinks and other liquid treatment •Preservation methods include Canning Pasteurization Cooking Refrigeration Freezing drying,/reducing water availability
- 14. Temperature Microorganisms growover a wide range of Temperatures Psychrotrophs Mesophiles Thermophiles Psychroduric Thermoduric
- 16. Heating foodsunder controlled conditions at high temperatures for short periods (HTST – 71.30 C FOR 10 min), LTHT- 62.30 C for 2 min and UHT 1300C for 2 sec Reduces number of spoilage organisms Does not alter taste of food significantly Cooking • Can destroy non-spore forming organisms • Alters characteristics of food • If heat is uneven some organisms may survive in undercooked portion of food
- 17. Refrigeration •Preserves food byslowing growth rate of spoilage organisms • Many organisms unable to multiply in low temperatures
- 18. Freezing Stopsmicrobial growth Water unavailable due to ice formation Portion of organisms can grow when food is thawed Drying/reducing water availability Inhibits microbial growth by decreasing available moisture Molds may grow eventually
- 20. Chemical-Based Preservation GRAS- chemicalagents “generally recognized as safe” pH of food impacts effectiveness of chemical preservative