Food as a substrate for microbial growth

FOOD MICROBIOLOGY

1. FOOD MICROBIOLOGY History Early Food Preservation 900 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 Growth of 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 (a w ) used to designate the amount of water available in foods Pure water has a w of 1.0 Most of the bacteria require a w of above 0.90 Most of the fungi require a w of above 0.80

5. Remove and/or Bind 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 1 Pure water No water Water activity lowered by: 1. Drying 2. Addition of salt or sugar Osmophilic microorganisms- prefer high osmotic pressure X erophilic microorganisms - prefer low water activity

6. pH Important in 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

9. Antimicrobial substances Coumarins – fruits and vegetables L ysozyme – cow’s milk and eggs A ldehydic and phenolic compounds – herbs and spices A llicin – garlic P olyphenols – green and black teas

10. Nutrients Nutrients present in 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 Extent of 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

12. Preservation of Food Spoilage

13. Extrinsic Factors Removal of Microorganisms Usually achieved 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 grow over a wide range of Temperatures Psychrotrophs Mesophiles Thermophiles Psychroduric Thermoduric

15. High Temperature Canning- Nicolas appert ( Appertization ) Foods are heated in special containers (retorts) to 115 °C for 25 to 100 minutes Kills microbes but not spores in food Spoilage of canned foods may due to leakage of contaminated water into cans during cooling process Destroys all nutrients and preservatives in the food Low acid foods require steam under pressure to destroy endospores Spore forming bacteria can’t grow or produce toxin in high acid environment

16. Pasteurization kills pathogens and substantially reduces the number of spoilage causing organisms Different pasteurization procedures used to preserve the food shorter heating time reduce the flavor Heating foods under controlled conditions at high temperatures for short periods (HTST – 71.3 C FOR 10 min), LTHT- 62.3 C for 2 min and UHT 130 C 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. 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 Refrigeration Preserves food by slowing growth rate of spoilage organisms Many organisms unable to multiply in low temperatures

18. Freezing Stops microbial 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

19. Radiation ultraviolet (UV) irradiation (Non ionising ) used to sterilize the surfaces of food-handling equipment does not penetrate foods Ionizing Radiation uses of ionizing radiation ( gamma, alpha and beta radiations) to extend shelf life or sterilize meat, seafoods , fruits, and vegetables

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