Food Microbiology

• Food Microbiology


  The study of microorganisms (their growth, characteristics, identification, and pathogenesis) associated with food, and their beneficial and harmful effects on the quality and safety of raw and processed foods
• Fermentation


  Metabolism, wherein it breaks down substances, usually glucose, by organisms anaerobically (but can occur in its presence)
• Beneficial aspects of microorganisms


  • Used as a food source/supplement
  • Agents of fermentation
• Harmful aspects of microorganisms


  • Causes food spoilage
  • Causes food poisoning or food-borne infection
  • Contaminants
• Role of Microorganisms in Dairy Products


  1. Cheese: Curd formation through enzymatic process of rennin, facilitated by lactic acid-producing bacteria

  2. Butter: Diacetyl production by lactic acid bacteria

  3. Yogurt: Thickening by Streptococcus thermophilus and Lactobacillus delbrueckii bulgaricus
• Role of Microorganisms in Nonbeverage Plant Products

  1. Cacao: Produce chocolate flavor

  2. Bread: Sugars fermented by Saccharomyces cerevisiae, causing dough expansion
• Role of Microorganisms in Alcoholic Beverages and Vinegar


  1. Beer: Conversion of sugars to alcohol and carbon dioxide by Saccharomyces cerevisiae and Saccharomyces uvarum

  2. Vinegar: Conversion of ethanol to acetic acid by Acetobacter and Gluconobacter
• Food Spoilage


  Process in which qualitative properties are lost in terms of color, texture, flavor, odor, and structure
• Criteria for Acceptable Foods

  • At a stage of development or maturity
  • Free from pollution at any stage in the production
  • Free from undesirable biological, chemical, and physical changes
  • Free from pathogenic microorganisms
• Classification of Foods Depending on Stability


  • Perishable Foods
  • Semiperishable Foods
  • Nonperishable Foods
• Classification of Food According to pH


  • High-acid Foods
  • Acidified Foods
  • Formulated Acid Foods
  • Low-acid Foods
• Causes of Food Spoilage

  • Growth and activity of microorganisms
  • Physical changes
  • Action of enzymes in plants or animals
  • Chemical reactions
  • Insects and rodents
• Types of Food Spoilage


  • Physical Spoilage
  • Chemical Spoilage
  • Microbial Spoilage
• Food Preservation


  The manipulation of intrinsic and extrinsic growth factors to limit or eliminate microbial growth, thereby extending the shelf life and safety of foods
• Significance of Food Preservation


  • Prevention of Foodborne Diseases
  • Prolonged Shelf Life
  • Sustainable Food Production
• Means of Preservation: Extrinsic Microbial Barriers


  • Freezing and Refrigeration
  • Storage and Packaging
• Freezing and Refrigeration


  1. Freezing: Makes water unavailable to microbes, kills parasites but not bacteria and viruses

  2. Refrigeration: Retards metabolism of microbes by keeping the food out of the temperature danger zone
• Storage and Packaging


  1. Storage: Keeps air at low temperatures and with a composition that allows the produce to stay fresher for extended periods

  2. Packing: Vacuum packaging removes air, modified atmosphere packaging excludes and replaces atmospheric air with other gases
• Freezing


  • Makes water unavailable to microbes, thereby controlling their growth
  • Kills parasites (e.g. in meats and fishes) but not bacteria and viruses
• Refrigeration


  Retards metabolism of microbes by keeping the food out of the temperature danger zone, especially during transport and storage
• Storage and Packaging


  Limits microbial growth by controlling atmospheric conditions during storage or controlling gas content inside the packaging to optimize the extrinsic microbial barriers (e.g., air temperature, humidity, or oxygen levels)
• Storage


  • Air is kept at low temperatures and with a composition that allows the produce to stay fresher for extended periods
  • Carbon dioxide level is usually kept at around 8% to 10%, which limits the growth of spoilage microbes
• Packing


  • Vacuum packaging: removes all air to make oxygen absent by reducing air pressure before sealing
  • Modified atmosphere packaging: some atmospheric air is excluded and replaced with other gases, usually nitrogen and CO2
  • Controlled atmosphere packaging: uses chemical scavengers (e.g. desiccants) that keep CO2 and oxygen levels at a constant level within the package
• Adding chemical additives


  Help to stabilize the product to keep it from separating or changing in consistency
• Antimicrobials


  • Potassium sorbate prevents mold in baked goods
  • Sodium benzoate prevents the growth of yeasts and molds in softdrinks
  • Curing agents restrict the growth of spore-producing bacteria in meats
• Antioxidants


  • Ascorbate
  • Hydroquinone family of chemicals (derivatives of benzene)
• Curing and smoking


  Prevents spoilage of meats and fish due to oxidation or rancidification and enhances the food's flavor, color, and texture
• Curing


  • Lowers the amount of available water and by reducing the pH in foods like bacon, ham, dry sausages, candied salmon, and jerky
  • Brining: uses salts and sugars to pull water out of microbial cells until those cells can no longer function and become inactive
• Smoking


  • Alters the acidity of foods, removes moisture, and creates preservatives to slow down bacterial growth and the chemical decomposition of fats known as rancidification
• Canning and pickling


  Involves the storage of foods in containers wherein thermal processes and changes in pH create hurdles to microbial growth
• Canning


  • Uses metal cans, aluminum pouches, or any other airtight packaging or container that can be subjected to heat and pressure
  • High-acid foods: canned without added pressure
  • Low-acid foods: canned with pressure
• Pickling


  • Soaks food in acidic solutions (often in vinegar with a pH of 2.4) to help prevent spoilage; may combine with other hurdle processes to effectively inactivate harmful microbes
  • e.g. pickled cucumbers processed by adding salt to the vinegar, heating to 74°C for 15 minutes, subsequent rapid cooling, and the possible addition of preservatives
• Pasteurization


  Heating food, usually liquids, to a specific temperature for a specified length of time, followed by sealing, to reduce the presence of potential pathogens to tolerable numbers
• Dehydration and freeze-drying


  Provide different ways of limiting water availability to allow storage at room temperature, shelf stability, and reduce microbial risks
• Dehydration


  • Uses heat to decrease water content by drying in the sun and air or with applied heat
  • e.g. dried fish, squids, etc.
• Freeze-drying


  • a.k.a lyophilization removes water through sublimation (from ice to water vapor), wherein food is pretreated, rapidly frozen, and then pressurized by vacuum-packing to reduce access to oxygen
• Irradiation


  Uses ionizing radiation in the form of gamma ray, electron beam, or X-rays to create transient reactive chemicals known as oxidants that damage the DNA of the microbes
• New technologies


  Processing of foods with less heat which result in minimal changes to the organoleptic and nutritional quality of foods
• New technologies


  • Bacteriophages: viruses are added to foods to suppress the growth of unwanted and harmful microbes
  • Pulsed electric field: short pulses of electricity are used to damage the protective outer layers of microbes
  • High-pressure processing: high pressure and mild heat are applied to effectively kill spore-forming microbes
  • UV light: for surface disinfection and damaging the DNA of microbes
  • Microwave and radiofrequency: uses electromagnetic heat within the food matrix to allow even heating and improve drying time
• Foodborne infection


  Consuming food that has sufficient numbers of viable pathogens to colonize and proliferate in the host, causes toxins leading to disease