Chapter 25
Microbiology of Foods
25.1 Food Spoilage
• Food Spoilage Comes from Many Microbial Sources
• The consumption of toxins or microorganisms can cause food poisoning
and infections
• Airborne microbes fall onto produce
• Crops carry soilborne microbes to food processing plants
• Shellfish concentrate microbes by filtering from contaminated water
• Rodents and arthropods carry microbes on their feet which can be
transmitted to foods
• Several Conditions Can Determine If Spoilage Will Occur
• The composition of food determines what microbes may be present
• For example, carbohydrate or protein content
• The Microorganisms Responsible for Spoilage Produce Specific Products
• Yeast converts carbohydrate to alcohol
• Some bacterial species break proteins into amino acids
• They then break down amino acids, yielding a foul odor
• Carbohydrate metabolism gives off acid and gas
• Some foods have antimicrobial properties
• Meats and Fish Can Become Contaminated in Several Ways
• Meats and fish usually begin without contamination because living muscle
tissue is sterile
• Microbes can enter during handling, processing, packaging, and storage
• Natural casings can retain bacteria from animal intestines
• Organ meats spoil quickly because they are less compact and can retain
bacteria from the animal
• Green pigment on meat surfaces is harmless discoloration due to bacterial
growth
• Fish tissues degrade quickly
Poultry and Eggs Can Spoil Quickly
• Contamination usually stems from infections in the bird, like
• Salmonella sp.
• Chlamydia psittaci
• Bacteria can penetrate egg membranes and infect the yolk
• Breads and Bakery Products Can Support Bacterial and Fungal Growth
• Spores can survive baking
• Cream fillings and toppings can contain contaminants
• Some Grains Are Susceptible to Spoilage
• Aspergillus flavus produces aflatoxins that accumulate in grains, nuts, and
corn
• Aflatoxins are linked to liver and colon cancers
• Ergotism occurs when Claviceps purpurea deposit toxins in
• rye
• wheat
• barley
•
Milk and Dairy Products Sometimes Sour
• Milk is an excellent source of nutrition for humans and microbes
• Milk sours when bacterial enzymes digest fats into fatty acids
• Curdling occurs when bacteria ferment lactose into
• lactic acid
• acetic acid
• Some bacterial species cause curdling by breaking down casein proteins
• This is a critical step in cheese production
• Coliform bacteria can
• curdle proteins
• produce so much gas that explosions occur
• Ropiness can occur if capsule-producing bacteria contaminate milk
• Some species produce lipase that attacks butterfats, causing
• sourness
• putrid smell
• Discoloration can occur with
• bacteria depositing different pigments
• contamination with yeast
• Milk is sterile in the udder but can be contaminated
• in the ducts
• by dairy plant equipment and employees
25.2 Food Preservation
• Heat Denatures Proteins
• Blanching produce with steam during canning destroys enzymes
• This prevents cellular metabolism
• Commercial sterilization is not as rigorous as true sterilization
• contamination can occur
• Anaerobic bacteria can
• produce gas
• cause bulging in the can
• Acid-producing bacteria can sour canned food
• The holding method of pasteurization exposes milk to a low, even temperature
for a long time
• Flash pasteurization exposes milk to a high temperature for a short time
• Ultrapasteurization involves exposure to an even higher temperature for 3
seconds
• Thermoduric organisms can survive the heat of pasteurizatrion
• Low Temperatures Slow Microbial Growth
• Enzymatic activity slows at low temperatures
• Freezing kills microbes through the ice crystal formation
• Food should not be thawed and refrozen due to the accumulation of toxins and
rapid microbial growth
• Drying and Osmotic Pressure Help Preserve Foods
• Dry foods cannot support microbial growth
• Lyophilization (freeze-drying) involves deep-freezing then vacuum
pumping off water
•
Osmotic pressure causes water to diffuse out of cells, causing dehydration
and death
• Chemical Preservatives Help Keep Foods Fresh
• Preservatives must
• inhibit microorganisms
• be easily broken down and eliminated by humans
• Organic acids like benzoic and propionic acids
• occur naturally
• damage microbial membranes
• By-products of smoke inhibit microbial growth
• Sulfur dioxide retards discoloration of dried fruits
• Ethylene oxide preserves spices, nuts, and dried fruits
• Radiation Can Sterilize Food
• UV light reduces surface contamination of meats
• Gamma rays extend shelf life of meats and produce
• Foodborne Disease Can Result from an Infection or Intoxication
• Food infections are caused by the contamination of food with an
organism
• Food intoxications are caused by the contamination of food with a toxin
produced by an organism
• Preventing foodborne disease involves
• Proper processing techniques
• Hand washing or wearing gloves
• Refrigeration
• HACCP systems attempt to identify potential contamination points
• Hazard Analysis and Critical Control Point regulation
• It applies to points in processing where food safety can be affected
25.3 Foods From Microorganisms
• Many Foods Are Fermented Products
• Fermentation produces a number of waste products
• Fermented foods are less vulnerable to spoilage and disease transmission
• This results from acid or alcohol content
• Fermentation can add to food
• proteins
• amino acids
• vitamins
• Taste, texture, and flavor can be enriched
• Sauerkraut is well-preserved and high in vitamin C
• Anaerobic bacteria ferment carbohydrates in cabbage cells
• Vinegar is created in the spontaneous souring of wine
• Yeasts ferment fruit juice to create alcohol
• Bacteria convert the alcohol to acetic acid
• Cucumbers are cured into pickles by three groups of microorganisms
• Two groups of bacteria produce acids that soften and sour the cucumbers
• Yeasts establish flavors in pickles
• Roasted soybeans inoculated with Aspergillus oryzae produce soy sauce
•
•
•
Fermented sausages, cocoa, and coffee all owe their flavor in part to
microorganisms
Many Milk Products Are the Result of Fermentation
• Fermentation of lactose by bacteria makes
• buttermilk
• acidophilus milk
• sour cream
• Dry milk solids containing active bacterial cultures are added to boiled
milk to form yogurt
• Cheese production begins when casein curdles out of milk
• The curd (unripened cheese) is sold as
– cottage cheese
– pot cheese
– cream cheese
Ripened cheese is made by washing, pressing, and maybe cooking the curd
• In Swiss cheese, bacteria ferment lactose and produce gas that
accumulates in holes
• Microbial enzymes can break down protein curds to produce soft cheeses
• Mold can ripen cheese, softening the curd by digesting proteins