2. Fermentation: Desirable Effects of Microbes17
Fermentation: Desirable Effects of Microbes

3. Objectives Describe the types of microbes that impact the food supply.
List factors that impact the growth of single-celled organisms.
Differentiate among yeast, bacterial, and mold fermentation.
Identify food products that are a result of fermentation.

4. Microorganisms
Microbiology is the study of living organisms too small to be seen by the unaided human eye
Organisms that are only visible through a microscope are called microorganisms or microbes
They multiply rapidly, and transfer easily from one surface to another

5. Types of Single-Celled Organisms
Microbes that have a positive use in foods are bacteria, yeasts, and molds
Bacteria are members of the Monera kingdom
Yeasts and molds are members of the Fungi kingdom
Microbes depend on outside sources of food to grow and multiply

6. Bacteria
Bacteria are single-celled organisms that multiply through cell division
Their cells have no nucleus and are filled with a gelatinous liquid called cytoplasm
Bacteria are
up to 3 micrometers long
classified by shape, cell wall structure, and oxygen needs
continued

7. Bacteria Bacteria have these basic shapes Bacilli have a rod shape
Cocci have a spherical shape
Spirilla have a spiral shape
©Sebastian Kaulitzki/Shutterstock.com
continued

8. Bacteria Bacteria have 2 types of cell wall structures
identified by the ability to be stained by a crystal violet dye, called Gram’s stain
Gram-positive bacteria turn violet; gram-negative turn red
continued
©Janice Haney Carr, Centers for Disease Control and Prevention

9. Bacteria Bacteria can be categorized by oxygen needs
Aerobic bacteria must have oxygen to function
Anaerobic bacteria function best in an oxygen-free environment
Facultative bacteria can function in either type of environment
continued

10. Bacteria Aerobic bacteria spoil cabbage, but
submerging cabbage in salt water does not allow aerobic bacteria to grow
the anaerobic bacteria can multiply and develop the flavor and texture of sauerkraut
Bacteria’s growth rate depends on pH, air, temperature, and food supply

11. Fungus A fungus is a plant that lacks chlorophyll Fungi
are not always single-celled
have a nucleus in their cells
are widely distributed in nature and help organic matter decay
Examples include mushrooms, molds, mildews, rusts, smuts, and yeasts
continued

12. Fungus
Fungi are classified by their structure and reproduction methods
Basic structure of most fungi is made of filaments or tubes called hyphae, which
have elongated cells or chains of cells that absorb nutrients from the environment
intertwine and form a branched network called a mycelium
continued

13. Fungus
Mycelium reproduces by creating spores, the reproductive cells of fungi
Spores develop in a sac- or balloon-like structure that explodes when full
Spores are microscopic and resistant to harsh environments
©Goodheart-Willcox Publisher
continued

14. Fungus
Molds are fungi that form a mycelium structure with a fuzzy appearance
Many give off an antibiotic that kills bacteria likely to be growing in the same area
Mushrooms, toadstools, and puffballs reproduce through a basiodiocarp, a spore case with a stem, a cap, and gills located under the cap
continued

15. Fungus
Yeasts are fungi with a single-celled structure that produce alcohol and carbon dioxide
They form mycelium structures on soil or plants, but reproduce in animals by budding
©Goodheart-Willcox Publisher

16. Common Characteristics of Microbes
grow rapidly and are good sources of edible protein for animals
can enhance or add to the nutritional value of food
can enter a dormant or inactive state to protect themselves from a harsh environment
reproduce when the environment supports growth needs
continued

17. Common Characteristics of Microbes
A pure culture is a large volume of one type of microbe grown purposely
At food processing plants, large volumes of microbes are used in the development of such foods as pickles, cheese, and beverages
When a pure culture is mixed with a food source, a starter is made

18. Microbial Enzymes
Enzymes produced by microbes are the source of most changes in food products
Microbes excrete digestive enzymes to break down large macromolecules for transport through the microbe’s cell wall
Microbes can then absorb the fragments and use them for fuel
continued

19. Microbial Enzymes
Microbes can be classified by the organic molecules they use as a food source
Proteolytic microbes release proteases that change proteins into amino acids and are used to clot milk, tenderize meat, and remove pulp from cacao beans
Lipolytic microbes produce enzymes that digest fats and are used to flavor cheese, remove egg yolk, and clean up oil spills
continued

20. Microbial Enzymes Halophilic microbes
require high concentrations of salt to function
are used to produce bean cake and many other Asian foods
©Andrew L./Shutterstock.com

21. Scientific Names for Microbes
Microbes are classified by 2 Latin names
The genus name
is always capitalized
indicates a family of organisms that have similar characteristics
The species name
is never capitalized
identifies the type of microbe within the family
continued

22. Scientific Names for Microbes
Example: Lactobacillus acidophilus
Lacto refers to milk
Bacillus refers to bacteria with a rod shape
Acidophilus indicates bacteria that give off acid
Lactobacillus acidophilus is a rod-shaped bacteria that lives in or feeds on milk and produces an acid

23. Factors Affecting Microbe Growth
Microbes need a food supply
Because microbes are composed of carbon, oxygen, nitrogen, and hydrogen, they need a food supply containing all 4 chemicals
The food supply needed depends on the enzyme systems the organism can make
Microbes also need minerals and vitamins to aid with enzyme activity
continued

24. Factors Affecting Microbe Growth
Microbes need water
Some microbes can remain alive, but dormant, in dried conditions
A safe water activity (aw) for most food storage is considered 0.70 or lower
Water activity can be lowered by removing water, adding solutes, or by freezing
continued

25. Factors Affecting Microbe Growth
Microbes need a certain pH
Each microbe has a preferred range, but can survive at pH levels above and below it
©Goodheart-Willcox Publisher
continued

26. Factors Affecting Microbe Growth
Microbes prefer a moderate temperature
Freezing slows enzyme activity and cell reproduction, while heating can kill microbes
Most microorganisms die at 5°C to 12°C above the temperature for maximum growth
Food processors balance temperatures needed to kill microbes against the affect heating has on food quality and production costs
continued

27. Factors Affecting Microbe Growth
Pasteurization is a process in which a liquid is heated until pathogens are destroyed
High temperatures for a brief period are less damaging to flavor
Food scientists can grow cultures of microbes if they know the preferred food supply, water activity, pH, and temperature

28. Fermentation
Fermentation is an enzymatically controlled change in a food product caused by the action of microorganisms
Changes result from
digestive enzymes released by the microbes
the release of by-products, such as carbon dioxide, acetic and lactic acids, and ethanol
continued

29. Fermentation The primary product of a microbial reaction is energy
By-products can change the color, texture, flavor, aroma, and pH of a food
Fermentation occurs when nutrients broken down by digestive enzymes are absorbed by microbes for growth and energy

30. Yeast Fermentation
Yeast is needed in all yeast breads, alcoholic beverages, and vinegars
Saccharomyces cerevisiae is often used
Sugar is its main source of energy
It also feeds on honey, molasses, or corn syrup
Quick-rising yeast may be used in some breads

31. Yeast Fermentation: Bread
Yeast is affected by steps in the bread-making process
Warm liquid activates yeast
Mixing distributes yeast evenly
Kneading the dough develops the gluten
Proofing allows the yeast to produce enzymes that break down sugars, releasing alcohol and carbon dioxide
continued

32. Yeast Fermentation: Bread
In the bread-making process
carbon dioxide becomes trapped in pockets throughout the dough, gluten stretches, and the dough rises
the dough continues to rise during baking and a crust forms
Bread products contain flour, yeast, salt, and water

33. Yeast Fermentation: Wine
Wine is the fermented juice of plant products such as grapes, honey, various fruits and dandelions
Sulfur dioxide (SO2) or potassium metabisulfite is added to inhibit unwanted microbe growth
Juices are held in anaerobic conditions for up to 4 weeks in oak barrels or stainless steel tanks
continued

34. Yeast Fermentation: Wine
Wine fermentation is complete when bubbling from CO2 production stops
Wine is then aged in barrels or vats to slowly develop the characteristic flavors
©Rusian Rizvanov/Shutterstock.com

35. Yeast Fermentation: Other Alcohols
Yeast is used in the fermentation of other alcoholic beverages
Beers are made from fermented barley
Bourbon is made from corn
Rum is made from sugar cane or molasses
Brandy is distilled wine or fermented fruit juice
Liqueurs and cordials have a brandy base with sugar and flavorings added

36. Bacterial Fermentation
Lactic acid fermentation
is one of 3 main types of bacterial fermentation
refers to the many fermented foods—vegetables, meats, and dairy products—produced by bacteria whose by-product is lactic acid
produces the other by-products of acetic acid, formic acid, and carbon dioxide

37. Bacterial Fermentation: Sauerkraut
Sauerkraut results from the lactic acid fermentation of cabbage submerged in brine, a salt and water solution
The salt controls water activity and pulls water with dissolved sugar and nutrients to the surface
Bacteria feed on the sugar and release carbon dioxide and lactic acid into the brine

38. Bacterial Fermentation: Pickles
Cucumbers are turned into pickles by
heating them in a spiced vinegar solution, refrigerating in an acid brine, or fermenting them with lactic acid bacteria
Commercial pickling involves
washing the cucumbers in a chlorine solution
placing them in a brine with a pure culture of Lactobacillus added

39. Bacterial Fermentation: Other Foods
Other foods that can be pickled include
vegetables such as watermelon rinds, beets, cauliflower, okra, and onions, and even eggs
The lactic acid fermentation of olives is a process similar to making pickles
except olives are washed in a lye solution to remove bitter flavor compounds

40. Bacterial Fermentation: Olives
Lactic acid neutralizes any lye remaining and sugar is added to start the fermentation process
Green or Spanish olives are picked before being ripe, then cured, pitted, and stuffed with pimento
Black or ripe olives are picked at a riper stage

41. Bacterial Fermentation: Meats
Lactic acid fermentation is used to make dry and semidry sausages
Increased acid tenderizes the meat and adds a tart flavor
Fermentation, smoking, and drying prevent spoilage
Sugar provides food for lactic acid, and spices and salt add desired flavor

42. Bacterial Fermentation: Cultured Dairy Products
Cultured dairy products include
sour cream, yogurt, and buttermilk
Streptococci are the fastest lactic acid producers, which enables milk to
acidify quickly, thus reducing prep time and contamination risk
Leuconostoc and Lactobacilli are added to produce the desired flavors
continued

43. Bacterial Fermentation: Cultured Dairy Products
Milk for cultured dairy products is pasteurized and bacteria is added
The action of bacteria on lactose forms lactic acid, which denatures the proteins and causes them to coagulate
Texture is also affected by temperature, pH, fermentation time, and added enzymes used
Cooling stops the fermentation process

44. Bacterial Fermentation: Cheese
Milk is pasteurized, then rennin and lactic acid bacteria are added to form curds
Curds are cut into small cubes and heated to help the whey separate from them
Whey is drained off and collected for use as an additive in other processed foods
Curds are rinsed and salted
continued

45. Bacterial Fermentation: Cheese
Curds are
mixed with cream to make cottage cheese
put in presses to squeeze out excess moisture for making aged cheese
During aging or ripening, cheeses wrapped with wax are placed in curing rooms
Different bacteria or molds are responsible for the characteristic flavor of cheeses
continued

46. Bacterial Fermentation: Cheese
The sharpness of cheese refers to the strength of flavor and aroma
Sharpness develops as cheese ages
Cheeses that age longer tend to
have firmer consistency
have more crumbly textures
melt into sauces more readily

47. Mold Fermentation Molds create a wide range of by-products
antibiotics, flavor compounds, and enzymes
Fermented foods important to Asian cuisine include
soy sauce, a fermented mix of soybeans and wheat
tempeh, a soybean cake of cooked, mashed soybeans pressed into blocks, inoculated with Rhizopus molds, and wrapped in banana leaves

48. Two-Step Fermentation
Cheese, soy sauce, and sourdough bread require lactic acid fermentation plus other microbes in a second step
For cheese, lactic acid is needed to form the product, then microbes develop the flavors and textures
For sourdough bread, lactic acid ferments the yeast starter, then yeast fermentation proceeds
continued

49. Two-Step Fermentation
Acetic acid fermentation follows yeast fermentation
Yeast release alcohol as they break down sugars, then acetic acid fermentation begins
Acetobacter bacteria feed on the alcohol and release acetic acid
Vinegar is produced by acetic acid fermentation
continued

50. Two-Step Fermentation
Acetic acid fermentation
produces candied citron from citron lemons for use in baked goods
turns cacao beans into chocolate
©stockcreations/Shutterstock.com

51. Benefits of Fermentation
Microbes help preserve some foods
Milk keeps for about a week under refrigeration
Cheese can keep for months when properly stored
Fermentation offers a wider range of food options

52. Nutritional Changes in Fermented Foods
Changes vary depending on the
microbes used
other ingredients added
impact of fermentation on bioavailability
Fat and calcium concentrations are higher in cheese than in milk
Pickling adds salt through the brine, which increases the sodium content

53. Recap Bacteria, yeasts, and molds have positive uses in foods
Factors that affect microbe growth are
food supply
water pH
temperature
continued

54. Recap Yeasts, bacteria, molds, or other microbes can ferment foods
Examples of foods products that result from fermentation are
yogurt, cheese, cottage cheese
pickles
soy sauce
sauerkraut