Chapter Summary

16.1

  • Fungal fruiting bodies such as mushrooms and truffles are consumed as a protein-rich food.
  • Edible algae include nori (toasted red algae), used to wrap sushi; and kelp (brown algae), which provides food additives.
  • Spirulina is an edible cyanobacterium, a source of single-celled protein. Most bacteria, however, are inedible because of their high concentration of nucleic acids.
  • Yeasts have been grown as an economical protein and vitamin supplement.

16.2

  • Fermentation of food enhances preservation, digestibility, nutrient content, and flavor.
  • Acidic fermentations lead to organic acid fermentation products, such as lactate and propionate.
  • Alkaline fermentations produce ammonia and break down proteins to peptides.
  • Ethanolic fermentation produces ethanol and carbon dioxide.
  • Lipids are relatively stable under anaerobic conditions of fermentation.

16.3

  • Milk curd forms by lactic acid fermentation and rennet proteolysis, rendering casein insoluble. The cleaved peptides coagulate to form a semisolid curd. The main fermentative organisms are lactic acid bacteria.
  • Cheese varieties include unripened cheese; semihard and hard cheeses that are cooked down and ripened; brined cheeses; and mold-ripened cheeses.
  • Cheese flavors are generated by minor side products of fermentation, such as alcohols, esters, and sulfur compounds.
  • Soy fermentation to tempeh and other products improves digestibility and decreases undesirable soy components such as phytates and lectins. The fermentative agent of tempeh is the bread mold Rhizopus oligosporus.
  • Vegetables are fermented and brined to make sauerkraut and pickles. Cabbage and supplementary foods are fermented and brined to make kimchi
  • Alkali-fermented vegetables include the soy product natto, the egg product pidan, and the locust bean product dawadawa. The main fermenting organisms are Bacillus species.

16.4

  • Bread is leavened by yeasts conducting limited ethanolic fermentation, producing enough carbon dioxide gas to expand the dough.
  • Injera bread dough undergoes more extensive fermentation by indigenous organisms and, as a result, generates multiple flavors.
  • Beer derives from alcoholic fermentation of grain. Barley grains are germinated, allowing enzymes to break down the starch to maltose for yeast fermentation.
  • Secondary products of grain fermentation, such as long-chain alcohols and esters, generate the special flavors of beer.
  • Wine derives from alcoholic fermentation of fruit, most commonly grapes. The grape sugar (glucose) is fermented by yeast to alcohol. A secondary product, malate, undergoes malolactic fermentation by Oenococcus oeni bacteria.

16.5

  • Food spoilage refers to chemical changes that render food unfit for consumption. Food spoils through degradation by enzymes within the food, through spontaneous chemical reactions, and through microbial metabolism.
  • Food contamination, or food poisoning, refers to the presence of microbial pathogens that cause human disease, or toxins produced by microbial growth. Food harvesting, processing, and shared consumption are all activities that spread pathogens.
  • Dairy products can be soured by excessive fermentation or made bitter by bacterial proteolysis.
  • Meat and poultry are putrefied by decarboxylating bacteria, which produce amines with noxious odors
  • Fish and other seafood spoil rapidly because their unsaturated fatty acids rapidly oxidize; they harbor psychrotrophic bacteria that grow under refrigeration; and their TMAO is reduced by bacteria to the fishy-smelling trimethylamine.
  • Vegetables spoil by excess growth of bacteria and molds. Plant pathogens destroy food crops before harvest.
  • Food preservation includes physical treatments, such as freezing and canning, as well as the addition of chemical preservatives, such as benzoates and nitrites.

16.6

  • Industrial microbiology includes the production of vaccines and clinical devices, industrial solvents and pharmaceuticals, and genetically modified plants and animals.
  • Microbial product molecules may be indigenous, or they may be cloned from nonmicrobial sources. Thermophiles and psychrophiles are particularly important sources of new strains with potentially interesting new properties.
  • Microbial products must be competitive with alternative technologies. Developing a competitive new molecular product requires identifying a useful molecule, isolating and engineering a strain to produce it, scaling up for production in quantity, developing a business plan, and testing for safety.
  • Bioprospecting is the mass screening of new microbial strains for potentially valuable protein and smallmolecule products.
  • Industrial strains, commonly Escherichia coli or Bacillus subtilis, are used to incorporate the newly discovered genes into a host microbe.
  • Upstream processing refers to the culturing of the industrial microbe to produce large quantities of product.
  • Downstream processing involves product recovery and purification.
  • Posttranscriptional processing of human or plant genes may be facilitated by use of a transgenic source or a virus-infected animal host.