1. Chapter 40
Microbiology of Food

2. Intrinsic Factors – control growth of microbes in food
composition pH
presence and availability of water
physical structure
presence of antimicrobial substances

3. Putrefaction – anaerobic breakdown of proteins
Carbohydrate – fungal , bread, jams, fruits, vegetables
Fat protein bacterial
proteolysis and anaerobic breakdown of proteins, yielding foul-smelling amine compounds, cadaverine, putrescine – meat, dairy products
pH impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food
Low ph yeasts and molds
Neutral alkaline bacteria

4. Water availability
in general, lower water activity inhibits microbial growth
water activity lowered by:
drying
addition of salt or sugar
Hypertonic environment

5. Table 40.2

6. Physical structure
grinding and mixing increase surface area and distribute microbes
promotes microbial growth
outer skin of vegetables and fruits slows microbial growth

7. Antimicrobial Substances
coumarins – fruits and vegetables
lysozyme – cow’s milk and eggs
phenolic compounds – herbs and spices -Cinnamon, mustard.
allicin – garlic

8. Extrinsic Factors temperature relative humidity atmosphere
lower temperatures retard microbial growth
relative humidity
higher levels promote microbial growth
atmosphere
oxygen promotes growth

9. Microbial Growth and Food Spoilage
results from growth of microbes in food
alters food visibly and in other ways, rendering it unsuitable for consumption
involves predictable succession of microbes
different foods undergo different types of spoilage processes
toxins are sometimes produced

10. Figure 40.2 Lactococcus -acids Lactobacillus Yeasts and molds
Dominant-
breakdown lactic acid
pH increases
Bacteria
breakdown proteins
Curds and whey
Left: fresh milk
Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey.
Figure 40.2

11. Figure 40.3 Left: fresh milk
Right: curdled mild. The curdled milk has undergone a natural 4-step sequence of spoilage organisms activity, resulting in separated curds and whey.
Figure 40.3

12. Toxins
ergotism
toxic condition caused by growth of a fungus in grains rye
Claviceps purpurea – fungus grows on grains rye
Ergot
Hallucinations, constriction of capillaries

13. disrupt synthesis and metabolism of sphingolipids
fumonisins
Figure 40.5

14. aflatoxins intercalate into DNA, causing frameshift mutations
The letter designations refer to the color of the compounds under ultraviolet light after extraction from the grain and separation by chromatography. The B1 and B2 compounds fluoresce with a blue color and the G1 and G2 appear green. The two type of M aflatoxins are found in the milk of lactating animals that have ingested type B aflatoxins.
Figure 40.4

15. Table 40.3

16. Controlling Food Spoilage
Table 40.4

17. Removal of Microorganisms
usually achieved by filtration
commonly used for water, beer, wine, juices, soft drinks, and other liquids

18. Low Temperature
refrigeration at 5°C retards but does not stop microbial growth
microorganisms can still cause spoilage with extended spoilage
growth at temperatures below -10°C has been observed

19. Pasteurization
kills pathogens and substantially reduces number of spoilage organisms
different pasteurization procedures heat for different lengths of time
shorter heating times result in improved flavor

20. Water Availability dehydration
freeze-dried foods is commonly used to eliminate bacterial growth
food preservation occurs as a result of free-water loss and an increase in solute concentration

21. Microbial Product-Based Inhibition
bacteriocins
e.g., nisin
used in low-acid foods to inactivate Clostridium botulinum during canning process

22. Food-Borne Diseases two primary types food-borne infections
food intoxications

23. Food-Borne Infection
ingestion of microbes, followed by growth, tissue invasion, and/or release of toxins
Salmonellosis

24. Food Borne Infections Salmonellosis Listeriosis
gastroenteritis from ingestion of contaminated meats, poultry or eggs
Listeriosis
pregnant women, the young and old and immunocompromised individuals most vulnerable
responsible for the largest meat recall in U.S.
at risk people should not eat soft cheeses, refrigerated smoked meats, deli meats and undercooked hot dogs

25. Food Borne Infections…
Escherichia coli
diarrhea caused by enteropathogenic, enteroinvasive and enterotoxigenic types
E. coli 0157:H7 is thought to have acquired enterohemorrhagic genes from Shigella, including genes for the shigalike toxin

26. Production of Breads
involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions
maximizes CO2 production, which leavens bread

27. Table 40.9

28. Other Fermented Foods…
sauerkraut (sour cabbage) – from wilted, shredded cabbage – Lactobacillus plantarum
pickles – from cucumbers

29. yogurt
Lactobacillus bulgaricus, Lactobacillus acidphilus, Streptococcus lactis
Cup of milk – warm
Add a spoon of yogurt, mix, cover, leave it at room temperature overnight
Bacteria ferment lactose – acids coagulate the milk protein casein. Milk becomes solid.

30. Cheese Various species of Lactobacillus bulgaricus, lactococcus lactis
Curd formation from milk
Watery part (whey) removed ripened ( a year)
Peniclillium camemberti, P. Roqueforti
Swiss cheese Propionibacterium makes holes by making gas

31. Microorganisms as Foods and Food Amendments
variety of bacteria, yeasts, and other fungi are used as animal and human food sources
probiotics
microbes added to diet in order to provide health benefits beyond basic nutritive value
Lactobcillus acidophilus in cattle feed reduces
Number of toxic E.coli in the intestinal tract of cattle

32. Benefits of probiotics
immunomodulation
control of diarrhea
anticancer effects
possible modulation of Crohn’s Disease
in beef cattle,
decrease E. coli
in poultry,
limit colonization of gut by the process of competitive exclusion

33. Wine production Saccharomyces cerevisiae added to grape juice.
Fermented 3 to 5 days at 20 to 28oC.