Copyright © The McGraw-Hill Companies. Permission required for reproduction or display. 1 Chapter...

Copyright © The McGraw-Hill Companies. Permission required for reproduction or display.

1

Chapter 41

Microbiology of Food


2

Microorganism Growth in Foods

Figure 41.1


3

Intrinsic Factors

• composition• pH• presence and availability of water• oxidation-reduction potential

– altered by cooking

• physical structure• presence of antimicrobial substances


4

Composition and pH

• putrefaction– proteolysis and anaerobic breakdown of

proteins, yielding foul-smelling amine compounds

• pH impacts make up of microbial community and therefore types of chemical reactions that occur when microbes grow in food


5

Water availability

• in general, lower water activity inhibits microbial growth

• water activity lowered by:– drying– addition of salt or sugar

• osmophilic microorganisms– prefer high osmotic pressure

• xerophilic microorganisms– prefer low water activity


6


7

Physical structure

• grinding and mixing increase surface area and distribute microbes– promotes microbial growth

• outer skin of vegetables and fruits slows microbial growth


8

Antimicrobial substances

• coumarins – fruits and vegetables

• lysozyme – cow’s milk and eggs

• aldehydic and phenolic compounds – herbs and spices

• allicin – garlic

• polyphenols – green and black teas


9

Extrinsic Factors• temperature

– lower temperatures retard microbial growth

• relative humidity– higher levels promote microbial growth

• atmosphere– oxygen promotes growth– modified atmosphere packaging (MAP)

• use of shrink wrap and vacuum technologies to package food in controlled atmospheres


10

Microbial Growth and Food Spoilage

• 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

• algal toxins may contaminate shellfish and finfish


11

Figure 41.2


12


13

Toxins• ergotism

– toxic condition caused by growth of a fungus in grains

• aflatoxins– carcinogens produced in fungus-

infected grains and nut products

• fumonisins– carcinogens produced in fungus-

infected corn


14

Figure41.4

intercalateinto DNA,causingframeshiftmutations

aflatoxins


15

Figure 41.5

disrupt synthesis and metabolismof sphingolipids

fumonisins


16


17

Controlling Food Spoilage


18

Removal of Microorganisms

• usually achieved by filtration

• commonly used for water, beer, wine, juices, soft drinks, and other liquids


19

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


20

High Temperature

• canning

• pasteurization


21

Canning• food heated in

special containers (retorts) to 115° C for 25 to 100 minutes

• kills spoilage microbes, but not necessarily all microbes in food Figure 41.6


22

Spoilage of canned goods

• spoilage prior to canning

• underprocessing

• leakage of contaminated water into cans during cooling process


23

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


24

Water Availability


25

Chemical-Based Preservation

• GRAS– chemical agents “generally recognized

as safe”

• pH of food impacts effectiveness of chemical preservative


26


27

Radiation• ultraviolet (UV) radiation

– used for surfaces of food-handling equipment

– does not penetrate foods

• radappertization– use of ionizing radiation (gamma radiation)

to extend shelf life or sterilize meat, seafoods, fruits, and vegetables

– kills microbes in moist foods by producing peroxides from water

• peroxides oxidize cellular constituents


28

Microbial Product-Based Inhibition• bacteriocins

– bactericidal proteins active against related species

– some dissipate proton motive force of susceptible bacteria

– some form pores in plasma membranes

– some inhibit protein or RNA synthesis

• e.g., nisin– used in low-acid foods to inactivate Clostridium

botulinum during canning process


29

Food-Borne Diseases

• two primary types– food-borne infections

– food intoxications


30

Food-Borne Infection

• ingestion of microbes, followed by growth, tissue invasion, and/or release of toxins

• raw foods (e.g., sprouts, raspberries, and seafood) are important sources


31


32

Food-Borne Intoxications

• ingestion of toxins in foods in which microbes have grown

• include staphylococcal food poisoning, botulism, Clostridium perfringens food poisoning, and Bacillus cereus food poisoning


33

Detection of Food-Borne Pathogens

• must be rapid and sensitive• methods include:

– culture techniques – may be too slow– immunological techniques - very

sensitive– molecular techniques

• probes used to detect specific DNA or RNA

• sensitive and specific


34Figure 41.10

comparison of PCR andgrowth for detection of Salmonella


35

Figure 41.7

nucleic acid can be detectedeven when plaque-formingability is lost


36

Surveillance for food-bornedisease• PulseNet

– established by Centers for Disease Control– uses pulsed-field gel electrophoresis under

carefully controlled and duplicated conditions to determine distinctive DNA pattern of each bacterial pathogen

– enables public health officials to link pathogens associated with disease outbreaks in different parts of the world to a specific food source


37

Surveillance…

• FoodNet– active surveillance network used to

follow nine major food-borne diseases

– enables public health officials to rapidly trace the course and cause of infection in days rather than weeks


38

Microbiology of Fermented Foods

• major fermentations used are lactic, propionic, and ethanolic fermentations


39


40

Fermented Milks• mesophilic – Lactobacillus and

Lactococcus• thermophilic – Lactobacillus and

Streptococcus• therapeutic – Lactobacillus and

Bifidobacterium• yeast lactic – yeasts, lactic acid bacteria,

and acetic acid bacteria• mold lactic – filamentous fungi and lactic

acid bacteria


41

Cheese Productionmilk

lactic acid bacteria and rennincurd

removal of whey ripening by microbial action

cheese


42


43

Meat and Fish

• sausages

• hams

• bologna

• salami

• izushi – fish, rice, and vegetables

• katsuobushi – tuna


44

Production of Alcoholic Beverages• begins with formation of liquid

containing carbohydrates in readily fermentable form– must

• juice from crushed grapes

– mashing• hydrolysis of complex carbohydrates in cereals

by addition of water and heating mixture• yields wort – clear liquid containing fermentable

carbohydrates


45

Figure 41.15racking – removes sediments

or

microbial oxidation of ethanol to acetic acid yields wine vinegar


46

Beers and ales• malt

– germinated barley grains having activated enzymes

• mash– the malt after being mixed with water in

order to hydrolyze starch to usable carbohydrates

• bottom yeasts– used in production of beers

• top yeasts– used in production of ales


47

pitched

Figure 41.16


48

Distilled spirits

• similar to beer-making process– begins with sour mash

• mash inoculated with homolactic bacterium

– following fermentation, is distilled to concentrate alcohol


49

Production of Breads

• involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions– maximizes CO2 production, which leavens

bread

• other microbes used to make special breads (e.g., sourdough bread)

• can be spoiled by Bacillus species that produce ropiness


50

Other Fermented Foods

• silages– fermented grass, corn, and other fresh

animal feeds


51

Figure 41.18

Making sauerkraut


52

Microorganisms as Foods and Food Amendments

• variety of bacteria, yeasts, and other fungi are used as animal and human food sources

• probiotics– microbial dietary adjuvants– microbes added to diet in order to

provide health benefits beyond basic nutritive value


53

Benefits of probiotics

• immunodilation• 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


54

Prebiotics

• oligosaccharide polymers that are not processed until they enter large intestine

• symbiotic system – combination of prebiotics and probiotics– results in production of certain acids that

may be responsible for possible beneficial effects of probiotics

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