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Chapter 41

Microbiology of Food

Microorganism growth in foods

Intrinsic factors

• Composition• pH• Presence and availability of water• Oxidation-reduction potential

– altered by cooking

• Physical structure• Presence of antimicrobial substances

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

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

Physical structure

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

• Outer skin of vegetables and fruits slows microbial growth

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

Extrinsic factors• Temperature

– lower temperatures retard microbial growth

• Relative humidity– higher levels promote microbial growth

• Atmosphere– oxygen promotes growth– modified atmosphere packaging

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

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• fungal toxins contaminate grains

Toxins

• Ergotism– toxic condition caused by growth of a fungus,

Claviceps purpura, in grains (← production of hallucinogenic alkaloids)

• Aflatoxins– carcinogens produced in fungus-infected grains

and nut products (Aspergillus flavus)

• Fumonisins– carcinogens produced in fungus-infected corn (Fusarium moniliforme)

Intercalateinto DNA,causingframeshiftmutations

aflatoxins

Disrupt synthesis and metabolism of sphingolipids

Fumonisins

FB1: R = OHFB2: R = H

Algal toxins

Controlling food spoilage

Removal of microorganisms

• Usually achieved by filtration

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

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

High temperature

• Canning

• Pasteurization

Canning

• Food heated in special containers to 115° C for 25 to 100 minutes

• Kills spoilage microbes, but not necessarily all microbes in food

Spoilage of canned goods

• Spoilage prior to canning

• Underprocessing

• Leakage of contaminated water into cans during cooling process

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

Chemical-based preservation

• GRAS– chemical agents “generally recognized

as safe”

• pH of food impacts effectiveness of chemical preservative

Radiation• Ultraviolet (UV) radiation

– used for surfaces of food-handling equipment

– does not penetrate foods

• Radappertization– use of ionizing radiation (e.g. gamma ray) 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

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 - produced by some strains of Streptococcus lactis - used in low-acid foods to inactivate Clostridium

botulinum during canning process

Food-borne diseases

• Two primary types– food-borne infections

– food intoxications

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

Nucleic acid can be detectedeven when plaque-formingability is lost

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

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

Comparison of PCR sensitivity and growth for detection of Salmonella

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

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

Microbiology of fermented foods

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

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

Cheese production

lactic acid bacteria and rennin Curd

removal of whey ripening by microbial action

Cheese

Meat and fish

• Sausages• Hams• Bologna• Salami• Izushi (fermentation of fish, rice, and

vegetables by Lactobacillus spp.)• Katsuobushi (fermentation of tuna by

Aspergillus glaucus)

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

racking – removes sediments

Microbial oxidation of ethanol to acetic acid yields wine vinegar

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

pitched

Distilled spirits

• Similar to beer-making process– usually begins with ‘sour mash’

• mash inoculated with homolactic bacterium: lower the mash pH

– following fermentation, is distilled to concentrate alcohol

Production of breads

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

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

• Can be spoiled by Bacillus species that causes ropiness

Other fermented foods

• Silages– fermented grass, corn, and other fresh

animal feeds

Making sauerkraut

Microorganisms as foods and food amendments

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

- mushrooms, Spirulina, etc.

• Probiotics– microbial dietary adjuvants– microbes added to diet in order to provide

health benefits beyond basic nutritive value

Benefits of probiotics

• Immunodilation• Control of diarrhea• Anticancer effects• Possible modulation of Crohn’s Disease• In beef cattle,

– Lactobacillus acidophilus decrease E. coli

• In poultry,– Bacillus subtilis limits Salmonella colonization of gut by the

process of competitive exclusion

Prebiotics

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

• Synbiotic system – combination of prebiotics and probiotics– results in an increase in production of

butyric acid and propionic acids that may be responsible for possible beneficial effects of probiotics

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