Extrinsic and Intrinsic Factors

Temperature rates

*Most important factor determining growth of mo's in food. Min. temp. of growth: 7-8C, fridge Optimum temp of growth: 39, fastest Max. temp of growth: 45-48C

Intrinsic Factor: Physical barrier role

-Limit access of mo's, no space or nutrient contained in space (exterior exposed, inside safe, until cut - meat) -same goes for water droplets in butter

Water activity spectrum.... (Aw)

0-0.1, Lipid oxidation 0.3-0.7, non-enzymatic browning rxns (maillard .6-.7), lipid oxidation No bacterial growth up to ~0.85 0.85-1.0, enzymatic activity, everything (mold,bacteria,yeast-no browning)

# magic numbers of food microbiology

10^6 , Less than this # = no noticeable change of food quality (looks and tastes same) 10^9 , up to this # = properties determined by microbial metabolites (fermented or spoilage) 10^12 , max # of bacterial cells per g

Danger Zone in both celsius and fahreinheit Temp of refrigeration, sterilization (C)

4-60C and 40-140F 7C and 121C

Temp. for death of spores and viable cells

>100-121C, most bacterial endospores >60-80C kills vegetative cells, no growth of mesophiles

Preservation of meat in aerobic Vs. anaerbic conditions

Aerobic storage = quick microbial growth, off-odour to putrefication (10^7) Pseudomonas spp. Anaerobic = slow microbial growth, (fermentation) souring Carnobacterium, Leuconostoc

Extrinsic Factor: Gaseous Environment -Define ambient conditions -How to modify atmosphere

Ambient conditions: 20% O2, 80% N2 MA: Take away oxygen (vacuum, N2) -add CO2 (10-80%) - inhibits aerobic growth, not lactic acid bacteria.

How cool does temp. need to be to inhibit most pathogen growth? what are the exceptions?

At or below 4C. Exceptions: Listeria monocytogenes, Yersinia enterocolitica, Clostridium botulinum group II (type E)

Survival of spores ( bacterial endospores and fungal spores)

Bacterial endospores are heat resistant. CANNOT be destroyed by household tools (pressure cooker, UV, radiation) fungal spores (conidiospores, ascospores), most are as heat sensitive as vegetative cells BUT some will withstand pasteurization.

Oxidation - Reduction potential

Basis for almost all biological rxns = the ease with which the substrate loses or gains electrons, measured in mV

Example foods not to store in fridge

Climateric fruits: tomatoes, melons, bananas Oil, honey, bread - spoilage by crystallization of fat, starch @4C

3 different techniques of decreasing water activity

Drying dry salting Brining (osmotic dehydration) -meats/fish -candied fruit

Enzymes that support microbial growth

Enzymes produced by mo's influece its ability to utilize different energy sources: -Starch needs amylolytic enzymes (bread) -lactose needs beta-galactosidase (milk) ----lactose is rare(mammary gland) -proteins need protease (milk)

Extrinsic Factors

Factors controlled by the storage environment, "where you put the food" -Temperature, presence and concentration of gases

Other nutrients mo's may use for growth

Inorganic nitrogen and CO2 (Pseudomonas spp. grow in deionized water) Fastidious quality : require sugars, a.a's , vitamins (lactic acid bacteria)

Antimicrobial compounds that defend against mo's

Lysozyme- egg white (saliva)- hydrolyses cell wall (peptidogycan) Lactoperoxidase (raw milk)- oxidative stress Isohumulones (from hops-elongate shelf life)-permeabilize bacterial membranes Essential oils (thymol +carvacrol) and phenolic compounds in plants Glucosinolates that generate isothiocyanates in garlic, onions and mustard - oxidise thiol groups in proteins ground beef mixed real well with 20% garlic or onions

What out-competes pH in inhibitory effect?

Organic acids (lactate, acetate)

Intrinsic Factor

Properties of the food (or plant/animal tissue), "what you put in the food" -pH, moisture content (Aw), oxidation, content of nutrients or antimicrobial constituents -physical structures

Classification of bacteria based on temp.

Psychrophiles optimum < 20C (based more in cold water, fish -not common in foods) Psychrotrophs optimum > 20C but grow at 5C (Listeria, pseudomonas spp.) Mesophiles ~30-40C, most foodborne pathogens (e.coli, staphylococcus aureus) Thermophiles > 45C , can be problem even in canning @ tropical temp.

Most mo's dont grow or germinate at <4.5pH. Which ones do and how do you prevent them?

Salmonella and E. coli Pasteurization prevent germination and refrigeration/sterilization keeps spores from growing

Water activity and pathogens

aw 0.92 or less inhibits growth of pathogens and toxin production by S. aureus *below 0.9, most spoilage bacteria inhibited Halophiles+halo-tolerant bacteria Aw > 0.75 Osmotolerant yeasts Aw > 0.61 Xerophilic molds (tolerate dry conditions) Aw > 0.61

How does acidity change during storage?

climacteric fruits increase 0.5 pH(more alkaline) units during storage Souring- milk and some dairy Alkalinization - acid + fermented foods as a result of microbial metabolism-mould (reduces organic acids on surface)

Extrinsic: Rh (relative humidity)

high Rh + refrigeration = condensation (supports local mould growth) low Rh, dries and keeps for long time

Hurdle concept

more sequential or simultaneous presence of stress (both intrinsic and extrinsic factors), the less likely mo's will grow.

What is more inhibitory, sugar or salt solution?

salt!