| Intrinsic factors | associated with the food product itself
- pH
- water activity
- O/R potential (Eh)
- nutrient content
- antimicrobial constituents
- biological structures |
| Extrinsic factors | associated with the storage environment
- temperature
- RH
- gaseous atmosphere
- activity of other microbes |
| pH | intrinsic factor
- can be inhibitory to microorganisms
- high acid foods (pH below 4.6): fruits, fruit juices, fermented fruits, salad dressing
- low acid foods (pH above 4.6): vegetables, red meats, poultry, sea foods, milk, soups
- foods with pH >= 4.6 are potentially hazardous: moist, nutrient rich, support rapid microbial growth
- pH 6.6-7.5 best for microbial growth
- most foodborne bacteria will not grow at pH < 4.6 |
| organisms & pH range | molds: 1.5-9
yeasts: 2-8.5
gram +: 4-8.5
gram -: 4.5-9 |
| when food pH is below lower limit for microbial growth... | - cessation of microbial growth
- inhibition of ATP synthesis and metabolic reactions
- loss of viability of microbes
- acidification of the cytoplasm
- depletion of ATP reserves
- denaturation of enzymes |
| Water activity (aw) | - measure of free water available for microbial growth
- total water = bound water + free water
- important factor in microbial and chemical shelf-life of foods
- no microbial growth occurs below a water activity of 0.6
- low aw --> very little effect on lipid oxidation |
| Ways to lower water activity of foods | - addition of solutes, ions, or hydrophilic colloids --> water becomes chemically bound
- freezing --> water transformed into solid
- drying --> free water actually removed |
| Importance of free water to microorgansims | - nutrient transport
- waste removal
- enzymatic reactions
- hydrolysis of polymers to monomers
- synthesis of cellular materials |
| Water activity (aw) and microbes | microbe minimum aw for growth
most molds 0.8
most yeasts 0.85
most gram + 0.9
most gram - 0.93
S. aureus 0.85
Halophilic bacteria 0.75
most foodborne bacteria have no growth below aw of 0.91
growth range also influenced by pH, temperature, oxidation-reduction potential |
| effects of low aw on microbes | - increase in lag phase of growth
- decrease in growth rate
- decrease in size of final population |
| Oxidation-reduction potential (Eh) | - ease with which food becomes oxidized (loses electrons) or reduced (gains electrons)
- oxidation --> also form addition of oxygen
- specific metabolic rxns for energy and growth of microbes in food extremely dependent on Eh
- transfer of electrons between components in food creates a potential difference which can be measured (mV) |
| Oxidation-Reduction Potential influencing factors | - chemical composition of food
- specific food processing treatment
- storage condition (in relation to air) |
| O/R potential and microbial growth | aerobes: 300-500 mV
fac anaerobes: -100-300 mV
anaerobes: -250-100 mV |
| Nutrient content | - water: req ranking --> molds < yeasts < g+ < g-
- energy source: sugars, alcohols, amino acids, complex carbohydrates
- nitrogen sources: amino acids, nucleotides, peptides, proteins
- vitamins & growth factors: many microbes req. B vitamins (g+ req more, g- can synthesize almost all req)
- minerals: req. in small amounts (P, Ca, Mg, Fe, Mn, K) |
| Antimicrobial constituents | some foods contain natural antimicrobial substances
ex. Cow's milk: lactoferrin, conglutinin, lactoperoxidase system |
| Lactoperoxidase Sytem | chemical method of milk preservation w/o refrigeration
inhibits g- and g+ bacteria, more effective against g-
three component system in raw milk
- lactoperoxidse (enzyme) 1-6%
- thiocyanate (SCN -) 0.1-1%
- hydrogen perodixe <0.1% |
| Lactoperoxidase | - enzyme naturally found in milk of most mammals
- converts thiocyanate (in the presence of H2O2) to hypothiocyanate
- must add activator (mix of thiocyanate and a source of H2O2)
- hypothiocyanate reacts with sulfhydryl group of enzymes
- hypothiocyanate inactivates bacterial metabolic enzymes
- microbial quality stabilized for 24hr @ 15 C, 6-8hr at 30 C |
| Natural antimicrobial constituents in eggs | - egg white: lysozyme (destroys peptidoglycan) and ovotransferrin (aka conalbumin, binds iron)
- egg yolk: phosvitin (strong metal ion-binding glycoprotein |
| Allicin | antimicrobial component in garlic |
| Thymol & Carvacrol | antimicrobial constituents in thyme and oregano |
| Eugenol | antimicrobial constituent in cloves |
| Citric acid | antimicrobial constituent in lemons |
| Commonly added antimicrobial constituents | - sodium chloride
- polyphosphates
- nitrite
- sodium lactate, diacetate
- herbs and spices
- smoke |
| Hydroxycinnamic acid derivatives | ex: p-coumaric, ferulic, caffeic, and chlorogenic acids
- antibacterial and antifungal activity
- found in fruits, vegetables, tea, and molasses |
| Glucosinolates | - present in cell vacuoles
- damaged cells release myrosinase, which converts glucosinolates to isothiocyanates
- isothiocyanates: antibacterial and antifungal activity, insoluble in water
- found in cruciferous plants (cabbage, brussels sprouts, broccoli, turnips) |
| Biological structures as microbial protection | damage to outer coverings allows entry of microorgansims and faster spoilage
- testa of seeds
- outer covering of fruits
- shell of nuts
- hide of animals
- shell of eggs |
