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Intrinsic and Extrinsic Parameters of Foods That Affect Microbial Growth As our foods are of plant and/or animal origin, it is worthwhile to consider those characteristics of plant and animal tissues that affect the growth of microorganisms. The plants and animals that serve as food sources have all evolved mechanisms of defense against the invasion and proliferation of microorganisms, and some of these remain in effect in fresh foods. 2INTRINSIC PARAMETERSThe parameters of plant and animal tissues that are an inherent part of the tissues are referred to as intrinsic parameter. These parameters are as follows:1. pH2. Moisture content3. Oxidationreduction potential (Eh)4. Nutrient content5. Antimicrobial constituents 6. Biological structures31. pHIt has been well established that most microorganisms grow best at pH values around 7.0 (6.67.5), whereas few grow below 4.0.Sensitivity to acidity: Bacteria > yeasts> molds 4

pH minima and maxima of microorganisms should not be taken to be precise boundaries, as the actual values are known to be dependent on other growth parameters.For example, the pH minima of certain lactobacilli have been shown to be dependent on the type of acid used, with citric, hydrochloric, phosphoric, and tartaric acids permitting growth at a lower pH value than acetic or lactic acids.

With respect to temperature, the pH of the substrate becomes more acid as the temperature increases. Concentration of salt has a definite effect on pH growth rate curves, where it can be seen that the addition of 0.2 M NaCl broadened the pH growth range of Escherichia coli .However, when the salt content exceeds this optimal level, the pH growth range is narrowed. Young cells are more susceptible to pH changes than older or resting cells.When microorganisms are grown on either side of their optimum pH range, an increased lag phase results.10Some food such as fruits and vinegar fall below the point at which bacteria normally grow. It is a common observation that fruits generally undergo mold and yeast spoilage, and this is due to the capacity of these organisms to grow at pH values 40C.Most thermophilic bacteria of importance in foods belong to the genera:BacillusPaenibacillusClostridiumGeobacillusAlicyclobacillusThermoanaerobacter

Molds have wider ranges of temperature than bacteria (like pH, osmotic pressure and nutrient content).Many molds are able to grow at refrigerator temperatures, notably some strains of Aspergillus, Cladosporium, and Thamnidium.Yeasts grow over the psychrotrophic and mesophilic temperature ranges but generally not within the thermophilic range.

Although it would seem desirable to store all foods at refrigerator temperatures or below, this is not always best for the maintenance of desirable quality in some foods.For example, bananas keep better if stored at 1317C than at 57C.A large number of vegetables such potato as are favored by temperatures of about 10C.Temperature of storage is the most important parameter that affects the spoilage of highly perishable Foods.The rate of spoilage of fresh poultry :at 10C is about twice that at 5Ca15C is about three times that at 5C

2. Relative humidity of environmentThe RH of the storage environment is important both from: The standpoint of aw within foods The growth of microorganisms at the surfaces When foods with low aw values are placed in environments of high RH, the foods pick up moisture until equilibrium has been established. Likewise, foods with a high aw lose moisture when placed in an environment of low RH. Relationship between RH and temperature:The higher the temperature, the lower the RH, and vice versa.Surface spoilage: In the refrigerator, surface spoilage occurred before deep spoilage occurs in food such as improperly wrapped beef cuts.Why?High RH of the refrigeratorMeat-spoilage biota is generally aerobic (deep sections of meat have often low O2).

3. Presence and concentration of gasesCarbon dioxide (CO2) is the single most important atmospheric gas that is used to control microorganisms in foods.O2 and O3 (Ozone) are important in modified atmosphere packaged (MAP) foods.O3 :has antimicrobial properties.strong oxidizing agent.should not be used on high-lipid-content foods since it would cause an increase in rancidity. is recognized GRAS (generally recognized as safe) in Australia, France, and Japan and USA.

4. Presence and activities of other microorganismsSome foodborne organisms produce substances that are either inhibitory or lethal to others; These include: Antibiotics Bacteriocins Hydrogen peroxide Organic acids