Microorganisms require a specific range of conditions to grow, including nutrients such as nitrogen, carbohydrates, minerals, and vitamins, as well as a specific temperature, water activity (aw), pH, and the presence of oxygen. In addition to using preservative measures, it's essential to process food products using high-quality raw materials and the highest hygienic conditions possible to minimize the initial microbial load and potential post-contamination.
 
 
Packaging (air/oxygen)
Aerobic microorganisms require oxygen to grow, while anaerobic and facultative anaerobic microorganisms can grow even when oxygen is absent or present in a very small amount. 
Temperature (T)
 
Temperature plays a big role in the preservation of food. Temperature increase leads to increased activity of microorganisms and enzymes. As a rule of thumb, for every increase in temperature by 10 °C, enzyme activity increases two-fold. On the other hand, decreasing temperature, by storage at refrigeration (4 to 6 °C) and freezing (-15 to -20 °C) conditions, results in growth inhibition of most of microorganisms as well as in the reduction of enzyme activity. Besides, the thermal resistance/sensitive of microorganisms determines the need for other hurdles. While most microorganisms and enzymes are destroyed or inactivated at temperatures above 60 °C, heat-resistant fungi and spore-forming bacteria can easily survive extreme heat treatments (> 90°C).
pH
 
Most microorganisms grow optimally around neutral pH (6.5 to 7.5). In contrast, some of them are classified as acidophilic as they require low pHs (<4) to optimally grow. As a general guideline, molds, yeasts and bacteria can grow on the following ranges of pH:
 
Table 1. pH range and microbial growth
 
pH Range
 
Microorganisms
1.5-11.0
Molds
1.5-9.0
Yeasts
3.6-9.5
Salmonella
4.2-9.6
Listeria monocytogenes
4.2-9.0
Yersinia enterocolitica
4.3-9.0
Escherichia coli
4.3-8.5
Clostridium botulinum
5.0-9.5
Bacillus cereus
5.0-9.0
Campylobacter
5.0-9.2
Shigella
5.0-11.0
Vibrio parahaemolyticus
5.0-9.5
Vibrio Cholerae
5.0-8.5
Clostridium refringent
2.0-6.0
Alicyclobacillus acidoterrestris
 
Water Activity (aw)
 
Water activit value (aw) is a measure for water available for microbial growth. A low aw will decrease microbial growth. A general guidelines for minimum aw values for microbial growth is presented as followed:
 
aw Levels
 
Effect
Above aw 0.90
Bacteria can grow          
Above aw 0.88
Yeasts can grow         
Above aw 0.80
Molds can grow     
 
Table 2 aw level and microbial growth
 
aw Levels
 
Effect
0.50 
No microbial proliferation 
0.60 to 0.65 
Osmophilic yeasts (S. rouxii), Few molds (A. echinulatus, Monascus bisporus
0.65 to 0.75 
Xerophilic molds (A. chevalieri, A. candidus, Wallemia sebi), S. bisporus
0.75 to 0.80 
Most halophilic bacteria, Mycotoxigenic aspergilli 
0.80 to 0.87 
Most molds (Mycotoxigenic penicillia), S. aureus, Saccharomyces, Debaromyces
0.87 to 0.91 
Many yeasts (Candida, Torulopsis, Hansenula, Micrococcus
0.91 to 0.95 
Salmonella, C. botulinum, Serratia, Vibrio parahaemolyticus, Lactobacillus, Pediococcus, some molds and yeasts (Rhodotorula, Pichia
0.95 to 1.00 
Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, C. perfringens, some yeasts