Microbial Growth

 Hypothetical bacterial growth curveThere are a number of factors that affect the survival and growth of microorganisms in food. The parameters that are inherent to the food, or intrinsic factors, include the following:

  • nutrient content
  • moisture content
  • pH
  • available oxygen
  • biological structures
  • antimicrobial constituents

 

 

 

 

 

 

 

Nutrient Requirements

 While the nutrient requirements are quite organism specific, the microorganisms of importance in foods require the following:

  • water
  • energy source
  • carbon/nitrogen source
  • vitamins
  • minerals

Milk and dairy products are generally very rich in nutrients which provides an ideal growth environment for many microorganisms.

Moisture Content

 All microorganisms require water but the amount necessary for growth varies between species. The amount of water that is available in food is expressed in terms of water activity (aw), where the aw of pure water is 1.0. Each microorganism has a maximum, optimum, and minimum aw for growth and survival. Generally bacteria dominate in foods with high aw (minimum approximately 0.90 aw) while yeasts and moulds, which require less moisture, dominate in low aw foods ( minimum 0.70 aw). The water activity of fluid milk is approximately 0.98 aw.

pH

Most microorganisms have approximately a neutral pH optimum (pH 6-7.5). Yeasts are able to grow in a more acid environment compared to bacteria. Moulds can grow over a wide pH range but prefer only slightly acid conditions. Milk has a pH of 6.6 which is ideal for the growth of many microoorganisms.

Available Oxygen

Microorganisms can be classified according to their oxygen requirements necessary for growth and survival:

  • Obligate Aerobes: oxygen required
  • Facultative: grow in the presence or absence of oxygen
  • Microaerophilic: grow best at very low levels of oxygen
  • Aerotolerant Anaerobes: oxygen not required for growth but not harmful if present
  • Obligate Anaerobes: grow only in complete absence of oxygen; if present it can be lethal

Biological Structures

Physical barriers such as skin, rinds, feathers, etc. have provided protection to plants and animals against the invasion of microorganisms. Milk, however, is a fluid product with no barriers to the spreading of microorganisms throughout the product.

Antimicrobial Constituents

As part of the natural protection against microorganisms, many foods have antimicrobial factors. Milk has several nonimmunological proteins which inhibit the growth and metabolism of many microorganisms including the following most common:

  1. lactoperoxidase
  2. lactoferrin
  3. lysozyme
  4. xanthine

More information on these antimicrobials can be found in the dairy microbiology textbook by Marth and Steele. See also the discussion on lactoperoxidase in this series at https://www.uoguelph.ca/foodscience/book-page/effects-milk-handling-quality-and-hygiene

Where the intrinsic factors are related to the food properties, the extrinsic factors are related to the storage environment. These would include temperature, relative humidity, and gases that surround the food.

Temperature

As a group, microorganisms are capable of growth over an extremely wide temperature range. However, in any particular environment, the types and numbers of microorganisms will depend greatly on the temperature. According to temperature, microorganisms can be placed into one of three broad groups:

  • Psychrotrophs: optimum growth temperatures 20 to 30° capable of growth at temperatures less than 7° C. Psychrotrophic organisms are specifically important in the spoilage of refrigerated dairy products.
  • Mesophiles: optimum growth temperatures 30 to 40° C; do not grow at refrigeration temperatures
  • Thermophiles: optimum growth between 55 and 65° C

It is important to note that for each group, the growth rate increases as the temperature increases only up to an optimum, afterwhich it rapidly declines.