Quality control

Dairy Microbiology

Laboratory analysis
Laboratory Tests
Testing Methods
Microorganisms: Bacteria, Yeasts, Moulds
Microbial Growth
Detection and Enumeration
Microorganisms in Milk: Spoilage & Pathogenic Microorganisms
Starter Cultures: Mesophilic and Thermophilic
Starter Cultures

Some are desirable but many are undesirable and can cause illness disease and even death. Bacteriophages attach the desireable organisms and can "kill" the product by stopping fermentation.

Many manufacturers as a routine will rotate their starter culture strains to minimise the risk of a phage attack.
Bacteria identification

Bacteria are single-celled organisms.

Cocci: spherical shape 0.4 - 1.5 µm in diameter

Eg: staphylococci - form clusters & streptococci - form chains
( staph aureus commonly found in the nose)

Rods: cylindrical shape 0.25 - 1.0 µm and 0.5 - 6.0 µm long
Eg: bacilli - straight rod

spirilla - spiral rod

Some bacteria when threatened by heat, cold, detergents, lack of moisture and nutrients etc form spores (sporulation) which are resistant to extremes of environmental conditions. When conditions suitable for growth return then the spores can re-germinate and begin to multiply
Depending on the products being manufactured there are varying spores that can cause difficulties. Milk powders are susceptible to Thermoduric spores and care has to be taken in plant design and processing to ensure that spores are not give the opportunity to multiply during processing.

i.e. in evaporators wher production runs can take up to 30 hours and temperatures in the range for thermophile germination particularly at times of high spore counts when animals are kept indoors and fed on sileage.

Bacteria reproduce asexually by division of the cell. The doubling is as short as 20 minnutes.

Viable bacterial populations are expressed as colony forming units (CFU) per gram or millilitre.

Bacterial growth:

Lag phase: time for microorganisms to become accustomed to their new environment. There is little or no growth during this phase.
Log phase: bacteria logarithmic growth begins; the rate of multiplication is the most rapid and constant.

Stationary phase: the rate of multiplication slows down due to lack of nutrients and build-up of toxins. At the same time, bacteria are constantly dying so the numbers seem to remain constant.
Death phase: the number of organisms decreases as growth stops and cells die off. The speed of change depends on the environmental conditions.


Yeasts are microorganisms called fungi

Their size varies greatly but are usually larger than bacterial cells.
Unlike bacterial spores, yeast form spores as a method of reproduction.

Yeasts are usually associated with rapid fermentation and thrive in warm damp conditions which is why food manufacturing plants whereever possible should be designed as dry areas.

Dairy products are extremely susceptible to yeas and mould contamination.


Moulds are filamentous, multi-celled fungi with an average size larger than both bacteria and yeasts (10 X 40 µ m). Each filament is referred to as a hypha. Moulds reproduce asexually or sexually, sometimes both within the same species.
Asexual Reproduction:
The hyphae separate into individual cells which are form in the tip of the hyphae or in swollen structures called sporangium
Sexual Reproduction:
Sexual spores reproduce by nuclear fission during unfavourable environmental conditions.

Microbial Growth

Growth is dependant upon a number of basic factors
  • nutrient content
  • available water / moisture content / osmotic pressure
  • pH / acidity
  • oxygen / inert atmosphere
  • biological structures / other organisms
  • antimicrobial constituent / natural inhibitors
Nutrient requirements of microbes:
  • water
  • nutrients

Milk and dairy products provides an ideal growth medium for microorganisms.

Food products are generally designed to preserve the food from spoilage organisms.
Most microorganisms have a neutral pH of 6-7.5.

Yeasts are able to grow in a more acidic environment.

Moulds grow prefer slightly acid conditions.

Milk has a pH of 6.6 which is ideal for the growth of many micro-organisms.

Available Oxygen: Micro-organisms are classified according to their oxygen requirements required for growthl:

Aerobes / Aerobic: oxygen liking / loving
Facultative: will multiply with and without oxygen
Microaerophilic: like low levels of oxygen
Anaerobes / Anaerobic: prefer no Oxygen but survive with Oxygen
Obligate Anaerobes: grow only in complete absence of oxygen; if present it can be lethal

Physical barriers such as skin or peel, shell, husk etc. provide protection to plants and animals against attack by microorganisms. Milk, however, has some natural protection with aAntimicrobial constituents / flora which is mostly destroyed by heat treatment, Pasteurised milk under the same temperature conditions as raw milk may deteriorate more rapidly.

Milk has several nonimmunological proteins which inhibit the growth and metabolism of many microorganisms including the following most common:

Psychrotrophs / Psychotrophic organisms / cold liking: optimum growth temperatures 20 to 30° capable of growth at temperatures less than 7° C and are involved in the spoilage of refrigerated dairy products.

Mesophiles / Mesophillic organisms: optimum growth temperatures 30 to 40° C and do not grow at refrigeration temperatures.

Thermophiles / thermophillic: optimum growth between 55 and 65° C the growth rate for each type of organism increases as the temperature increases up to an optimum, afterwhich it rapidly declines.
Detection and Enumeration of Microorganisms

There are many methods for detection and enumeration of microorganisms in food.

Direct Enumeration:

Using direct microscopic counts (DMC), allows a rapid estimation of all viable cells.

Viable Enumeration:

The use of standard plate counts, most probable number (MPN), membrane filtration, plate loop methos, spiral plating etc., allows the estimation of only viable cells.

Metabolic Activity Measurement:

An estimation of metabolic activity of the total cell population is possible using dye reduction tests such as resazurin or methylene blue dye reduction, acid production, electrical impedence etc. The level of bacterial activity can be used to assess the keeping quality and freshness of milk. Toxin levels can also be measured, indicating the presence of toxin producing pathogens.

Cellular Constituents Measurement:

The luciferase test to measure ATP is a rapid tests available that will indicate the presence pathogenic bacterial cells.

Rapid tests are now widely used in swab tests on equipment and surfaces prior to use.

Microorganisms in Milk
Mastisis bacteria at the point of milking are few in number. Gross contamination of milk by microorganisms can take place during milking, handling, storage.

Some examples of organisms present in milk are:

  • Streptococci
  • Lactococci
  • Lactobacilli
  • Leuconostoc
  • Coliforms / Coli: coliforms are facultative anaerobes with an optimum growth at 37° C. Coliforms are indicator organisms; which closely associated with poor hygiene. They cause rapid spoilage of milk by fermenting lactose and producing acid and gas. Coliforms are killed by heat treatment. Escherichia colii s E0157 strain belongs to this group and is associated with faecal contamination.

Some species and strains of Bacillus, Clostridium, Cornebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus , and Streptococcus can survive heat treatment and grow at refrigeration temperatures which can cause spoilage problems.

Pathogens associated with milk and dairy products:

Bacillus cereus
Listeria monocytogenes
Yersinia enterocolitica
Salmonella spp.
Escherichia coli O157:H7
Campylobacter jejuni

Starter Cultures

Micro-organisms used in the production of cultured dairy products such as yogurt and cheese.

The primary function of lactic starters is the production of lactic acid from lactose and starters are selected on the basis of the temperatures of processing and flavours and acidity etc resulting.

Starter cultures are categorized as mesophilic or thermophilic:
Mesophilic / cold liking

Lactococcus lactis subsp. cremoris
L. delbrueckii subsp. lactis
L. lactis subsp. lactis biovar diacetylactis
Leuconostoc mesenteroides subsp. cremoris


Streptococcus salivarius subsp. thermophilus (S.thermophilus)
Lactobacillus delbrueckii subsp. bulgaricus
L. delbrueckii subsp. lactis
L. casei
L. helveticus
L. plantarum

Probiotic Starter cultures are specific cultures which are known to provide health benefits in the gut of the consumer

Starter cultures are generally Freeze dried


Bacteriophages are viruses that require bacteria host cells for growth and reproduction. The bacteriophage attaches itself to the bacterial cell wall and injects nuclear material into the cell. Inside the cell, the nuclear material produces shells, or phage coats, for the new bacteriophage which are quickly filled with nucleic acid. The bacterial cell ruptures and dies as the new bacteriophage are released. Bacteriophages generally enter the milk processing plant from farm milk / tankers. They are inactivated by heat treatments or by the use of chemicals. Bacteriophages concern issue in cheese making as they attack the desireable starter cultures ans may result in a slow of complete vat failiure Phages generally multiply in the whey so handling and storage of whey is critical in a cheese manufacturing plant.

Manufacturers supply starter cultures in liquid, freeze-dried, frozen or spray-dried.

Bulk starter culture - Many manufacturers still use bulk starter cultures but with reduced costs of manufacture of freeze dried culture coupled with the security and consistency and capital cost of equipment, many manufacturers are moving to freeze dried cultures

For more information or to discuss your requirements please contact us.

John Watson
Office: +44 1224 861 507
Mobile: +44 7931 776 499

We are a longstanding member of the Society of Dairy Technology and have Fellowship of the Institute of Food Science and Technology.
Member of the Society of Dairy Technology and have Fellowship of the Institute of Food Science and Technology IOD


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John Watson
Office: +44 1224 861 507
Mobile: +44 7931 776 499

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We are a longstanding member of the Society of Dairy Technology and have Fellowship of the Institute of Food Science and Technology.


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