Bacteria related to foodborne illness are destroyed by proper pastaurization.  Recent outbreaks of salmonellosis and listeriosis in pasteurized milk have been linked to post-pasteurization contamination.   Post-pasteurization contamination with L. monocytogenes and Yersinia enterocolitica is of major concern to the dairy industry since these organisms grow at refrigeration temperatures.     

Recommended tests:  

Aerobic plate count.
Total coliform count.  

Additional tests:

Psychrotrophic count
Moseley Keeping Quality test
Salmonella.
Listeria.
Campylobacter.
Yersinia.

Dried products are often used as ingredients of other foods and are subject to further processing.  Yet, dried milks are considered sensitive products from a public health aspect because they are often consumed after reconstitution without additional heating.  It is well known that dried milk can be a source of foodborne illness because of contamination with Salmonella (environmental contamination) and Staphylococcus intoxication (due to growth of this microorganism and toxin production prior to the heating processing).  Because of their low water activity, dried products rarely spoil or deteriorate because of microbial growth.  Improper storage of dry milk will promote the growth of molds.  Molds can cause discoloration, musty flavors, breakdown of milk components resulting in off-flavors, and possible production of mycotoxins.

Recommended tests:  

Aerobic plate count.
Total coliform count.  
Yeast and mold count.

Additional tests:

Thermoduric count.
Psychrotrophic count.
Aerobic spore count.
Salmonella.
Staphylococcal enterotoxin.

Butter must contain 80% milkfat and can be salted or unsalted.  Butter is manufactured by creating a water-in-oil emulsion.  Storage stability is provided by the addition of salt, which can result in a salt-in-water concentration as high as 16%.  The microflora of butter reflects the quality of the crea, the sanitary conditions of the equipment and the environment. Bacteria such as Pseudomonas and Lactococcus can cause rancid, putrid, malty and fishy off-flavors.  Yeast and molds such as Geotrichum and Candida can develop on the surface causing discoloration and flavor problems.  Butter has been implicated in an outbreak of food poisoning caused by staphylococci.

Recommended tests:  

Aerobic plate count.
Total coliform count.  
Yeast and mold count.

Additional tests:

Lipolytic count.
Proteolytic count.
Psychrotrophic count.
Salmonella.
Staphylococcus count

Frozen dairy products include ice cream, sherbet, novelties, and frozen yogurt.  The microbial content of frozen dairy products reflects the quality of the ingredients used for their manufacture.  In addition, post-pasteurization contamination can occur from poorly cleaned equipment, air incorporation, poor use of product rerun, and personnel.    Although there is no growth in frozen dairy products, many types of bacteria, including pathogens, can survive.  Ice cream has been implicated in Salmonella infections and staphylococcal intoxications.  Failure to store ice cream scoops in flowing water allows the accumulation of ice cream solids which provides a rich environment for bacterial growth.  All bulk ingredients used in manufacturing frozen dairy products should be tested before use.

Recommended tests:  

Aerobic plate count.
Total coliform count.  
 

Additional tests:

Yeast and mold count.
Thermoduric count.
Salmonella.
Listeria.
Campylobacter.
Yersinia.

Concentrated milk products include evaporated milk, concentrated milk, sweetened condensed milk, and condensed sour products.  They are manufactured by the removal of moisture by heat and differ in the amount of heat treatment given during processing, the degree of concentration, and in the ingredients added to increase shelf stability or to influence the characteristics of the final product.  The microflora of plain concentrated milk products will consist of gram-positive thermoduric bacteria that survive pasteurization and the heat treatment applied during concentration.  The presence of psychrotrophic bacteria, coliform bacteria, yeast and most molds in these products indicates contamination of the product during or after manufacturing.  

Sweetened condensed milk must contain 28% total milk solids and a sugar cncentration enough to prevent spoilage (42-43% sucrose).  The main type of spoilage is caused by osmophilic sucrose-fermenting yeasts such as Torula spp. and molds such as some Aspergillus  and Penicillium spp.  The yeast and mold content of the sucrose used as an ingredient must be monitored.  Evaporated milk is sterilized in a can by batch or continuous retort procedures and is considered a commercially sterile, low-acid food.  Spoilage occurs from inadequate heat treatment or from leakage of the cans after packaging.  Spoilage by Bacillus stearothermophilus may occur if the product is stored at abnormally high ambient temperatures.

Recommended tests:  

Aerobic plate count.
Total coliform count.  
Yeast and mold count.

Additional tests:

Thermoduric count.
Thermophilic count.

Fermented dairy products are cheese, yogurts, and fermented milks.  They are fermented by the addition of a starter culture such as lactic acid bacteria responsible for the production of lactic acid from lactose that gives the product its flavor characteristics.  

In cheese production, the initial microflora of the raw or heated milk is rapidly overshadowed by the active started microorganism(s).  The primary functions of the starter culture(s) include production of lactic acid, which promotes curd formation in conjunction of coagulating enzymes such as rennet, lowering of the redox potential, and destruction or prevention of the growth of pathogens and spoilage microorganisms.  They also contribute to the flavor characteristics of the product.  Microbial spoilage in cheese is limited because of the combined effects of acid and salt, and is less likely in the lower moisture cheeses.  The presence of heat-stable enzymes from psychrotrophic bacteria can be detrimental to the quality of both fresh and ripened products by causing bitter or rancid flavors.  Fresh cheeses such as cottage cheese may be subject to spoilage by gram-negative psychrotrophic bacteria (Pseudomonas, Flavobacterium, Alcaligenes), coliforms, yeast, and molds that enter as post-pasteurization contaminants.  Ripened cheese is prone to surface growth of yeast and molds, particularly if exposed to atmospheric oxygen.  A slow starter culture can allow growth of foodborne pathogens such as Staphylococcus, Salmonella, Listeria, and enteropathogenic E. coli which enter with raw milk or as post-pasteurization contaminants.

Yogurt, fermented milks (buttermilk), and cultured cream (sour cream) are unripened, cultured dairy products.  In a normal fermentation, a final pH of < 4.5 is developed.  This low pH generally prevents the growth of most spoilage and pathogenic organisms.  Microorganisms that cause deterioration can enter products through poor sanitation techniques or can be introduced by the addition of flavoring materials and other ingredients.  Yeast and molds that tolerate lower pH are the more predominant spoilage organisms.  Bacillus subtillis and b. cereus can cause bitter flavors if large numbers survive pasteurization.  Yeast, molds and coliforms may be used as indicators of poor sanitation.  Enterococci are probably better indicators of improper sanitation than coliforms since they are less sensitive to high acid conditions.   

Recommended tests:  

Aerobic plate count.
Total coliform count.  
Yeast and mold count.

Additional tests:

Psychrotrophic bacteria count.
Staphylococcus enterotoxin.
Salmonella.
Campylobacter.
Listeria.
Yersinia.

Adapted from:

Richter, R. L. and Vedamuthu, E. R.  2001.  Milk and Milk Products, p. 483.  In F. P. Downes and K. Ito (eds.), Compendium of Methods for the Microbiological Examination of Foods.  American Public Health Association, Washington, DC.