Raw Milk Quality

Test Procedures for Identifying Sources of Heat-Resistant Psychrotrophic Bacteria

plant-app-bag-samplingA challenge for the dairy processing industry is to monitor and control heat-resistant bacteria. These bacteria are significant for the fluid milk industry because they survive pasteurization and grow at refrigeration temperatures, causing product defects.

Heat-resistant psychrotrophic bacteria in milk were first recognized by Mikolajcik in the 1970’s. He and his fellow researchers pointed out that these bacteria are capable of causing product defects in fluid milk products.

Recent changes to the dairy industry have created situations favorable for the growth of heat-resistant psychrotrophs. The dairy industry has improved plant sanitation procedures, improved equipment and continued regulatory involvement and effective quality control and quality assurance programs. These have drastically reduced post-pasteurization contamination in fluid milk processing plants. This has enabled the dairy industry to increase shelf life (code).

With the extended codes and control of post-pasteurization contamination, heat-resistant psychrotrophs are able to grow resulting in product defects. The growth of heat- resistant psychrotrophs result in product defects in 18-21 days.

To further improve dairy product quality, fluid milk processors need to determine what effect heat-resistant Psychrotrophs have on their products and to identify sources of these bacteria.

To determine the presence and effect these bacteria would have on dairy product quality, QMI suggests the following procedures:

  1. Using the QMI 2 Liter Composite Bag and Aseptic Sampler, aseptically obtain a 2L sample of product at the High Temperature Short Time (pasteurizer),
  2. Incubate the bag for 18-24 days (2 days beyond code) at 45ºF,
  3. Conduct a Standard Plate Count, and
  4. Identify the bacteria using gram-staining procedures for any counts greater than 1,000,000/ml (gram positive bacteria will indicate heat resistant psychrotrophs).


composite-bag-in-water-bath-cropTo determine the source of heat resistant psychrotrophs, QMI suggests the QMI Heat Resistant Psychrotroph Test. This test involves:

  1. Aseptically collecting a sample of raw milk using the 250ml QMI Bag and the QMI Aseptic Sampler,
  2. Lab Pasteurize the sample in the bag at 75ºC for 20 minutes,
  3. Place in a 45ºF incubator, and
  4. Determine Standard Plate Counts at the end of code. Identify bacteria using gram-staining procedures.


QMI speculates that the primary source of heat-resistant psychrotrophic bacteria is from raw milk handling equipment. Several factors lead us to believe that:

  1. Bacillus bacteria, the primary organism involved, tend for form biofilms,
  2. The cold environment of the raw milk handling equipment favors psychrotrophic bacteria,
  3. Stress, such as the removal of reduction of nutrients and water can cause the bacteria to sporulate,
  4. Effective sanitation of raw milk handling equipment is often neglected,
  5. The high humidity conditions favor sporulation, and
  6. Contamination rates as low as 1 bacteria per liter could result in quality defects in pasteurized milk.


Table 1 shows data collected by DQCI using the QMI Heat-Resistant Psychrotroph Test:


While this is not a complete study, it does indicate heat-resistant psychrotrophic bacteria can be found in milk samples taken from farm bulk tanks and plant raw milk silos.