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Regulation, Risk and Return: A Food Systems Approach to Dairy Product Safety

Jayarao, Bhushan; Demirci, Ali; Jaenicke, Edward (Ted); Floros, John
Pennsylvania State University
Start date
End date
The seven specific objectives of the proposed research can be classified as Pre-Harvest Milk Safety, Post-Harvest Dairy Product Safety and Post-Process Dairy Product Safety. Nevertheless, it should be recognized that some of the objectives include more than one area.

Specific objectives include:

  1. Determination of the mechanisms by which predominating Staphylococcus aureus genotypes are able to resist neutrophil phagocytosis and killing
  2. Development of a farm-based systems approach for improving pre-harvest milk quality and safety, measurement
  3. Model development and validation of temperatures and fluid flow in HTST regenerative heating systems
  4. Measurement of microbial growth and toxin formation in HTST regenerative heat exchangers during extended runs
  5. Evaluation of novel technologies for milk pasteurization and enterotoxin detection
  6. Evaluation of how particular changes in production or processing technologies will affect consumer demand for dairy products
  7. Evaluation of the potential for economic benefits and costs associated with meeting stringent milk safety requirements that may accompany alternative milk processing procedures.
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NON-TECHNICAL SUMMARY: As the dairy industry of the United States becomes more concentrated in terms of both production and processing activities, the safety of the entire production and processing and distribution chain, that is the dairy food system, must be clearly understood. This project seeks to understand the influence of pre-harvest, post-harvest and post-processing factors on the safety and consumer perception of safety of milk and dairy products. In the area of pre-harvest milk safety this project will lead to a greater understanding of how Staphylococcus aureus organisms cause mastitis, and to a farm level approach to elimination of pathogenic bacteria in milk. Post-harvest goals are to understand heat and mass transfer in pasteurization systems, to determine the potential for growth and toxin production during processing of milk and development of novel methods for pasteurization of milk. Post-process activities will focus on determining how consumers perceive various processing technologies with respect to safety and will also evaluate the potential economic benefits of novel processing strategies.

APPROACH: Pre-harvest objectives will be achieved by aanalysis and comparison of cell surface and secreted protein profiles of common and rare Staphylococcus aureus genotypes isolated from field cases of bovine mastitis. Proteins unique and selectively synthesised by most prevalent genotypes of Staphylococcus aureus to will be isolated identified. To determine the incidence and routes of microbial contamination on the farm, a survey will be conducted to determine the prevalence and distribution of food borne pathogens in lactating cattle and bulk tank milk. Results of the survey will be used to identify farm management practices related "risk factors" that lead to contamination of bulk tank milk with foodborne pathogens. Post-Harvest objectives focus on understanding the implications of extended process run times to the safety of milk. A predictive model will be developed by measuring the temperature profile of fluid milk within a plate heat exchanger for realistic thermal and fluid flow property estimation. This data will be used to generate a generalized time-dependent model lumped mathematical model for describing the fluid milk flow and temperature variation within the plate heat exchanger. This model will integrate microbiological growth models with the dynamic temperature computational model to simulate S. aureus growth and toxin development during extended process runs. During simulated extended time processing runs, HTST process sensor fouling will be monitored. Finally, microbial growth in the regenerative heating section of HTST pasteurization systems during extended processing runs (greater than 24 h) will be monitored to determine these procedures result in increased risk of S. aureus enterotoxin formation in the regenerative heating section of HTST pasteurization system. In addition, alternative non-thermal pasteurization techniques including pulsed UV-light sterilization and infrared heating systems will be evaluated for efficacy. Post-process milk objectives seek to quantify how consumer demand for particular dairy products relates to attributes of safety and quality. Evaluation of consumer mental models of traditional milk products when compared to milk processed using newer technologies and verification of mental models using realistic product exposure coupled with a preference survey will be employed. In addition, the relative benefits of private, incentive-based contracts between processors and farmers as a way of improving economic efficiency and effectiveness while ensuring milk safety will be examined as will potential firm-level economic benefits of extended run processing against the economic risks of failing to meet stringent milk safety requirements.

PROGRESS: 2002/07 TO 2005/06
Milk temperatures and fouling deposits at multiple locations in channels between the plates of a plate heat exchanger (PHE) system were measured. A simplified heat transfer model provided useful insights into heating behavior of milk in the PHE system. Integrated flow and reaction kinetics equations permitted real time visualization of fouled mass distribution over the plate surface. The model developed enabled optimization of processing conditions. The fouling model was capable of incorporating bacterial survival and growth behavior. Fifteen strains of S. aureus isolated from mastitic cows were characterized and evaluated for enterotoxin production. Growth and toxin production rates were determined for three toxin producing strains in milk at temperatures ranging from 21-45C. These results were used to calculate the possible level of enterotoxin production in the regenerative heating section of a high temperature short time pasteurizer. Inactivation of S. aureus in milk using pulsed UV-light as an alternative to pasteurization was investigated. Log reductions from 0.16 to 8.55 log10 CFU/ml were obtained within 3 minutes. A Continuous flow system was set up and evaluated for inactivation of S. aureus. Milk at 20, 30, or 40 ml/min rate through a quartz glass tube was treated under pulsed UV-light at 5, 8, or 11 cm distance from the UV-strobe. Complete inactivation of S. aureus was obtained in several cases (reduction of 7.09 log10 CFU/ml). Sensory analysis was also conducted. Experiments were also performed using infrared (IR) heating system to inactivate S. aureus in phosphate buffer. IR treatment (lamp temperature: 700C) of 5 ml of phosphate buffer inoculated with S. aureus yielded 0.93, 1.43, and 4.61 log10 CFU/ml reductions for 5, 10, 20 min treatment times, respectively. A surface plasmon resonance protocol was developed to detect S. aureus in buffer systems. Experiments were conducted using poly and monoclonal antibodies to assess the efficacy of the sensor for detection. Studies indicated detection could be done at a sensitivity of 104 CFU/ml. The effect of primary and secondary antibody concentrations on detection was investigated using a surface response model. Specificity of anti-S. aureus to detect S. aureus in a mixture with Escherichia coli O157:H7 was also studied. Both the surfaces could detect S. aureus at levels of 105 CFU/ml in a sandwich assay, with good specificity against E. coli O157:H7. Results indicate that mono and dithiol self-assembled monolayers can be employed for rapid detection of S. aureus. The possible impact of alternative processing technologies on milk consumption and other dairy products were conducted with Chinese consumers (N=60). Participants completed a short food frequency form assessing dairy product intake and a demographic form. Dairy product consumption was low across all groups. The most acceptable products for all participants were fluid milk, ice cream and yogurt but preference for whole vs. reduced fat milk varied by sex. Current flavors and sweetness of ice cream were acceptable. However yogurt was valued for its ease of digestion and tartness.

IMPACT: 2002/07 TO 2005/06
The feasibility of a fouling computational model for PHEs capable of minimizing the occurrence of milk fouling during pasteurization was demonstrated. This approach enables dairy research engineers and dairy plant processors to make the process more cost effective. Coupling the model developed with data regarding growth and toxin production by S. aureus allowed us to determine that even taking a worst possible case approach it was unlikely that sufficient toxin could be produced in the HTST regenerator to lead to illness. Alternative techniques for milk pasteurization may reduce the operational and maintenance costs associated with milk processing. Pulsed UV-light system may be cheaper than and at least as effective as conventional pasteurization method on inactivation of pathogens upon optimization of parameters. The detection methods developed have the potential to be extended to examining pathogens in real food systems. to enhance the safety of milk.To increase intention to eat dairy products, processors need to address Asian taste preferences and packaging characteristics that fit each segments lifestyles.

Funding Source
Nat'l. Inst. of Food and Agriculture
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Escherichia coli
Bacterial Pathogens