The physical removal of microorganisms and somatic cells from raw milk using membrane separation can significantly improve the safety, quality, and shelf life of milk and dairy products. The objective of this work was to develop a membrane separation process able to efficiently remove microorganisms from milk and extend the microbiological quality and shelf life of raw skim milk. High cross-flow velocities and low transmembrane pressures were conducive to high permeate fluxes and also resulted in nearly complete removal of the raw milk microflora and a composition of the microfiltered milk very close to the composition of the initial raw skim milk. In order to test the sensory acceptance of the microfiltered milk, triangle tests were performed, using a panel comprised of 90 panelists, which found no significant difference between the microfiltered and traditionally processed skim milk. A strategy involving CO2 gas surges was developed in order to further increase the permeate flux. The CO2 gas surging technique resulted in a 20 percent increase in final permeate flux, a 13 percent in the total amount of permeate, as well as a less pronounced decrease in flux after three hours of microfiltration. The use of CO2 had no negative effect on permeate composition or quality. Moreover, permeate samples obtained using this technique and further stored under low CO2 pressure at refrigeration temperatures conserved the integrity of caseins for longer periods of time when compared to samples of permeate simply stored at refrigeration temperatures. We have also continued our efforts to elucidate the mechanisms responsible for the severe membrane fouling in the cold microfiltration of raw skim milk, using a range of physical and chemical methods.

The safety, quality, and shelf life of fluid milk and dairy products is of extreme importance both to consumers and the dairy industry. Pathogenic and spoilage microorganisms contaminate raw milk at various points during and after milking. The microflora in raw milk continue to grow between the dairy farm and the processing plant, particularly if the temperature during transportation and storage of milk is not maintained below 45 degrees Fahrenheit. The existing legislation mandates that milk be pasteurized at the processing plant. While pasteurization kills the harmful bacteria, it is not effective against spores and somatic cells. Also, the dead bacteria left in pasteurized milk can limit its shelf life, due to the activity of the thermally resistant enzymes that they secrete. The physical removal of bacteria and spores using membranes has the potential to avoid these problems and lead to a significant increase in the shelf life of milk and dairy products, particularly if done early in the process, prior to pasteurization. This would avoid the need for excessive heat treatment of the milk, resulting in milk and dairy foods with enhanced freshness and nutritional attributes. Such a treatment would also allow the transportation of raw milk over longer distances, benefiting New York milk producers, who could thus export milk to other areas of the country.

A strategy involving CO2 gas surges was developed in order to counteract the non-uniform transmembrane pressure and to physically disrupt the fouling layer formed at the areas of higher transmembrane pressure, thus allowing for a more uniform and efficient separation. The surging technique was optimized to gas surges at every minute for duration of 10 - 15 seconds at a pressure equivalent to the inlet pressure, which yielded maximum flux enhancements. The use of CO2 as a processing aid did not lead to any protein denaturation. Moreover, permeate samples obtained using this technique and further stored under low CO2 pressure at refrigeration temperatures conserved the integrity of caseins for longer periods of time when compared to samples of permeate simply stored at refrigeration temperatures. In order to test the sensory acceptance of the microfiltered milk, triangle tests were performed using a panel of 90 panelists. Results of the sensory test indicated no significant difference between the microfiltered and traditionally processed skim milk. The results for the 24-day old samples were not conclusive, and therefore the sensory tests need to be repeated. We have also continued our efforts to elucidate the mechanisms responsible for the severe membrane fouling in the cold microfiltration of raw skim milk, using a range of physical and chemical methods. Preliminary results suggest some preferential retention in the fouling layer of certain milk proteins, which have to be positively identified in the next phase of our research.

The physical, nonthermal removal of bacteria, spores, and somatic cells from raw milk by membrane separation (microfiltration) leads to a significant enhancement of its microbiological quality and shelf life. Optimum process parameters coupled with the developed carbon dioxide (CO2) techniques have the potential to make raw skim milk microfiltration an economically feasible process. Due to its bacteriostatic effects, the use of CO2 is also expected to help extend the shelf life of the microfiltered milk. The results of this work will also help understand the mechanism of fouling in cold milk, which is critical for the development of an efficient process. It is expected that this will allow us to develop a highly efficient microfiltration process that could be used to significantly increase the quality and shelf life of raw milk and dairy products. This will directly impact NYS dairy plants, which will have immediate and direct access to the know-how and technical solutions that result from this research. Overall, this process has the potential to become economically attractive and gain acceptance in the dairy industry for applications such as microbial removal at the farm level for milk being used for fluid milk production as well as milk being used for cheese production.