Improving the safety and shelf-life of milk using a membrane separation strategy
CALS Impact Statement
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 understand the factors that limit the effectiveness of microfiltration and to evaluate the effect of this process on the quality and shelf life of milk. Membrane fouling was identified as the main factor responsible for flux reduction. This suggested that the destabilization of the fouling layer is the appropriate solution for increasing the yield of the microfiltration process. To evaluate the potential of microfiltration to increase milk shelf life, a 14-week refrigerated shelf life study was conducted, in which the rate of proteolysis and microbial growth were quantified. The microbial counts for microfiltered and high-temperature short-time (HTST) pasteurized milk remained close to zero for 14 weeks, while for HTST pasteurized samples the counts exceeded the Pasteurized Milk Ordinance (PMO) limit after about 40 days of storage. Raw, microfiltered milk exceeded the PMO standards after 21 days, and raw milk after only seven days. Despite the low microbial counts, a significant degree of proteolysis was observed in the microfiltered and HTST pasteurized milk. Overall, microfiltration led to a significant increase in microbial shelf life of milk. The observed proteolysis is a limitation that needs to be further investigated.
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 continues 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 to 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 longer transportation and storage of raw milk, benefiting New York milk producers. Cold microfiltration could also benefit processors, by minimizing microbial fouling of the microfiltration membranes.
This research focused on elucidating the reasons for membrane fouling in the cold microfiltration of raw skim milk, which could help make this process economically attractive. The mechanisms responsible for the membrane fouling in cold microfiltration of skim milk were investigated using a range of physical and chemical methods. It was found that fouling by proteins and the high viscosity of the fouling layer lead to low fluxes in cold microfiltration. This suggested that destabilization of the fouling layer is the appropriate solution for increasing the yield of the process. To evaluate the potential of microfiltration to increase milk shelf life, a 14-week refrigerated shelf life study was conducted. The rate of proteolysis and microbial growth were quantified. The microbial count for microfiltered and high-temperature short-time (HTST) pasteurized samples remained close to zero for the entirse duration of the study. For HTST pasteurized milk, the counts exceeded the Pasteurized Milk Ordinance (PMO) limit after 40 days. Raw, microfiltered milk exceeded the PMO standard after 21 days, and raw milk after only seven days. For raw and pasteurized milk, the degree of proteolysis was correlated with microbial growth. For microfiltered and pasteurized milk, the degree of proteolysis was significant, despite the very low microbial counts. Further studies will focus on understanding the reasons for the observed proteolysis in the milk subjected to both microfiltration and pasteurization.
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 techniques have the potential to make raw skim milk microfiltration an economically feasible process. The results of this work will also help understand the mechanism of fouling in cold milk milk fat (MF), which is critical for the development of an efficient MF 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. This process will also benefit dairy processors by minimizing microbial fouling of membranes. This will reduce cleaning costs and increase the life time of membranes, which are critical aspects of operating membrane separation systems in processing plants.
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New York State Department of Agriculture and Markets