Dairy processing plants commonly use the reverse-osmosis filtration technique to remove water from milk for further processing, such as for cheese or milk powder. However, many resources would be saved if it was possible to move this process to dairy farms, since the amount of water to be transported would then be reduced.

In cooperation with Danish dairy company Arla, doctoral student Ida Sørensen and associate professor Lars Wiking from the department of food science at Aarhus University in Denmark have examined how milk quality is affected when concentrating the milk is carried out on the farm.

Sørensen explained, "Even though the reverse-osmosis technique is widely used, we only have limited documentation of how it affects milk quality. Milk is a rather delicate raw material, so it is important to identify how key quality parameters such as protein breakdown and level of free fatty acids (FFAs) and total bacterial count are affected. These parameters are important to the taste, smell and look of the final product.

"It is important that on-farm concentration does not deteriorate the quality," she added. "Normally, it is not a problem that the fat globules in the milk are damaged in the process at the dairy (plant), as the milk is subsequently pasteurized. However, when carrying out milk concentration at the farm, the milk is left unpasteurized, and all the enzymes and microorganisms are still there, and therefore, the milk is more delicate."

Sørensen and Wiking analyzed experiments with both the so-called ultra-filtration — which is supposed be gentler on the milk — and with the reverse-osmosis technique — which requires higher pressure on the milk but also retains the lactose that may be an advantage in, for example, milk powder. Neither the total bacterial count, FFA levels nor protein breakdown were negatively affected by reverse osmosis; the concentrated milk could very well be used for both cheese and milk powder, the researchers said.

Analyses also showed that the quality and durability of milk powder made from concentrated milk is the same as for powder made from ordinary milk; in cheese, however, there is a minor difference as to how the enzymes react, Sørensen and Wiking said, and in the experiments, concentrated milk coagulated approximately 10 minutes later than regular milk.

"My theory is that it is more difficult for the enzymes to 'get through,'" Sørensen said, explaining that concentrated milk is thicker — more like coffee cream — which might explain why coagulation takes a little longer.

Interest is significant, but is it worthwhile?

Concentration of milk on the farm — or during transport from farm to dairy plant — is carried out in some regions where both herds and distances are large, like in Texas, for example.

Different models exist as to how on-farm milk concentration may become a reality, the researchers suggested. The farmer may buy the filtration equipment and achieve an additional price for the milk; perhaps the dairy processor could buy, maintain and service the filtration installation, or it could be acquired through some kind of leasing agreement, the researchers said.

Herd size and distance to the dairy plant, in particular, are of major importance when considering resources and profitability, because small installations typically use more power than one large installation, said Sørensen, who just presented the results of the studies at a major conference in Dublin, Ireland.

"While farmers have shown significant interest in the idea, the dairies seem more skeptical, especially in relation to finances as well as the handling of this new type of milk. Arla is currently making calculations in relation to the profitability of this project, as energy consumption and investments should be balanced in proportion to cost savings," she said.

"New Sustainable Milk Concentration Technology for Dairy Herds" is a five-year project that ends this year. Project participants include Arla, the Danish Cattle Research Center and Aarhus University. In addition, GEA Process Engineering is affiliated as an external consultant.