“Why Membranes?”

Welcome to the first installment of Membrane Matters, a quarterly column here in Dairy Foods where I am pleased to share my thoughts about the ongoing, special relevance of spiral crossflow membranes in the production of dairy foods and ingredients.

My plan for this 2026 series is to begin with some basic information about spiral membranes, describing the most basic principles of how and why they work in this first installment; and then progressing into some deeper, more specific explorations later in the year, showcasing in greater detail some select applications where membranes make special contributions to dairy; and finally, we will wrap the year with some thoughts about the future role of membranes in dairy processing.

If you walk the floor of a dairy plant, you will encounter all sorts of processing equipment: heat exchangers, pasteurizers, cream separators, cheese vats, brine tanks, evaporators, spray dryers, you name it. Odds are, nestled somewhere in that mix, you will also find a bank of membranes, distinguished by their parallel arrangements of long, shiny, tubular stainless-steel vessels, interconnected at the inlet and outlet, concealing a daisy-chained series of membrane modules that are working together as a unit to add value to the operation. What are spiral crossflow membranes? The name itself reveals several important characteristics of the modules themselves:

First, they are constructed by rolling layers of materials tightly around a perforated central tube, resulting in a compact cylindrical unit that has a spiral cross-section. Think of the way that paper towels are rolled around a central cardboard tube, except in this case the central tube is also perforated with circular openings placed at regular intervals.

Second, the flow of fluid material through a membrane module isn’t quite from one end to the other, like liquid through a pipe; rather, the feed for a membrane modules is pumped into the housing vessel around the outside of the unit and flows across the membrane from outside to in, through the cross-section of the spiral-wound layers. A fraction of the feed liquid makes this journey all the way through and into the perforated central tube, to become the permeate stream, while the fraction that does not pass completely through the membrane layers will flow onward through the system as the retentate.

Finally, the key to the module’s processing value comes in large part from the membrane itself, the most critical functional component of the module. The membrane is composed of a very thin layer of a carefully engineered polymer, deposited on the outermost surface of a supporting material, and rolled tightly along with a polymer spacer around that central permeate tube. While not visible to the naked eye, the polymer flat sheet is dotted with microscopic pores whose exact dimensions, pore size distribution, and other key characteristics determine the exact kinds of applications where the membrane can be useful. It is this special polymer material that works the magic of a crossflow membrane module.

Why spiral membranes?

Now we come to the purpose of using spiral membranes: separation. Plain and simple. Dairy processors use membranes to separate one or more constituents of a fluid stream from others, and it is by this simple act of separation that value is added, in a myriad of ways.

One of the simplest and yet greatest ways that membranes can deliver value is by reduction of water. Fluid milk itself is 87% water, and for purposes of manufacturing most dairy products or ingredients, removing some or almost all of this water is a major (if not the biggest) single processing cost.

Evaporators are a reasonably efficient way to remove water from milk on its way to a spray dryer to become milk powder, but energy to operate the evaporators is increasingly costly. The water removal load that must be handled by the evaporators can be optimized by first removing some of that water using membranes, which demand relatively little energy in comparison. One or more stages of specific membranes, installed before the evaporators, keeps those high-energy units fed continuously while reducing the water load by half or more, resulting in substantial energy savings.

A potential bottleneck in the manufacture of cheese is the level of milk solids that can be achieved at the vat. Membranes are often used to raise the milk solids simply by removing water, allowing the same size vat to yield more pounds of curd, increasing plant capacity without installing expensive, space-consuming new vats. And milk isn’t necessarily consistent in composition from farm to farm or season to season, meaning that vat solids can vary to an undesirable degree, resulting in inconsistent cheese yield and quality. Membranes can be employed to standardize the input, resulting in a much more consistent process and product.

Water separation opportunities abound, but the separations capabilities of membranes really start to shine when you consider the amazing range of their suitability for other purposes.

Remember that the membranes have pores, and that the size and size distribution of those pores is carefully controlled. This means that membranes can be purpose-made to accomplish a wide range of separations, depending in large part on that carefully engineered porosity.

Microfiltration (MF) membranes reject only the very largest of structures found in fluid, such as bacteria, milkfat globules, and casein micelles. They can be used to recover/regenerate salt brine in cheesemaking, recover milkfat globules for production of whey protein phospholipid concentrate (WPPC), or to separate and concentrate micellar casein and native whey.

Ultrafiltration (UF) membranes have a smaller pore size than MF, allowing lactose and minerals to pass through while retaining everything else. These modules are especially useful in raising and standardizing cheese vat solids, separating and reducing lactose content, or producing high-protein ingredients such as milk protein and whey protein concentrates and isolates.

Nanofiltration (NF) membranes are “tighter” still, passing monovalent (single charge) mineral ions while retaining everything else. Sometimes described as “loose RO,” these are used to manufacture specialized ingredients such as demineralized whey for infant formula applications; in conjunction with other membranes for reducing or removing lactose; and for recovery of costly clean-in-place (CIP) agents for multiple cycles of reuse.

Finally, reverse osmosis (RO) membranes have such small pores that only water molecules can pass, concentrating everything else in the fluid stream (such as for evaporator feed as described above). They excel at the exclusive removal of water, or the purification of water for further use/reuse at the plant, when the highest degree of specificity or purity is demanded.

This concludes the crash course in basic membranes technology! If you take away anything from this month’s column, the answer to the question “why membranes?” is that spiral crossflow membranes are incredibly versatile, providing specific, cost-effective solutions to many of dairy’s special processing challenges. Next time, join me where I will add some further details to the tech and where we will begin to examine a handful of use cases where membranes are driving bottom-line value. DF

Andy Powers is vice president of technical services at the American Dairy Products Institute, where he applies over 30 years of technical, commercial, and scientific experience to manage ADPI’s library of Ingredient Standards and supporting Analytical Methods, along with his liaison role for three of the organization’s standing committees and his position on the Center of Excellence.

Andy earned his bachelor’s and master’s degrees in molecular biology and business administration from the University of Illinois at Urbana-Champaign, and he holds various certifications in international quality systems and U.S. food safety. Andy is a member of several groups and committees supporting the dairy industry including the U.S. Technical Advisory Group to ISO / TC 34 / SC 5 on Milk and Milk Products and the IDF/ISO Standing Committee on Analytical Methods for Composition; and he is a member of the American Society for Quality (ASQ), the American Chemical Society (ACS), and AOAC International.

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