[Image above] New graphene oxide membranes developed by researchers in Japan and Mexico appear to filter lactose out of milk better than current commercial membranes. Credit: Morelos-Gomez et al., Carbon (CC BY 4.0)
When I think back to my childhood trips to the grocery store, my most vivid memories are not of the times when my parents let me add a rare special treat to the shopping cart. Instead, I remember how we checked the ingredient list on various products every week to ensure the manufacturer had not changed the formulation to include milk.
My dad has lactose intolerance, meaning he is unable to digest lactose, a sugar present in milk. When he was originally diagnosed, lactose intolerance was a rather obscure diagnosis. Now, the condition is widely recognized and is actually very common—experts estimate that about 68% of the world’s population have difficulties digesting lactose.
The increased awareness of lactose intolerance and milk allergy—which is a different digestive problem—has led to the development and improvement of many dairy alternatives, including soy, almond, and coconut products. Lactose-free milk is an option as well (if you do not have a milk allergy).
Lactose-free milk refers to milk with a very low concentration of lactose (some countries require a reduction to less than 0.01% lactose by volume before the milk can be called lactose-free). The main way that lactose-free milk is created is by adding the enzyme lactase to regular milk, which breaks the lactose molecules down into simple sugars that are easier to digest.
Overall, this method is quite effective. However, the process does have some tradeoffs—it increases sweetness, changes coloring, and may decrease the nutritional value of the milk.
An alternative method for creating lactose-free milk is to filter milk using membrane technology. This process typically involves using ultrafiltration to separate most lactose and proteins from the milk and then using nanofiltration to separate just the lactose.
Studies on membrane technology show great potential for filtration. For example, one study in 2020 achieved 100% protein recovery with 10% lactose remaining in the concentrate. But membrane technologies, particularly nanofiltration membranes, do face a big challenge—fouling.
Fouling refers to when proteins clog the membrane and reduce its ability to filter lactose. (A 2018 study suggests that adsorption of proteins is the main mechanism for fouling rather than pore blocking.)
Many current nanofiltration membranes are polymer-based and prone to fouling by organic compounds, such as milk proteins. So new membranes are needed to make filtration a cost-effective method for creating lactose-free milk.
In a recent open-access study, researchers from labs in Japan and Mexico explored whether membranes made of graphene oxide would offer antifouling properties.
Graphene oxide is a highly functionalized 2D material derived from graphite by chemical, thermal, or electrochemical reduction processes. Graphene oxide membranes exhibit nanofiltration level selectivity—the ability to separate molecular constituents from liquids—as well as excellent antifouling properties against oil, surfactants, sodium alginate, and proteins.
The researchers fabricated graphene oxide membranes by spray coating graphene oxide onto polysulfone (PS) and polytetrafluoroethylene (PTFE) supports. To evaluate membrane efficiency, they used attenuated total reflection–Fourier transform infrared spectroscopy and thermogravimetric analysis to determine lactose content in the part of the milk that made it through the membrane, called the permeate.
Based on the spectroscopic and thermogravimetric analyses, the researchers determined that graphene oxide membranes—particularly the ones created on PTFE supports—have several advantages compared to current commercial ultrafiltration and nanofiltration membranes.
For one, the graphene oxide membranes more efficiently extracted lactose from the milk, both in terms of concentration and speed of extraction. In particular,
- Concentration—Lactose content in unfiltered milk was estimated to be 7.7 wt.%. Permeate solutions of the PS and PTFE graphene oxide membranes were 1.4 and 4.9 wt.% lactose, respectively, whereas permeate solutions of the nanofiltration and ultrafiltration membranes exhibited 1 and 2.3 wt.% of lactose, respectively.
- Speed of extraction—Using lactose concentration values, the lactose permeate flux for PS and PTFE graphene membranes was calculated to be 0.2 and 2.9 kg m−2 day−1, respectively, whereas that for nanofiltration and ultrafiltration membranes was 0.64 and 1.60 m−2 day−1, respectively.
In addition, graphene oxide membranes exhibited less irreversible fouling than commercial membranes, as well as thinner and less dense removable fouling layers.
To better understand these results, the researchers carried out a molecular dynamics simulation following the filtration experiment. The simulation revealed that the spacing between the graphene oxide sheets and their surface charge led to a weak interaction between lactose and graphene oxide—thus allowing the lactose to diffuse out through the membrane while keeping the milk’s fats and proteins intact. It also revealed that the water naturally present in milk acts as a “driving force” for diffusion, pushing the lactose through the membrane’s nano-sized pores.
“The developed membrane technology allows the use of [graphene oxide] membranes in the dairy industry for lactose removal and concentration of fat and proteins, with better performance than that of typical polymeric [nanofiltration] membranes,” the researchers write. “This unique selectivity can help preserve the original milk flavor, and the developed technology is expected to remove sugars from other beverages to improve the nutrition content of diverse commercial products.”
The open-access paper, published in Carbon, is “Graphene oxide membranes for lactose-free milk” (DOI: 10.1016/j.carbon.2021.05.005).