Improving ultrafiltration efficiency of acidified skim milk using bipolar membrane electrodialysis: Impact on protein concentrate composition, process performance, and fouling

The efficiency of ultrafiltration (UF) of acidified skim milk (SM) is impaired by protein aggregation and mineral scaling. The aim of this study is to assess the potential of acidification by electrodialysis with bipolar membranes (EDBM), in comparison with citric acid (CA), prior to the UF process on filtration performance, fouling and composition of the protein concentrates. Electro-acidification, facilitated by a water-splitting reaction, decreased the pH of milk to ∼ 5.7 and caused partial demineralization (∼21.9 % ash removal), which increased protein concentration and reduced UF fouling. This resulted in a ∼ 34.7 % increase in average permeate flux and ∼ 9.5 % more efficient removal of calcium from the UF retentates compared to CA. The final ash content of the produced protein concentrates showed that the EDBM acidification resulted in an ash content of 5.76 ± 0.23 % on a dry basis, while the citric acid method resulted in an ash content of 6.63 ± 0.27 %, showing a reduction of ∼ 13.1 %. Additionally, electro-chemical and spectroscopic methods were employed to evaluate the ion-exchange membranes (IEMs). Minor changes were observed in the specific resistivity and permselectivity of the cation-exchange membranes (CMs), indicating the formation of fouling and inorganic scaling precipitates on the membrane surface due to the process. The FTIR analysis of both CMs and bipolar membranes (BMs) showed sorption of proteins on the surface. The FTIR and atomic force microscopy (AFM) results of UF membranes confirmed that acidification using CA led to increased fouling and reduced permeate flux, attributed to the aggregation of proteins and lipid residues compared to the EDBM acidification method. This study provides valuable insights into improving and enhancing UF performance while significantly reducing membrane fouling during the filtration of partially acidified dairy streams, by employing chemical-free green technologies.