Nanofiltration and nanodiafiltration of acid whey as a function of pH and temperature

Valorisation of acid whey (AW) is a challenge for the dairy industry largely due to processing difficulties associated with the inability of lactose to crystallise in the presence of lactic acid (LA), which can require removal of LA from AW prior to further processing. The aim of the present study was to compare the performance of three commercially available nanofiltration (NF) membranes in the processing of AW. NF membrane types HL, DL and DK, were assessed under industrially relevant conditions including adjusted pH levels (3, 4.5 or 7.3) and two processing temperatures (25 or 40 °C). Additionally, nano-diafiltration (NDF) was employed at 25 °C. In both NF and NDF, higher permeate fluxes were obtained with HL membranes. Lactose retention appeared temperature dependent only, while LA transmission depended on pH, membrane type and temperature. Both, pH and temperature influenced the divalent and phosphate ion transmissions. Formation of calcium complexes with phosphates and lactates, which may consequently induce attraction with proteins, was responsible for the pH and temperature effects on transmission of components. NF with the HL membrane at 40 °C and adjusted pH 3.0 showed the highest permeate transmission (∼50%) for LA and was able to retain ∼93% lactose. This system also had the highest flux (average of 60 L/m2/h). In contrast, NDF showed only ∼34% LA transmission while ∼97% lactose was retained. Thus removal of almost half of the LA content of AW was possible in the best case, which indicates that NF may have a useful role to play in the valorisation of this industrial by-product.