Minerals: A Need of Developing Feasible Desalination Processes
An important limiting factor in whey permeate applications is the high mineral content, especially for WPP. Jaros et al. [44] examined the replacement of sucrose in stirred yogurt both with WPP and nanofiltered whey permeate. Sensory studies showed that 25% of sucrose can be substituted with WPP and 30% with nanofiltered WPP. Further increase in sucrose substitution was accompanied by decreased consumer acceptance due to the high mineral content, being associated with a more pronounced salty taste. Partial removal of mineral salts by nanofiltration enabled a slight increase in the addition of WPP (25 vs. 30%).
In many applications, the minerals have a negative impact on either the process itself or also on the application and functionality of the final product. For example, during the evaporation to concentrate whey, calcium salts can precipitate on heat exchanger surfaces and furthermore contaminate lactose crystals during crystallization [56]. Desalination technologies could be utilized to increase the purity of lactose solutions obtained from whey permeate, or to improve the water reclamation from delactosed permeate, all across the food processing industry.
Electrodialysis has been investigated as a method to increase the purity of lactose in whey permeate (up to 98.5% on a solids basis) since the 1970s (101). Although a 90% reduction in conductivity has been obtained in approximately one hour, high energy costs for electrodialysis render the process uneconomical [102]. Further investigations concerning optimal feedstock concentration are needed for the process to become feasible at an industrial scale [103].
Emerging technologies offer promising resource-efficient alternatives to energy-intensive reverse osmosis for desalination of whey permeate. In particular, the development of microbial desalination cells (MDCs) holds potential to replace or reduce the need for reverse osmosis [104, 105]. In their most basic format, MDCs consist of a three chamber system wherein a central desalination chamber is flanked by anode and cathode chambers that operate similarly to a microbial fuel cell. Anion- and cation-exchange membranes separate the desalination chamber from the anode and cathode chambers, respectively.
In the anode chamber, exoelectrogenic bacteria consume organic matter, producing electrons and protons as products. Electrons are delivered to the cathode chamber on the opposite side of the desalination chamber via an external circuit, creating electricity that can be used to do work. Moreover, the electric field generated between the anode and cathode chambers imposes electrostatic forces on any ions within the solution located in the desalination chamber.
These forces drive anions and cations out of the desalination chamber, across the ion exchange membranes, and into anode and cathode chamber, respectively, resulting in desalination of the central chamber without any externally supplied pressure or energy. In addition to whey permeate, cheese processing generates other materials that could promote the use of MDCs. Specifically, the organic nutrient-rich wastewater from processing facilities might serve as the anode chamber substrate, ultimately providing the energy for desalination. The prospective three-fold benefit of renewable energy generation, wastewater biochemical oxygen demand reduction, and whey permeate desalination warrants further research to study the application of MDCs to whey permeate management.
Microbial desalination cell studies have primarily used synthetic wastewater solutions as substrate in the anode chamber and simple salt solutions in the desalination chamber. For salt solutions with 20 to 35 g/L of sodium chloride, near complete desalination was achieved over a period of several days when ample refreshing of anode substrate was used [104]. Although these studies have demonstrated the efficacy of using microorganism-derived electrical current to drive desalination, additional work is needed to examine MDC compatibility with streams relevant to cheese processing and whey permeate desalination. In particular, while domestic wastewater has been used successfully in MDCs [89], no work to date has studied cheese processing wastewater as anode substrate for MDCs. The organic matter content, microbial load, salinity, pH, and bacterial inhibitor content of cheese processing wastewater will affect MDC power output and desalination potential.
Furthermore, the composition of whey permeate will also influence desalination. Transfer of material other than salts through the ion exchange membranes separating the desalination chamber from the cathode and anode chambers must be considered. This is especially important because whey permeate desalination may be coupled with downstream purification of high-value compounds, such as bioactive oligosaccharides. Retention of these compounds during desalination in an MDC must be investigated. Similarly, potential contamination of whey permeate in the desalination chamber due to diffusion of material from the flanking anode and cathode chambers must be understood, particularly when wastewater is used in the anode chamber.
Aside from the technical considerations specific to whey permeate desalination, additional research is needed to make MDCs suitable for industrial use in general. While MDCs have been run continuously at scales up to one liter (106), much larger scales will be required to process commercially relevant volumes of wastewater and whey permeate. Concurrent with scale-up work, improvements in desalination kinetics are necessary to be comparable to reverse osmosis, as desalination in MDCs currently requires retention times of several days.
Advancements in MDC materials and designs will be needed to address these challenges while making MDCs cost-effective at larger scales [105]. The accelerating pace of research for MDCs and bioelectrochemical systems in general, along with growing incentives for energy efficiency and sustainable wastewater management, will likely drive the translation of these technologies to the dairy processing industry.
