Liquid concentration minimizes packaging, transportation, handling and storage costs where the most commonly-used process is a thermal concentration process, such as evaporation and freeze concentration. The use of a thermal process results in substantial product losses through the degradation of flavors, aromas, and color. Steps may be taken to strip and recover volatile essences and oils but can add substantial costs. Utilizing a phase change to separate water from juice results in a hefty energy requirement and poses a continuing challenge in minimizing energy costs while maintaining product quality. Prior research has shown that reverse osmosis using standard commercial polyamide membranes can retain desirable solutes found in fruit juices. However, reverse osmosis can be viewed as energy-intensive relative to direct osmosis. Direct osmosis mimics energy-efficient osmotic processes utilized by biological systems avoiding the need for high-pressure or high-temperature and their material requirements such as stainless steel. This allows low-pressure plastics to be substituted for the stainless steel pumps and piping. A direct osmosis-based process can be lighter because of its low-pressure plastic components and quieter because it does not require high-pressure reverse osmosis pumps. Direct osmosis operates by using specifically-selected driving (a.k.a. draw) solutes to cause water to flow from the juice solutions through a semi-permeable membrane into the driving solutes. The driving solutes are in solution with an osmotic pressure higher than that of the juice solution and both solutions operate at atmospheric pressures. The high osmotic pressure from high concentrations of driving solutes can concentrate the juice solutions, potentially offering very high recoveries, an advantage compared to reverse osmosis where recovery is limited by operating pressure. High recovery means minimization of juice volume to be packaged, transported, handled and stored. The proposed research satisfies the USDA's strategic goal to enhance the competitiveness and sustainability of rural and farm economics by developing a non-thermal, membrane-based, liquid concentration process to replace existing thermal concentrating processes for fruit juices. The proposed non-thermal, membrane-based liquid concentration process requires the development of a direct osmosis polyamide thin-film composite hollow-fiber membrane. The direct osmosis process, when utilized with a direct osmosis membrane, will replace energy-intensive thermal concentration processes while producing high-quality concentrated juices by avoiding thermal degradation. OBJECTIVES: This project's goal is to develop the first direct osmosis, polyamide thin-film composite hollow-fiber membrane for use in a non-thermal, membrane-based, liquid concentration process for fruit juices. The required membrane must be specifically-tailored for direct osmosis processes to reduce concentration polarization on both sides of the membrane. A membrane meeting these specifications does not commercially exist. A polyamide thin-film composite hollow-fiber membrane with a semi-permeable barrier on the outside of the hollow-fiber will be developed. The juice solution will flow on the outside of the hollow-fiber membrane and the concentrated driving solute will flow on the inside of the hollow-fiber membrane. By having the driving solutes in a solution with an osmotic pressure higher than that of the juice solution the net water flows from the outside-in of the hollow-fiber membrane. The hollow-fiber microporous support membrane must have the proper characteristics for a successful direct osmosis process. It must have the proper substructure to aid in the mixing of concentrated solutions to reduce concentration polarization where the substructure is characterized by its porosity, tortuosity and thickness. It must have a porous outer skin upon which to form the polyamide thin-film semi-permeable layer by an in-situ interfacial polymerization. In a second step, the polyamide thin film will be deposited onto the porous outer skin of the hollow-fiber membrane by focusing on the interfacial reaction of trimesoyl chloride with m-phenylene diamine. Phase I will demonstrate the feasibility of depositing a polyamide thin-film layer onto a microporous hollow-fiber membrane and identifying prospective driving (a.k.a. draw) solutes. Potential driving solutes must be selected based on its ease of removal and recycle, high-rejection by the direct osmosis membrane, low toxicity because small amounts may leak through the membrane and some may also remain in the final purified product, and low cost because of likely losses through the membrane and in the final product. This project will further the understanding of direct osmosis processes. Future work will focus on optimizing the process parameters by which these membranes were made to further improve transport properties of various compounds such as aromatics, sugars, and acids. APPROACH: To develop and improve the membrane properties, the principal investigators will implement an organized, efficient hollow-fiber membrane development procedure consisting of sequential trials with the following objectives in mind: 1) A thin, porous backing structure supporting a semi-permeable barrier that minimizes concentration polarization on both sides of the membrane. 2) A membrane sufficiently strong to be supported in a direct osmosis system. 3) A thin semi-permeable barrier with a high water transport coefficient (A) and low solute transport coefficients (B's) for desirable solutes. 4) A thin semi-permeable barrier with a low solute transport coefficient (B's) for prospective driving solutes. With these objectives the following steps are implemented to design experiments: 1) Select process variables for testing. 2) Select levels of each process variable. 3) Use fractional factorial designs to prescribe process conditions. 4) Measure response variables (see objectives 1-4 above). 5) Evaluate effects of the selected process variables and process variable interactions on response variables by statistical analysis. 6) Review and discuss results in preparation for the next trial. Through a factorial design the levels are varied orthogonally which help minimize the empirical aspects that cause bias in experimentation while the main objectives are kept in focus. Key process variables are identified and its effects and interactions with other variables on membrane properties are determined. The construction and analysis of factorial designs minimize and control confounding, increase redundancy, and allows for an increase in design resolution with minimal experiments leading to an optimum combination in an organized, efficient manner. The direct osmosis, polyamide thin-film composite hollow-fiber membrane will be the main component of the concentrating process. It is anticipated a concentrating process will reduce capital costs through reduced size and reduced material requirements, minimize energy usage through use of an efficient direct osmosis process potentially leading to reduced operations and maintenance costs
