An official website of the United States government.
The overall goal of this competitive research program is to develop an innovative CNT surface technology, which exhibits a low surface energy and prevents electrochemical reactions and fouling during juice pasteurization using PEF, ohmic heating and PHE systems. It is intended to quantitatively analyze the relation between surface characteristics and interfacial reactions of food materials, to effectively grow the CNTs on the metal surface, and to test the surface in the lab scale systems.
<P>Specific objectives leading to this goal are:
<OL> <LI> Develop and integrate a polytetrafluoroethylene (PTFE)-based carbon nanotube (CNT) coated plate heat exchanger (PHE) test setup for juice pasteurization. <LI> Validate the functionalities of the polytetrafluoroethylene (PTFE)-based carbon nanotube (CNT) coated surface for minimizing juice fouling. <LI> Develop and integrate a polytetrafluoroethylene (PTFE)-based carbon nanotube (CNT) coated ohmic heating system for juice pasteurization. <LI> Test the ohmic heating system for efficacy in minimizing the electrochemical reaction and fouling occurrence.
</ol>
The expected outcomes are: <ol> <LI> The electrochemical reactions from ohmic heating systems with PTFE-based CNT coated electrodes will be a minor and negligible. <LI> The PTFE-based CNT coated electrodes or plates will reduce the fouling mass by at least 30% compared to the control stainless steel plates, being less contamination of processing units. <LI> The CNTs characteristics can be clearly correlated with the electrical double layer and the surface chemistry such as hydrophobicity and interfacial energy. <LI> Selective electric field treatment for food components might be possible using the PTFE-based CNT coated electrodes. It has been recently reported that only electrons with the right wavelength are allowed to pass, depending upon the circumference and chirality of the nanotubes (Mamalis et al., 2004). <LI> The initial capital investment for nanotube coating is estimated to be high; however, the mass production of CNTs surface treatment is expected to reduce the production cost to within $20 to $50 per square inch (vendor estimates), which is very minimal investment when compared with the expected benefits, i.e. a rapid return on investment (< 2 years) for a minimal amount of initial capital. <LI> For PEF and ohmic heating, the undesired reactions in the vicinity of the electrode surfaces can cause changes to the chemical properties of the liquid, produce toxic chemicals (H2O2). Fouling is a common problem for both conventional (PHE) and emerging (PEF and ohmic heating) pasteurizers since the product after treatment may become contaminated with pieces of the fouling layer that detach from the electrode or walls. Note that developed biofilms could not only provide favorable growth conditions for pathogens but cause the juice products under-processed. </ol>
If successful, the PTFE-based CNT surface coating will ensure the microbiological quality of fluid food products by keeping biofilms from developing, furthermore, providing the potential and promise to pave the way for new microbial safety practices in the US tropical juice industry.
NON-TECHNICAL SUMMARY: Currently 98 percent of all tropical juice concentrates, such as orange, pineapple, and guava, sold in the USA are pasteurized. This process is necessary to prevent microbial contamination in juices. Undesirable reactions at the interface between juice products and contact elements of juice pasteurizers have been a common juice industry-wide issue. Thermal processing such as using plate heat exchangers (PHEs) produces serious juice fouling problems on the surface, which induces hydraulic and thermal disturbances and creates the need for cleaning operations every 5 to 10 hours. Emerging alternatives, including pulsed electric field (PEF) and ohmic heating, may cause electrochemical reactions at the interface. Due to the frequency and economic cost of the cleaning-in-place (CIP) process, numerous research efforts have been made; however, none have been fully satisfactory. Hence, the PD proposes an innovative approach of surface coating with carbon nanoparticulates in tube form to effectively address electrochemical reactions and fouling problems in both electrical/electro-thermal and PHE pasteurization systems. Carbon nanotubes (CNTs) have excellent anti-fouling properties including favorable attributes, i.e. strength, flexibility, and thermal/electrical conductivities. This project will describe the fundamental effect of the surface energy and nanoscale roughness on the electrochemical reactions and nucleation of foulants in fluid juices, as well as the process of alignment of the CNTs. Additional polytetrafluoroethylene (PTFE) layer will provide a sturdy platform to stabilize aligned CNTs. The PD will compare stainless steel surfaces with CNT coated surfaces by quantifying gas generation, migrated metal ions, and fouled masses after juice pasteurization, which will validate significant thermal and electrical energy savings, ensuring food quality and safety.
<P>APPROACH: Task 1. Develop and integrate a polytetrafluoroethylene (PTFE)-based carbon nanotube (CNT) coated plate heat exchanger (PHE) test setup for juice pasteurization. 1. In an interactive collaboration with a cooperator, Dr. Chung in University of Washington, the PD will work together with Omega Piezo Technologies, Inc. (State College, PA) and GVD Corporation (Cambridge, MA) due to their competitiveness and expertise in nanofabrication. The CNT forest will be planted using plasma enhanced chemical vapor deposition (PECVD) technique. 2. A simple test setup of a PHE system will be designed for fouling tests and wear test. Food samples for testing will be pineapple and guava juices, which are representative fruit juice products in Hawaii. Task 2. Validate the functionalities of the PTFE-based CNT coated surface for minimizing juice fouling. 1. Measurement of contact angle and topographic image: The static contact angles of different surfaces, i.e. PTFE-based CNT coated surface and control will be measured by the sessile drop technique. The measured values at five points on the same surface will be averaged. The scanning electron microscopy will be used to image the topography of the standard and CNT treated stainless steel surfaces. 2. The test setup will be opened after different heating times (0.5, 1, 2, 3, and 6 hrs) of juice pasteurization to record distribution of the deposits and weigh them in a dry form. The measured value will be compared with the control data (stainless steel 316). 3. Wear resistance test: The elemental carbon concentrations in juice products are determined by inductively coupled plasma - optical emission spectrometer (ICP-OES) monitoring the emission spectra near 193.03 nm. 4. The nucleation process to initiate juice fouling will be investigated in relation to surface characteristics. Adhesion theory vs. interfacial energy between foulant and surface will be discussed. Task 3. Develop and integrate the PTFE-based CNT coated ohmic heating system for juice pasteurization. 1. The PD will design a static ohmic heating system for tropical fruit juice pasteurization. The system will be energized by the conventional (60 Hz) sinusoidal alternating current and pulsed alternative current with a high frequency (10 KHz) for comparison. 2. As requested by the PD and cooperator, the identified vendor will align PTFE-based nanotubes on one circular electrode made of stainless steel (2.5 cm in diameter) for the electrochemical reaction test. Task 4. Test the ohmic heating system for efficacy in minimizing the electrochemical reaction and fouling occurrence. 1. A series U hydrogen detector will be used to measure headspace hydrogen gas generated during electric field treatment. Concentrations of Fe and Cr migrated into the heating media are taken as measures of electrode corrosion. Quantitative analysis of the metal ions is performed by ICP-MS. Fouling mass in a dry form after pasteurization will be collected by rubbing off and weighed from the electrode. 2. The concentration of ascorbic acid in pineapple and guava juices under pasteurization will be determined using a reverse-phase high performance liquid chromatograph (RP-HPLC).