The relationship of dielectric response and water activity in food

This study has deduced a correlation between points of inflection of water activity and loss factor with respect to moisture content. A point of inflection in loss factor with respect to moisture content was found to coincide with the sorption isotherm point of inflection that defines the transition from multilayer to solution in every instance analysed, with an average difference of just 0.01kg.kg-1. Food can support microbial growth and chemical reactions in water activity levels above this critical transition. This correlation was discovered using published dielectric and sorption data for specific foods at similar temperatures. It was found that low sugar foods containing high levels of hydrocolloids generally exhibited different behaviour from fruits. This shows that microwave heating behaviour will be different in fruits compared to low sugar foods with high hydrocolloid content when drying to achieve a certain water activity and therefore shelf life

Characterisation of potato crisp effective porosity using micro-CT

Background The effective porosity is an important quantitative parameter for food products that has a significant effect on taste and quality. It is challenging to quantify the apparent porosity of fried potato crisps as they have a thin irregularly shaped cross section containing oil and water. This study uses a novel micro-CT technique to determine the solid volume fraction and hence the effective porosity of three types of potato crisps: standard continuously fried crisps, microwaved crisps, and continuously fried ‘kettle’ crisps. Results It was found that continuously fried kettle crisps had the lowest effective porosity at 0.54, providing the desired crunchy taste and lower oil contents. Crisps produced using a microwave process designed to mimic the dehydration process of standard continuous fried crisps had an effective porosity of 0.65, which was very similar to the effective porosity of 0.63 for standard continuously fried crisps. The results were supported by the findings of a forced preference consumer test. Conclusion The effective porosity affects the product taste and is therefore a critical parameter. This study shows that micro-CT analysis can be used to characterise the change in effective porosity of a thin irregularly shaped food product, caused by a change of cooking procedure

Enhanced 'In-situ' catalysis via microwave selective heating: catalytic chain transfer polymerisation

An extremely facile, single stage, ‘in-situ’, Catalytic Chain Transfer Polymerisation (CCTP) process has been identified, where the optimal polymerisation process was shown to depend upon a combination of catalyst characteristics (i.e. solubility, sensitivity, activity) and the method of heating applied. In comparison to the current benchmark catalyst, the preparation of which is only about 40 % efficient, this represents a significant increase in waste prevention/atom efficiency and removes the need for organic solvent. It was also shown possible to significantly reduce the overall ‘in-situ’ reaction cycle time by adopting different processing strategies in order to minimise energy use. The application of microwave heating was demonstrated to overcome system diffusion/dilution issues and result in rapid, ‘in-situ’ catalyst formation. This allowed processing times to be minimised by enabling a critical concentration of the species susceptible to microwave selective heating to dominate the heat and mass transfer involved

Methodology for the synthesis of methacrylate monomers using designed single mode microwave applicators

© 2019 The Royal Society of Chemistry. A novel single-well prototype high throughput microwave reactor geometry has been produced and shown to be capable of synthesizing an array of non-commercially available methacrylate monomers. The reactor, which delivers the energy required via a dedicated coaxial line, has been shown experimentally to outperform other conventional/microwave formats. It is demonstrated to achieve significantly higher conversions than the alternative reactor types, whilst requiring (a) low levels of input power, (b) no additional energy for agitation/mass transfer, (c) no solvent and (d) no environmentally impacting thermos-fluids

Combining continuous flow oscillatory baffled reactors and microwave heating: Process intensification and accelerated synthesis of metal-organic frameworks

We have constructed a continuous flow oscillatory baffled reactor (CF-OBR) equipped with a homogeneous and controllable microwave applicator in an entirely novel design. This affords a new route to chemical production incorporating many of the principles of process intensification and allows, for the first time, investigation of the synergistic benefits of microwave heating and CF-OBRs such as; faster and continuous processing; improved product properties and purity; improved control over the processing parameters; and reduced energy consumption. The process is demonstrated by the production of a metal-organic framework (MOF), HKUST-1, a highly porous crystalline material with potential applications in gas storage and separation, catalysis, and sensing. Our reactor enabled the production of HKUST-1 at the 97.42 g/h scale, with a space time yield (STY) of 6.32 × 105 kg/m3/day and surface area production rate (SAPR) of 1.12 × 1012 m2/m3/day. This represents the highest reported STY and fastest reported synthesis (2.2 seconds) for any MOF produced via any method to-date and is an improvement on the current SAPR for HKUST-1 by two orders of magnitude owing to the superior porosity exhibited by HKUST-1 produced using our rig (Langmuir surface area of 1772 compared to 600 m2/g)

Developing a sustainable route to environmentally relevant metal-organic frameworks: ultra-rapid synthesis of MFM-300(Al) using microwave heating

NO2, SO2 and CO2 are major air pollutants causing significant environmental and health problems. Metal-organic frameworks (MOFs), in particular [Al2(OH)2(C16O8H6)](H2O)6 (trivial names: NOTT-300/MFM-300(Al)), have shown great promise for capturing these gases. However MOF syntheses often involve toxic solvents and long durations which are inherently energy intensive, an environmental burden, and have serious safety risks. There is a pressing need to develop environmentally-friendly routes to MOFs that require less energy and implement safer solvents particularly when considering scale-up beyond the laboratory for industrial application. We report the rapid synthesis of MFM-300(Al) in aqueous conditions and 10 minutes using microwave heating. This is the fastest reported synthesis of MFM-300(Al) to date with a 99.77 % reduction in reaction time compared to the current reported 3-day conventional heated route. The microwave synthesized sub-micron crystalline material exhibits gas uptake capacities of 8.8 mmol g-1 at 273 K and 1.0 bar for CO2, 8.5 mmol g-1 at 298 K and 0.17 bar for SO2, and 1.9 mmol g-1 at 298 K and 0.01 bar for NO2. These are 26 %, 70 %, and 90 % greater for CO2, SO2, and NO2, respectively, when compared to previously reported MFM-300(Al) materials produced via a 3-day conventionally heated route demonstrating the production of high quality materials at a fraction of the time with enhanced gas properties. Crucially, this offers an opportunity to move from batch to continuous processing owing to reduced reaction times underpinned by targeted heating

Realising the environmental benefits of metal–organic frameworks: recent advances in microwave synthesis

Metal–organic frameworks (MOFs) are a broad class of porous crystalline materials that show great potential for a wide-range of applications in areas such as energy and environmental sustainability. MOFscan show significant advantages in gas selectivity and separation over traditional adsorbents such as zeolites and activated carbons since they are tuneable both in terms of porosity and chemical functionality. The ability to control the pore environment of the MOF is one of their remarkable advantages and affords control over the structure and properties required for specific applications. Despite these advantages, the industrial adoption of MOFs is slow owing to the paucity of scalable, environmentally sustainable manufacturing methods and higher costs compared to zeolites. Microwave (MW) technology is an extremely promising method of MOF production owing to significantly reduced reaction times and subsequently lower process energy consumption, control over MOF properties, and the ability to produce MOFs and MOF-hybrids otherwise difficult to isolate or unobtainable through other synthetic routes. However, the ability to produce the multiple kilogram or even tonne quantities of MOFs required by industry using MW technology is yet to be achieved owing to little or no understanding of the interaction(s) of reactants and MOFs with the electric field, and crucially, how this informs the design of the scale up processes. This review aims to bridge this gap in knowledge by (1) highlighting recent advances in understanding of MW–MOF interactions and areas for future focus; (2) providing an up-to-date and comprehensive summary of literature on MW synthesis of MOFs, focusing on examples where MW heating has facilitated novel and unique results in the laboratory; and (3) emphasising the advantages, challenges and current steps and methodologies required towards industrial-scale MW production of MOFs

Formation of Metallurgical Coke within Minutes through Coal Densification and Microwave Energy

This paper shows how feedstock densification gives rise to a step change in the time required to create a metallurgical grade coke using microwave energy. Five densified coking and non-coking coals were heated in a multi-mode microwave 2450 MHz cavity for varying treatment times (2-20 minutes) with a fixed power input (6 kW). Proximate analysis, intrinsic reactivity, coke reactivity, dielectric properties, and petrographic analysis of the coals and microwave produced lump cokes were compared to a commercial lump coke. Densifying the sample prior to microwave treatment enabled a dramatic acceleration of the coking process when combined with targeted high microwave energy densities. It was possible to form fused coke lump structures with only 2 minutes of microwave heating compared to 16-24 hours via conventional coking. Anisotropic coke morphologies (lenticular and circular) were formed from non-coking coal that are not possible with conventional coking and increasing treatment time improved overall coke reflectance. Three of the coals produced coke with equivalent coke reactivity index values of 20-30, which are in the acceptable range for blast furnaces. The study demonstrated that via this process, non-coking coals could potentially be used to produce high quality cokes, potentially expanding the raw material options for metallurgical coke production

State-of-the-art in microwave processing of metals, metal powders and alloys

Discovery of the capacity of microwave energy to heat materials was a major milestone in the history of technology and scientific research. This discovery led to the development of many devices and processes that have replaced conventional heating methods, thereby reducing reliance on fossil fuels and reducing atmospheric CO2 emissions. Over the past two decades the use of microwave heating for processing of non-dielectric metals, metal powders alloys have increased markedly and includes sintering, melting, joining, cladding, drilling, and 3D printing. These developments have used a wide range of experimental procedures, and the use of microwave power has resulted in significant benefits over conventional heating methods. These benefits include a many-fold decreases in processing times and energy consumption, as well as improved microstructural characteristics and mechanical properties of the processed material. This review provides a comprehensive overview of the state-of-the-art of microwave processing of metals, metal powders and their alloys and focuses on important process parameters such as heating mechanisms, electromagnetic properties of metals, the factors affecting these parameters and applications to metal processing. Requirements for efficient metal processing using microwave power are presented including metal properties, microwave susceptors, insulators, ceramic containment structures and temperature measurement methods that all play roles in the development of microwave processing of metals and metal alloy materials. Current challenges and issues in equipment design parameters and various processing methods to facilitate commercial implementation of metal processing at larger scales are investigated