Long term food stability for extended space missions: a review.

At present, human spaceflight is confined to low Earth orbit but, in future, will again go to the Moon and, beyond, to Mars. The provision of food during these extended missions will need to meet the special nutritional and psychosocial needs of the crew. Terrestrially grown and processed food products, currently provided for consumption by astronauts/cosmonauts, have not yet been systematically optimised to maintain their nutritional integrity and reach the shelf-life necessary for extended space voyages. Notably, space food provisions for Mars exploration will be subject to extended exposure to galactic cosmic radiation and solar particle events, the impact of which is not fully understood. In this review, we provide a summary of the existing knowledge about current space food products, the impact of radiation and storage on food composition, the identification of radiolytic biomarkers and identify gaps in our knowledge that are specific in relation to the effect of the cosmic radiation on food in space.

Perspectives from CO+RE: How COVID-19 changed our food systems and food security paradigms.

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The pasting properties of sonicated waxy rice starch suspensions.

The effect of sonication on the pasting properties of waxy rice starch solutions (5 wt%) was investigated. It has been found that the functionality of starch granules was significantly influenced by the length of sonication and the solution temperature. A comparison of the pasting behaviour showed that the peak and final viscosities of the starch dispersions sonicated at temperatures near the onset temperature of gelatinisation were lower than those of the non-sonicated dispersions. The particle size measurements showed that the size of the heated and sonicated granules were smaller than that of the heated non-sonicated starch granules. Scanning electron microscopy (SEM) observations showed that the starch granule surface was not affected by sonication, and the size exclusion chromatography did not show any reduction in the size of the starch molecules. Based on these observations, the change in the pasting behaviour is explained in terms of the solubilisation of the swollen starch granules and starch aggregates induced by sonication.

Advances in high frequency ultrasound separation of particulates from biomass.

In recent years the use of high frequency ultrasound standing waves (megasonics) for droplet or cell separation from biomass has emerged beyond the microfluidics scale into the litre to industrial scale applications. The principle for this separation technology relies on the differential positioning of individual droplets or particles across an ultrasonic standing wave field within the reactor and subsequent biomass material predisposition for separation via rapid droplet agglomeration or coalescence into larger entities. Large scale transducers have been characterised with sonochemiluminescence and hydrophones to enable better reactor designs. High frequency enhanced separation technology has been demonstrated at industrial scale for oil recovery in the palm oil industry and at litre scale to assist olive oil, coconut oil and milk fat separation. Other applications include algal cell dewatering and milk fat globule fractionation. Frequency selection depends on the material properties and structure in the biomass mixture. Higher frequencies (1 and 2MHz) have proven preferable for better separation of materials with smaller sized droplets such as milk fat globules. For palm oil and olive oil, separation has been demonstrated within the 400-600kHz region, which has high radical production, without detectable impact on product quality.

Ultrasound pressure distributions generated by high frequency transducers in large reactors.

The performance of an ultrasound reactor chamber relies on the sound pressure level achieved throughout the system. The active volume of a high frequency ultrasound chamber can be determined by the sound pressure penetration and distribution provided by the transducers. This work evaluated the sound pressure levels and uniformity achieved in water by selected commercial scale high frequency plate transducers without and with reflector plates. Sound pressure produced by ultrasonic plate transducers vertically operating at frequencies of 400 kHz (120 W) and 2 MHz (128 W) was characterized with hydrophones in a 2 m long chamber and their effective operating distance across the chamber's vertical cross section was determined. The 2 MHz transducer produced the highest pressure amplitude near the transducer surface, with a sharp decline of approximately 40% of the sound pressure occurring in the range between 55 and 155 mm from the transducer. The placement of a reflector plate 500 mm from the surface of the transducer was shown to improve the sound pressure uniformity of 2 MHz ultrasound. Ultrasound at 400 kHz was found to penetrate the fluid up to 2 m without significant losses. Furthermore, 400 kHz ultrasound generated a more uniform sound pressure distribution regardless of the presence or absence of a reflector plate. The choice of the transducer distance to the opposite reactor wall therefore depends on the transducer plate frequency selected. Based on pressure measurements in water, large scale 400 kHz reactor designs can consider larger transducer distance to opposite wall and larger active cross-section, and therefore can reach higher volumes than when using 2 MHz transducer plates.

Design parameters of stainless steel plates for maximizing high frequency ultrasound wave transmission.

This work validated, in a higher frequency range, the theoretical predictions made by Boyle around 1930, which state that the optimal transmission of sound pressure through a metal plate occurs when the plate thickness equals a multiple of half the wavelength of the sound wave. Several reactor design parameters influencing the transmission of high frequency ultrasonic waves through a stainless steel plate were examined. The transmission properties of steel plates of various thicknesses (1-7 mm) were studied for frequencies ranging from 400 kHz to 2 MHz and at different distances between plates and transducers. It was shown that transmission of sound pressure through a steel plate showed high dependence of the thickness of the plate to the frequency of the sound wave (thickness ratio). Maximum sound pressure transmission of ∼ 60% of the incident pressure was observed when the ratio of the plate thickness to the applied frequency was a multiple of a half wavelength (2 MHz, 6mm stainless steel plate). In contrast, minimal sound pressure transmission (∼ 10-20%) was measured for thickness ratios that were not a multiple of a half wavelength. Furthermore, the attenuation of the sound pressure in the transmission region was also investigated. As expected, it was confirmed that higher frequencies have more pronounced sound pressure attenuation than lower frequencies. The spatial distribution of the sound pressure transmitted through the plate characterized by sonochemiluminescence measurements using luminol emission, supports the validity of the pressure measurements in this study.

Enhancement of convective drying by application of airborne ultrasound - a response surface approach.

Drying is one of the oldest and most commonly used processes in the food manufacturing industry. The conventional way of drying is by forced convection at elevated temperatures. However, this process step often requires a very long treatment time, is highly energy consuming and detrimental to the product quality. Therefore, an investigation of whether the drying time and temperature can be reduced with the assistance of an airborne ultrasound intervention is of interest. Previous studies have shown that contact ultrasound can accelerate the drying process. It is assumed that mechanical vibrations, creating micro channels in the food matrix or keeping these channels from collapsing upon drying, are responsible for the faster water removal. In food samples, due to their natural origin, drying is also influenced by fluctuations in tissue structure, varying between different trials. For this reason, a model food system with thermo-physical properties and composition (water, cellulose, starch, fructose) similar to those of plant-based foods has been used in this study. The main objective was, therefore, to investigate the influence of airborne ultrasound conditions on the drying behaviour of the model food. The impact of airborne ultrasound at various power levels, drying temperature, relative humidity of the drying air, and the air speed was analysed. To examine possible interactions between these parameters, the experiments were designed with a Response Surface Method using Minitab 16 Statistical Software (Minitab Inc., State College, PA, USA). In addition, a first attempt at improving the process conditions and performance for better suitability and applicability in industrial scale processing was undertaken by non-continuous/intermittent sonication.

Separation of suspensions and emulsions via ultrasonic standing waves - a review.

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.

Effect of acoustic frequency and power density on the aqueous ultrasonic-assisted extraction of grape pomace (Vitis vinifera L.) - a response surface approach.

Aqueous ultrasound-assisted extraction (UAE) of grape pomace was investigated by Response Surface Methodology (RSM) to evaluate the effect of acoustic frequency (40, 80, 120kHz), ultrasonic power density (50, 100, 150W/L) and extraction time (5, 15, 25min) on total phenolics, total flavonols and antioxidant capacity. All the process variables showed a significant effect on the aqueous UAE of grape pomace (p<0.05). The Box-Behnken Design (BBD) generated satisfactory mathematical models which accurately explain the behavior of the system; allowing to predict both the extraction yield of phenolic and flavonol compounds, and also the antioxidant capacity of the grape pomace extracts. The optimal UAE conditions for all response factors were a frequency of 40kHz, a power density of 150W/L and 25min of extraction time. Under these conditions, the aqueous UAE would achieve a maximum of 32.31mg GA/100g fw for total phenolics and 2.04mg quercetin/100g fw for total flavonols. Regarding the antioxidant capacity, the maximum predicted values were 53.47 and 43.66mg Trolox/100g fw for CUPRAC and FRAP assays, respectively. When comparing with organic UAE, in the present research, from 12% to 38% of total phenolic bibliographic values were obtained, but using only water as the extraction solvent, and applying lower temperatures and shorter extraction times. To the best of the authors' knowledge, no studies specifically addressing the optimization of both acoustic frequency and power density during aqueous-UAE of plant materials have been previously published.

Production of particulates from transducer erosion: implications on food safety.

The formation of metallic particulates from erosion was investigated by running a series of transducers at various frequencies in water. Two low frequency transducer sonotrodes were run for 7.5h at 18kHz and 20kHz. Three high frequency plates operating at megasonic frequencies of 0.4MHz, 1MHz, and 2MHz were run over a 7days period. Electrical conductivity and pH of the solution were measured before and after each run. A portion of the non-sonicated and treated water was partially evaporated to achieve an 80-fold concentration of particles and then sieved through nano-filters of 0.1µm, 0.05µm, and 0.01µm. An aliquot of the evaporated liquid was also completely dried on strips of carbon tape to determine the presence of finer particles post sieving. An aliquot was analyzed for detection of 11 trace elements by Inductively Coupled Plasma Mass Spectroscopy (ICPMS). The filters and carbon tapes were analyzed by FE-SEM imaging to track the presence of metals by EDS (Energy Dispersive Spectroscopy) and measure the particle size and approximate composition of individual particles detected. Light microscopy visualization was used to calculate the area occupied by the particles present in each filter and high resolution photography was used for visualization of sonotrode surfaces. The roughness of all transducers before and after sonication was tested through profilometry. No evidence of formation of nano-particles was found at any tested frequency. High amounts of metallic micron-sized particles at 18kHz and 20kHz formed within a day, while after 7day runs only a few metallic micro particles were detected above 0.4MHz. Erosion was corroborated by an increase in roughness in the 20kHz tip after ultrasound. The elemental analysis showed that metal leach occurred but values remained below accepted drinking water limits, even after excessively long exposure to ultrasound. With the proviso that the particles measured here were only characterized in two dimensions and could be nanoparticulate in terms of the third dimension, this research suggests that there are no serious health implications resulting from the formation of nanoparticles under the evaluation conditions. Therefore, high frequency transducer plates can be safely operated in direct contact with foods. However, due to significant production of metallic micro-particulates, redesign of lower frequency sonotrodes and reaction chambers is advised to enable operation in various food processing direct-contact applications.

Creaming enhancement in a liter scale ultrasonic reactor at selected transducer configurations and frequencies.

Recent research has shown that high frequency ultrasound (0.4-3 MHz), can enhance milkfat separation in small scale systems able to treat only a few milliliters of sample. In this work, the effect of ultrasonic standing waves on milkfat creaming was studied in a 6L reactor and the influence of different frequencies and transducer configurations in direct contact with the fluid was investigated. A recombined coarse milk emulsion with fat globules stained with oil-red-O dye was selected for the separation trials. Runs were performed with one or two transducers placed in vertical (parallel or perpendicular) and horizontal positions (at the reactor base) at 0.4, 1 and/or 2 MHz (specific energy 8.5 ± 0.6 kJ/kg per transducer). Creaming behavior was assessed by measuring the thickness of the separated cream layer. Other methods supporting this assessment included the measurement of fat content, backscattering, particle size distribution, and microscopy of samples taken at the bottom and top of the reactor. Most efficient creaming was found after treatment at 0.4 MHz in single and double vertical transducer configurations. Among these configurations, a higher separation rate was obtained when sonicating at 0.4 MHz in a vertical perpendicular double transducer setup. The horizontal transducer configuration promoted creaming at 2 MHz only. Fat globule size increase was observed when creaming occurred. This research highlights the potential for enhanced separation of milkfat in larger scale systems from selected transducer configurations in contact with a dairy emulsion, or emulsion splitting in general.

Multiphysics modelling of the separation of suspended particles via frequency ramping of ultrasonic standing waves.

A model was developed to determine the local changes of concentration of particles and the formations of bands induced by a standing acoustic wave field subjected to a sawtooth frequency ramping pattern. The mass transport equation was modified to incorporate the effect of acoustic forces on the concentration of particles. This was achieved by balancing the forces acting on particles. The frequency ramping was implemented as a parametric sweep for the time harmonic frequency response in time steps of 0.1s. The physics phenomena of piezoelectricity, acoustic fields and diffusion of particles were coupled and solved in COMSOL Multiphysics™ (COMSOL AB, Stockholm, Sweden) following a three step approach. The first step solves the governing partial differential equations describing the acoustic field by assuming that the pressure field achieves a pseudo steady state. In the second step, the acoustic radiation force is calculated from the pressure field. The final step allows calculating the locally changing concentration of particles as a function of time by solving the modified equation of particle transport. The diffusivity was calculated as function of concentration following the Garg and Ruthven equation which describes the steep increase of diffusivity when the concentration approaches saturation. However, it was found that this steep increase creates numerical instabilities at high voltages (in the piezoelectricity equations) and high initial particle concentration. The model was simplified to a pseudo one-dimensional case due to computation power limitations. The predicted particle distribution calculated with the model is in good agreement with the experimental data as it follows accurately the movement of the bands in the centre of the chamber.

Numerical evaluation of lactoperoxidase inactivation during continuous pulsed electric field processing.

A computational fluid dynamics (CFD) model describing the flow, electric field and temperature distribution of a laboratory-scale pulsed electric field (PEF) treatment chamber with co-field electrode configuration was developed. The predicted temperature increase was validated by means of integral temperature studies using thermocouples at the outlet of each flow cell for grape juice and salt solutions. Simulations of PEF treatments revealed intensity peaks of the electric field and laminar flow conditions in the treatment chamber causing local temperature hot spots near the chamber walls. Furthermore, thermal inactivation kinetics of lactoperoxidase (LPO) dissolved in simulated milk ultrafiltrate were determined with a glass capillary method at temperatures ranging from 65 to 80 °C. Temperature dependence of first order inactivation rate constants was accurately described by the Arrhenius equation yielding an activation energy of 597.1 kJ mol(-1). The thermal impact of different PEF processes on LPO activity was estimated by coupling the derived Arrhenius model with the CFD model and the predicted enzyme inactivation was compared to experimental measurements. Results indicated that LPO inactivation during combined PEF/thermal treatments was largely due to thermal effects, but 5-12% enzyme inactivation may be related to other electro-chemical effects occurring during PEF treatments.

A computational modeling approach of the jet-like acoustic streaming and heat generation induced by low frequency high power ultrasonic horn reactors.

High power ultrasound reactors have gained a lot of interest in the food industry given the effects that can arise from ultrasonic-induced cavitation in liquid foods. However, most of the new food processing developments have been based on empirical approaches. Thus, there is a need for mathematical models which help to understand, optimize, and scale up ultrasonic reactors. In this work, a computational fluid dynamics (CFD) model was developed to predict the acoustic streaming and induced heat generated by an ultrasonic horn reactor. In the model it is assumed that the horn tip is a fluid inlet, where a turbulent jet flow is injected into the vessel. The hydrodynamic momentum rate of the incoming jet is assumed to be equal to the total acoustic momentum rate emitted by the acoustic power source. CFD velocity predictions show excellent agreement with the experimental data for power densities higher than W(0)/V ≥ 25kWm(-3). This model successfully describes hydrodynamic fields (streaming) generated by low-frequency-high-power ultrasound.

C. botulinum inactivation kinetics implemented in a computational model of a high-pressure sterilization process.

High-pressure, high-temperature (HPHT) processing is effective for microbial spore inactivation using mild preheating, followed by rapid volumetric compression heating and cooling on pressure release, enabling much shorter processing times than conventional thermal processing for many food products. A computational thermal fluid dynamic (CTFD) model has been developed to model all processing steps, including the vertical pressure vessel, an internal polymeric carrier, and food packages in an axis-symmetric geometry. Heat transfer and fluid dynamic equations were coupled to four selected kinetic models for the inactivation of C. botulinum; the traditional first-order kinetic model, the Weibull model, an nth-order model, and a combined discrete log-linear nth-order model. The models were solved to compare the resulting microbial inactivation distributions. The initial temperature of the system was set to 90 degrees C and pressure was selected at 600 MPa, holding for 220 s, with a target temperature of 121 degrees C. A representation of the extent of microbial inactivation throughout all processing steps was obtained for each microbial model. Comparison of the models showed that the conventional thermal processing kinetics (not accounting for pressure) required shorter holding times to achieve a 12D reduction of C. botulinum spores than the other models. The temperature distribution inside the vessel resulted in a more uniform inactivation distribution when using a Weibull or an nth-order kinetics model than when using log-linear kinetics. The CTFD platform could illustrate the inactivation extent and uniformity provided by the microbial models. The platform is expected to be useful to evaluate models fitted into new C. botulinum inactivation data at varying conditions of pressure and temperature, as an aid for regulatory filing of the technology as well as in process and equipment design.