Direct conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone in zinc chloride solution under microwave irradiation

Conversion of cellulose to 1-(furan-2-yl)-2-hydroxyethanone has been demonstrated in concentrated zinc chloride solution under microwave irradiation. Compared with the conventional oil-bath heating mode, microwave irradiation significantly reduced the reaction time and increased the yield of 1-(furan-2-yl)-2-hydroxyethanone. A typical degradation reaction with cellulose produced 1-(furan-2-yl)-2-hydroxyethanone in 12.0% molar yield in ZnCl₂ solution (ZnCl₂–H₂O ratio = 2.25:1, w/w) with microwave irradiation at 600 W for 5 minutes at 135 °C.     

Modelling the Effects of Debranching and Microwave Irradiation Treatments on the Properties of High Amylose Corn Starch by Using Response Surface Methodology

Response surface methodology was applied to determine the effects of pullulanase debranching, microwave irradiation time (2–4 min) and power (20–100%) on resistant starch (RS) formation and in-vitro glycemic index (GI) values in high amylose corn starch, Hylon VII. Starch:water (1:10) suspensions were cooked and autoclaved, debranched with pullulanase (1000 PUN/g; 1500 U/kg starch) at 60 °C and then different microwave-storing cycles and drying (oven or freeze drying) processes were applied. In order to describe the relationship between the dependent and independent variables (microwave power and irradiation time), the response values were fitted by first order polynomial regression models. Significance analysis showed that microwave irradiation time had significant effect on RS content and GI value of the samples treated with one cycle of microwave-storing prior to freeze-drying. Microwave power had significant factor on the GI value of the samples that were oven-dried after one cycle of microwave-storing. Solubility and water binding capacity values of all heat treated samples were higher than those of native starch. On the other hand, RVA viscosity values were lower than native starch for oven-dried samples. Water binding capacity, solubility and final viscosity values of the freeze-dried samples were higher than those of oven-dried ones.     

Microwave-assisted pretreatment of woody biomass with ammonium molybdate activated by H2O2

Pretreatments for enzymatic saccharification are crucial for the establishment of lignocellulosic biorefineries. In this study, we focused on ammonium ions and peroxometal complexes as potential delignifying agents. We first examined the pretreatment of beech wood with nine different ammonium salts in the presence of H₂O₂. Significant pretreatment effects were found only for ammonium molybdate, which is transformed to a peroxometal complex on reacting with H₂O₂. Since microwave sensitizer effects are expected for (peroxo)molybdate, beech wood was pretreated using external heating and microwave irradiation. As a result, a maximum sugar yield of 59.5% was obtained by microwave irradiation at 140 °C for 30 min, while external heating in an autoclave gave a sugar yield of 41.8%. We also found that an ammonium ion is the key counterion accelerating the pretreatment with molybdate. These results highlight the powerful selective delignifying capability of the H₂O₂-activated ammonium molybdate system energised by microwave radiation.     

Regional brain heating during microwave exposure (2.06 GHz), warm-water immersion,environmental heating and exercise

Nonuniform heating may result from microwave (MW) irradiation of tissues and is therefore important to investigate in terms of health and safety issues. Hypothalamic (T_hyp), cortical (T_ctx), tympanic (T_ty), and rectal (T_re) temperatures were measured in rats exposed in the far field, k-polarization (i.e., head pointed toward the transmitter horn and E-field in vertical direction) to two power densities of 2.06 GHz irradiation. The high-power density (HPM) was 1700 mW/cm² [specific absorption rate (SAR): hypothalamus 1224 W/kg; cortex 493 W/kg]; the low-power density (LPM) was 170 mW/cm² (SAR: hypothalamus 122.4 W/kg; cortex 49.3 W/kg). The increase (rate-of-rise, in °C/s) in T_hyp was significantly greater than those in T_ctx or T_re when rats were exposed to HPM. LPM produced more homogeneous heating. Quantitatively similar results were observed whether rats were implanted with probes in two brain sites or a single probe in one or the other of the two sites. The qualitative difference between regional brain heating was maintained during unrestrained exposure to HPM in the h-polarization (i.e., body parallel to magnetic field). To compare the temperature changes during MW irradiation with those produced by other modalities of heating, rats were immersed in warm water (44 °C, WWI); exposed to a warm ambient environment (50 °C, WSED); or exercised on a treadmill (17 m/min 8% grade) in a warm ambient environment (35 °C, WEX). WWI produced uniform heating in the regions measured. Similar rates-of-rise occurred among regions following WSED or WEX, thus maintaining the pre-existing gradient between T_hyp and T_ctx. These data indicate that HPM produced a 2–2.5-fold difference in the rate-of-heating within brain regions that were separated by only a few millimeters. In contrast, more homogeneous heating was recorded during LPM or nonmicrowave modalities of heating. Bioelectromagnetics 19:341–353, 1998. © 1998 Wiley-Liss, Inc.     

Impact of microwave heating on the physico-chemical properties of a starch–water model system

The objective of this work was to understand the physico-chemical changes induced in a wheat starch model system as a result of microwave heating. Wheat starch dispersions in water, with final solids content of 33%, 40% or 50%, were heated in a microwave oven. Following heating the samples were stored at 25 °C for up to 120 h and analyzed periodically. Microwave heated gels were significantly different from conduction heated gels in all parameters measured. The differences in properties are a reflection of the differences in the heat and mass transfer of the different modes of heating. The lack of granule swelling and the resulting soft gel are two key observations. The results of this study suggest a different mechanism of starch gelatinization compared to conduction heating. The vibrational motion and the rapid increase in temperature also result in granule rupture and formation of film polymers coating the granule surface.     

Fluorescent carbon dots synthesized by waste wind turbine blade for photocatalytic degradation

Developing novel waste recycling strategies has become a feasible solution to overcome environmental pollution. In this work, a method of using waste wind turbine blade (WTB) as a carbon source to synthesize blue fluorescent carbon dots (B-CDs) by hydrothermal treatment is proposed. B-CDs are spherical and have an average particle size of 5.2 nm. The surface is rich in C–O, C=O, −CH₃, and N–H bond functional groups, containing five elements: C, O, N, Si, and Ca. The optimal emission wavelength of B-CDs is 463 nm, corresponding to an excitation wavelength of 380 nm. Notably, a relatively high quantum yield of 29.9% and a utilization rate of 40% were obtained. In addition, B-CDs can serve as a photocatalyst to degrade methylene blue dye, with a degradation efficiency of 64% under 40-min irradiation conditions. The presence of holes has a significant influence on the degradation process.     

POLYMERIZATION OF SILICA IN ACIDIC SOLUTIONS: A NOTE OF CAUTION TO PHYCOLOGISTS1

Concentrated stock solutions of sodium metasilicate (Na₂SiO₃.9H₂O), prepared in deionized-distilled water and acidified to pH 2 as suggested by McLachlan (1973), were found to be polymerized; stocks kept at their alkaline pH of dissolution were not polymerized. Depolymerization studies using the acidified stock added to deionized-distilled water, 3% NaCl or artificial saltwater medium showed that complete depolymerization took longer than 2 days. Studies of Si uptake, using the marine diatom Thalassiosira pseudonana (Hust.) Hasle and Heimdal, showed that use of an acidified Si stock for experiments can lead to anomolous results. It is recommended that concentrated Si stocks be stored at their pH of dissolution and not be acidified prior to addition to media. In instances where the pH must be adjusted prior to addition (e.g. in some Si-uptake experiments), it may be necessary to take into account polymerization / depolymerization effects.     

Solid acid and microwave-assisted hydrolysis of cellulose in ionic liquid

Solid acid-catalyzed hydrolysis of cellulose in ionic liquid was greatly promoted by microwave heating. H-form zeolites with a lower Si/Al molar ratio and a larger surface area showed a relatively higher catalytic activity. These solid catalysts exhibited better performance than the sulfated ion-exchanging resin NKC-9. Compared with conventional oil bath heating mode, microwave irradiation at an appropriate power significantly reduced the reaction time and increased the yields of reducing sugars. A typical hydrolysis reaction with Avicel cellulose produced glucose in around 37% yield within 8 min.     

Microwave-assisted synthesis of long-chain alkyl glucopyranosides

Long-chain (C₆ to C₁₆) alkyl glucopyranosides have been synthesized using microwave irradiation. Yields and anomeric ratios can be controlled under precise and short irradiation times (few min). Comparison with classical heating showed a better efficiency of microwaves.     

Effects of protein on crosslinking of normal maize, waxy maize, and potato starches

Channels of maize starch granules are lined with proteins and phospholipids. Therefore, when they are treated with reagents that react at or near the surfaces of channels, three types of crosslinks could be produced: protein–protein, protein–starch, starch–starch. To determine which of these may be occurring and the effect(s) of channel proteins (and their removal) on crosslinking, normal and waxy maize starches were treated with a proteinase (thermolysin, which is known to remove protein from channels) before and after crosslinking, and the properties of the products were compared to those of a control (crosslinking without proteinase treatment). After establishing that treatment of starch with thermolysin alone had no effect on the RVA trace, three reaction sequences were used: crosslinking alone (CL), proteinase treatment before crosslinking (Enz-CL), proteinase treatment after crosslinking (CL-Enz). Two crosslinking reagents were used: phosphoryl chloride (POCl₃), which is known to react at or near channel surfaces; STMP, which is believed to react throughout the granule matrix. Three concentrations of POCl₃ (based on the weight of starch) were used. For both normal maize starch (NMS) and waxy maize starch (WMS) reacted with POCl₃, the trends were generally the same, with apparent relative degrees of crosslinking indicated to be CL-Enz = CL > Enz-CL, but the effects were greater with NMS and there were differences when different concentrations of reagent were used. The basic trends were the same when potato starch was used in the same experiments. Crosslinking with STMP was done both in the presence and the absence of sodium sulfate (SS). Both with and without SS and with both NMS and WMS, the order of indicated crosslinking was generally the same as found after reaction with POCl₃, with the indicated swelling inhibition being greater when SS was present in the reaction mixture. Examination of the maize starches with a protein stain indicated that channel protein was removed by treatment with thermolysin when the proteinase treatment occurred before crosslinking with either POCl₃ or STMP, but only incompletely or not at all if the treatment with the proteinase occurred after crosslinking. Because the crosslinking reactions were less effective when the protein was removed, the results are tentatively interpreted as indicating that they involved protein molecules, although there may not be a direct relationship.     

Microwave-assisted methylation of cassava starch with dimethyl carbonate

A novel and environmentally friendly process for the methylation of cassava starch with dimethyl carbonate (DMC) could be accelerated by employing a combined strategy: using disodium hydrogen phosphate (Na₂HPO₄) as the catalyst (chemical means) and microwave irradiation as the energy source (physical means). By varying the volume of 5% sodium chloride aqueous solution between 50 and 150 mL, the amount of Na₂HPO₄ between 0 and 1.25 g, the volume of DMC between 75 and 200 mL, and the microwave time from 5 to 20 min, methyl cassava starch with degree of substitution (DS) values in the range of 0.033 and 1.087 was prepared. The chemical structure of methyl cassava starch was analyzed by ¹H NMR spectroscopy.     

Safranin O Staining Using a Microwave Oven

We investigated the effects of microwave irradiation on a safranin O staining method for paraffin sections of formalin fixed rabbit larynx. The control sections were stained according to the conventional method, and the experimental sections were stained in microwave oven for 10 sec at 360 W in Weigert's iron hematoxylin, and for 30 sec at 360 W in fast green and 0.1% safranin O staining solutions. Light microscopic examination of the sections revealed that the microwave heating did not adversely affect the staining properties of cartilage tissue compared to the conventional staining method. Small differences such as darker staining of the matrix and shrinkage of the cytoplasm was observed in some microwave treated sections. The present study revealed that microwave application can be used safely for the safranin O method with the advantage of reduced staining time.     

Effects of millimeter-wave radiation on monolayer cell cultures. III. A search for frequency-specific athermal biological effects on protein synthesis

A method recently developed in this laboratory has been used to directly expose BHK-21/C13 cells to high levels of microwave radiation without significant microwave-induced heating (? 0.1 °C). Monolayer cultures were grown on microwave-transparent polystyrene coverslips, placed on the open end of a wave guide, and maintained at 37.2 °C during irradiation at frequencies in both the E- and U-bands (average power densities 292 and 177 mW/cm², respectively). Effects of microwave radiation were assessed at 0.1 GHz increments in the ranges of 38–48 GHz and 65–75 GHz. Protein synthesis was measured in quadruplicate cultures that were allowed to incorporate labeled methionine during the 15-minute period of microwave irradiation. Autoradiographs of each monolayer culture were scanned along the region corresponding to the longer axis of the wave guide aperture using a microdensitometer to quantify incorporation. Since microwave power incident on the cells was previously shown to vary along this axis according to a cosine² relationship from zero at each edge of the wave guide to twice the average power density at the center of the wave guide, this technique should reveal biological effects that might only be manifested in narrow amplitude domains or “power windows.” Observations of protein synthesis in monolayer cultures irradiated at 202 closely spaced frequencies in the E- and U-bands failed to reveal changes associated with microwave exposure. Thus no evidence was obtained in support of the existence of frequency-specific athermal biological effects of microwaves. In addition, no support was found for the existence of amplitude-specific “power windows”.     

Fourier-transform infrared spectroscopy of Pediastrum duplex: characterization of a micro-population isolated from a eutrophic lake

Fourier-transform infrared (FTIR) spectroscopy was carried out on single colonies of Pediastrum duplex present in air-dried preparations of mixed phytoplankton samples isolated from a eutrophic freshwater lake. FTIR absorption spectra had 12 distinct bands over the wavenumber range 3300–900?cm^?1 which were tentatively assigned to a range of chemical groups, including –OH (residual water, wavenumber 3299?cm^?1), –CH2 (lipid, 2924), –C=O (cellulose, 1739), amide (protein, 1650 and 1542), >P=O (nucleic acid, 1077) and –C–O (starch, 1151 and 1077). Measurement of band areas identified residual water, protein and starch as the major detectable constituents. Areas of single bands and combined bands of –CH₂, –C–O and >P=O species normalized to protein (to correct for differences in specimen hydration and thickness) showed wide variation between colonies, indicating environmental heterogeneity. Correlation analysis demonstrated close statistical associations between different molecular species. Particularly high levels of correlation between bands 3/4 (CH₂), 6/7 (amide) and 8/9 (–CH₃) was consistent with their joint origin from the same molecular species. The isolation of bands 11 and 12 in the correlation pattern was confirmed by factor analysis, suggesting that variation in the level of starch is statistically unrelated to other macromolecules being monitored. The use of FTIR spectroscopy to characterize an algal micro-population within mixed phytoplankton has potential for future studies on biodiversity and environmental interactions at the species level.     

Effect of microwave irradiation on the structure of glucoamylase

In the present study, we describe changes in the primary and secondary structural patterns of glucoamylase during starch hydrolysis under microwave irradiation using SDS-PAGE and circular dichroism (CD) spectroscopy. Our SDS-PAGE results show that the primary structure of glucoamylase did not change after microwave irradiation. According to the CD spectra, the positive peak height (λ = 193 nm) of the microwave-irradiated samples decreased by 36.4–68.2% compared to those without irradiation, whereas the double negative peak height (λ = 206 nm, λ = 220 nm) increased by 10.8–31.4%. In addition, the positive peak (λ = 193 nm) shifted by 0.2–3 nm. After treatment of glucoamylase with microwave irradiation, the α-helical content of glucoamylase decreased sharply, whereas the β-sheet, β-turn and random coil content increased gradually. The conformational changes of glucoamylase after microwave irradiation provide theoretical support for the mechanism whereby microwave irradiation accelerates starch hydrolysis catalyzed by glucoamylase.     

Effect of DMSO on the dielectric properties of canine kidney tissue

The dielectric permittivity and conductivity of canine kidney tissue samples were measured at R_f frequencies between ?20 °C and +20 °C. Some of the kidneys had been perfused with DMSO (10%) in canine plasma, others with physiological saline alone. The DMSO greatly increases the conductivity of frozen tissue above that of tissue not treated with this cryoprotectant. Apparently, the chief reason for nonuniform heating of a partially frozen organ in a microwave field is the great change in tissue conductivity as it thaws. We suggest that the effect on the conductivity of tissue should be considered in the choice of a cryoprotectant for tissues which are to be thawed by microwave or radiofrequency irradiation.     

Thin-layer liquid crystal thermometry of cells in vitro during hyperthermal microwave irradiation

A nonperturbing technique of thin-layer liquid crystal thermometry was developed to quantitate heating of Chinese hamster ovary cells and the bacterium Serratia marcescens when exposed to 2450-MHz microwave fields at 0.2–0.5 W/cm². Cells suspended in culture medium were injected into 5-cm glass microcapillary tubes coated on the inside with a thin layer of liquid crystal. The tubes were sealed and placed parallel to the electric field in a watertight waveguide exposure chamber where they were heated by circulating temperature-controlled water. Even at high circulation rates, liquid crystal color changes indicated local microwave capillary tube heating of 0.1–0.25 °C. Precision of measurement was 0.02 °C. Observations during microwave heating were significantly different from observations without microwaves at the 1% level, and heating increased as circulating water flow was reduced from 300 ml/s to 100 ml/s. The results of a cell survival assay following hyperthermal treatment were in good agreement with expectations based on the observations of microwave heating using liquid crystals.     

Safranin O Staining Using a Microwave Oven

We investigated the effects of microwave irradiation on a safranin O staining method for paraffin sections of formalin fixed rabbit larynx. The control sections were stained according to the conventional method, and the experimental sections were stained in microwave oven for 10 sec at 360 W in Weigert's iron hematoxylin, and for 30 sec at 360 W in fast green and 0.1% safranin O staining solutions. Light microscopic examination of the sections revealed that the microwave heating did not adversely affect the staining properties of cartilage tissue compared to the conventional staining method. Small differences such as darker staining of the matrix and shrinkage of the cytoplasm was observed in some microwave treated sections. The present study revealed that microwave application can be used safely for the safranin O method with the advantage of reduced staining time.     

Super-absorbent polymers from starch-polyacrylonitrile graft copolymers by acid hydrolysis before saponification

Graft copolymers (SPAN) of polyacrylonitrile (PAN) onto starch were prepared from gelatinized starch varieties with ammonium ceric nitrate as an initiator. The molecular weight of the PAN branches increased for the varieties of starches in the order high amylose maize starch < maize starch < waxy maize starch. SPAN samples were saponified with aqueous NaOH, and the aqueous solution of the resulting polymer (HSPAN) was cast into film in a forced-air oven at 35°C. The water absorbency of the HSPAN film formed from waxy maize starch was the highest (1200 g H₂O (g dry sample)⁻¹) and that from high amylose maize starch was the lowest (530 g g⁻¹). SPAN samples from maize starch were partially hydrolyzed with dilute hydrochloric acid. The resulting polyacrylonitriles with low molecular weight starch end groups (LSPAN) were also saponified. The resulting saponified product (HLSPAN) was cast into film. The absorbencies of HLSPAN films were found to be far larger (up to 6000 g g⁻¹) than those of the corresponding HSPAN films. The absorbency increased with increasing molecular weight of PAN in the initial SPAN up to a molecular weight of 1−1·5 × 10⁶. The absorbency decreased significantly when HSPAN and HLSPAN films were subjected to heat treatment at 135°C or above. The crosslinks present in HSPAN and HLSPAN films prepared at 35°C and those formed during heat treatment were considered to have different structures: the former formed between carbohydrate alkoxide ions and nitrile groups at the early stages of saponification and the latter formed between carbohydrate and copoly(acrylate-acrylamide) chains and/or between copoly(acrylate-acrylamide) chains.     

Evidence for a specific microwave radiation effect on the green fluorescent protein

We have compared the effect of microwave irradiation and of conventional heating on the fluorescence of solution-based green fluorescent protein. A specialized near-field 8.5 GHz microwave applicator operating at 250 mW input microwave power was used. The solution temperature, the intensity, and the spectrum of the green fluorescent protein fluorescence 1), under microwave irradiation and 2), under conventional heating, were measured. In both cases the fluorescence intensity decreases and the spectrum becomes red-shifted. Although the microwave irradiation heats the solution, the microwave-induced changes in fluorescence cannot be explained by heating alone. Several possible scenarios are discussed.