Methodology for enhancing drying rate and improving maize quality in a fluidised-bed dryer

A theoretical diffusion model for a single maize (Zea mays L.) kernel has been developed to describe moisture migration during drying and tempering stages. The effective diffusivity was determined experimentally at a range of temperatures between 90°C and 170°C with initial moisture contents of 37% and 43% dry-basis. The functional relationship between the diffusivity and temperature is analogous to a modified Arrhenius equation.Two-stage drying including tempering between stages provides not only faster removal of moisture, but also a reduced number of stress cracks when compared to single-stage drying. However, the colour of the maize appears darker and more intense with longer tempering periods. To maintain maize quality and to reduce energy consumption, the results from the experiments and simulations are limited to temperatures of less than 150°C; high-moisture maize should be dried to 23% dry-basis and then tempered for 40 min.The ambient-air velocity for cooling the tempered maize in a range between 0.075 and 0.375 m/s does not affect the maize quality as measured by the number of stress cracks and colour.     

Pasteurization process development for controlling Salmonella in in-shell almonds using radio frequency energy

Radio frequency (RF) treatment holds potential as a pasteurization method to control Salmonella in almonds without causing a substantial loss of product quality. Thermal resistance of Salmonella can be reduced by increasing water activity, thus a soaking process was designed prior to RF treatments. A pilot-scale 27 MHz, 6 kW RF heating system was used to rapidly heat 1.7 kg washed in-shell almonds with hot air heating at 55 °C. To achieve appropriate heating rate, constant drying temperature and short time cooling, the RF treatment protocol was obtained using an electrode gap of 13 cm for heating, 14 cm for drying, and followed by forced room air cooling of 5-cm thick samples. The results showed that almond temperatures above 75 °C at 23% moisture contents for 2-4 min RF heating could meet the requirements to achieve 5-log reduction of Salmonella. The RF treatment process for 20 min reduced the moisture content to 5.7% w.b. Peroxide value, fatty acids values and kernel colors of the RF treated almonds met good quality standard used by nut industry.     

A wild strain of Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) from farm-stored maize in South Carolina: Development under different temperature, moisture, and dietary conditions

The purpose of this study was to determine the duration of immature development and survivorship of Plodia interpunctella (Hübner) on maize over a range of temperatures and grain moisture contents encountered in maize stored on farms in the southeastern states (USA). Laboratory cultures were established with moths collected from farm-stored maize in South Carolina and maintained on cracked maize at 30 °C and 60% r.h. The incubation period and percentage hatch of eggs was determined at 18 combinations of temperature and r.h. Hatch was <1% at 15 and 40 °C. In the range 20-35 °C, percentage hatch declined as temperature increased, and the mean incubation period ranged from 3.1 to 8.5 d. Neither percentage hatch nor incubation period were affected by r.h. between 43% and 76%. The relationship between mean developmental period (oviposition to adult eclosion) and temperature was well described by a quadratic polynomial that predicted a decline from 67.6 to 30.1 d as temperature increased from 20 to 31.1 °C, followed by an increase to 38.5 d as temperature increased further to 35 °C. The results suggest a lower temperature threshold for development near 15 °C and an upper limit slightly greater than 35 °C. Moisture content had a significant effect on developmental period at all the temperatures studied, but the pattern of variation with moisture depended upon the temperature.     

Impact of limited drying on Momordica cochinchinensis Spreng. aril carotenoids content and antioxidant activity

Food product based on gac fruit (Momordica cochinchinensis Spreng.) arils have a high potential due to the high carotenoids content of this fruit. Drying is a key preparation step for carotenoids extraction from gac fruit in a economically viable process. The impact of different drying technics, temperature, final product moisture content on the carotenoid content, hydrophilic and lipophilic antioxidant activity (evaluated with three methods) and color of the gac arils is discussed based on laboratory scale experimental tests. The results highlight an optimal temperature between 50 °C and 60 °C to conserve the color, the carotenoid content and the antioxidant activity. Also, these properties are better preserved by limiting the drying to dry based moisture content between 15% and 18% while the advantages of drying for further processing and for refrigerated conservation for a few months are achieved.     

TG–DTA approach for designing reference methods for moisture content determination in food powders

There is no systematic procedure described in the literature to establish a robust and accurate reference method for determining the moisture content in any solid food product. In this paper, we are proposing a new approach based on simultaneous thermogravimetry and differential thermal analysis (TG–DTA), with data for several amorphous food powders that result from spray-drying, freeze-drying or extrusion. In the first step, by heating a representative sample of about 20 mg at 2 °C/min we would detect the temperature and the mass loss at the inflection point that characterises, if there is an inflection, the end of the drying and the onset of chemical reactions. In cases of not too much sensitive products, the mass loss at the inflection may be considered as a good estimation of the moisture content. At 2 °C/min heating rate, the inflection temperature T_i is an indicator that allows estimating the optimal isothermal drying temperature T_d about 15–30 °C below T_i, depending on the product sensitivity to heat treatments and the kinetics of water molecule diffusion through the amorphous matrix. Then, a series of three isothermal drying are performed at about T_d−12 °C, T_d and T_d+8 °C, and a simple multilinear model allows calculating the best oven temperature to achieve the optimal moisture content determination in 2 h. This procedure is described and results are shown for several dehydrated food products: milk, coffee, cereal and pet food. This fast procedure may be applied either for establishing optimal oven conditions for most amorphous new products or for revising conditions that have been established in the past but are not robust enough for several ones.     

Extrusion of a hard-to-cook bean (Phaseolus vulgaris L.) and quality protein maize (Zea mays L.) flour blend

Heated extrusion was tested as an alternative process for incorporating “hard-to-cook” beans into food products. A 3² factorial design was used to evaluate extrusion conditions for a 40/60 (w/w) blend of “hard-to-cook” beans and quality protein maize. Tested extrusion variables were temperature (155, 170 and 185 °C) and moisture content (15.5, 17.5 and 19.5 g/100 g). Screw speed was fixed at 130 rpm. The extrudates obtained at 155 and 170 °C with 15.5% moisture had the best physical characteristics and were chosen for comparative analysis of nutritional changes between the unprocessed “hard-to-cook” bean/quality protein maize flour blend and the resulting extrudates. In vitro protein digestibility was higher in the extrudates (80%) than in the flour blend (76%). In vitro starch digestibility was higher at 155 °C (89%) and 170 °C (92%) than in the flour blend (12%). Processing conditions decreased dietary fibre content by 38% at 155 °C and 44% at 170 °C.     

Effect of drying and tempering on rice fissuring analysed by integrating intra-kernel moisture distribution

The fissure generation of one short- and two long-grain rough rice dried at different air conditions for 15, 30 and 60 min, and respectively, tempered at 20, 30, 40 and 50 °C for tempering duration ranging from 0 to 330 min was analysed by integrating moisture gradients in rice kernels. Results indicated that drying and tempering processes had significant effects on the moisture gradient and rice fissuring. The percentage of fissured kernels increased with increasing drying time and decreasing tempering time. The tempering duration required for long-grain rice was shorter than that for short-grain rice to prevent kernel fissuring. Higher tempering temperatures can effectively eliminate moisture gradients and then lower the rice fissuring generation. Tempering at 50 °C reduced fissuring incidence by 32 to 50% compared to tempering at 20 °C. Moreover, short-grain rough rice was very susceptible to fissure whereas long-grain rough rice with a high amylose content was much more fissure-tolerance.     

Investigations on head-rice yield and operating time in the fluidised-bed drying process: experiment and simulation

This analysis of the drying process is based on experiments and simulations of the influence of temperature in the fluidised-bed dryer, moisture content and tempering period on head-rice yield, and operating time. A mathematical model incorporating the head-rice yield equation for predictions of the moisture content and the grain temperature in each drying stage and the final head-rice yield, is in good agreement with experimental data. The moisture content after first-stage drying and tempering have a dominant effect on head-rice yield and operating time in reducing high-moisture contents to a safe level. The simulation results recommend that the allowable temperature should be not higher than 150 °C for the first stage and the moisture content after first-stage drying should be not lower than 22.5% dry basis, with subsequent tempering for at least 25 min.     

Drying kinetics of whole durum wheat pasta according to temperature and relative humidity

Drying kinetics of bran-free and bran-rich pasta (whole durum) was determined according to temperature (40, 60 or 80 °C) and relative humidity (65%, 75% or 85%). Compared to temperature, relative humidity in drying chamber had a greater effect on pasta effective moisture diffusivity (α<0.01), and both parameters responded to a modified Arrhenius-type equation. Activation energy of pasta (11.4 kJ mol⁻¹) was lower than reported in the literature. Bran changed the course of pasta drying, depending on temperature and relative humidity. When relative humidity was higher than 75%, effective moisture diffusivity of bran-rich pasta decreased but the reverse was observed below 75%. Above 76 °C, equilibrium moisture content of bran-rich pasta was higher than bran-free pasta. In conclusion, optimal drying conditions for bran-rich pasta were different than standard (bran-free) pasta. Close control of relative humidity in pasta drying unit would be critical, especially under high relative humidity and high temperature conditions.     

Drying of yellow pea starch on inert carriers: Drying kinetics,moisture diffusivity,and product quality

This study aimed at investigating the drying of yellow pea starch dispersions on inert solid carriers and determining the drying kinetics, moisture diffusivity and the product quality, quantified through damage index and final moisture content. Drying kinetics accomplished in a convective drying tunnel show that the overall mass transfer is controlled by internal migration of moisture within the starch particles. For a given inlet air temperature from 100 to 180 °C, the apparent diffusion coefficient derived from the drying curves increases exponentially with the instantaneous moisture content, with values ranging from 4 × 10⁻¹¹ to 3 × 10⁻⁸ m²/s. Due to low diffusivity and the thin coat formed on the surface of solid carriers, the resistance to internal diffusion is negligible as compared to the overall mass transfer resistance when drying of starch dispersions takes place on inert solid carriers. Drying of yellow pea starch dispersion on Teflon particles as inert carriers was studied in laboratory and pilot fast spouted bed dryer for inlet air temperatures from 140 to 240 °C, and initial solid content of 38%, mass (d.b.). The starch damage index for targeted product moisture content was below 2.5% in the inlet air temperature range from 120 to 210 °C, when atomizing from the bottom of dryer.     

A comparative evaluation of methodologies for water content determination in green coffee

The main objective of this study was to compare methods for mass loss evaluation in green coffee to water content determination by Karl Fischer titration (KFT). The following methodologies were tested: (i) ISO 6673 (oven drying at 105 °C for 16 h); (ii) the reference method employed by the Brazilian Agriculture Ministry (oven drying at 105 °C for 24 h)—BRAMw, employing whole beans and BRAMg, employing ground beans; and (iii) infrared drying (IRD). Reference oven drying methodologies ISO 6673 and BRAMw presented results statistically equivalent (p>0.05) to those from KFT in the moisture content range that is of interest for green coffee commercialization (8-13 g/100 g), whereas IRD results were lower than those for KFT. ISO 6673 and BRAMw also presented the highest values of correlation coefficients to KFT. Differences in moisture content determination became more significant for lower moisture content values (4-7 g/100 g), probably due to loss of organic volatile substances during drying and occurrence of moisture loss during sample grinding.     

Effect of drying method on the moisture sorption isotherms for Inonotus obliquus mushroom

Moisture sorption isotherms of Inonotus obliquus mushroom were studied over a selected temperature range (20-50 °C). Sigmoid sorption isotherms were observed for these samples. The sorption data were analyzed using various conventional models. The Oswin model was found to be the best model for predicting the equilibrium moisture content of mushroom in the range of water activity 0.08-0.96. The monolayer moisture content decreased as temperature increased and was affected by the drying method used. The net isosteric heat of sorption was determined using the Clausius-Clapeyron equation and the value decreased with increase in moisture content of mushroom.     

Drying kinetics and quality changes during drying of red pepper

A mathematical model is proposed to simulate the process of drying of individual pieces of red pepper under constant external conditions and to predict changes in some nutritional and organoleptic attributes of the product. The model was solved numerically to obtain moisture content and temperature as well as ascorbic acid and carotenoids concentration in the product as a function of time. A good agreement between predictions and experimental data at different drying temperatures was obtained.Water sorption isotherms of red pepper were determined in the range 20-50 °C and represented by two different sorption equations. Drying kinetics were represented by a diffusive model, the effective moisture diffusivity ranging from 5.01 to 8.32×10⁻¹⁰ m²/s with an activation energy of 23.35 kJ/mol. Degradation kinetics for ascorbic acid and total carotenoids were measured in the range 50-70 °C and modelled as first-order reactions. The rate constants increased with temperature and product moisture content. Average activation energies for carotenoids and vitamin C degradation were 50.1 and 26.9 kJ/mol, respectively.     

Mathematical model of pork slice drying using superheated steam

Superheated steam has received much attention as an effective technique for drying purposes because it produces dried products with high quality attributes. Although currently there are a number of works reporting the development of a mathematical model of superheated steam drying, they do not use a numerical method to estimate the effective moisture diffusivity value (D_eff value) of the product. The purposes of this work, therefore, were to develop a semi-empirical model for estimating the D_eff value of pork, and for predicting the evolution of the moisture content and the center temperature of sliced pork during superheated steam drying. The model was based on mass and energy-balance equations and was divided into three periods: heating up, constant drying rate and falling drying rate period. It was solved using an explicit finite difference method and used a grid search method to estimate the D_eff value of pork. The predicted results were compared with the experimental data of superheated steam drying of seasoned and unseasoned pork with slice thicknesses of 1 and 2 mm at a drying temperature of 140 °C. The comparison results showed that the developed model could estimate the ranges of the D_eff value of pork fairly well (D_eff = 3.311-12.471 × 10⁻¹⁰ m²/s for seasoned pork, and 4.200-15.056 × 10⁻¹⁰ m²/s for unseasoned pork) and could reasonably predict the evolution of the moisture content of the sliced pork. The predicted center temperature of the sliced pork was higher than the experimental data in the heating up period and in the first 5 min of the falling drying rate period, but it agreed well in the constant drying rate period and after the drying time of 10 min. Moreover, it was found that the slice thickness and the seasoning had an influence on the drying curves only in the constant drying rate and falling drying rate period.     

Carbohydrate composition and color development during drying and roasting of macadamia nuts (Macadamia integrifolia)

Research was conducted to determine whether variability in sugar content contributes to differences in kernel browning during processing of macadamia nut (Macadamia integrifolia) cultivars, Kau (HAES 344), Keaau (HAES 660), Keauhou (HAES 246), and Kakea (HAES 508). At harvest, total sugar content of fresh macadamia kernels varied from 2.9 to 5.6 g/100 g dry weight basis (db), and the average moisture content ranged from 15.6 to 23.6 g/100 g fresh weight. Cultivars differed in kernel sucrose content, but not reducing sugar content. Reducing sugars decreased during drying, and kernel centers darkened slightly. An incremental drying process limited sucrose hydrolysis, minimizing the amount of glucose and fructose available for browning reactions. Therefore, the centers of roasted kernels were not darker than dried kernels. The variability in sugar composition in fresh kernels had a minimal impact on color quality when low-temperature drying and roasting at 125 °C were used. However, when roasted kernels received from a processor were separated based on color quality, kernels with internal or external browning had higher reducing sugar concentrations (0.24-0.27 g/100 g db) than cream-colored kernels (0.03 g/100 g db). Immature kernels had higher sucrose and reducing sugar contents and more browning than mature kernels. During commercial processing, optimal conditions may not be achieved and the presence of immature nuts can contribute to kernel browning.     

Fat oxidation in freeze-dried grouper during storage at different temperatures and moisture contents

The effects of moisture content levels and storage temperature on the lipid oxidation in freeze-dried grouper were studied. Three trends were observed in the changes of PV (peroxide value) depending on the moisture content and storage temperature. An exponential rise of PV was observed (p < 0.05) at lower temperatures (−20 °C and 5 °C for samples at moisture 5 g/100 g; and −20 °C for samples at moisture 20 g/100 g). At temperatures 25 °C for samples at moisture 5 g /100 g, and 5 and 25 °C for samples at 20 g moisture/100 g peaks of PV were observed (p < 0.05). At higher temperatures (40 °C for both moisture content levels) an exponential decay instead of increase were observed (p < 0.05). Moisture content levels had no significant effect (p > 0.05) on PV stored at temperature of −40 °C. The monolayer moisture for freeze-dried grouper was estimated as 6.2 g/100 g dry solids using BET-isotherm. Differential scanning calorimetry (DSC and modulated DSC) thermogram line showed two shifts (i.e. glass transition) and two endothermic peaks, one for melting of oil and another for decomposition. The validity of the water activity and glass transition concepts were evaluated. It is concluded that glass transition concept may be used to explain the process more adequately.     

Moisture adsorption isotherms for mushroom

The moisture adsorption isotherms of Agaricus bisporus and Pleurotus florida mushrooms were determined at temperatures (30-70°C) typically found in drying and storage. The samples were equilibrated above saturated salt solutions. Equilibrium moisture content of mushroom decreased with an increase in temperature at constant water activity. The data was adjusted to 11 sorption models to ascertain the best fit. Comparisons were based on mean relative error, standard deviation and coefficient of determination. Of the models tested, the Chung and Pfost model showed the best fit. The net isosteric heat of sorption was determined using the Clausius-Clapeyron equation. The net isosteric heat of sorption decreased from 13 to about 1 kJ/mol when the moisture content increased from 5 to 50 g /100 g dry matter.     

Optimization of oven drying conditions for lycopene content and lipophilic antioxidant capacity in a by-product of the pink guava puree industry using response surface methodology

Response surface methodology (RSM) was applied to optimize the oven drying conditions for lycopene content (Y₁) and lipophilic antioxidant capacity (Y₂) in decanter, a by-product of the pink guava puree industry. Two-factor central composite design was employed to determine the effects of two independent variables, namely temperature (X₁: 50-80 °C) and drying time (X₂: 4-6 h). Lycopene content and lipophilic antioxidant capacity were measured using high-performance liquid chromatography (HPLC) and the ABTS radicals scavenging assay, respectively. A β-carotene bleaching assay was also applied to measure the antioxidant activity. Response surface plots showed that an increase in temperature and time significantly reduced the response variables. The optimum oven conditions for drying of decanter with minimum lycopene degradation were 43.8 °C for 6.4 h, with a predicted lycopene content of 14 mg/100 g and antioxidant capacity of 21 μmol LE/100 g. To validate the optimized model, the experimental values were compared with the predicted values to check the adequacy of the model. The experimental values were found to be in agreement with those predicted, indicating the suitability of the model for optimizing the oven drying conditions for decanter.     

Superheated steam drying characteristic and moisture diffusivity of distillers’ wet grains and condensed distillers’ solubles

Samples of distillers’ spent grains were prepared by blending different proportions of distillers’ wet grains (DWG) with condensed distillers’ solubles (CDS). Such samples when dried with superheated steam (SS) at 110, 130 and 160 °C showed typical behaviour of drying in the falling rate period. The overall moisture diffusivity (D_m) increased with a decrease in moisture content under all drying conditions. An increase in moisture diffusivity with respect to the SS temperature was also observed. For all drying conditions, the values of average diffusivities (D_m)_avg non-corrected for shrinkage ranged from 0.52 × 10⁻⁹ to 3.08 × 10⁻⁹ m²/s. For distillers’ spent grains of different ratios of DWG to CDS, the decrease in SS temperature from 160 to 110 °C caused a decrease in (D_m)_avg by 69-82%. Increasing the amount of CDS added to the DWG from 0% to 100% caused an increase in (D_m)_avg by 14-35% for the temperature range tested in this study. The values of (D_m)_avg corrected for shrinkage ranged from 0.17 × 10⁻⁹ to 0.86 × 10⁻⁹ m²/s for all drying conditions studied. The decrease in SS temperature from 160 to 110 °C caused a decrease in (D_m)_avg by 70-74%. Not much differences were observed for the same drying temperatures and different ratios of DWG to CDS. The differences between the values of average overall moisture diffusivity (D_m)_avg corrected and non-corrected for shrinkage were significant, nearly one order of magnitude.     

Effect of continuous and intermittent ultrasound on drying time and effective diffusivity during convective drying of apple and red bell pepper

A new approach of ultrasound-assisted convective drying was tested in this study using a screen as product support and sound transmitting surface. Red bell pepper and apple cubes were dried at 70 °C in a laboratory scale hot-air drying oven. In addition to continuous ultrasound used for both products, intermittent ultrasound was applied for apple drying reducing net sonication time to 50% and 10%. Significant improvements of drying characteristics were observed for continuous sonication and intermittent ultrasound at 50% net sonication time. Experimental data were fitted with eight models, among which the Midilli model resulted in the best fit with R² > 0.9988 and reduced χ² ? 0.0002. The influence of ultrasound on drying time to reach 20% and 30% residual moisture content as well as on effective moisture diffusivities and drying periods is discussed.