Drying Of Slices Research Articles

Non-isothermal drying of garlic slices (Allium sativum, L.): Wave period and initial temperature of the heating/cooling effect

Intermittent drying is a common technique for food conservation. The objective of this work is to evaluate the effect of non-isothermal drying and the wave periods using the effective diffusivity (Deff), the glass transition temperature (Tg), their relationship to moisture content and the temperature of the product, as parameters for controlling quality deterioration. The heating–cooling profile was developed as a trapezoidal wave form using three different wave periods (τ:60, 120 and 160min), with a midpoint temperature (Tm) of 50°C and ∆T=10°C. The ascending (AC) heating–cooling treatment began at 40°C, and the descending (DC) heating–cooling treatment began at 60°C. The drying temperature (Ta), the surface temperature of the garlic (Ts), the internal temperature of the garlic (Ti), and the weight of the garlic slices were measured and recorded in real time. The Deff was calculated using the slope method. A higher drying rate was observed in the AC drying treatment. The Tg variations were associated with the Ti and moisture content of the garlic. The drying time (ts) for the AC and the DC heating–cooling profiles was found to be between 180 to 238min.

Evolution of the driving forces during convective drying of carrot slices

To explore the drying characteristics, a model combining the Fick’s second law of diffusion with a convective boundary condition on the material surface is solved numerically. And the evolutions of the moisture gradient within the sample (internal driving force) and the relative humidity difference between the bulk air and the material surface (external driving force) with drying time are obtained. These obtained results show that the relative humidity difference remains its maximum for some period at the beginning of drying, and then falls progressively to zero for samples with high initial moisture content. However, it falls from its maximum to zero without constant period for samples with low initial moisture content. And the moisture gradient in the outer is always greater than that in the inner within the sample. Correspondingly, the moisture migration flux in the outer is always greater than that in the inner within the sample. These observations indicate that both the external and the internal driving force simultaneously control the whole drying process, and the transferring rate of internal moisture cannot be equal to or higher than that of surface moisture. Thus, the magnitude of drying rate is directly dominated by the external driving force.

Contacting ultrasound enhanced hot-air convective drying of garlic slices: Mass transfer modeling and quality evaluation

A self-designed hot-air convective dryer coupled with contacting ultrasound system was used for the dehydration of garlic slices. Contacting ultrasound significantly accelerated the drying process. Compared with non-ultrasound treated samples, the drying time of garlic samples sonicated at 1513.5 W/m2 was shortened by 35.0% at 50 °C, 48.5% at 60 °C and 50.0% at 70 °C. According to water transport simulation using a diffusion model considering sample shrinkage, both water effective diffusivity and external mass transfer coefficient were enhanced when garlic samples contacted ultrasound directly during drying. In the meantime, internal water diffusion was more affected by contacting ultrasound. Furthermore, contacting ultrasound had a positive effect on the preservation of organosulfur compounds in garlic slices during drying. The browning problem of garlic products during hot-air drying was also alleviated if contacting ultrasound treatment was performed properly. Besides, no over-heating problem was observed during contacting ultrasound enhanced drying within the studied experimental range.

Convective and microwave drying of onion slices regarding texture attributes

The textural characteristics of onion slices (Allium cepa L.) of 3- and 7-mm thicknesses undergoing convective drying (50, 60, and 70°C) and microwave drying (68, 204, and 340 W real effective power levels) techniques with or without pre-treatment were evaluated. Pre-treatment consisted of dipping into brine solution (8% NaCl). Texture profile analysis at 25% compression was applied and hardness, chewiness, springiness and gumminess values of onions were recorded. As the temperature (convective) or real effective power level (microwave) increased, the hardness and chewiness levels of dried onion slices were enhanced. The values of these parameters were higher in response to microwave application compared to convective drying. Pre-treatment had an additive effect on hardness and chewiness in convectively dried samples. Maximum springiness was observed in response to convective drying. Dipping in brine solution generally caused no significant changes among slices of the same thickness (P < 0.05). With respect to gumminess, the results were quite similar to those for hardness and chewiness, i.e., markedly higher in microwave-dried onions.

Drying characteristic, enzyme inactivation and browning pigmentation kinetics of controlled humidity-convective drying of banana slices

Investigating the kinetics of enzyme activities and browning indexes in food are very essential in understanding the enzyme inactivation and browning pigmentation reaction during drying processing. In order to understand and predict accurately the enzyme inactivation and browning pigmentation of banana slices using Relative Humidity (RH)-convective hot air dryer aided by ultrasound (US) pretreatment, this study was conducted. Drying was carried out with 20 kHz frequency of US-pretreatment using three durations (10 20 and 30 min) and RH (10 20 and 30%) conditions at 70 °C and 2.0 m/s air velocity. The kinetic study of both enzyme inactivation and browning pigmentation results were compared to their relevance of fit in terms of coefficient of correlation (R2), the root mean square error (RMSE) and the reduced chi-square (χ2). First order and second-order polynomial kinetic model fitted well for enzyme inactivation and browning indexes respectively. Both enzymes inactivation kinetics and enzymatic browning index (EBI) declined significantly (p < 0.05) with increasing drying time in all drying conditions and rate of decrease intensified in longer US-pretreatment duration and lower RH conditions. However, shorter US-pretreatment duration and higher RH conditions reduced the non- enzymatic browning index (NBI) significantly. Again, longer US-pretreatment duration and lower RH shortened the drying time but adversely created more microspores from the micrograph study. Longer US pretreatment and lower RH decrease significantly (p < 0.05) the L* and b* values whereas the a* values was increased.

The kinetics and thermodynamics study of bioactive compounds and antioxidant degradation of dried banana (Musa ssp.) slices using controlled humidity convective air drying

Investigating the kinetics of bioactive and antioxidant compounds in food are very crucial in understanding the degradation reaction during storage and processing. To understand the ameliorative effect of relative humidity (RH) and predict accurately the degradation of bioactive and antioxidant compounds of banana slice using RH-convective hot air dryer, this study was conducted. Drying was investigated under three RH (10, 20 and 30%) and temperatures (60, 70 and 80 °C) at 2.0 m/s air velocity. Two mathematical models describing degradation of food properties were employed and results were compared to their goodness of fit in terms of coefficient of correlation (R2), the root mean square error (RMSE) and the reduced Chi square (\({\chi ^2}\)). First-order model could satisfactorily describe degradation bioactive and antioxidant compounds of drying of banana slices with highest R2, and lowest RMSE and\(~{\chi ^2}\). The enthalpy changes were significantly (p < 0.05) difference among RH conditions. Again, non-spontaneous reaction and lower structural freedom of the transition state compared with reactant were observed in the degradation bioactive and antioxidant as a result of positive and negative values of Gibbs free energy and entropy changes respectively. The results reveal that a range of 4.5–10.7% of these compounds in dried banana slices were retained by 10% increase in RH. This suggests that higher drying temperatures can be applied to achieve higher retention of nutrients and shorten drying time when higher RH drying conditions are considered in the food industry.

Mass Transfer and Color Changes Kinetics of Infrared-Vacuum Drying of Grapefruit Slices

ABSTRACTThe effects of combined infrared-vacuum drying parameters, including infrared power (300–400 W), pressure (5–25 kPa) and time (0–140 min) on the drying kinetics of grapefruit slices were investigated. Both the infrared lamp power and vacuum pressure influenced the drying time of grapefruit slices. The regression results showed that the quadratic model satisfactorily described the drying behavior with highest R and lowest SE values. The effective moisture diffusivity increases as power increases and range between 5.83 × 10–10 and 2.13 × 10–9 m2/s. The color scale parameters, including redness (a*), yellowness (b*) and lightness (L*) values of dried grapefruit slices increased during drying. The rise in infrared power has a negative effect on the color change intensity (∆E) and with increasing in infrared radiation power it was increased. Different kinetic models were used to fit the experimental data and the results revealed that the power model was the most suitable to describe the ∆E.

Microwave vacuum drying and multi-flash drying of pumpkin slices

In this study, the effect of different drying methods on the drying kinetics, product structures, rehydration kinetics and rehydration indices of dehydrated pumpkin slices was evaluated. For that, pumpkin slices were dehydrated by five different methods: i) Microwave multi-flash drying (MWMFD), ii) Microwave vacuum drying (MWVD), iii) Conductive multi-flash drying (KMFD), iv) Freeze-drying (FD), and v) Air-drying (AD). For reaching a moisture content of 0.022 g g−1 (dry basis, db) and water activity of 0.438, the drying times were 1.28 h for MWVD, 1.32 h for MWMFD, 3.50 h for KMFD, 16.33 h for AD and 16.67 h for FD. All the drying methods presented a constant drying rate period and a falling rate period. The highest constant drying rate was observed for MWVD, which was 1.3, 8, 22 and 53 times higher than the observed for MWMFD, KMFD, AD, and FD, respectively. Stereo micrographs of samples from multi-flash drying processes (KMFD and MWMFD) and from MWVD showed large pores in a highly porous structure. Rehydration of dried pumpkin slices was studied at 25 °C and 80 °C. Water temperature influenced samples rehydration ratio (RR = mass of rehydrated sample/mass of dried sample). RR up to 15 was observed at 25 °C, while RR up to 12 was observed at 80 °C. Samples from MWMFD and MWVD, rehydrated at 25 °C presented higher moisture after rehydration than those observed from AD, FD and KMFD samples. At 80 °C, FD samples showed the highest RR. From the whole results, one can conclude that MWVD and MWMFD are the suitable processes for producing dehydrated pumpkin slices in very short drying times.

Tomato slices drying in a liquid desiccant-assisted solar dryer coupled with a photovoltaic-thermal regeneration system

A liquid desiccant-assisted solar dryer was developed for tomato slices drying. In this dryer a photovoltaic-thermal solar collector was used to supply the required electrical energy and regenerate the desiccant. Experimental analysis of the dryer indicated that electrical energy needed for tomato drying was ranged from 0.65 to 1.4 kW h. The maximum specific moisture extraction rate was around 0.275 kg/kW h. It was also revealed that both solar heat fraction and ratio of solar electricity to consumed electricity increased with decreasing the drying temperature. The designed dryer was able to supply the required electricity independent of grid with drying at the temperature of 60–65 °C and regeneration’s pump activation relative humidity (RH) of 28%. Color analysis of the dried tomato indicated that the increase in the temperature of the drying air increased the lightness (L∗), and yellowness (b∗) values, but the increase in the RH decreased the values of lightness and redness (a∗). In addition, the hue angle values of dried tomatoes increased with increase of the activation RH and drying temperature, producing dry slices with slightly light red color. Based on the fair value of solar electricity generation and good color qualities of the final products, the temperature of 60 °C and activation RH of 23% was recommended for tomato slices drying in this dryer.

Nondestructive quality evaluation of banana slices during microwave vacuum drying using spectral and imaging techniques

ABSTRACTMicrowave vacuum drying of banana slices was investigated in this study. Near-infrared hyperspectral imaging (NIR-HSI, 950–1650 nm with 7 nm increments) and chemometrics were used to predict moisture content, hardness, and fracturability of banana slices during the entire drying process. A computer vision system was also used to evaluate browning index (BI) of slices at different drying stages; dark brown spots observed in slices were subjected to over 21-min total microwave heating. In addition, thermal imaging results showed that higher temperature occurred in the central part of banana slices. Calibration models were developed based on partial least square regression and support vector machine regression (SVM-R) using mean spectra with standard normal variate pretreatment. The results showed that SVM-R models performed better in the prediction of the three quality attributes with of 0.996, 0.927, and 0.961, respectively. The moisture distribution maps were generated using NIR-HSI; uniformly distributed moisture patterns were observed in each drying batch with small variations observed within individual slices. This study demonstrated the potential of spectral and imaging techniques for nondestructive quality evaluation of food products during drying processes.

Influence of Drying Methods on Flour Quality and Cyanide Content of Cassava Root Tuber

The aim of this particular work was to study effect of different drying methods on the quality attributes of cassava flour focusing on physico-chemical properties (moisture, water activity, pH and titratable acidity), functional properties (water and oil absorption capacity, bulk density, foaming capacity and stability) and also antinutritional factor specially cyanide content in dried cassava tuber. Losses were determined in flours after drying of slices. Prior to drying, samples were washed with clean water to remove adhering soil and other undesirable materials. The samples were sorted and hand-peeled using clean, sharp knives and then sliced into sizes of 2 to 2.5cm in thickness. Drying methods like tray drying, oven drying and sun drying were investigated. The physico-chemical properties were 11.45, 9.67, 9.23 of moisture content, 0.59, 0.54, 0.48 of water activity, 6.55, 6.74, 6.92 of pH, 2.35, 2.36, 2.33 of titratable acidity for tray dried, oven dried and sun dried cassava flours respectively using different drying time. The low moisture observed for all prepared cassava flours is a good indicator of their potential to have longer shelf life. All prepared flours predict lower water activity (< 0.5). These shows using these drying methodologies and the respective drying time of the prepared flours were safe from microbial growth during storage time. The functional properties were 0.58, 0.55, 0.49g/ml, 0.64, 0.78, 0.65ml/g, 2.11, 2.03, 1.85ml/g, 2.45, 2.73, 2.55%, and 2.34, 2.13, 1.97% of bulk density, water absorption capacity, oil absorption capacity, foam capacity and foam stability for tray dried, oven dried and sun dried cassava flours respectively. Sun dried cassava flour samples shows a lower cyanide content whereas tray dried cassava flour samples had a good functional property.

Mathematical modeling of infrared assisted hot air drying of ginger slices

Infra-red assisted hot air drying studies were conducted on ginger slices of diameter 20±2.5 mm and thickness 5±0.5 mm. Drying experiments were executed under infra-red and hot air temperature of 60°C with two levels of air velocities: 0.8 m/s and 1.4 m/s. the drying rate was found to increase proportionately with the drying air velocity, thereby minimising the total drying time. Time taken for drying ginger slices from an initial moisture content of 442 per cent (d.b.) to a final moisture content of 8.4 per cent (d.b.) at 0.8 m/s air velocity was 300 min. Whereas, it took 210 min to lower the moisture content of ginger from 433.33 per cent (d.b.) to 6.67% (d.b.) under the drying air velocity of 1.4 m/s. Infrared drying temperature of 60°C in combination with air velocity of 0.8 m/s showed better results for quality evaluation with reduced drying times. Logarithmic model fitted the experimental data well for the whole range of temperatures (R 2 = 0.9989, RMSE= 0.0119 and λ 2 = 0.000140574).

EFFECT OF PRE-TREATMENT AND AIR TEMPERATURE ON DRYING TIME OF CHERRY TOMATO

The effects of pre-treatment and air temperature with on drying and rehydration characteristics of cherry tomato slices were studied. Drying experiments are carried out with the air temperature of 55, 65, and 75°C. Drying time decreased with pre-treatment, and it also decreased considerably with increase in air temperature. The results indicated that the cherry tomatoes which were pre-treated with potassium solution were explored within the shortest time. Furthermore, superior rehydration was observed the samples which were pre-treated with potassium solution. The moisture effective diffusivity calculated from the second Fick’s law of diffusion ranged from 2.26 to 6.22 ´ 10 -9 m 2 /s over the temperature range studied. Activation energy was estimated by an Arrhenius type equation and the activation energy values varied from 26.51 to 32.79 kJ/mol.

Postharvest monitoring of organic potato (cv. Anuschka) during hot‐air drying using visible–NIR hyperspectral imaging

The potential of hyperspectral imaging (500-1010 nm) was evaluated for monitoring of the quality of potato slices (var. Anuschka) of 5, 7 and 9 mm thickness subjected to air drying at 50 °C. The study investigated three different feature selection methods for the prediction of dry basis moisture content and colour of potato slices using partial least squares regression (PLS). The feature selection strategies tested include interval PLS regression (iPLS), and differences and ratios between raw reflectance values for each possible pair of wavelengths (R[λ1 ]-R[λ2 ] and R[λ1 ]:R[λ2 ], respectively). Moreover, the combination of spectral and spatial domains was tested. Excellent results were obtained using the iPLS algorithm. However, features from both datasets of raw reflectance differences and ratios represent suitable alternatives for development of low-complex prediction models. Finally, the dry basis moisture content was high accurately predicted by combining spectral data (i.e. R[511 nm]-R[994 nm]) and spatial domain (i.e. relative area shrinkage of slice). Modelling the data acquired during drying through hyperspectral imaging can provide useful information concerning the chemical and physicochemical changes of the product. With all this information, the proposed approach lays the foundations for a more efficient smart dryer that can be designed and its process optimized for drying of potato slices. © 2017 Society of Chemical Industry.

Indirect forced solar drying of banana slices: phenomenological explanation of non-isotropic shrinkage and color changes kinetics

ABSTRACTSolar drying technology is a noteworthy technique as it uses the renewable solar energy. In this study, thin slices of banana were dried by using an indirect forced solar dryer at air mass flow rates of 0.016, 0.041, and 0.082 kg s−1. In order to assess the kinetics of shrinkage and color changes, image processing technique was applied for determining area, volume, density, total color difference and browning index. Shrinkage factor of the samples was less than 1 during drying indicating non-isotropic shrinkage with contraction of inner voids. Furthermore, product shrinkage showed two descending drying steps in which the volume change was more than the evaporated water volume in the first step and equal to that in the second step. The dimensionless evaporated water volume with respect to the dimensionless volume difference of the product also revealed that two steps of volume change existed during drying separated at critical moisture ratio 0.23. The area and volume changes were only related to the product moisture content and were independent of the air mass flow rate, and hence air temperature. In contrary to the browning index, the total color difference was not influenced by air mass flow rate and the least change in browning index occurred at mass flow rate of 0.041 kg s−1.

Convective drying of osmo-dehydrated apple slices: kinetics and spatial behavior of effective mass diffusivity and moisture content

The main objective of this study is the presentation of a numerical model of liquid diffusion for the description of the convective drying of apple slices submitted to pretreatment of osmotic dehydration able of predicting the spatial distribution of effective mass diffusivity values in apple slabs. Two models that use numerical solutions of the two-dimensional diffusion equation in Cartesian coordinates with the boundary condition of third kind were proposed to describe drying. The first one does not consider the shrinkage of the product and assumes that the process parameters remain constant along the convective drying. The second one considers the shrinkage of the product and assumes that the effective mass diffusivity of water varies according to the local value of the water content in the apple samples. Process parameters were estimated from experimental data through an optimizer coupled to the numerical solutions. The osmotic pretreatment did not reduce the drying time in relation to the fresh fruits when the drying temperature was equal to 40 °C. The use of the temperature of 60 °C led to a reduction in the drying time. The model that considers the variations in the dimensions of the product and the variation in the effective mass diffusivity proved to be more adequate to describe the process.

Effect of microwave blanching and brine solution pretreatment on the quality of carrots dried in solar‐biomass hybrid dryer

The effect of microwave blanching time (0–3 min), followed by dipping in brine solution (sample to solution ratio 0–4) prior to drying of carrot slices and shreds in solar-biomass hybrid dryer on the final moisture content of the samples, drying rate, rehydration ratio, color change, textural firmness change, and beta carotene content was studied using two-factor 3 level factorial design. Pretreatment parameters were optimized using response surface methodology. As the microwave blanching time increased, there was an increase in the drying rate, rehydration ratio, and beta carotene content, with minimum changes in textural firmness, color, and final moisture content. Increase in brine solution to sample ratio impacted the drying rate and beta-carotene content in the samples, positively. Microwave blanching time of 3 min and brine solution to sample ratio of 2 were found to be the optimum pretreatment parameters for drying of carrot slices and shreds. Practical applications Solar energy is abundantly available in tropical countries like India and this renewable energy can be utilized to preserve food materials, economically. Carrots being seasonal in nature require be drying and preserving for further use during off seasons, and potential of a solar-biomass hybrid dryer can be tapped to reduce the energy requirement for a high energy requiring processing operation like drying. The study recommends pretreatments to carrot slices and shreds like dry microwave blanching, followed by brine solution dipping of carrot slices and shreds for better quality in terms of faster moisture removal leading to lower drying period, beta-carotene retention, better rehydration characteristics as compared to control samples, where no such pretreatments were given. This recommendation can be very well be utilized to reduce the energy demand in food processing industry.

Heat and mass transfer coefficients and modeling of infrared drying of banana slices

ABSTRACT Banana is one of the most consumed fruits in the world, having a large part of its production performed in tropical countries. This product possesses a wide range of vitamins and minerals, being an important component of the alimentation worldwide. However, the shelf life of bananas is short, thus requiring procedures to prevent the quality loss and increase the shelf life. One of these procedures widely used is drying. This work aimed to study the infrared drying process of banana slices (cv. Prata) and determine the heat and mass transfer coefficients of this process. In addition, effective diffusion coefficient and relationship between ripening stages of banana and drying were obtained. Banana slices at four different ripening stages were dried using a dryer with infrared heating source with four different temperatures (65, 75, 85, and 95 ºC). Midilli model was the one that best represented infrared drying of banana slices. Heat and mass transfer coefficients varied, respectively, between 46.84 and 70.54 W m-2 K-1 and 0.040 to 0.0632 m s-1 for temperature range, at the different ripening stages. Effective diffusion coefficient ranged from 1.96 to 3.59 × 10-15 m² s-1. Activation energy encountered were 16.392, 29.531, 23.194, and 25.206 kJ mol-1 for 2nd, 3rd, 5th, and 7th ripening stages, respectively. Ripening stages did not affect the infrared drying of bananas.

Simultaneous application of microwave energy and hot air to whole drying process of apple slices: drying kinetics, modeling, temperature profile and energy aspect

Drying kinetics, modeling, temperature profile and energy indices were investigated in apple slices during drying by a specially designed microwave-hot air domestic hybrid oven at the following conditions: 120, 150 and 180 W microwave powers coupled with 50, 60 and 70 °C air temperatures. Both sources of energy were applied simultaneously during the whole drying processes. The drying process continued until the moisture content of apple slices reached to 20% from 86.3% (wet basis, w.b). Drying times ranged from 330 to 800 min and decreased with increasing microwave power and air temperatures. The constant rate period was only observed at low microwave powers and air temperatures. Two falling rate periods were observed. Temperature of apple slices sharply increased within the first 60 min, then reached equilibrium with drying medium and finally increased at the end of the drying process. In order to describe drying behavior of apple slices nine empirical models were applied. The Modified Logistic Model fitted the best our experimental data (R 2 = 0.9955–0.9998; χ 2 = 3.46 × 10−5-7.85 × 10−4 and RMSE = 0.0052–0.0221). The effective moisture and thermal diffusivities were calculated by Fick’s second law and ranged from 1.42 × 10−9 to 3.31 × 10−9 m2/s and 7.70 × 10−9 to 12.54 × 10−9 m2/s, respectively. The activation energy (Ea) values were calculated from effective moisture diffusivity (Deff), thermal diffusivity (α) and the rate constant of the best model (k). The Ea values found from these three terms were similar and varied from 13.04 to 33.52 kJ/mol. Energy consumption and specific energy requirement of the hybrid drying of apple slices decreased and energy efficiency of the drying system increased with increasing microwave power and air temperature. Apples can be dried rapidly and effectively by use of the hybrid technique.

Simulation and validation of a suitable model for thin layer drying of ginger rhizomes in an induced draft dryer

ABSTRACTVariation in drying material and their biological differences, coupled with heat supply method in different dryers, makes mathematical modeling of drying complicated. Attempt was made to simulate a drying process and to identify best suitable model out of six selected drying models, for drying of ginger slices in a solar-biomass integrated drying system designed and developed for spice drying. Moisture content data were converted into the moisture ratio (MR) expressions and curve fitting with drying time for the selected drying models was analyzed. Sigma Plot software was used for nonlinear regression to the data obtained during drying and for modeling of drying curves. The suitability of the models was evaluated in terms of statistical parameters such as coefficient of determination (R2), mean percentage error (P), and standard error estimate. Drying air temperature was in the range of 47–55°C and air velocity was between 1.0 and 1.3 m s−1. Ginger slices were dried from 88.13% to 7.65 ± 0.65% (wb) in 16 h. Trays were interchanged in a predetermined matrix sequence from 4 h onwards when moisture content was reduced to 60–70% (wb), for uniformity in drying. Highest value of R2 (0.997), lowest value of SEE (0.020), and P value < 0.0001 established Page model as the best suitable model for the developed drying system. The predicted MRs were in good agreement with the experimental values and the effective moisture diffusivity for ginger was found to be 2.97 × 10–7 m2 s−1.