KINETICS ON COLOR CHANGES DURING DRYING OF SOME FRUITS AND VEGETABLES

The effect of drying conditions on color changes of apple, banana, carrot and potato during conventional and vacuum drying was investigated. The Hunder color scale parameters redness, yellowness and lightness were used to estimate color changes during vacuum and conventional drying at 50, 70 and 90°C. Air humidity during conventional drying was regulated at 15, 30 and 40%. Air temperature and humidity affected redness and yellowness, but not lightness. A first order kinetic model was fitted to experimental data adequately for both redness and yellowness. The rate of color deterioration was found to increase as temperature increased and air humidity decreased, for both drying methods and all the examined materials.     

EFFECT OF OSMOTIC DEHYDRATION ON COLOR AND SORPTION CHARACTERISTICS OF APPLE AND BANANA

ABSTRACT

Color and sorption characteristics of osmotically treated and air dried apple and banana were studied during air drying at 70^°C. The color parameters: Lightness (L), Redness (a) and Yellowness (b) were studied, using a Hunter Lab chromatometer. A first order kinetic model was fitted to the experimental data adequately for color parameters, while sorption data for treated and air dried products were fitted to the GAB model. Untreated fruits showed an extensive browning which was monitored by a significant drop of the lightness (L) and an increase of redness (a) and yellowness (b). Osmotically pretreated samples did not brown as much as the untreated samples and the lightness L decreased only slightly while a, b increased slightly. Osmotic pretreatment resulted in a shift in sorption isotherm for both fruits. Osmotic dehydration prevented color damages and decreased the sorption capacity of dehydrated products.     

Effect of Air-Drying Temperature on Kinetics of Quality Attributes of Lemon (Citrus limon cv. lunari) Peels

The aim of this work was to determine the effect of air drying temperature (40 °C, 50 °C, and 60 °C) on the degradation kinetics of quality parameters of lemon peels. Drying at 40 °C was in favor of maximal water and oil retention capacities and lowest color loss (35%), but it also induced lower phenolic and flavonoids residual contents. First-order reaction model showed a good fit for lightness and yellowness color parameter. Zero-order reaction model fitted well redness color parameter and total color difference. A higher value of activation energy indicated greater heat sensitivity of lightness and redness during air drying.     

Drying Kinetics and Quality of Soy Residue (Okara) Dried in a Jet Spouted-Bed Dryer

ABSTRACT

The objective of this study was to investigate experimentally the effects of various drying parameters, i.e., inlet air velocity, inlet air temperature, initial bed height and heating duration, on both the drying kinetics and various quality attributes of dried okara viz. percentage changes of the total protein content, color, urease index, as well as the specific energy consumption during drying in a jet spouted-bed dryer. It was observed that all drying conditions tested could reduce the amount of urease to an acceptable level and increasing the heating duration, air velocity, and hot air temperature led to a significantly higher rate of reduction of urease activity. The percentage change of the total protein content of okara undergoing different drying conditions was not significantly different, however. After drying, the redness of okara was the highest changing color index, but its absolute value was still much lower than those observed for the lightness and yellowness. Hence, dried okara appeared light-brown. The specific energy consumption of the process was found to be in the range of 3.69 to 5.89 MJ/kg evaporated water.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

EFFECT OF OSMOTIC PRE-TREATMENT AND INFRARED RADIATION ON DRYING RATE AND COLOR CHANGES DURING DRYING OF POTATO AND PINEAPPLE

A combination of intermittent infrared and continuous convection heating was used to dry various osmotically pretreated sample of potato (in solutions of 10%, 20% and 30% NaCl) and pineapple (in solutions of 30%, 50%, 70% Brix). The effect of drying conditions on color changes of potato and pineapple was investigated. The Hunter color scale parameters (redness, yellowness and lightness) were measured to quantify the color changes. With appropriate choice of infrared intermittency as well as osmotic pretreatment, it is possible to reduce the overall color change while maintaining high drying rates. As expected, osmotic pretreatment resulted in a shift in the sorption isotherms for both products.     

Drying Kinetics and Physicochemical Characteristics of Laboratory-Prepared Corn/Wheat Distillers Grains and Solubles Dried with Superheated Steam and Hot Air

The effect of superheated steam (SS) drying and hot air (HA) drying on drying kinetics and changes in the color, crude protein, and amino acid concentrations (in particular, lysine content) of corn/wheat wet distillers grains (WDG) and centrifuged solubles (CS) was evaluated. An inversion temperature was reached at 139°C for WDG and 132°C for CS, above which moisture evaporation rate and qualitative changes under SS drying conditions exceeded the values noted in HA, and below which the reverse was observed. A significant decrease (from 8 to 50%) in the lysine content of WDG and CS was reported during SS and HA. The overall changes in the color (ΔE*) of corn/wheat WDG and CS ranged from 7.9 ± 2.6 to 27.2 ± 1.9 during SS drying and from 11.9 ± 3.7 to 32.0 ± 0.5 during HA drying. The observed deterioration in color was attributed mainly to changes in lightness (L*) and yellowness (b*) of dried samples. The values of L* and b* were reliable predictors of the lysine content of corn/wheat distillers co-products.     

Color Changes of Birch Wood During High-Temperature Drying

In low-temperature drying, a high drying rate has been found to be adequate for silver birch (Betula pendula) and European white birch (B. pubescens) timber as far as wood colour is concerned. During high-temperature drying, however, birch timber darkens significantly if steam is used as the drying medium. The objective of this research was to study the effects of drying force (wet-bulb depression), timber thickness, and initial moisture content on the color of high-temperature-dried birch wood. The reflectance spectrum of wood was measured and transformed to the CIEL?a?b? color scale.

The increase in drying force increased the lightness and decreased the redness and the yellowness of wood. At the same time, the difference in color between the surface layer and the interior of boards increased. Increase in thickness and in initial moisture content increased the difference in color between the surface and the interior of boards. Pretreatment of timber with water soaking decreased the difference in color between the surface layer and the interior of boards when low drying force was used, but this difference was increased when high drying force was used.     

EFFECT OF OSMOTIC DEHYDRATION ON COLOR AND SORPTION CHARACTERISTICS OF APPLE AND BANANA

Color and sorption characteristics of osmotically treated and air dried apple and banana were studied during air drying at 70^°C. The color parameters: Lightness (L), Redness (a) and Yellowness (b) were studied, using a Hunter Lab chromatometer. A first order kinetic model was fitted to the experimental data adequately for color parameters, while sorption data for treated and air dried products were fitted to the GAB model. Untreated fruits showed an extensive browning which was monitored by a significant drop of the lightness (L) and an increase of redness (a) and yellowness (b). Osmotically pretreated samples did not brown as much as the untreated samples and the lightness L decreased only slightly while a, b increased slightly. Osmotic pretreatment resulted in a shift in sorption isotherm for both fruits. Osmotic dehydration prevented color damages and decreased the sorption capacity of dehydrated products.     

Vacuum Frying of Carrot Chips

Abstract

Vacuum frying is an alternative method to produce high quality vegetable or fruit chips. The effect of frying temperature and vacuum degree on moisture content, oil content, color, and texture of fried carrot chips were investigated. During the early stage of vacuum frying, the rate of moisture removal and oil absorption increased with increasing frying oil temperature and degree of vacuum. Statistical analysis of the color data showed that there were no significant differences P > 0.05) in lightness (L), redness (a), and yellowness (b) of carrot chips as a function of vacuum degree and temperature. The breaking force of carrot chips decreased with increasing frying temperature and vacuum degree. The statistical analysis also showed that there were no significant differences P < 0.05) in the breaking force as a function of temperature, but the breaking force was significantly (P < 0.05) affected by the vacuum degree.     

Drying Kinetics and Quality of Soy Residue (Okara) Dried in a Jet Spouted-Bed Dryer

The objective of this study was to investigate experimentally the effects of various drying parameters, i.e., inlet air velocity, inlet air temperature, initial bed height and heating duration, on both the drying kinetics and various quality attributes of dried okara viz. percentage changes of the total protein content, color, urease index, as well as the specific energy consumption during drying in a jet spouted-bed dryer. It was observed that all drying conditions tested could reduce the amount of urease to an acceptable level and increasing the heating duration, air velocity, and hot air temperature led to a significantly higher rate of reduction of urease activity. The percentage change of the total protein content of okara undergoing different drying conditions was not significantly different, however. After drying, the redness of okara was the highest changing color index, but its absolute value was still much lower than those observed for the lightness and yellowness. Hence, dried okara appeared light-brown. The specific energy consumption of the process was found to be in the range of 3.69 to 5.89 MJ/kg evaporated water.     

Effect of Variety on Drying Characteristics and Selected Quality Attributes of Dried Carrots

The effects of variety on drying characteristics, color, and water absorption of carrots were investigated. Six different varieties of carrots, viz. Kazan, Maxima, Nandor, Nektarina, Simba, and Tito were evaluated. The hot air drying characteristics of carrot cubes dried under forced convection conditions were determined and drying data were analyzed to obtain parameters of Page and first-order kinetic models as well as moisture diffusivity. Color characteristics were determined for fresh, dried, and dehydrated samples by measuring lightness (L*), redness (a*), and yellowness (b*). Water absorption data were analyzed for ground samples. It was observed that drying characteristics, such as color and water absorption were significantly influenced by variety. The varieties of Kazan and Nektarina were found to be characterized by highest and lowest moisture diffusivity of 7.52 × 10^?9 and 3.31 × 10^?9m²/s respectively. Kazan variety was also characterized by shortest drying time. The lowest changes in color caused by drying were observed for Tito variety. The variety of Kazan was characterized by the highest resistances to color changes affected by drying followed by rehydration. Nandor and Tito varieties displayed the highest water absorption near to 560 g/100 g. The best drying characteristics and good water absorption accompanied by the high color attributes of dried and rehydrated samples implies that Kazan variety is expected to be the most useful to drying industry.     

Process-Based Drying Temperature and Humidity Integration Control Enhances Drying Kinetics of Apricot Halves

The effects of drying temperature (50, 60, and 70°C) and absolute humidity (65, 90, 115, 140 g/kg at the initial stage) on drying kinetics and color attributes of apricot halves under process-based drying temperature and humidity integration control (PDTHIC) were investigated. Results indicated that appropriate PDTHIC could reduce the drying time by 18.75% compared to the control group. The absolute humidity parameter should be controlled well rather than continuously dehumidify for traditional practices. The moisture effective diffusivity (D_eff) at 70°C presented drastic fluctuations with increasing absolute humidity parameters, calculated using the Weibull distribution model. The activation energy (E_a) of samples treated by continuous dehumidification and PDTHIC were 31.40 and 74.18 kJ/mol, respectively. The variation trend of color parameters was different from the conventional hot air drying probably due to the dehydration mechanism. Observation of the microstructure of the dried samples indicated that the PDTHIC process can enhance the drying rate of apricot through generating a larger pore network compared to continuous dehumidification at the same temperature. The findings of the current work clearly indicated that process-based drying temperature and humidity integration control enhances the drying kinetics of apricot halves, leading to a promising technology for energy savings.     

A Vacuum Drying Model for Mango Pulp

Abstract

Vacuum drying of mango pulp at varying conditions of pulp thickness (2, 3, and 4 mm) and vacuum chamber plate temperature (65, 70, and 75°C) was carried out under 30–50 mm of mercury absolute pressure. A model based on moisture diffusivity was found to give close prediction to moisture content of the pulp at different times of drying with correlation coefficient varying between 0.98–0.99 for pure mango pulp and pulp with ingredients. Color change of reconstituted pulp made from mango powder was found to depend more on pulp thickness than plate temperature. For getting low color change vacuum drying should be carried at maximum pulp thickness of 2.6 mm and vacuum chamber plate temperature of 72.3°C.     

Effect of Drying Conditions and Level of Condensed Distillers Solubles on Protein Quality of Wheat Distillers Dried Grain with Solubles

Reduced protein quality is one of the concerns currently confronting the supply and utilization of wheat distillers dried grain with solubles (DDGS) as an animal feed ingredient. This study assessed the protein quality of wheat DDGS, expressed as acid detergent insoluble crude protein (ADICP) and lysine content, by blending wet distillers grain (WDG) with varying condensed distillers solubles (CDS) levels and drying using forced air convection, microwave, and microwave–convection methods. As the CDS level was increased, the protein content of wheat DDGS generated from the three drying methods increased. Interactions of CDS level with drying air temperature, microwave power, and microwave–convection settings had a significant effect (p < 0.05) on average ADICP and lysine contents. Higher ADICP and lower lysine contents were observed in samples dried at higher temperature, microwave power, and microwave convection settings. Further, the CDS level significantly affected the color parameters of microwave- and microwave–convection-dried samples and the drying air temperature–CDS level interaction significantly affected the color of forced air convection–dried samples. Significant lysine content–redness, ADICP–lightness color parameter, and ADICP–total color difference correlations were found in forced air convection–, microwave-, and microwave–convection-dried samples, respectively. Microwave and microwave–convection drying achieved desirable protein quality associated with low-temperature drying at much shorter times.     

Pulsed vacuum drying (PVD) of wolfberry: Drying kinetics and quality attributes

The effects of drying temperature (50, 53, 56, 59, 62, and 65°C) and pulsed vacuum ratio defined as the vacuum pressure duration versus atmosphere pressure duration (3:3, 6:6, 9:2, 12:5, 15:1, 18:4?min/min) on pulsed vacuum drying (PVD) characteristics and quality attributes of wolfberry in terms of polysaccharide content, color parameters (L^*, a^*, b^*, ΔE, and C), rehydration ratio and microstructure were investigated. Results revealed that appropriate PVD can reduce drying time by 73.2% compared to hot air drying at the same drying temperature. The moisture effective diffusivity (D_eff) ranged from 5.23?×?10^?10 to 9.73?×?10^?10?m²/s, calculated using the Weibull distribution model. The polysaccharide content, L^* (lightness), a^* (redness/greenness) of the PVD products were higher than those of the hot air-dried samples at the same drying temperature. The total color difference (ΔE) and color intensity (C) of PVD samples were close to those of the fresh ones. The retention rate of total polysaccharide content of PVD samples was about 49–77%, which was significantly higher than 30% of the hot air-dried samples. The surface of PVD wolfberry was highly porous, which may enhance moisture transfer during drying as well as rehydration processes. The results of current work indicate that PVD is a promising technology for wolfberry process, for the reason that PVD can reduce drying time significantly as well as enhance the quality attributes in terms of the total polysaccharide content, color parameters and rehydration ratio.     

Variation in Drying Behavior and Final Moisture Content of Wood during Conventional Low Temperature Drying and Vacuum Drying of Betula pendula Timber

Conventional and vacuum drying experiments were conducted on Betula pendula timber, which was sawn from trees felled during three different seasons. The influence of the wood procurement season on drying behavior differed, on the one hand, between the drying phases above and below 30% moisture content in the conventional drying, and, on the other hand, between the conventional and vacuum drying methods. During the first steps of the conventional drying process, relative humidity in the kiln, as well as drying time and drying rate, varied according to the felling season. Variations in environmental conditions outside the kiln and the seasonal variation in the physical properties of the wood were presumed to be the reasons for differences in drying behavior. The difference in moisture content gradient, i.e., the difference in final moisture content between the inner wood and the surface layer of boards, was greater in conventionally dried timber than in vacuum-dried timber. In conventionally dried timber there was a clear seasonal variation in the gradient of final moisture content, which was greatest for winter-felled wood. The premature drying of the surface layer during the first steps of the conventional drying process of winter-felled wood was the reason for the higher gradient of moisture content. Storage of wood as logs decreased the standard deviation of the final moisture content.     

DEVELOPMENT OF A NEW DRYING PROCESS – DEHYDRATION BY CYCLICAL PRESSURE DROPS (D.D.S.): APPLICATION TO THE COLLAGEN GEL

ABSTRACT

Experiments to dry a collagen gel in order to obtain a homogenous film were carried out using a new process: dehydration by successive decompression “DDS” (Déshydratation par Détentes Successives). This process, particularly suited to thermosensitive products, involves a series of cycles during which the collagen gel is placed in desiccated air at a given pressure then subjected to an instantaneous (200 ms) pressure drop to a vacuum (7–90 kPa). This paper assesses the effect of four parameters of this drying process, namely high pressure (P _h) and vacuum pressure (P _v) and the effect of the duration at these pressures (t _h and t _v) on the drying rate, and the temperature of the collagen gel during drying. A comparative study between this new drying process and conventional methods indicated that the drying time was reduced from 480 and 700 min for vacuum and hot air drying respectively to 270 min for our process.     

Influences of Drying and Storage of Lycopene-Rich Carrots on the Carotenoid Content

ABSTRACT

The stability of β-carotene and lycopene was investigated during convective air and inert gas drying, microwave vacuum drying, and freeze-drying for lycopene containing carrots (Daucus carota L. cv. Nutri Red). After convection drying at temperatures below 70°C, β-carotene and lycopene contents remained unchanged independent of the drying medium. Freeze-drying did not show any advantage to convection-drying regarding carotenoid retention. Microwave vacuum–drying led to dry products with high carotenoid retention within very short drying times of about 2 h. Storage in air and in inert gas (nitrogen) containers was studied for convection-dried products, observing a better retention of carotenoids when using inert gas for a period of up to 6 months. After convection- and microwave vacuum–drying, an even better carotenoid extractability could be observed. No changes in the isomere fractions could be detected in any case.     

Kiln-, Solar-, and Air-Drying Behavior of Lumber of Tectona grandis and Gmelina arborea from Fast-Grown Plantations: Moisture Content,Wood Color,and Drying Defects

Tectona grandis and Gmelina arborea are common in commercial reforestation in the tropics. However, color variations, moisture content, and drying defects are also present in dried lumber. Moisture content variations, drying defects, and color changes were evaluated in the present work for three drying methods (kiln, solar, and air drying) during three seasons (dry, rainy, and transition season) in Costa Rica. According to the results, kiln drying had the fastest drying times, regardless of the season. On the other hand, air drying had slower drying time and higher final moisture content. With regard to defects, kiln drying produced the highest number and magnitude of defects in both species, whereas air drying showed the lowest quantity and severity of defects. No variations due to the drying methods or the season were observed in check and split, though solar drying presented intermediate values in all drying defects. The seasons of the year did not present any effect on drying defects. Finally, T. grandis dried lumber is darker than green lumber, and dried G. arborea wood is clearer. In addition, there is an increase in red ( a ^* ) and yellow ( L ^* ) tonalities, and color changes (Δ E ^* ) are considered perceptible or very perceptible in both species. No differences were found among the three drying methods in Δ E ^* , although the season of the year affected dif L ^* and dif C ^* significantly.