Color Characteristics of Carrots: Effect of Drying and Rehydration

Abstract

The aim of this study was to observe a change in color and a degradation of total carotenoids and β-carotene in carrots subjected to spout-fluidized bed drying at 60, 70, 80, and 90°C. Additionally, the color of carrots rehydrated at 95°C (10 min) was measured in order to estimate the reversibility of color changes caused by drying. Significant changes in a*, b*, ΔE*, and ΔC* were observed during drying and rehydration of carrots. The total difference in color and saturation ranged from 23.67 to 30.28, and from 22.97 to 29.90, depending on drying air temperature. The rate of deterioration of total carotenoids and β-carotene content influenced the color parameters of carrots. The first-order kinetic model adequately described evolution of a*, b*, ΔE*, and ΔC* derived for carrots during drying at 60, 70, 80, and 90°C. It was found that the spout-fluidized bed drying followed by rehydration changed the color of carrots irreversibly.

Keywords:

• Carrots
• Blanching
• Drying
• Rehydration
• Color characteristic
• Modeling

INTRODUCTION

Carotenoids are organic pigments that naturally occur in the chloroplasts and chromoplasts of plants. They are important factors in human health and essential for maintaining a healthy vision as they can act as antioxidants.[1,2] β-Carotene as a carotene with beta-rings at both ends is the most common form of carotene. It is a precursor (inactive form) of vitamin A. The health benefits provided by β-carotene include anti-cancer activity, cataract prevention, protection against cardiovascular disease, and enhanced immune response.[3–7] The composition and concentration of carotenoids are important factors, affecting evaluation of product quality due to their attractive color and biological properties.

Fruits and vegetables provide most of the carotenoids in the human diet and carrot (Daucus carota L.) has the highest carotenoids content among food products. In Poland, carrots are consumed in large quantities, with many varieties used for industrial purposes. They are indispensable sources of essential dietary nutrients, such as β-carotene and vitamin B complex, organic acids, fiber, pectin, and mineral compounds.[8,9] To make carrots convenient in handling, transportation, storage, and further meal preparation they need to be preserved by drying.[10] The above-mentioned unique properties of dried carrots make them desirable as an ingredient in many processed or ready-to-eat food products, such as instant soups, meals, snacks, and sauces.

Dehydration is one of the oldest methods of food preservation and it represents a very important aspect of food processing.[11] Drying has been successfully applied in food processing in order to prevent biochemical, chemical, and physical deterioration of wet biological material.[12] The moisture removal from wet food product helps extend the shelf-life, prolong the storage time, reduce volume, restrain the microbial growth, and inactivate enzymes.[13] However, the reduction of nutritional value and development of undesirable color, flavor, taste, and texture (shrinkage, case hardening) are usually observed during drying of food products exposed to high temperatures.[11] Hot air drying in a stationary layer under forced convection is the most popular drying technique applied in the fruit and vegetable industries.[14] However, fluidized or spout-fluidized bed drying techniques offer some advantages. Due to high heat and mass transfer coefficients, an increase of drying rate is usually observed. Also, mixing improves a homogeneity of the process and finally positively affects the product quality.[12] Other types of drying methods, e.g., convective air drying, fluidized-bed drying, microwave drying, vacuum-microwave drying, and freeze drying, have been used to dry carrots.[11,15–20] Special attention has been paid to the quality evaluation of dehydrated carrots.[11,15,19] The quality of dehydrated carrots does not only depend on the drying method and drying conditions used for food preservation but also on the other processing operations carried out before and after drying.[21] Blanching is carried out prior to dehydration to inactivate the enzyme peroxidase, which may lead to formation of unacceptable colors and flavors.[22,23] An initial pretreatment followed by hot water blanching is usually carried out before drying, not only to minimize the enzymatic reactions, but also to diminish changes in tissue structure, obtain shorter drying time, increase drying rate, and improve the rehydration capacity as well as the overall acceptability of the final products.[11,24] Blanching before drying of carrots was also reported to enhance the stability of carotenoids during storage.[25,26] It has also been proved that weak chemical treatment prior to carrots drying affects the rate of drying and helps to improve the acceptability of final product by avoiding deterioration in the color.[27] After drying, most of the dehydrated vegetables are used in preparation of different types of ready-to-eat meals. Therefore, they require rehydration for the reconstitution to a state that is similar to the fresh product. The dried food particles should have a short preparation time (between 5 and 15 min) and retain the characteristics of fresh vegetables, such as color, taste, odor, shape, and size.[28]

Several deteriorative reactions that affect the color, nutrient properties, texture, and flavor of dehydrated products are initiated during processing (blanching, drying, rehydration), and continue during storage. Color is one of the most important attributes of dried food products, since it influences consumer acceptability. The first quality judgment made by a consumer on food materials is their visual appearance, which is usually negatively affected by drying.[29] The color changes of thermally treated food materials occur because of chemical changes, such as pigment degradation (especially carotenoids and chlorophyll), browning reactions, such as Maillard condensation of hexoses and amino components, and finally oxidation of ascorbic acid. Dehydrated vegetables also change their color due to the oxidation of highly unsaturated molecules upon exposure to air during processing.[30] Particularly, carotenoids are subjected to rapid decomposition in the presence of oxygen.[31] In case of carrots, the color is determined by the concentration of principle components: α- and β-carotene, as they constitute over 90% of all carotenoids.[32] Therefore, a loss of natural color of carrot tissue depends on thermal destruction of these key carotenoids.[29] Degradation of carotenoids affects not only the color of food products but also their nutritive value and flavor.[33] The color characteristics of foodstuffs are complex and depend on both chemical composition and physical properties of the material. Water evaporation results in changing concentration of solubles, and in consequence, color properties of foodstuffs. Moreover, considerable shrinkage of the tissue causes surface texture changes and leads to a compact structure. Therefore, a perception of color of dried products may be different from that of raw materials.[34]

Maintenance of satisfactory color in the final product is a challenging aspect of thermal processing of carrots. A measurement of tri-stimulus color co-ordinates is widely used as a quality indicator for monitoring of color changes during food processing.[2] Tristimulus colorimetry has been used to evaluate color changes in carrots.[2,11,15,35] The kinetics of color changes and carotenoids degradation required to minimize the undesired changes and optimize the quality of dehydrated food products have been studied by several authors.[2,34–36 However, there is no report on color changes in carrot tissue during spout-fluidized bed drying and rehydration. The determination of kinetic parameters for color change is important to optimize the thermal processes.[37] Also, the execution of mathematical modeling in the design and control of operating parameters during processing, and performing simulations using kinetic models, can contribute to the optimization of the process.[38]

Color denotes the visual appearance of the product, whereas pigments are the chemical compounds that impart the observed color. In this context, the aim of our study was to observe the color change and degradation of the total carotenoids and β-carotene in carrots subjected to spout-fluidized bed drying. Moreover, the color of rehydrated material was measured in order to estimate the reversibility of changes in color. The effect of dehydration, and rehydration of carrot cubes on: (1) the kinetics of color changes; (2) the total carotenoids and β-carotene degradation; and (3) the kinetic model parameters was studied. It seem worth to underline that there is the lack of published results on the modeling the kinetics of changes in color indices during drying of carrots as affected by drying temperature. To this end, a series of studies were carried out on different colorimetric parameters, total carotenoids, and β-carotene content. Kinetic parameters of color loss during drying and rehydration of carrots were assessed using tristimulus colorimetry. Kinetic models for changes in redness, yellowness, total color difference, and total saturation difference of carrots were applied.

MATERIALS AND METHODS

Materials

Carrots (Daucus carota cv. Macon F1) were obtained from experimental plots of the Agricultural Research Institute in Skierniewice, Poland. After harvesting, they were kept refrigerated for 1 month prior to drying operation. The initial moisture content of samples was determined to be 7.07 ± 0.13 kg/kg d.b.[39]

Blanching

Just before the drying experiments, carrots were washed, peeled, cut into 10 × 10 × 10 mm cubes and blanched in hot water at 95°C (4 min). Blanching was carried out in distilled water solution of sodium thriphosphate (2 g/100 g). After blanching, the samples were blotted with tissue paper to remove excess water. The initial moisture content of blanched carrot cubes was determined to be 8.34 ± 0.04 kg/kg d.b.[38]

Drying

The fundamental drying experiments were carried out in a spout-fluidized bed dryer designed and fabricated in the Department of Agri-Food Process Engineering at the University of Warmia and Mazury in Olsztyn, Poland. The drying system was fully described in a previous study.[12] Drying experiments were carried out at atmospheric pressure at the inlet air temperatures of 60, 70, 80, and 90°C. Temperature of the air at the inlet and outlet, temperature inside the bed, and the temperature of material were recorded every 30 s with an accuracy of ±1°C using five J-type thermocouples connected to the data acquisition system to provide stable operating conditions during drying. The experiments were conducted under forced convection with airflow velocity 40 ± 0.1 m/s. The airflow velocity was measured at the inlet of the conical part of the drying chamber. The nozzle diameter of the spout-fluidized bed was 0.065 m. The moisture content was calculated according to AOAC standards.[39] Under each set of drying conditions, the tests were conducted in triplicate and arithmetic average was taken for calculation.

Rehydration

To determine the rehydration kinetics of dried samples, 1 g of dried carrots was weighed and then soaked in hot water. The rehydration was carried out at 95°C (10 min) in water bath filled with distilled water. Each sample was removed from the water every 2 min, blotted with tissue paper, and weighed. The final moisture content of carrot cubes after reconstitution was determined according to AOAC standards.[39] Each measurement was carried out in triplicate. An arithmetic average was taken for the data interpretation.

Color

Instrumental measurements of color characteristic were performed on raw, blanched, dried and rehydrated material. A spectrophotometer Miniscan XE Plus (Hunter & Associates Laboratory, HunterLab, Resfon, VA, USA) for a standard illuminant D65, observer 10° and 8° diaphragm was used to describe color parameters of carrots. During processing, carrots were removed at re-specified time intervals (0, 10, 20, 40, 60, 80, 100, 120, 140, 160, 180 min) to describe their color on the CIELab scale: L* (lightness), a* (redness), and b* (yellowness). The indices: ΔE* (total color difference), ΔC* (total saturation difference), and ΔH* (total hue difference) were calculated according to Eqs. (1) to (3):[40]

(1)

(2)

(3)

(4)

Results were analyzed using method described in literature.[29] The criterion established by the International Commission on Illumination (CIE) was applied.[40] It corresponds with color perception by human sight and the total color difference between 0 and 2 is insignificant and unrecognizable by an experienced and qualified observer, whereas the total color difference bigger than 5 is significant and recognizable even by an inexperienced observer. The calculated indices of color were averaged over 35 measurements results.

Total Carotenoids and β-Carotene Content

The total carotenoids and β-carotene content were determined according to the PN-90/A-75101/12.[41] Carotenoids were extracted from carrot tissue with a mixture of alcohol and ether. After filtration the alcohol-water fraction was rinsed with petroleum benzin until discoloring. The ether extracts obtained were mixed, washed with water, and dried with odor-free sodium sulfate. The total carotenoids content of this extract was measured directly with a spectrophotometer, and their concentration was determined on the basis of an analytical curve plotted for β-carotene. At the next stage, β-carotene was separated from other dyes by column chromatography. The absorbance of a β-carotene solution was measured with a spectrophotometer at a wavelength of 467 (nm), and β-carotene concentration was determined using an analytical curve. The tests were conducted in triplicate and arithmetic average was taken into consideration.

Kinetic Model for Color Changes

The first-order kinetics model (Eq. 5) was previously applied by Koca et al.[2] to study degradation and color loss in dehydrated blanched and unblanched carrot slices during storage at different temperatures.

(5)

A similar equation was used to describe changes in color of pear puree during thermal processing at high temperatures and to study the color changes of strawberry juice.[36,42] A modified first-order kinetics model was applied to describe discoloration of dried powder of carrots.[35] In order to study the influence of temperature of drying on changes in color indices (a*, b*, ΔE*, ΔC*) of carrots during drying as the first-order kinetics Eq. (5) was used.

Statistical Analysis

The calculations were done using STATISTICA 7.1 (StatSoft Inc., Tulsa, OK, USA) software. The experimental data were subjected to a one–factor analysis of variance (ANOVA). The significance of differences was determined by the Duncan multiple range test at p ≤ 0.05. Parameters of kinetic models were estimated by a non-linear regression analysis to obtain relation of color parameters with drying time and temperature.

RESULTS AND DISCUSSION

In the present study changes in visual color parameters, total carotenoids and β-carotene content were evaluated. Color of carrots was estimated using L*, a*, b* parameters, and ΔE*, ΔC*, ΔH* were calculated for measuring color differences and tracking color changes during processing. The L*, a*, b* values for unblanched samples were: 55.35 ± 1.63, 28.90 ± 0.85, 40.05 ± 2.05, respectively. In order to prevent the enzymatic reactions, diminish changes in the tissue structure, and improve the final product quality, raw carrots were subjected to a blanching operation. Blanching, used as a pretreatment operation, significantly ( p ≤ 0.05) influenced color parameters, such as L* and a*. Presumably, the increase in the lightness indicate that blanching caused carotenoids isomerization changing the structure from all trans to partially cis and creating lighter and less biologically active derivatives, i.e., so-called procarotenoids. However, no significant differences were observed in b* (yellowness) values. The measured L*, a*, b* parameters for blanched carrots were 66.91 ± 0.62, 36.64 ± 0.62, and 40.59 ± 0.40, respectively. The results demonstrate that blanching significantly ( p ≤ 0.05) influenced the total difference in color, i.e., ΔE* was 10.77 ± 1.42. According to the criterion established by the International Commission on Illumination, which corresponds with color perception by human sight, the total difference in color was visible and recognizable even by inexperienced observer.[40] However, it was observed that the total difference in saturation and hue was insignificant ( p ≤ 0.05), i.e., ΔC* and ΔH* were: 1.44 ± 0.84 and 2.74 ± 0.88, respectively.

The effect of hot-air drying on the changes in color of carrots was studied. Visual color parameters of rehydrated material were measured in order to estimate the reversibility of color changes (the blanched carrots were used as a reference). The mean and standard deviations of color parameters for blanched, dried, and rehydrated samples were calculated. Figure 1a shows the effect of drying and rehydration on color changes in L*, a*, and b*. To obtain the best color of dried and rehydrated carrots, parameters L*, a*, and b* should gain high values. Significant influence of drying on the color changes in a* (redness) and b* (yellowness) parameters was noticed. However, marginal differences in parameter L* (lightness) were observed (Fig. 1a). The color of dehydrated carrots was seen as much different from that of raw ones and it corresponded with different values of a* and b* parameters, which were in the range of 21.32 ± 2.53 and 20.15 ± 1.22, respectively. Rehydration mostly affected two color parameters of investigated samples and resulted in relatively high values of a* (redness) and b* (yellowness). Then, a* and b* values were 65.42 ± 1.34, 29.31 ± 1.37, and 29.45 ± 2.38, respectively (Fig. 1a). The changes in color parameters during drying and rehydration depended also on the temperature of drying air. The higher air drying temperature the more visible changes in color were noticed. The relative importance of the main effects, such as drying and rehydration on the three color parameters (L*, a*, b*), at the level of significance ( p ≤ 0.05), is presented in Table 1. It can be easily observed that drying and rehydration significantly influenced the two quality indicators a* and b*, but had a small effect on parameter L* (Table 1). Based on the magnitudes of the F-values, it was found that each of two steps of carrot processing more significantly affected b* value. No significant ( p ≤ 0.05) differences in L* were observed for drying temperatures of 60 and 70°C.

Figure 1 Effect of drying and rehydration on (a) color parameters: L*, a*, b*; (b) color indices: ΔE*, ΔC*, ΔH*. Means ± standard error.

Table 1 The statistical analyses of the main effects (drying and rehydration) on the color parameters (L*, a*, b*) of processed carrots

Sample	L*	a*	b*
D60	—	x	x
D70	—	x	x
D80	—	x	x
D90	x	x	x
R60	—	x	x
R70	—	x	x
R80	x	x	x
R90	x	x	x
x = significant difference between the experimental and reference samples (p ≤ 0.05).

Moreover, the total difference in ΔE*, ΔC*, and ΔH* was studied (Fig. 1b). Drying significantly affected ( p ≤ 0.05) the values of ΔE* and ΔC*. For the dried samples, the total difference in color and saturation were 25.68 ± 2.67 and 25.33 ± 2.69, respectively. Smaller differences in ΔE* and ΔC* were observed for rehydrated samples. The calculated mean values for ΔE* and ΔC* were 13.54 ± 2.46 and 13.10 ± 2.15, respectively. On the other hand, drying and rehydration did not significantly ( p ≤ 0.05) influence the total difference in hue. The results indicate that rehydrated material had a different color than that of raw one. The color of rehydrated carrots moved toward the color of reference sample. However, it was observed that carrot cubes dried in the spout-fluidized bed drier and rehydrated did not recover their original color.

Kinetics of Color Changes during Drying

The kinetics of color degradation in carrot cubes were evaluated during thermal treatment by spout-fluidized bed drying at 60, 70, 80, and 90°C (Fig. 2). The initial stage of drying resulted in significant increase in redness and yellowness of carrots. The initial increase in a* and b* was also reported by other authors for different food products, e.g., broccoli, beans, apples, bananas, and carrots.[43,44] However, it was observed that except this initial period, carrot cubes lost their redness, yellowness, and turned brownish with increasing drying time and temperature. The decrease in redness and yellowness, after the maximum was reached, was probably due to the water removal, internal structure alterations, changes in surface texture, and concentration of dry matter.[36] Significant ( p ≤ 0.05) degradation in redness during drying corresponded with a considerable decrease of parameter a* (34–53%) respect to reference sample. This could be explained due to degradation of carotenoids in the carrot tissue affecting the product color.[45] The loss in b* value indicates that the yellowness of the sample decreased when subjected to drying and it may be due to partial decomposition of carotenoids and generation of brown pigments.[46] Parameter b* showed a significant decrease ( p ≤ 0.05) in its value (47–56%) as drying air temperature and time of processing increased. The differences in a* (redness) and b* (yellowness) of carrots among drying air temperatures have found to be significant (Fig. 2a and 2b). The higher drying air temperature, the lower values of the parameter a* were noted. The samples dried at 60°C exhibited the highest values of the parameter a* (24.19 ± 0.26), which was attributable to the highest content of carotenoids. The lowest average values of parameter a* (17.29 ± 0.46) were determined in carrots dried at 90°C. The differences in L* (lightness) were also dependent on drying air temperature as well as drying time. However, the effect was not as evident as in the case of a* (redness) and b* (yellowness). The small decline followed by the raise in L* (lightness) of carrot cubes was observed during drying, except the rapid degradation in L* (8%) during drying at 90°C. Then, carrots dried at 90°C displayed remarkably lower L* values (62.38 ± 0.66) as compared to the other one dried at less rigorous drying temperatures. The parameter L* presented higher values for carrots dried at 60, 70, and 80°C and they were 68.28 ± 0.56, 67.03 ± 0.55, and 66.86 ± 0.66, respectively (Fig. 2c).

Figure 2 Kinetic changes of color parameter: (a) a* (redness); (b) b* (yellowness); and (c) L* (lightness) for carrot cubes during drying. Means ± standard error.

One of the best parameters for describing the color variation is the total difference in ΔE*, since it is a combination of parameters L*, a*, and b*. Figure 3a shows the variation of this parameter with the treatment time for the different work temperatures. The variation of total difference in saturation and hue over drying time under different operating conditions were also evaluated. It was found that the drying significantly influenced values of color indices, such as ΔE* and ΔC* (Figs. 3a and 3b). The indices of ΔE* and ΔC* followed a significantly increasing trend with drying time at all the drying temperatures (Figs. 3a and 3b). This corresponded with a total difference in color and saturation of carrots during drying. The total difference in color and saturation ranged from 23.67 ± 1.69 to 30.28 ± 1.85, and from 22.97 ± 0.89 to 29.90 ± 0.96, depending on drying air temperature. According to the criterion established by the CIE, the difference in ΔE* was significant and recognizable even by an inexperienced observer. On the other hand, no significant differences were observed in hue of carrot cubes (Fig. 3c).

Figure 3 Kinetic changes of color indices: (a) ΔE* (total color difference); (b) ΔC* (total saturation difference); and (c) ΔH* (total hue difference) for carrot cubes during drying. Means ± standard error.

Modeling of Color Changes during Drying

Table 2 shows the parameters of model (5) applied to study changes in color parameters as dependent on drying temperatures. It was found that first-order kinetic model adequately described the evolution of a* and b* parameters, as well as ΔE* and ΔC* indices derived for carrots during drying at 60, 70, 80, and 90°C. The values of determination coefficient close to 1 confirmed high quality of estimation. It is evident from Table 2 that kinetic parameter of model (5) applied to a* color index derived for samples dried at 90°C is significantly lower than those derived for other samples, while there were not any significant differences in a* color index observed between drying temperatures of 60, 70, and 80°C. The values of b* color index derived at 60, 80, and 90°C did not differ significantly and were significantly higher than the one received at 70°C. It was also found that the values of ΔE* color index derived at 60, 70, and 90°C did not differ significantly and were significantly lower the one received at 80°C, while ΔC* color index did not depend significantly on temperature of drying. The data shown in Table 2 confirm that changes in a*, b*, and ΔE* color parameters of carrots during drying depended on temperature applied.

Table 2 The parameters of model (5) of color kinetics of dried carrots

	a*				b*				ΔE*			ΔC*
			K × 10^2				K × 10^2			K × 10^2			K × 10^2
T, °C	X 0	Xe	(1/min)	R ^2	X 0	Xe	(1/min)	R ^2	Xe	(1/min)	R ^2	Xe	(1/min)	R ^2
60	46.3^a	23.1	2.29^b	0.990	50.4^b	23.1^a	2.53^b	0.999	27.2^a	1.40^a	0.954	27.6^a	1.24^a	0.926
70	39.8^a	21.2	2.01^b	0.945	40.3^a	19.3^a	1.48^a	0.985	26.6^a	1.55^a	0.996	27.3^a	1.41^a	0.987
80	41.7^a	20.4	2.44^b	0.979	54.8^b	21.1^a	3.65^b	0.977	27.2^a	1.87^b	0.930	30.2^a	1.38^a	0.935
90	42.5^a	13.6	1.57^a	0.969	49.9^b	18.8^a	3.33^b	0.977	37.8^b	1.31^a	0.975	40.2^b	1.14^a	0.969
The values with the same letters in columns did not differ significantly (p ≤ 0.05).

Kinetics of Color Changes during Rehydration

The kinetics of changes in color parameters L*, a*, b* of carrots vs. time of rehydration were studied (Fig. 4). It was found that redness and yellowness of carrot cubes significantly changed over time of rehydration. The longer the rehydration time used, the higher values of redness and yellowness were observed (Figs. 4a and 4b). The changes corresponded with the significant ( p ≤ 0.05) increase in a* and b* values (Fig. 4a). The final a* values varied from 27.69 ± 0.66 to 31.39 ± 0.70, as the drying time increased. An increase in the b* value (Fig. 4b) was also observed during rehydration. The final b* values varied from 25.65 ± 0.97 to 32.18 ± 1.00 as the drying time increased. Additionally, the color of rehydrated material was diversified by the drying air temperature. The higher the drying temperature used, the lower the values of redness and yellowness were found. The color of carrots dried at 60°C was characterized by the highest values of color parameters, which were significantly ( p ≤ 0.05) different than those observed for the samples dried at higher temperatures. However, no significant ( p ≤ 0.05) differences in the color parameters were observed for the higher drying air temperatures in the range of 70–90°C. The values of parameter a* and b* moved toward the values that were characteristic for the reference sample. There was an increase of parameter a* (21–24%) and b* (14–27%) in respect to dried samples ( p ≤ 0.05). However, drying followed by rehydration changed the color of carrot cubes irreversibly. The clear tendency of changes in lightness during rehydration of carrots was not observed and only the slight difference in parameter L* was noted (Fig. 4c). The lightness of rehydrated carrots ranged from 64.11 ± 0.51 to 67.63 ± 0.62, depending on drying air temperatures.

Figure 4 Kinetic changes of color parameters: (a) a* (redness); (b) b* (yellowness); and (c) L* (lightness) for carrot cubes during rehydration. Means ± standard error.

Kinetics of changes in ΔE*, ΔC*, and ΔH* over rehydration time were illustrated in Fig. 5. The rehydration significantly (p ≤ 0.05) influenced the total difference in color of carrots dried in a spout-fluidized bed dryer. The final values of ΔE* ranged from 9.94 ± 1.84 to 16.76 ± 1.94. The total difference in color was found to be dependent on both time of rehydration and the drying air temperature (Fig. 5a). Rehydration also resulted in a considerable increase in ΔC* (Fig. 5b), and the final values of ΔC* ranged from 9.72 ± 1.00 to 15.75 ± 1.01. No significant differences in ΔH* were noted during carrot rehydration (Fig. 5c). Finally, the changes in ΔE*, ΔC*, and ΔH* noted during rehydration were found to be lower than those observed during drying.

Figure 5 Kinetic changes of color indices: (a) ΔE* (total color difference); (b) ΔC* (total saturation difference); and (c) ΔH* (total hue difference) for carrot cubes during rehydration. Means ± standard error.

Total Carotenoids and β-Carotene Content

The degradation in total carotenoids and β-carotene content during carrot processing was investigated. The results are presented in Table 3. The significant decrease (p ≤ 0.05) in total carotenoids and β-carotene content was observed during processing. Total carotenoids and β-carotene content for raw samples was 191.05 ± 0.27 mg/100 g d.b. and 117.29 ± 0 23 mg/100 g d.b., respectively. After blanching, slightly lower values were observed and they were 189.34 ± 0.26 mg/100 g d.b. and 116.35 ± 0.55 mg/100 g d.b., respectively. It may be concluded that blanching prevents nonenzymatic browning reaction, which results in a relatively high value of total carotenoids and β-carotene content as well as redness and yellowness. Drying significantly influenced the total carotenoids and β-carotene content. The higher the drying air temperature, the lower total carotenoids and β-carotene content was observed. The highest total carotenoids (158.70 ± 0.41 mg/100 g d.b.) and β-carotene (96.60 ± 0.21 mg/100 g d.b.) content was noted for carrots dried at temperature of 60°C. The lowest total carotenoids (122.68 ± 0.63 mg/100 g d.b.) and β-carotene (66.85 ± 0.47 mg/100 g d.b.) content were noted for carrots dried at 90°C, which corresponded with the largest decrease in redness and yellowness. It might be due to the reaction of the product of carotenoid degradation, namely carbonylic compounds, with other food components (i.e., amine) and including them into the chain of Maillard reaction, which accelerate non-enzymatic browning. The degradation in lightness of carrots dried at 90°C corresponded with degradation of thermo-labile components (carotenoids) under the influence of rigorous operating conditions. It confirms the effect of both high temperature and long time processing on the color degradation of dried carrot cubes.

Table 3 Total carotenoids content for raw, blanched, and dried carrot samples

Sample	Total carotenoids (mg/100 g d.b.)	β-carotene (mg/100 g d.b.)
R	191.05 ± 0.27	117.29 ± 0 53
Bl	189.34 ± 0.26	116.35 ± 0.55
D60	158.70 ± 0.41	96.60 ± 0.21
D70	140.41 ± 0.42	79.29 ± 0.44
D80	134.52 ± 0.55	78.08 ± 0.20
D90	122.68 ± 0.63	66.85 ± 0.47

CONCLUSIONS

Knowledge of kinetics of color change can be used as a tool for the improvement in color of dried carrots. Therefore, the color change and degradation of the total carotenoids and β-carotene in carrots subjected to spout-fluidized bed drying were investigated. The color of rehydrated material was measured in order to estimate the reversibility of the changes in color. The results show significant degradation in a* and b* during drying. The indices of ΔE* and ΔC* followed significantly increasing trend with drying time at all the drying temperatures. The first-order kinetic model adequately described the evolution of a* and b* parameters, as well as ΔE* and ΔC* indices derived for carrots during drying at 60, 70, 80, and 90°C. The significant increase in a*, b*, ΔE*, and ΔC* over time of rehydration was noted. Additionally, the color of dried and rehydrated material was diversified by the drying air temperature and the time of carrot processing. The rate of deterioration of total carotenoids and β-carotene content influenced the color parameters of carrots. Carrots dried at 60°C exhibited the highest values of the parameter a*, which was attributable to the highest content of carotenoids. Carrot cubes did not recover their original color after heat treatment processing, such as drying and rehydration. In conclusion, spout-fluidized bed drying followed by rehydration changed the color of carrots irreversibly.

NOMENCLATURE

L*, a*, b*	=	Lightness, redness, yellowness
ΔE*, ΔC*, ΔH*	=	Total differences in color, saturation, and hue
C, C 0, Ce	=	Actual, initial, and equilibrium value of a color parameter in Eq. (5)
K	=	Rate constant in Eq. (5) (1/min)
t	=	Time (min)
R	=	Raw, untreated sample
Bl	=	Blanched sample (not dried)
D60	=	Dried sample (drying air temperature of 60°C)
D70	=	Dried sample (drying air temperature of 70°C)
D80	=	Dried sample (drying air temperature of 80°C)
D90	=	Dried sample (drying air temperature of 90°C)
R60	=	Rehydrated sample (drying air temperature of 60°C)
R70	=	Rehydrated sample (drying air temperature of 70°C)
R80	=	Rehydrated sample (drying air temperature of 80°C)
R90	=	Rehydrated sample (drying air temperature of 90°C)
Subscripts	=
Samp	=	Experimental sample
Ref	=	Reference sample

ACKNOWLEDGMENT

The study was financially supported by the Polish Ministry of Science and Higher Education through the program entitled “Supporting International Mobility of Researchers.”