Influence of Drying Temperature and Rehydration on Selected Textural Properties of Carrots

Abstract

Cubes of carrots dried at 60, 70, 80, and 90°C were rehydrated up to 180 min at 20°C and up to 10 min at 95°C. Kinetics of cutting strength were determined for the samples soaked at 95°C. Compression stress relaxation behavior of the samples was studied on carrots rehydrated at 20 and 95°C. Kinetics of cutting strength were satisfactorily described by means of a first-order kinetic model. The compression stress relaxation of carrots was analyzed using five element Maxwell and empirical Peleg models. Both of the models were found to be appropriate for characterizing viscoelastic properties of dried and rehydrated carrot cubes. The results indicated that the use of low drying temperatures was suitable for preserving the quality attributes of carrots after rehydration, and drying air temperature of 60°C was chosen as the most appropriate for hot air processing of carrots.

Keywords:

• Carrots
• Drying
• Rehydration
• Cutting strength
• Compression stress relaxation

INTRODUCTION

Carrots are one of the most important crops due to their high nutritive value as they contain an appreciable amount of vitamins B complex besides being rich in β-carotene.[1] Both raw and processed carrots are used commercially. As consumers move toward functional foods with specific health effects, scientists and food manufacturers have also taken an interest in the potential of the antioxidant constituents of processed carrots to maintain health.[2] Dried carrots are currently used in flavoring, instant soups, meals, and sauces preparation, and their unique properties make them an excellent and desirable ingredient in many processed or ready-to-eat foods in place of fresh foods; they are also used for their convenience in handling, transportation, storage, and further meal preparation. Longer shelf life, product diversity, and volume reduction are the reasons for the popularity of dried carrots.

Air-drying is the most widely employed method for preserving food materials. Much research has been done in recent years on the processing of carrots by convective air drying,[3] fluidized-bed drying,[4,5] microwave drying,[6] vacuum-microwave drying,[1] and freeze drying.[1] Most of the dried food materials must be rehydrated by immersion in water until use. During rehydration, the dry material submerged in water undergoes several simultaneous changes in moisture and solids content, porosity, volume, and temperature. Rehydration parameters of food materials indicate the degree of alterations occurring during processing and are usually used to determine final product quality.[7] Although food drying and rehydration research is extensive, there is still a need for studying the kinetics of food quality changes during rehydration.

The mechanical properties are considered one of the most important four parameters, which reflect the quality of food material.[8] Those parameters include texture, firmness, and chewability. Texture attributes are usually correlated to rheological parameters, which are important in understanding the structure of food and biological materials and how they are affected by the drying and rehydration processes.[9] Chenoll et al.[10] applied texture analysis to determine the period during which the seed coat of chickpea acts as a barrier to water penetration during soaking. Moreira et al.[11] studied kinetics of water absorption, texture, and color of air-dried chestnuts during rehydration and showed that textural kinetics were satisfactorily modeled by a second-order kinetic model. Marabi et al.[12] used a trained panel to study the sensory texture attributes and differences in carrots dried by vacuum drying and hot air drying methods and rehydrated for different times. They found that samples dried with hot air were significantly harder than those dried with the vacuum drying method for all of the times rehydration was tested. They also observed that rehydration time had a significant effect only on the overall acceptability of the samples dried with hot air and the acceptability of hot air-dried samples was significantly lower due to its hardness. To describe the kinetics of changes in textural parameters of foods during rehydration, several models based on expressions for reaction rate are available in the literature. Generally, a first-order model[13] and second-order model[11] are employed. Cunningham et al.[14] observed that the rehydration characteristics and textural degradation kinetics of potatoes were dependent on processing temperature, blanching, drying method, and rehydration. They also found that the fractional conversion equation may be used as an accurate analytical tool to model textural degradation of potato during the soaking. In the present study, the first-order model was applied to describe the textural properties of carrots during rehydration. Viscoelastic material exhibits stress relaxation phenomena, which is one of the most important factors in characterizing agricultural materials. The relaxation time measured shows how fast the material dissipates stress after receiving a sudden deformation. Viscoelastic properties for carrot and potato were experimentally determined by stress relaxation tests of cylindrical specimens at various deformation rates as noted by Krokida et al.[15] They found two different modes of viscoelastic properties: (i) at high moisture content of fresh or slightly processed materials, that lose their elasticity, and (ii) at low moisture, when the solid material has collapsed regaining its elasticity. Hassan et al.[16] applied generalized Maxwell and Peleg models to predict stress relaxation behavior of dates. They observed that Maxwell model was the best in predicting experimental data of dates. Telis-Romero et al.[17] studied viscoelastic behavior of rehydrated papaya. The results obtained showed that dehydrated papaya exhibited a more pronounced elastic behavior at lower moisture contents and higher drying air temperatures. The samples showed more viscous and less rigid behavior at higher moisture contents and lower drying air temperatures.

In this study, the rehydration behavior of hot-air-dried carrots as a function of the drying air temperature and the bath temperature was analyzed, by considering textural properties of rehydrated material. Negligible published results were found that were related to the effect of drying temperature and the temperature of the rehydration medium on texture kinetics during the soaking of carrots; the effect of the temperature was also studied.

MATERIAL AND METHODS

Material

Fresh carrots (Daucus carota cv Macon) obtained from the experimental field of the Agricultural Research Institute in Skierniewice, Poland were used. After harvesting, the carrots were stored in a refrigerator at 3 ± 1°C for about 1 month before experiments. The initial moisture content of the raw carrots was 7.07 ± 0.13 kg/kg d.b.

Pretreatment

One hour before the experiments, the carrots were washed, peeled, and cut using a stainless steel die into 1 × 1 × 1 cm cubes. Prior to the drying operation, they were blanched in a water bath at 95°C for 4 min. The bath was filled with aqueous solution of sodium thriphosphate (2 g/100 g) in order to limit changes in color and texture of carrot samples during further processing. After blanching, the cubes were immediately cooled and blotted with tissue paper to remove superficial water.

Drying

Hot-air-drying of carrot samples was performed using a laboratory-scale spout-fluidized-bed drier.[18] The dryer consisted of a centrifugal fan, an electric heater, an airflow regulating valve, a processing chamber, a data acquisition with a computer controlled system, an anemometer, a mass balance, and thermocouples. Drying experiments were carried out in a transparent, vertical cylinder 1 m in height and 0.172 m in diameter. The air was introduced from a fan through a pipe (0.065 m in diameter) to the conical section of a drying chamber (0.25 m in height) and its velocity was 40 m/s at the inlet to the conical part and 7 m/s at the outlet from the cylindrical part of the drying chamber. The supporting mesh with 4-mm-square orifices was placed as a gas distributor at the bottom of the cylindrical drying chamber. The initial height of a stationary layer was 0.1 ± 0.01 m, whereas the height of a spout was 0.5 ± 0.05 m. Drying experiments were carried out at 60, 70, 80, and 90°C in triplicate and arithmetic average was taken for calculation. The drying apparatus consisted of conical and cylindrical sections. The moisture content of carrots was calculated based on the measured mass changes according to AOAC standards.[19] The initial moisture content of blanched carrot cubes was 8.35 ± 0.03 kg/kg d.b. All samples were dried to the same final moisture content of 0.06 ± 0.03 kg/kg d.b. The carrot cubes were dried until no changes in mass of the samples were observed in three consecutive measurements. All drying tests were conducted in triplicate.

Rehydration

Fifty dried samples in a wire-netting basket were immersed during a pre-determined time into water at different temperatures (20 and 95 ± 1°C) using a glass vessel containing 500 mL of water. Ten samples were used separately in order to determine volume changes and moisture content. The rest of the samples were used to determine texture. Rehydration times were 2, 4, 6, 8, 10, and 60 min at 95°C, and 10, 20, 30, 40, 50, 60, 90, 120, 150, and 180 min at 20°C. At these specified intervals, samples were carefully removed, blotted with paper towel to remove superficial water, and weighed. Finally, dry solids content was determined according to AOAC standards[19] to determine leaching of solutes. Each rehydration experiment was performed in duplicate.

Cutting Strength

The cutting tests were performed to study the textural properties of carrots rehydrated at 95°C. All the tests were performed using an Instron Universal Testing Machine (High Wycombe, Bucks, UK) model 4301 equipped with a 100 N load cell. The force and energy required to cut through an individual rehydrated carrot cube at a given time of rehydration was measured using a Warner-Bratzler (WB) device. The cutting tests were performed with a cross-head speed of 50 mm/min. The shear stress was calculated as the ratio of the force required to cut the cube to its cross-section area. The toughness was calculated as the ratio of the energy required to cut the sample to its volume prior to fracture. The first-order kinetic equation was employed for the modeling of shear strength and toughness values during rehydration. The integrated equation is given by:

(1)

where S can be the maximum shear stress or toughness at time t (min), and K (1/min) is the corresponding rate constant of the model obtained by fitting of the experimental data of an individual carrot cube.

Compression Stress Relaxation

The compression stress-relaxation tests of carrots rehydrated at 20 and 95°were carried out using an Instron Universal Testing Machine (Instron, Norwood, MA, USA) fitted with a parallel plate fixture for uniaxial compression. Cross-head speed was 25 mm/min during the loading phase of force relaxation tests. Randomly chosen individual carrot cubes were positioned between two plates and squeezed to attain 20% deformation. The residual stress required to maintain constant strain was measured as a function of time for 60 s. All the reported values of textural properties were the mean of 10 replications.

For small deformations, solid foods can be assumed to behave as linear viscoelastic materials.[20] A typical experimental technique for studying the effect of time on the mechanical properties of foods is the stress relaxation test. In the present study, the time-dependent changes in stress during compression stress-relaxation tests were studied, and stress-relaxation curves for an individual carrot cube were described using the generalized Maxwell model. The model consisted of two Maxwell elements in parallel with a residual spring and was described after Steffe:[21]

(2)

Parameters σ₁ and σ₂ (Mpa) in the generalized Maxwell model (1) were assumed to be proportional to the apparent elastic modulus of the material. The last term, σ_e, (MPa) in Eq. (2)(2) describes a hypothetical value of the asymptotic stress determined according to the Maxwell model. The relaxation times, τ₁ and τ₂, (min) of each parallel Maxwell element are defined as:[21]

(3)

The changes in compression stress in carrot cube versus time of relaxation can be calculated after Steffe[21] from the Peleg model:[22,23]

(4)

The initial decay rate in the relaxation stress was calculated from:

(5)

The remaining stress was calculated based on the equation:

(6)

Equations (4)–(6) may be applied for studying the viscoelastic properties of foodstuffs in the case the large deformation, usually over 10% in strain.

Statistical Analysis

The quality of the fit of the models to the experimental data (X_i ) was evaluated with the coefficient of determination, R ², using the percent root mean square error (RMSE). If the value of RMSE is below 5%, then it could be stated that fitting of a model to experimental data is very good. The values of RMSE from the range of 5–10% indicate good fitting. Generally, as RMSE is closer to zero and R ² is closer to unity, the closer the prediction is to experimental data. The nonparametric Kolmogorov-Smirnov test was applied to study the significance of the differences between means. All the statistical analyses were performed using STATISTICA 9.0 (StatSoft Inc., Tulsa, OK, USA) software.

RESULTS AND DISCUSSION

Shear Test

Textural properties of dried carrot cubes during rehydration at 95°C for 10 min were measured as the shear strength and toughness, which are presented in Fig. 1 and for each time of rehydration—in Fig. 2. The shear strength indicates the resistance of the material to the applied load and it is an indicator of the toughness of the product when consumed in the rehydrated state. The toughness indicates the energy absorbed by the material prior to rupture. The initial values of both parameters, observed for dry samples, were the highest and they decreased as moisture content of samples increased. When the rehydration process progressed and the samples show an increase in moisture content, shear strength significantly decreased. The values of shear strength of the samples dried at 60, 70, and 80°C and rehydrated at 95°C for 10 min were in the same range as those obtained for raw and blanched carrots. The shear strength observed for the samples dried at 90°C was significantly lower than that received for raw and blanched carrots (Fig. 1). These results can be explained based on the fact that the final ratio of moisture uptake to the total solid dissolved in water calculated for the samples dried at lower temperatures was higher than those observed for the samples dried at 90°C. On the other hand, Markowski and Zielińska[24] found that the higher the drying temperature that is applied, the highest solid loss in rehydrated carrots is observed. This indicates that the higher the drying temperature applied, the more pronounced decline in mechanical strength of rehydrated carrots was observed. The same explanation can be made for the variations in the toughness of rehydrated samples. These results can also indicate that the state of the water absorbed during carrots rehydration was different from that of the water absorbed by the fresh product, which is in agreement with results received by Moreira et al.[11] for chestnuts.

Figure 2 (a) Kinetics of shear strength and (b) toughness of an individual carrot cube during rehydration at 95°C.

Figure 1 Shear strength and toughness of raw, blanched and rehydrated at 95°C for 10 min individual carrot cubes.

Table 1 shows the corresponding values of the rate constant for shear strength and toughness of carrots in Eq. (1)(1) as well as adjusting parameters R ² and RMSE. In both cases, the rate constant was significantly dependent on the drying temperature with the lowest values observed for the samples dried at 60 and 90°C, while the highest ones were observed for the samples dried at 70 and 80°C. The equilibrium values of shear strength and toughness of carrots were estimated based on Eq. (1)(1) and are presented in Table 1. In both cases, these parameters were dependent on drying temperature with lowest and highest values found for the samples dried at 90 and 70–80°C, respectively. This means that the kinetics of texture degradation during rehydration were dependent on the damage in microstructure of carrot tissue caused by the previous drying step.

Table 1  K and S _e values of Eq. (12)

	Shear strength				Toughness
T (°C)	K (1/min)	Se (MPa)	R ^2	RMSE (%)	K (1/min)	Se (MPa)	R ^2	RMSE (%)
60	1.100^b	0.368^b	0.999	2.43	1.047^b	0.239^b	0.999	2.73
70	0.991^ab	0.523^c	0.998	3.99	0.890^a	0.381^c	0.998	3.44
80	0.890^a	0.503^c	0.997	4.10	0.776^a	0.321^c	0.996	4.87
90	1.402^c	0.219^a	0.999	1.38	1.208^b	0.161^a	0.999	3.16
Values followed by different letters in the same column are significantly different at the 5% level.

Table 2  Temperature dependent rheological parameters of the Maxwell model (2)–(3) for carrots

Processing temperature (°C)		τ1 × 10^2 (min)	τ2 (min)	σ1 × 10^2 (MPa)	σ2 × 10^2 (MPa)	σe × 10^2 (MPa)	R ^2	RMSE (%)
Raw		1.85^b	0.329^b	24.1^d	18.3^d	62.7^d	0.996	0.59
Blanching 95		2.73^c	0.262^a	23.0^c	16.5^c	6.55^c	0.998	2.21
Drying 60	Soaking 20	2.07^b	0.373^b	1.38^b	1.39^b	2.42^b	0.996	0.90
70		1.91^b	0.333^b	1.55^b	1.57^b	2.06^b	0.997	1.08
80		1.81^b	0.322^b	1.50^b	1.43^b	2.55^b	0.996	0.93
90		1.86^b	0.333^b	1.56^b	1.54^b	2.78^b	0.995	0.99
60	95	1.58^a	0.335^b	0.663^a	0.658^a	1.23^a	0.996	0.92
70		1.80^b	0.351^b	0.755^a	0.718^a	1.41^a	0.995	0.93
80		1.67^a	0.338^b	0.668^a	0.626^a	1.44^a	0.995	0.83
90		1.69^a	0.355^b	0.645^a	0.672^a	1.35^a	0.996	0.83

Table 3  Temperature dependent rheological parameters of the Peleg model (4)–(6) for carrots

Processing temperature (°C)		k 1 (min)	k 2 (-)	σ0 × 10^2 (MPa)	σ e  × 10^2 (MPa)	KR 0 (MPa/min)	R ^2	RMSE (%)
Raw		0.115^c	2.51^c	105.0^f	63.0^f	9.00^e	0.954	1.88
Blanching 95		0.056^a	1.13^a	45.5^e	5.38^e	8.11^d	0.991	4.42
Drying 60	Soaking 20	0.118^c	1.88^b	6.14^c	2.87^c	0.520^b	0.954	3.22
70		0.096^b	1.65^b	6.54^c	2.59^c	0.681^c	0.956	3.96
80		0.100^b	1.85^b	6.97^c	3.21^c	0.698^c	0.953	3.25
90		0.112^c	1.89^b	7.43^d	3.50^d	0.665^c	0.954	3.14
60	95	0.101^b	1.93^b	2.35^a	1.13^a	0.232^a	0.938	3.47
70		0.099^b	1.95^b	2.38^a	1.16^a	0.241^a	0.941	3.30
80		0.104^b	2.11^b	2.25^a	1.18^a	0.217^a	0.939	2.93
90		0.113^c	2.02^b	2.84^b	1.44^b	0.251^a	0.937	3.24

Stress Relaxation

Stress relaxation curves obtained for fresh, blanched, and rehydrated carrots were shown in Fig. 3. The solid lines in the curves represent the generalized Maxwell model. The bars show the standard error of the experiments. Since each experimental point represents the average of ten measurements, the heterogeneity of the samples caused a considerable dispersion in some experimental points. The curves shown in Fig. 3 are typical force relaxation curves of viscoelastic solids where stress decreases exponentially with time. Tables 2 and 3 present the viscoelastic properties of raw and processed carrot cubes as a function of processing temperature for both the generalized Maxwell model and the Peleg model, and adjusted parameters R ² and RMSE. It is evident from Fig. 3 and Table 2 that every processing step decreased the strength of the carrot tissue. For both the temperatures of rehydration, the samples dried at 90°C were characterized by the highest resistance to the compression. The determination coefficient, R ², of the generalized Maxwell model ranged between 0.995 and 0.997 for all drying temperatures. In the Peleg model, R ² varied from 0.937 for the samples dried at 90°C and soaked at 95°C to 0.956 for the samples dried at 70°C and soaked at 20°C. It can be deduced from Tables 2 and 3 that both Maxwell and Peleg models predicted the same (p ≤ 0.05) amount of force that remained unrelaxed. A comparison between the determination coefficient R ² for the force relaxation curves received on the basis of the generalized Maxwell model (1) and the Peleg model (3) indicates that the generalized Maxwell model predicted the experimental data slightly better than the Peleg model. However, taking into account the ability of both models to satisfy good prediction of the experimental results, it should be stated that both models, as well as three-element Maxwell Peleg ones, can be considered appropriate for characterizing viscoelastic properties of hot-air-dried and rehydrated carrot cubes.

Figure 3 Influence of temperature of drying and rehydration on (a) stress relaxation curves for raw and blanched carrots and (b) dried carrots rehydrated at 20 and 95°C for 180 and 10 min, respectively.

No significant differences in resistance of rehydrated samples to the compression were observed between drying temperatures of 60, 70, and 80°C. This may be the indication that carrots dried at lower air temperatures were characterized after rehydration by more viscous and less rigid behavior, which is in agreement with the findings of Telis-Romero et al.[17] received for rehydrated papaya. Data presented in Table 2 shows that both parameters, the maximum and residual stress measured during relaxation of processed carrots, were higher for the samples dried at higher and rehydrated at lower temperatures. Furthermore, the resistance to the compression of the rehydrated carrot samples was found to be very low in comparison with the resistance of the raw and blanched ones. Ciurzynska and Lenart[25] showed that osmotically pretreated freeze-drying of strawberries modifies their microstructure and thus also modifies their mechanical properties. Similarly, the present findings proved that thermal processing of carrots caused significant damage in their microstructure.

CONCLUSION

Different soaking conditions were examined to rehydrate hot-air-dried carrot cubes. Shear strength and toughness of carrot cubes dried with hot air at 60°C and rehydrated for 10 min at 95°C did not differ significantly from those observed for fresh carrot cubes. However, rehydrated carrots exhibited significantly lower resistance to the compression of raw and blanched ones. Samples dried at 90°C prior to rehydration exhibited the highest resistance to the compression during the stress-relaxation tests. As the air drying at 60°C ensured relatively high values of the rehydration quality attributes and the acceptable parameters related to the quality of dried and rehydrated carrot cubes, a temperature of 60°C was chosen as the most appropriate for drying of carrots. Both the three-element Maxwell model and Peleg model can be considered appropriate for characterizing viscoelastic properties of hot-air-dried and rehydrated carrot cubes. Although modeling textural properties of dried foods has been the subject of substantial research over the last decades, further investigations should be performed on modeling of the quality of foods dried using different techniques based on the lows of mass, heat, and momentum transfer. Fluidized-bed drying of carrots should be optimized towards the collapse prevention, in addition to safety and nutritional aspects, to satisfy the increasing consumer demands for higher products quality.

NOMENCLATURE

K	=	Rate constant in Eq. (1) (1/min)
k1, k2	=	Coefficients in Eqs. (4)–(6)
M	=	Moisture content (kg H2O/kg d.b.)
N	=	Number of degree of freedom
n	=	Number of samplings (−)
R 2	=	Coefficient of determination (−)
S	=	Cutting strength or toughness
t	=	Time (min)
Greek Symbols	=
Δ	=	Difference (−)
σ	=	Stress (MPa)
Subscripts	=
0	=	Fresh sample (or initial value)
Corr	=	Corrected (soluble solids are taken into consideration)
e	=	Equilibrium (residual) value
exp	=	Experimental
sim	=	Simulated
τ	=	Time of relaxation (min)

ACKNOWLEDGMENT

The authors are grateful for the financial support from grant No. 2 P06T 024 26 from the Polish State Committee for Scientific Research.