Biochemical Quality Assessment of Dehydrated Carrots

Abstract

Quality assessment of carrot cubes preserved through batch dehydration introducing tempering cycles was performed in this study. The parameters considered were: the percentage of carotene retention, peroxidase inactivation and retention of invertase activity, as well as the degree of rehydration. Although, a similar final moisture content was always reached through the different carrot dehydration process, that with tempering cycles had better qualities than those without tempering. Carotene and invertase activity retention showed higher values, and peroxidase showed lower values compared to a control samples in all cases. Respect to the rehydration tests, no differences were found in all samples. As of the treatments with tempering cycles, the best scheme was 20 min drying time and 20 min tempering time at 70°C.

Keywords:

• Dried carrot, Carotene, Fluidized bed drying, Tempering

INTRODUCTION

Carrots are considered some of the best carotene suppliers (as a source of vitamin A), they contain from 15 mg (yellowish) to 31 mg (reddish) carotene per 100 g fresh carrots, mainly as β-carotene.[1,2] Today, carrots are regarded as one of the vegetables with a great benefits when consumed as food. As well as other nutrients, high carotenes confer a good nutritional value; however, many authors report a reduction in these characteristics when carrots are dehydrated through conventional processes.

One of the drying methods that may be recommended for foods and other biomaterials that are basically dried during the decreasing rate period, is drying with tempering cycles.[3,4,5] The tempering process can be carried out by removing the solid load from the dryer and introducing it either in a container with controlled temperature or in a container at room temperature. In the latter, tension caused not only by temperature gradients within the solids, but also because of the tension caused by the humidity gradient or the combination of both, are generated during carrot cooling and reheating.[6,7,8] This process, therefore, favors the occurrence of drying conditions resembling the initial ones at the beginning of a new cycle, but at progressively lower average humidity content each time, as the number of cycles increase.[9,1,10]

Irrespective of a conventional or intermittent drying, there is always the risk that quality deteriorates, as food products are multi-component systems consisting of biomolecules such as proteins, carbohydrates and lipids.[6,11,12] Nutrient losses as a function of temperature, moisture content, and presence of catalysts such as enzymes, lead to its deterioration.

This study aims to preserve carrot cubes using fluidized bed drying as a work methodology preserving the dehydrated product quality. The dehydrated product quality was assessed in terms of biochemical (percentage of carotene retention), enzymatic (retention of peroxidase and invertase activity), and physical (percentage of rehydration) parameters.

MATERIALS AND METHODS

Fresh carrot (Daucus carota var. Nantes) was obtained from a local market, with 88% mean moisture content (Brainweigh MB-300 moisture balance). Before starting each drying experiment, carrot was peeled and diced into 1cm³, blanched in a 0.10% Na₂S₂O₅ solution at 80°C for 5 min, drained, and left at room temperature for 2 h.

The fluidized-bed dryer consists of a 0.2 m squared-section stainless steel tunnel for air circulation. It has a funnel coupled at one end allowing placing a detachable 0.10 m inner-diameter acrylic tube, with a 80 mesh-size, used as drying area. Air is extracted from the environment by a fan with a capacity of 0.118 m³/sec. The electric motor (Siemens) is connected to a Micromaster Vector voltage transformer to enable the selection of different air velocities.[3] Air was heated using L.P. gas. Air velocity was 9m/s, equivalent to six times the minimum fluidizing velocity (1.5 m/s) in a particle - bed height corresponding to a packed bed height /dryer diameter ratio (L/D ratio) of 0.5 (0.150 kg). The incoming air temperatures were conditioned to 60, 70, and 80°C measured with a digital recorder including a J-type copper-constantan thermocouple (Cole Parmer). The drying process was carried out in two ways: batch with tempering periods and, as control, simply batch. For the first one, the experimental design is shown in Table 1 using the parameter α[5] Eq. 1:

(1)

Table 1 Tempering cycles at several α and temperature levels for carrot dehydration in the fluidized bed dryer

	Temperature(°C)
	60	70	80
	Cycles: ts × tr (min × min)
0.4	30 × 45	20 × 40	15 × 20
0.5	30 × 30	20 × 20	15 × 15
0.6	15 × 10	15 × 10	15 × 10
ts: drying time; tr: tempering time.

where t_d is the effective resident time of product in dryer, and t_t is the tempering time. Humidity losses were recorded every minute in a digital balance (Mettler Toledo PB302) during drying and reported as percentage. The packed bed height/dryer diameter ratio and drying air speed were determined according to the drier dimensions. The test temperatures chosen were those that did not affect the quality of the product when drying with tempering cycles. During the tempering periods (t_t) the load was removed from the dryer and placed in a closed container, keeping it inside a chamber at 28°C (Hotpack Incubator controlled-temperature storage chamber) for the required period of time.

The response variables (quality parameters) considered were: carotene retention[13] was determined according to AOAC 941.15 Official Method; peroxidase activity[14] was tested as a rate of hydrogen peroxide reduction using O-dianisidine as hydrogen donor and recorded using a spectrophotometer (Thermo Spectronic Genesis 10 UV) at 460 nm. Results were expressed in peroxidase-activity units. Invertase activity[15] was assessed considering the grade of sucrose hydrolysis due to this enzyme. Samples were read adjusting with the control at 540 nm in the spectrophotometer. Sucrose together with 1.9 mL of 0.05M acetate buffer pH 4.7 was used as control evaluated in the same way like the test samples. Rehydration capacity[16] indicates the percentage of moisture that a particular dried product can absorb. Eighty to one hundred fifty mL of distilled water is poured to a 250 mL beaker, covered with a watch glass, heated to boil and then 2 to 10 g of the dry sample is added to it, boiled for 20 min, poured in a 7.5 cm diameter Buchner funnel to drain and finally weighed. The moisture percentage absorbed in the rehydrated sample was calculated as in Eq. 2.

(2)

The statistical analysis was conducted using the Minitab Statistical Software 14 program, using a 2² with a central point factorial design. The evaluated variables were temperature and the α relationship.

A pilot plant fluidized bed dryer was employed to repeat some laboratory schemes for comparative purposes. The pilot plant fluidized bed dryer consists of a tunnel where through which hot air circulates. A 0.382 m³/s capacity 7.5-HP turbo-fan (Evisa) with frequency inverter (Mitzubishi) to select different air velocities is coupled to one end. A 0.25m inner-diameter acrylic tube with an aluminum air distribution disk was coupled to the opposite end used as drying area.[17] Air is heated using L.P.-gas (Eclipse, Model 40AH). The dryer was filled with 3 kg of carrots for an L/D = 0.50. Drying experiments at 70°C were carried out. Six samples were sampled at different drying times conventional drying process (batch). Samples at the beginning and at the end of each cycle were tested when drying with tempering method. In the last case, dryer was stop to temper the product and starting again as soon as the cycle was finished.

To calculate the effective diffusion coefficient, given the diffusive nature of carrot drying, the solution of the second Fick's Law is applied[18] for the analysis of the fluid-bed drying curve, assuming an equilibrium state at the solid-gas interface and considering that carrot cube has an equivalent spherical geometry, then d_e = (6 V_C/π)^1/3, where V_C is an average volume of the carrot considered as an sphere and d_e the equivalent diameter.

For long drying times, this equation applies for a single term in the series. The effective diffusion coefficient is calculated from the slope derived when the abovementioned equation is linearized.

(3)

In the case of tempering drying, the same model is applied for each drying cycle “i,” thus deriving the corresponding effective diffusion coefficient. Finally, the mean effective diffusion coefficient, corresponding to the total drying process including “i” tempering cycles, is calculated from Eq. (4):

(4)

where y_i is the solid moisture percent loss during the drying cycle “i”, and D_eff,i is the effective diffusion coefficient corresponding to this cycle.

RESULTS AND DISCUSSION

Table 2 shows the total duration for each process (drying time x tempering time) and the effective residence time of carrot in the dryer. It can be seen how the effective diffusion coefficients have increased with temperature, especially with tempering cycles compared to conventional drying (batch). This can infer that the temperature gradient, due to tempering periods, favors water redistribution and migration inside of the product.

Table 2 Total and actual or effective process duration to dehydrate carrots to a final 9% moisture content in the laboratory fluidized bed dryer for various α values

	60°C				70°C				80°C
	Process time (min)				Process time (min)				Process time (min)
α	Total	Effective		Deff × 10^8(m^2/s)	Total	Effective		Deff × 10^8(m^2/s)	Total	Effective		Deff × 10^8(m^2/s)
0.4	255∗	120	0.333	4.8391 ± 0.001	309∗	109∗	0.273	4.8871 ± 0.002	187	87	0.330	6.4635 ± 0.001
0.5	210	120	0.333	5.0682 ± 0.001	166	86	0.427	6.2102 ± 0.001	162	87	0.330	6.8753∗ ± 0.003
0.6	190	120	0.333	5.4029 ± 0.002	159	99	0.34	6.1470 ± 0.001	156	99	0.238	6.2767 ± 0.003
Batch	180		0	2.5379∗ ± 0.001	150		0	3.1274∗ ± 0.001	130		0	3.7169∗ ± 0.002
∗Significantly different values (p < 0.05); Total process duration (X ± δ^2 = 185 ± 32.30); Drying time (X ± δ^2 = 102 ± 14.58); Deff (X ± δ^2 = 5.1293 ± 1.3912).

Typical drying curves are shown in Figures 1 and 2. The curves typically demonstrated smooth diffusion-controlled drying behavior under all experimental conditions. A constant rate drying period was not observed in any of the experiments of this study; therefore, diffusion was accepted to be the main mechanism during moisture transportation to solid surface. The values for the effective moisture diffusivity were calculated from the slope in Eq. 4, moisture ratio as a function of time. Values of D_eff are show in Table 2. All the diffusivities from tempering experiments varied between 4.6 × 10⁻⁸ to 6.87 × 10 ⁻⁸ m²/s compared to those of conventional drying experiments: 2.53 × 10 ⁻⁸ to 3.71 × 10⁻⁸ m²/s. The differences in effective moisture diffusivity among schemes (tempering and conventional) were significant (p ≤ 0.05). The effective diffusion coefficient associated with tempering drying increased two times compared to conventional drying values. All the effective diffusion coefficients can be compared with those reported by other authors.[19]

Figure 1 Carrot conventional fluidized bed drying curve corresponding to decreasing rate period at 70°C.

Figure 2 Carrot fuidized bed drying curve with tempering cycles corresponding to decreasing rate period at 70°C

Table 3 shows that the changing effect between lab and pilot plant, beside of the dimension changes between one to the used equipment, there is also a dry air mass to dry solids different relations to be considered that, finally becomes in a pilot plant drying time reduction. This could be considered, very interesting, from the energy save point of view. It can also be observed some increase in their effective diffusion coefficient values. It can be observed that total drying processing were longer, with tempering cycles, than that of batch; however, for the first one, the effective drying times were reduced leading to a substantial energy savings. It can be observed that the 150 min of batch effective drying time (100%) at 70°C was reduced to 86 min for α = 0.5 (20 × 20), representing a 57.3%, which can be considered as a 42.7% of energy saving. Similar consideration can be taken in account for α = 0.4 and 0.6. As it was mention before, shortest and longest experimental laboratory schemes at 70°C were chosen to compare their results by pilot fluidized bed dryer.

Table 3 Total and actual or effective process duration to dehydrate carrots to a final 9% moisture content in the pilot and laboratory fluidized bed dryers at 70°C for different schemes compared to batch (conventional)

	Pilot dryer					Laboratory dryer
	Process time (min)					Process time(min)
α	Total	Effective		Moisture (d.b.)	Deff × 10^8(m^2/s)	Total	Effective		Moisture (d.b.)	Deff × 10^8(m^2/s)
0.4	240	80	0.407	2.45	5.1042 ± 0.002	309∗	109	0.273	4.85	4.8871 ± 0.001
0.5	160	80	0.407	2.46	6.3256 ± 0.001	166	86	0.427	4.94	6.2102 ± 0.001
Batch	135		0	2.43	4.5860 ± 0.002	150		0	4.58	3.1274∗ ± 0.001
∗Significantly different values (p < 0.05); Total process duration (X ± δ^2 = 193.33 ± 67.37); Drying time (X ± δ^2 = 106.66 ± 30.11); Deff (X ± δ^2 = 5.1762 ± 1.1600).

Also, it can be seen that even changing to pilot plant no different figures in biochemical data where found comparing with those from laboratory test. No matter how longer the total process was from a biochemical point of view, these scheme involved a greater advantages, as shown in Table 4 where the peroxide activity was reduced to 21.9, carotene and invertase activity retention were 97.65 and 67.5 respectably, where 54.82, 91.76, and 39.68 correspond to batch. Almost same values can be observed for pilot fluidized bed tests in Table 5. Concerning to moisture absorbed in samples rehydration test, no significant differences were found.

Table 4 Biochemical and physical parameters in the quality assessment of carrots dehydrated using tempering and non tempering (batch) processes

	Temperature(°C)
	Carotene retention (%)			Invertase activity (%)			Peroxidase activity (%)			Moisture absorbed by the dehydrated sample (%)
α	60	70	80	60	70	80	60	70	80	60	70	80
0.4	90.59	97.65∗	88.23	33.8	67.5∗	42.1	85.8∗	21.9∗	49.85	77.99	74.70	75.89
0.5	82.35	95.29∗	86.47	33.81	53.17	41.27	61.19	24.97∗	59.40	76.80	76.12	75.18
0.6	81.18	93.53	85.88	42.06	45.40	39.68	54.63	41.29	67.66	76.80	70.00∗	75.50
Batch	79.41∗	91.76	85.29	31.74∗	39.68	34.92	95.22∗	54.82	81.09	75.02	74.60	75.50
Control	100%			100%			100%			—	—	—
∗Significantly different numbers (p < 0.05); % Carotene retention (X ± δ^2 = 88.18 ± 5.75); %Invertase Ret. (X ± δ^2 = 42.09 ± 9.94); %Peroxidase Ret. (X ± δ^2 = 58.15 ± 22.48); %Moisture in rehydrated samples (X ± δ^2 = 75.34 ± 1.95).

Table 5 Biochemical and physical parameters in the quality assessment of carrots dehydrated at 70°C, using different fluidized bed drying schemes at pilot and laboratory scales compared to batch

	Carotene retention (%)		Invertase activity (%)		Peroxidase activity (%)		Moisture absorbed by the dehydrated sample (%)
α	Pilot	Lab	Pilot	Lab	Pilot	Lab	Pilot	Lab
0.4	97.65	97.65	75.55∗	67.46	22.99	21.9∗	75.6	74.70
0.5	94.41	95.29	64.29	53.17	25.77	24.97∗	76.8	76.12
Batch	91.28	91.76	42.06	39.7∗	56.46∗	54.82∗	75.2	74.6
Control	100%		100%		100%		—	—
∗Significantly different numbers (p < 0.05); %Carotene Ret. (X ± δ^2 = 94.67 ± 2.76); %Invertase Ret. (X ± δ^2 = 57.04 ± 14.45); %Peroxidase Ret. (X ± δ^2 = 34.48 ± 16.45); %Moisture in rehydrated samples (X ± δ^2 = 75.50 ± 0.85).

Biochemical Parameters

Table 4 shows the biochemical and physical parameters tested to evaluate the quality of the final product.

Carotene Content

As it can be observed in Table 4, higher carotene retention was found in all tempering drying process schemes at 70°C compared to that of conventional drying. It seems that tempering reduces carrot temperature and oxygen availability, leading to a reduction in carotene degradation. Carrot carotene content was increased to 135.03 μg/g d.b. after blanching and was recorded as 100% control. Considering fresh carrot with 89.73 μg carotene/g d.b then carotene increments could be due to easy carotenoides are extracted from processed samples compared to fresh. In the last case carotenoids that are either physically protected or combined with other components, reducing their extraction.[20,21] Furthermore, the increments observed may be due to moisture content that was not accounted for, along with a loss of soluble solids, both of which would concentrate and increase provitamin A levels.

Table 5 shows a difference less than 0.2% between the percent carotene retention at a pilot scale relative to lab scale. This outlines to an advantage in favor of the pilot scale, given that a greater amount of product is obtained, along with energy savings and carotene retention levels similar to those obtained in the lab.

Invertase Activity

Invertase activity was observed to increase with temperature. This may be due to the fact that carrot texture changes as temperature rises, thereby facilitating enzyme extraction; however, further temperature increments inhibit invertase activity to a certain extent, as noted in Table 5, where a high enzyme activity retention was found at 70°C. Higher retention of this enzyme was attained at pilot scale compared with that of the laboratory, with 11% difference for the 20 × 40 scheme to 20.9% for the 20 × 20 scheme. This may derived from carrots that were dehydrated in a shorter time at pilot scale than in the lab, where, in the first case, there is a minor process temperature invertase damage, resulting in more activity retention.

Peroxidase Activity

Given its thermal resistance, this enzyme is not completely inactivated, but it decreases as a result of both the blanching and tempering processes. Table 4 shows that peroxidase activity in dehydrated samples is significantly lower compared with fresh samples, the 70°C sample being the one with the lowest enzyme activity. At 60°C peroxidase activity remains virtually unaffected (95% retention), whereas at 70°C peroxidase activity decreases 40% versus the conventional 60°C-drying process. This is beneficial from the perspective of higher product stability, but nonetheless at 80°C this enzyme attains a 26% reactivation compared with the 70°C drying process;[22] however, this enzyme is likely to be reactivated at high temperatures, as shown in Table 4, with 21.89% and 49.85% of enzyme activity retained at 70°C (20 × 40) and 80°C (15 × 20), respectively. At a pilot scale, there is a 3% increase in this enzyme activity compared to the lab scale drying process, as shown in Table 5.

Rehydration Test

The moisture percent absorbed by the dehydrated sample indicates the moisture that can be reincorporated into it after the drying process. As noted in Table 4, up to 77.99% moisture can be incorporated, which corresponds to the 60°C (15 × 10) scheme, and more than 70% moisture can be incorporated into the final product in all cases. Table 5 shows that more than 75% moisture can be incorporated at a pilot scale, since the drying process is shorter, causing less damage to the carrot structure.

CONCLUSIONS

Drying with tempering cycles reduced on 42.7% carrot residence time in the dryer compared with batch drying. This may represents an energy savings, favoring the product quality. Effective diffusion moisture coefficients were increased two times drying with tempering cycles than those of conventional process (batch). The 70°C, 20 × 20 lab achieved the highest efficiency presenting 97.65% carotene retention, 53.17% invertase activity retention, and 24.97% on peroxidase activity reduction. For pilot plant drying, there were 94.41%, 64.29%, and 25.77% same parameters respectively, whereas batch drying were 91.76%, 39.7%, and 54.82%, respectively. It could be said that a better carrot dehydrated quality, with substantial energy saving, can be prepared by tempering drying process than the conventional method.

Notes

7. Fletcher, J. Food Dehydration. Technical note N° 51. Campe en Food Preservation Research Association. Chipping Camdem, Gloucestershire, U. K., 1982, 33–44.

13. Association of Official Analytical Chemists. Determination of Carotenes, Official Method 941.15.

14. Worthington Ed. William Horwithz. Washington, D.C., 1989. Manual. No. 11 1 Ed. Worthington Biochemical Corporation: Lakewood, New Jersey, 1961.