Degradation kinetics of carotenoids and visual colour in pumpkin (Cucurbita maxima L.) slices during microwave-vacuum drying

ABSTRACT

The degradation kinetics of colour and individual carotenoids including lutein, α-carotene and β-carotene in pumpkin slices was investigated during microwave-vacuum drying (MVD) at selected microwave powers (4–8 W/g). The contents of lutein, α-carotene and β-carotene significantly decreased, while visual colour parameters such as the Hunter L* value decreased and the total colour difference (ΔE) value increased with the increase of processing time and microwave power. Degradation of lutein, α-carotene, β-carotene and Hunter L* value followed the first-order reaction kinetics; however, zero-order reaction kinetics was found adequate to describe changes in ΔE with R² ≥ 0.9027. The relationship between individual carotenoid content and Hunter L* value was found to be more consistent through one-dimensional linear regression analysis with R² ranging from 0.8697 to 0.9701. These results indicated that visual colour might be used to predict the lutein, α-carotene and β-carotene contents during drying pumpkin slices.

KEYWORDS:

• Carotenoids
• Colour
• Degradation kinetics
• MVD
• Pumpkin

Introduction

The pumpkin (Cucurbita maxima L.), the member of Cucurbitaceae family, is a tropical and subtropical plant cultivated throughout the world.^[1] The reddish yellow colour of pumpkin solely depends on the varieties and contents of carotenoids, and the main carotenoids present in pumpkin include β-carotene (more than 80% of the total carotenoid content), α-carotene and a few xanthophylls.^[2–4] Carotenoids in the diet are mostly associated with the beneficial effects on human health. Several studies have indicated that carotenoid intake may reduce the risk of certain types of cancer, degenerative and cardiovascular diseases, cataracts and macular degeneration, which plays an important role in disease prevention.^[5,6] Carotenoid pigments are sensitive to light, heat, oxygen and acid.^[7] Pumpkins and pumpkin products are major sources of β-carotene and contribute significantly to carotenoid intake for humans. However, the thermal processing of pumpkin products usually causes carotenoid degradation and colour change in the final products, which is associated with various factors such as isomerization and oxidation reactions.^[7,8]

Drying process assists in the extension of shelf life as well as a reduction in the volume of fruits and vegetables.^[9] Microwave-vacuum drying (MVD) is one of the advanced methods used for drying fruits and vegetables.^[10] However, various microwave drying conditions, such as the applied microwave power, vacuum levels and drying time, might have adverse effects on the colour and carotenoid contents in the processed products.^[11,12] Thus, it is necessary to identify the optimal kinetic parameters to predict the change in colour and quality parameters during drying. Several researchers have reported the model of thermal degradation kinetics of carotenoid and colour during drying. Demiray and Tulek^[13] demonstrated that the degradation of β-carotene of carrot slices followed the first-order reaction kinetics model in convective drying, and the degradation rate is proportional to the drying temperature, which was consistent with the results for the degradation of β-carotene and the order of reaction found by Demiray et al.^[14] in hot-air drying of tomatoes. Similar results were described by Hadjal et al.,^[15] who reported that first-order reaction kinetics agreed well to describe the degradation of lutein and cryptoxanthin on microwave heating of orange Juice. However, Suvarnakuta et al.^[16] found that the degradation of β-carotene was in accordance with the third-order degradation kinetics in the vacuum microwave, hot air and low-pressure steam drying process. The difference in reaction order may be related to the fruit and vegetable matrix of carotenoids and the thermal processing. Carotenoid degradation is usually accompanied by changes in colour. Correlations between visual colour coordinates with carotenoids content have been widely discussed in the literature. Saxena et al.^[17] found that the degradation of colour parameters Hunter L*×b* value and the total carotenoids of jackfruit bulb slices agreed well with the first-order reaction kinetics during hot air drying. Moreover, there was a certain correlation between total carotenoids and the L*×b* value, and the combination of Hunter L*×b* value could replace carotenoid content for online quality control adequately. However, not all of the colour parameters in the system were correlated with carotenoids; Albanese et al.^[18] stated in their research that the degradation of colour parameters L* and C* of apricot are in line with the first-order reaction kinetics in the drying process, but with lack of correlation between β-carotene and colour. This could be explained by the assumption that the non-enzymatic browning reaction has more effect on the change of colour parameters of apricot than the degradation of carotenoid. Other reactions such as the Maillard reaction will affect the colour parameters, so it is necessary to examine whether there is a correlation between colour and carotenoid, which may be used as a substitute for the determination of carotenoid content.^[17,19]

The objective of this study was to determine the kinetic parameters for the degradation of lutein, α-carotene, β-carotene and visual colour of pumpkin slices during MVD at different microwave powers as well as to evaluate the relationship between visual colour parameters and total carotenoids, which contributed to optimize the process through the control of processing parameters during drying and perform simulations using accurate kinetic parameters.

Materials and methods

Fresh material

The pumpkin used in this study is a Cucurbita maxima L. ‘Benmi’ variety, which was bought from a local market in Nanjing, China, washed in running tap water and then hand peeled. The washed pumpkins were cut into slices with a thickness of 6 mm using a chip cutter (FC-312, Zhaoqing Fengxiang Food Machinery Co., Ltd., Guangdong, China) and immersed in boiling water for 40s and then air-cooled to room temperature.

Chemicals and reagents

Carotenoid standards, namely lutein (95% pure, Sigma-Aldrich, catalogue no. C-9750) and β-carotene (≥95% pure, Sigma-Aldrich, catalogue no. C4582), were obtained from Sigma (St. Louis, MO). High performance liquid chromatography-grade (HPLC-grade) methyl tert-butyl ether (MTBE) and methanol were purchased from Tedia (Fairfield, USA). Analytical-grade hexane, anhydrous sodium sulphate, acetone, toluene, and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Purified water was obtained from a Milli-Q system (Millipore, Milford, MA, USA). All other chemicals and reagents were of analytical grade.

Microwave-vacuum drying

MVD was performed with a microwave-vacuum dryer (MVD-1, Nanjing Xiaoma Electrome-chanical Equipment Factory, Nanjing, China). Pumpkin slices were evenly placed on the turntable in the vacuum vessel, and subjected to a negative pressure of 90 KPa. MVD was intermittently conducted with 3 min heating-on followed by 1 min heating-off. The drying was continued until the final moisture content was approximately 5.0%. Pumpkin sampling under different microwave powers (4 W/g, 5 W/g, 6 W/g, 7 W/g and 8 W/g) at different dehydration times was carried out. Meanwhile, the core temperature of the pumpkin slices during MVD was detected and recorded by an optic fibre (Supplementary figure). All experiments were carried out in triplicates.

Colour measurement

Colour measurement was performed by our previous method.^[20] Visual colour of dried pumpkin slices was assessed using a colorimeter (WSC-S, Shanghai Precision & Scientific Instrument Co., Ltd., China) and reported as L*, a* and b* values. The L* value represents lightness, ranging from 0 (black) to 100 (white), and indicates how dark/light the sample is; the a* value exhibits redness (positive) and greenness (negative); and the b* value exhibits yellowness (positive) and blueness (negative). The total colour difference (ΔE) is used to characterize the overall change in colour during the drying process. The ΔE value is calculated by the following equation:

(1)

where L₀, a₀ and b₀ are the control values for freeze-dried pumpkin.

Analysis of carotenoids

The method for the analysis of carotenoids in pumpkin slices is based on a previously published method.^[20–22]

Carotenoid extraction

The slice samples were first ground into a fine powder using a herb grinder (FW100, Tianjin Taisite Instrument Co., Ltd, China), then a 0.5 g sample of pumpkin powder was mixed with 30 mL of acetone/petroleum ether (2:1, v/v) mixture in a 100-mL volumetric flask, followed by shaking for 4 h. Thereafter, 2 mL of 10% methanolic KOH was added for saponification at 25^ºC in the dark under nitrogen gas for 12 h. After saponification, 30 mL of hexane was added for the partitioning of carotenoids, followed by shaking for 1 min, after which a 10% sodium sulphate solution was added, and the sample was diluted to volume. The mixture was allowed to stand until two phases separated clearly. The upper layer containing carotenoids was collected, evaporated to dryness, redissolved in 10 mL of methanol, and filtered through a 0.45-µm membrane filter for HPLC analysis.

HPLC-DAD-MS/MS analysis

The HPLC analysis was performed using an analytical-scale C₃₀ reversed-phase column (250 mm×4.6 mm i. d.) with 5 µm particle size (YMC, Wilmington, MA, USA). Eluent A used consisted of methanol/MTBE/water (70: 25: 5, v/v/v), and eluent B consisted of MTBE/methanol/water (85: 10: 5, v/v/v). The separation was performed at a column temperature of 25^ºC using gradient elution conditions within 30 min at a flow rate of 0.6 mL/min. Aliquots of 20 µL were used for HPLC analysis. The solvent gradient elution program employed was as follows: 0–4.5 min, linear gradient 95%A–80%A; 4.5–12.5 min, linear gradient 80%A–50%A; 12.5–18 min, linear gradient 50% A–25%A; 18–24 min, linear gradient 25%A–5%A; and finally, a return to the initial setting.

The MS procedure was carried out on an Agilent 1290 Infinity LC/Agilent Technologies 6530 Q-TOF MS (Agilent Technologies, Santa Clara, CA, USA). An atmospheric pressure chemical ionization (APCI) interface in positive ion mode was used to detect the carotenoids, with a total ion current (TIC) scanning range of 80–1000 m/z, a corona current of 4 μA, a capillary voltage of 2500 V, nitrogen of the highest purity (99.9%) as a nebulizer gas (flow rate 4 L/min) and a vaporizer temperature of 350^ºC. Quantification was performed using the external standards method, after the preparation of a five-point external standard calibration curve for each available standard; standard calibration curve correlation coefficient (R²) values were in the range of 0.9991–0.9995.

Kinetic data analysis

To describe the changes in colour and carotenoid contents, zero-order (Eq. 2), first-order (Eq. 3) and second-order kinetic models (Eq. 4) were used. The R² was one of the primary criteria to select the best equation to account for variation. In addition to R², root mean square error (RMSE) was also used to determine the quality of the fit. Higher R² value and lower RMSE value were chosen as the criteria for goodness of fit.^[23,24] The above-mentioned parameters can be calculated as follows:

(2)

(3)

(4)

where C_t is the concentration (%) at time t; C₀, the concentration (%) at time zero; k₀, the zero-order rate constant (min⁻¹); k₁, the first-order rate constant (min⁻¹); k₂, the second-order rate constant (min⁻¹); and t, the drying time (min). Moreover, the half time (t_½), defined as the time required for carotenoids concentration in dried pumpkins to decay decreased to 50% of its initial concentration, was calculated by Eq. (5). The decimal reduction time (D values), defined as the time required for carotenoids concentration in dried pumpkins to decay decreased to 90% of its initial concentration, was calculated by Eq. (6).

(5)

(6)

where k is the reaction rate constant.

Statistical analysis

All experiments were performed in triplicate. Kinetic data were analysed by regression analysis using origin version 9.0 (Washington, USA). Regression results include the RMSE and coefficient of determination, R².

Results and discussion

Predominant carotenoids of pumpkins

An HPLC chromatography profile of carotenoids from Cucurbita maxima L. ‘Benmi’ is shown in Fig. 1. The identification is discussed according to the combined information obtained from the retention times, UV-Visible absorption maxima, electron ionization mass spectroscopy (EIMS) and chemical ionization mass spectroscopy (CIMS) fragmentation patterns.^[25] Among the 12 different carotenoid compounds identified in this pumpkin sample, all-trans-α-carotene and all-trans-β-carotene, which was accompanied by four minor cis-isomers (15-cis-β-carotene, 13-cis-β-carotene, 9-cis-β-carotene and 9-cis-α-carotene), were the most abundant components (97%), followed by all-trans-lutein and traces of neoxanthin, neochrome, violaxanthin, α-cryptoxanthin and β-cryptoxanthin. In our study, zeaxanthin and antheraxanthin were not detected, which is not completely consistent with the literature.^[26] This might be due to the different composition of carotenoids in different varieties of pumpkin, but also related to the determination methods such as different extraction solvent and detection methods.^[4,27] However, the major trans carotenoids, including all-trans-β-carotene, all-trans-α-carotene and all-trans-lutein, were similar to the results reported by others.^[26] The analysis of carotenoids in pumpkins during MVD focussed on the degradation of β-carotene, α-carotene and lutein in the following research.

Figure 1. HPLC chromatograms of carotenoids in fresh pumpkin pulp. The following peaks were identified: 1. neoxanthin, 2. neochrome, 3. violaxanthin, 4. all-trans-lutein, 5. α-cryptoxanthin, 6. β-cryptoxanthin, 7. 15-cis-β-carotene 8. 13-cis-β-carotene, 9. all-trans-α-carotene, 10. 9-cis-α-carotene, 11. all-trans-β-carotene, 12. 9-β-carotene.

Fitting of kinetic modelling of carotenoids degradation

The reduction observed for lutein, α-carotene and β-carotene was, respectively, described by zero-, first- and second-order kinetic models in different microwave powers (4 W/g, 5 W/g, 6 W/g, 7 W/g, and 8 W/g). The R² and RMSE used to determine the goodness of fit of the models are shown in Tables 1 and 2. At the fitting of the zero-order reaction rate model Eq. (2) (Table 1), the values of R² showed 0.9461, 0.8802 and 0.9494 for lutein, α-carotene and β-carotene, respectively, but the values of RMSE for the zero-order kinetic models, which varied from 1.60391 to 22.82657, were the highest for all the models considered. To obtain higher R² and lower RMSE, the first- (Eq. (3)) and second-order reaction rate models (Eq. (4)) were chosen in place of the zero-order reaction kinetic model. It was observed that the first-order kinetic model was found to better describe the carotenoids data. The R² ranged from 0.9405 to 0.9527, while the RMSE values were less than 0.062 for all cases. Similar results have been reported by Fratianni et al.^[28] during apricots drying by hot air and microwave. By the time of second-order development, the R² was decreased to 0.7867, 0.8191 and 0.9069 for lutein, α-carotene and β-carotene, respectively. Therefore, in the following discussion only the coefficients relating to the first-order kinetics will be considered (Fig. 1).

Table 1. Values of reaction rate constant and its coefficient of zero-, first- and second-order of the main trans carotenoids during microwave-vacuum drying.

Carotenoids	Zero			First			Second
	k/min^−1	R^2	RMSE	k/min-^1	R^2	RMSE	k/min^−1	R^2	RMSE
Lutein	1.4477	0.9461	1.60391	0.0445	0.9405	0.06156	0.0011	0.8191	0.00183
α-carotene	14.1706	0.8802	22.82657	0.0407	0.9527	0.04658	0.0020	0.7867	0.00037
β-carotene	9.7395	0.9494	8.79045	0.0412	0.9461	0.04658	0.0020	0.9069	0.00037

Degradation kinetics of lutein

The lutein content of fresh pumpkin samples was found to be 47.51 μg/g DW. When the pumpkin slices were dried at the selected microwave power, its lutein content was decreased. The lutein content decreased to 28.03 μg/g DW after 24 min of drying at 4 W/g, accounting for approximately 41% loss. A similar trend was observed during drying of the pumpkin slices at higher power. The degradation of lutein increased with increase in microwave power and it lost more than 55% after only 6 min of drying at 8 W/g. Similar behaviours of lutein loss were observed by Aparicio-Ruiz et al.^[29] during heat treatment of virgin olive oils. Degradation occurs due to various reasons such as high temperature, long processing time, light and oxygen.^[7] Subagio et al.^[30] reported that the decline in lutein was due to destruction by isomerization and oxidation, resulting in cis isomers, epoxides, hydroxyl oxidation containing aldehyde or ketone group. The first-order model (Eq. (3)) for degradation of lutein during drying was fitted and the degradation parameters were obtained (Table 2). It was observed that as the microwave power increased, the degradation rate constant increased, with corresponding reduction in the half-life (t_1/2) and the value of D. For example, the degradation rate constant of lutein was 0.0391 min⁻¹ at 4 W/g, but it increased up to two times with increase in microwave power to 8 W/g. Results indicated that lutein was highly affected by microwave-induced tissue heating. The predicted values of lutein were obtained from the model and shown as a solid line in Fig. 2A. The R² values ranged from 0.9381 to 0.9942, whereas RMSE varied from 0.02117 to 0.06325, which adequately validated the first-order model for explaining the degradation behaviour. Similar studies on the thermal and oxidative degradation of lutein in safflower seed oil and virgin olive oils also concluded that the degradation of lutein fitted the first-order model with degradation rate and increased with increasing temperature.^[29,31] The present findings concluded that the degradation of lutein followed the first-order model and the degradation rate constant increased with increase in microwave power and these results were similar to earlier reported observations.

Table 2. The kinetic parameters of three kinds of carotenoids, L* and ΔE at different microwave powers.

Parameter	Reaction order	Power (W/g)	K (min^−1)	R^2	RMSE	t1/2 (min)	D (min)
lutein	First	4	0.0391	0.9381	0.0412	17.74	58.93
		5	0.0445	0.9405	0.0616	15.58	51.74
		6	0.0556	0.9504	0.0444	12.48	41.44
		7	0.0624	0.9592	0.0633	11.11	36.89
		8	0.0756	0.9942	0.0212	9.17	30.45
α-carotene	First	4	0.0353	0.8280	0.0654	19.66	65.32
		5	0.0407	0.9527	0.0466	17.05	56.63
		6	0.0479	0.9270	0.0468	14.46	48.05
		7	0.0556	0.9084	0.0792	12.47	41.41
		8	0.0650	0.9295	0.0752	10.66	35.40
β-carotene	First	4	0.0363	0.9299	0.0382	19.09	63.43
		5	0.0412	0.9461	0.0466	16.82	55.89
		6	0.0490	0.9298	0.0458	14.14	46.97
		7	0.0603	0.9731	0.0469	11.50	38.21
		8	0.0686	0.9382	0.0716	10.11	33.58
Hunter L* value	First	4	0.0033	0.9513	0.0000	210.04	697.75
		5	0.0069	0.9783	0.0000	100.46	333.71
		6	0.0103	0.9556	0.0001	67.30	223.55
		7	0.0129	0.9647	0.0000	53.73	178.49
		8	0.0215	0.9730	0.0001	32.24	107.10
ΔE	Zero	4	0.2119	0.9027	0.3202	3.27	10.87
		5	0.4868	0.9655	0.1691	1.42	4.73
		6	0.6691	0.9678	0.1634	1.04	3.44
		7	1.0846	0.9806	0.1163	0.64	2.12
		8	1.4652	0.9726	0.2291	0.47	1.57

Figure 2. First-order degradation kinetics of lutein (A), α-carotene (B) and β-carotene (C) in pumpkin at a different microwave power with (□) 4W/g, (○) 5W/g, (△) 6W/g, (▽) 7W/g and (×) 8 W/g, respectively.

Degradation kinetics of α-carotene

Pumpkins are a good source of α-carotene and its content may vary. For example, Murkovic et al.^[4] reported that the α-carotene content of fresh pumpkins ranged from 0 to 7.5 mg/100 g, whereas de Carvalho et al.^[3] reported that the α-carotene content of landrace pumpkins (Cucurbita moschata Duch) ranged from 67.07 to 72.99 μg/g. In the present study, the most abundant carotenoid found in fresh pumpkin samples was α-carotene, the content of which was 386.22 μg/g DW, accounting for nearly 50% of the total carotenoid content. However, the thermal processing of pumpkins has been reported to have a very adverse effect on their α-carotene content. The α-carotene content of pumpkins dried at 4 W/g changed from 386.22 to 252.55μg/g DW at the end of drying. However, at 8 W/g, it dropped to 186.43μg/g DW. The results indicated that the degradation of carotenoids in tomatoes was highly influenced by the power and length of drying. Previous studies, such as Fratianni et al.,^[32] reported a significant loss of 56% in the α-carotene content of orange juice after 10 min microwave heating at 70°C. However, Dueik et al.^[33] found only 10.5% loss in α-carotene when carrots were dehydrated by vacuum frying at 60°C for 5 min, which may be due to the low temperatures employed and minimal exposure to oxygen under vacuum condition. The degradation reaction of α-carotene followed a first-order kinetic model, and this was in good agreement with the first-order degradation of α-carotene reported in previous studies.^[32] The α-carotene degradation at various powers, reported as the natural logarithm plot of the carotenoid content, as a function of drying time, is indicated in Fig. 2B. The linear R² was 0.8280–0.9527, confirming that the reaction of α-carotene degradation is of the first-order. The calculated rate constants (k) and other kinetic parameters are given in Table 2 for various drying conditions. The reaction rate constants for α-carotene loss were in the range of 0.0353–0.0650 min⁻¹ and were significantly influenced by drying power. The degradation rate of α-carotene increased gradually with microwave power. The half-life time (t_1/2) of α-carotene in pumpkins was 19.66 min at 4 W/g microwave power, but it decreased to 10.66 min at 8 W/g.

Degradation kinetics of β-carotene

A large range of β-carotene content in pumpkins was reported in the literature, depending on the variety, maturation stage and extraction procedure as well as instruments used for analysis.^[27,34] The initial β-carotene content was found to be 320.53 μg/g DW in fresh pumpkins. The β-carotene contents of pumpkin at 4, 5, 6, 7 and 8 W/g after completion of drying were experimentally determined to be 214.77, 190.39, 180.21, 153.22 and 149.52 μg/g, respectively. According to Fratianni et al.^[28] when apricot was hot dried at 60°C and 70°C, losses of carotenoids were approximately 20% and 40%, respectively. Several studies report the decrease of all-trans-carotenoid and the concomitant increase in cis-isomers in vegetables subjected to thermal processes.^[35–37] Our results suggest that the β-carotene decreased, whereas three cis-isomers (15-cis-β-carotene, 13-cis-β-carotene and 9-cis-β-carotene) increased on prolonging the heating time, which is accelerated at higher microwave power. Therefore, suitable kinetics for carotenoids degradation was evaluated at different drying conditions to minimize its loss during drying.

As shown in Fig. 2C, degradation of β-carotene was found to follow first-order reaction kinetics because the linear R² was higher than 0.9299 and the mean square error was lower than 0.07155, which was in agreement with previous studies.^[8,13,16] First-order kinetic rate constants at different drying conditions are presented in Table 2. The range of reaction rate constants for β-carotene loses was 0.0363–0.0686 min⁻¹. The reaction rate constant of β-carotene degradation was affected by several factors such as food matrix, pretreatment and processing method. For example, the degradation rate of β-carotene of dehydrated carrots was higher in unblanched carrots compared with blanched samples when they were stored at the same temperature, which also showed that the enzymes in fruits and vegetables could affect the kinetic parameters.^[38] The degradation rate of total carotenoids and β-carotene in apricot by microwave heating was higher than that by hot-air drying.^[8]

Degradation kinetics of visual colour

With the increase of drying power and time, pumpkin slices become darker. This corresponds to a decrease in the L* value of the colour scale (Fig. 3A). At the end of drying, the Hunter L* value of the pumpkin slice decreased exponentially from the initial values of 80.11–73.74, 72.50, 72.54, 71.85 and 70.61 at 4, 5, 6, 7 and 8 W/g, respectively. However, the changes of a* and b* values were not obvious with the increase of microwave power and time. This phenomenon can be attributed to the fact that β-carotene and α-carotene, the most abundant pigments present in the pumpkin, are yellowish carotenoids, and their degradation produced a higher impact in L* compared with the a* and b* values. However, Ahmed et al.^[39] observed that apart from the Hunter b* value, both L* and a* values also decreased with time at a given temperature as the mango puree turned brown. Chutintrasri and Noomhorm^[40] and Saxena et al.^[17] both found that the Hunter L* and b* values decreased and the a* value increased during heat-treated pineapple puree at 70–110^ºC and hot-air drying for jackfruit bulb slices. The different results may be due to the changes in the yellow and red colour of the pumpkin slices, which may be caused not only by the variation of carotenoids content but also by a series of complex Maillard reactions in the drying process.^[19] The total colour difference ΔE, which is a combination of the L*, a* and b* values, as given by Eq. (1), is a colorimetric parameter extensively used to characterize the variation of colour in pumpkins during processing. The results presented in this work suggest that the values of ΔE increased during dry processing (Fig. 3B).

In the current study, it was observed that the first-order kinetic model fitted well to L* (Table 2). In all cases, a significant (p < 0.05) linear regression with the coefficient of determination values (R²) ranging between 0.9513 and 0.9783 was obtained whereas the RMSE values were less than 0.00006. The same order of reaction was found by Chutintrasri and Noomhorm^[40] in pineapple puree and by Avila and Silva^[41] in peach puree. In addition, it was found that the degradation of ΔE value followed zero-order reaction kinetics with R² values of 0.9027–0.9806, which is in agreement with previous reports.^[17,40]

Figure 3. The degradation kinetics of L* (A) and ΔE (B) in pumpkin at a different microwave power with (□) 4W/g, (○) 5W/g, (△) 6W/g, (▽) 7W/g and (×) 8W/g, respectively.

Relationship between visual colour and carotenoids content

The decrease in Hunter L* value was correlated with the carotenoids concentration of pumpkin slices during drying. The relationship between visual colour and carotenoid concentration has been found to be well described using the linear equation (Eq.7):

(7)

where k_a and k_b are the coefficients, ‘X’ represents Hunter L* value and ‘Y’ represents lutein, α-carotene and β-carotene concentrations (μg/g DW), respectively. The regression equation and the R² of Eq. (7) are presented in Table 3. From the table, there was a certain correlation between lutein, α-carotene and β-carotene concentration and the L* value, with the R² ranging between 0.8697 and 0.9701, which showed that the fitting degree of the one-dimensional linear regression equation was better. These results suggested that as the carotenoids degradation occurred with processing, the visual colour of the product got affected. Correlations between visual colour coordinates with the carotenoids content are widely discussed in the literature. Ahmed et al.^[42] found that the combination of the Hunter a* × b* value adequately represented thermal colour change. Correlation of the Hunter a* × b* values was also reported to agree with the total carotenoids and lycopene contents in heated watermelon juice by Sharma et al.^[43]. Saxena et al.^[17] described a good relationship of the total carotenoids content and the Hunter L* × b* value and suggested that the combination of Hunter L* × b* value could represent the colour change adequately. Thus, determination of the L* value can be a good predictor of lutein, α-carotene and β-carotene losses during the drying of pumpkin slices in our study.

Table 3. Correlation of brightness value L*and carotenoids content in pumpkin at different microwave powers.

Microwave power (W/g)	Lutein	α-carotene	β-carotene
4	y = 2.8703x-182.64	y = 22.896x-1433.3	y = 20.334x-1305.4
	(0.9701)	(0.9364)	(0.8838)
5	y = 2.6954x-165.35	y = 26.43x-1677.6	y = 18.828x-1153.1
	(0.9064)	(0.8802)	(0.9201)
6	y = 2.4136x-146.67	y = 19.537x-1173.1	y = 17.014x-1032
	(0.9449)	(0.9609)	(0.9512)
7	y = 3.7341x-250.47	y = 29.247x-1941.1	y = 26.485x-1778.1
	(0.9267)	(0.8772)	(0.9066)
8	y = 2.5363x-250.47	y = 22.437x-1396.7	y = 18.534x-1177.6
	(0.8697)	(0.9418)	(0.9377)

Conclusions

In this paper, the degradation kinetics of lutein, α-carotene, β-carotene and colour degradation in pumpkin slices were studied during MVD. It was observed that the first-order reaction agreed well with the degradation of lutein, α-carotene, β-carotene and Hunter L* values, with the reaction rate constants in the range of 0.0391–0.0756, 0.0353–0.0650, 0.0363–0.0686 and 0.0033–0.0215 min⁻¹, respectively, while the degradation of ΔE values followed zero-order reaction kinetics and the reaction rate constant was in the range of 0.2119–1.4652 min⁻¹. The lutein, α-carotene and β-carotene were correlated with the Hunter L* value, and thus could predict the carotenoids degradation in the pumpkin during drying.

Funding

This work was supported by Grants-in-Aid for scientific research from the National Natural Science Foundation of China (No. 31301534), the special fund for agro-scientific research in the public interest (No. 201503142-5), the basal research fund for the Jiangsu Academy of Agricultural Sciences (major cultivation project No. ZX (15)1008) and this project for Key Laboratory of Horticultural Crop Genetic Improvement of Jiangsu Province (2014019).

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Additional information

Funding

This work was supported by Grants-in-Aid for scientific research from the National Natural Science Foundation of China (No. 31301534), the special fund for agro-scientific research in the public interest (No. 201503142-5), the basal research fund for the Jiangsu Academy of Agricultural Sciences (major cultivation project No. ZX (15)1008) and this project for Key Laboratory of Horticultural Crop Genetic Improvement of Jiangsu Province (2014019).