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Combined treatments of blanching and dehydration: study on potato cubes
Journal of Food Engineering 68 (2005) 289–296 www.elsevier.com/locate/jfoodeng
Combined treatments of blanching and dehydration: study on potato cubes Carla Severini, Antonietta Baiano *, Teresa De Pilli, Barbara F. Carbone, A. Derossi Faculty of Agriculture, Department of Food Science, University of Foggia, Via Napoli 25, 71100 Foggia, Italy Received 23 January 2004; accepted 31 May 2004
Abstract Samples of Solanum tuberosum var. Primura were submitted to combined treatments of blanching and dehydration. Blanching was alternatively performed in hot distilled water, hot sugary-saline solution, by microwaves in distilled water or by microwaves in saline solution. Drying was alternatively carried out in an air cabinet, a microwave oven or a belt drier. In terms of process speed, colour retention and water absorption capacity, the best results were obtained combining microwave blanching with dehydration on the belt drier. In particular, dehydration on the belt drier levelled eventual negative effects determined by the blanching treatments. 2004 Elsevier Ltd. All rights reserved. Keywords: Blanching; Colour; Dehydration; Potato; Rehydration
1. Introduction In the production of canned, frozen and dehydrated vegetables, the main purpose of a blanching treatment is the inactivation of enzymes such as polyphenoloxidases, responsible for browning (Mapson, Swain, & Tomalin, 1963; Collins & McCarty, 1969), peroxidase, catalase and phenolase which leads to the development of off-flavours (Pinsent, 1962; Bizzarri, Andreotti, & Massini, 1981). In particular, thanks to their heat resistance or regeneration capacity, peroxidase, catalase and phenolase are considered as indices of the heat treatment efficacy. Among them, peroxidase is the most heat resistant. Smith (1977) found that it is necessary to performed a blanching treatment at 100 C for 3 min, to obtain its complete inactivation in cubed potatoes destined for dehydration. Bizzarri et al. (1981) obtained the complete and irreversible inactivation of peroxidase by blanching potatoes for 4 min at 97 C.
*
Corresponding author. Tel.: +39 881 589242; fax: +39 881 740211. E-mail address: [email protected] (A. Baiano).
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.05.045
Blanching can be performed by exposing vegetables to hot water (the most common method), hot and boiling solutions containing acids and/or salts, by steam (Kidmose & Martens, 1999) or by microwaving product dipped in water or solutions (Chen, Collins, McCarty, & Johnston, 1971; Ramaswamy & Van de Voort, 1990; Ponne, Van Remmen, & Bartels, 1991; Severini, De Pilli, Baiano, Mastrocola, & Massini, 2001) for several seconds or minutes. The effectiveness of a drying process depends on different factors: method of heat transfer, continuity or discontinuity of the process, direction of the heating fluids with respect to the product, pressure (atmospheric, low, deep vacuum). Dehydration can be performed by using different kinds of equipment: air cabinet, belt drier, tunnel drier, fluidized bed, spray drier, drum drier, foam drier, freeze-drier, microwave oven. An interesting drying method is represented by osmotic dehydration. Krokida, Maroulis, and Saravacos (2001) investigated the effects of different drying methods on the colour of the obtained products. They found that colour characteristics are significantly affected by the drying methods and that the
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changes in redness (a*) and yellowness (b*) follow a first order kinetic model. In particular, air-, vacuum and microwave drying caused extensive browning in fruit and vegetables with a decrease in L* value and an increase in a* and b* values whereas freeze-drying improved colour characteristics. Osmotically pre-treated samples were preserved from browning to a certain measure. Drying methods also affect the behaviour of dehydrated product during rehydration. Krokida, Kiranoudis, and Maroulis (1999) and Krokida and Maroulis (2000, 2001) studied the viscoelastic behaviour of dehydrated products during rehydration. They found that dehydrated foods do not keep their viscoelastic behaviour after rehydration due to structural damages that occur during drying. Freeze-dried products showed the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pre-treatment helped freeze-dried materials to keep their elastic nature probably thanks to the solid gain. Air and vacuum dried foods showed the smallest hysteresis tendency, keeping their viscoelastic characteristics during rehydration close to those of dried materials. The purpose of this work was to study the effects of different combined systems of blanching and dehydration on dehydration speed, colour characteristics and the behaviour during rehydration of cubed potatoes.
2. Materials and methods
By immersion in a corn syrup solution diluted to 70 Brix with distilled water and containing sodium-chloride (3%, w/w) at 100 C for 5 min. The undiluted corn syrup (commercial name ‘‘Glicosa’’, supplied by Roquette Italia, Cassano Spinola, Italy) had the following characteristics and chemical composition: refractometer grade 80.6 Brix, pH value 5.1 at 20 C, 30% dextrose, 46% maltose, 10% trysaccharides, 14% polysaccharides. By microwaves in a domestic oven (De Longhi, Milan, Italy) upon immersion in distilled water at 850 W for 5 min. By microwaves in a domestic oven (De Longhi, Milan, Italy) upon immersion in a sodium-chloride solution (3%, w/w) at 850 W for 5 min. The product–solution ratio was always 1:5 (w/w). After blanching, the samples were cooled in tap water, drained and wiped with blotting paper. Unblanched potato cubes were kept as controls. 2.4. Dehydration Blanched and unblanched potato samples were submitted to three different methods of dehydration: in an air cabinet (I.S.C.O., Milan, Italy) at 100 C; in a microwave oven (De Longhi, Milan, Italy) at 85 W; in a monolayer on a concurrent belt drier (pilot plant, Sandvik Process Systems, Milan, Italy) at 100 C.
2.1. Raw material Trials were performed on potato tubers (Solanum tuberosum var. Primura) purchased at a local market. This Italian variety, characterized by an early ripening time, is the most commonly used in food industry thanks to the regular shape and the good resistance to cooking of its big tubers. 2.2. Sample preparation After grading, tubers were washed under running water, wiped with blotting paper, hand-peeled and cut into cubes (1 cm side). 2.3. Blanching Blanching treatments were performed in four different ways found as the optimal blanching conditions (in terms of texture retention and enzyme inactivation) in a previous screening: By immersion in a sodium-chloride solution (3%, w/ w) at 98 C for 5 min.
During dehydration, samples were withdrawn at regular intervals (15 min) until the samples showed constant weights. 2.5. Rehydration The liquids used for rehydration trials were distilled water and a vegetable stock obtained by dissolving 22 g of stock cubes in distilled water. Exactly weighed potato cubes were dipped in the hot (initial temperature 80 C) rehydration liquids (1:5 w/v ratio) for 0.5, 1, 2.5, 5, 7 and 10 min, recovered, drained and weighed. Rehydration tests were also performed at room temperature in water for 120 min with regular withdrawing. 2.6. Analyses Colour analysis: a tristimulus colorimeter (Chromameter-2 Reflectance, Minolta, Osaka, Japan) equipped with a CR 300 measuring head was used. Colour was expressed as L*, a* and b* (luminosity, red value and yellow value, respectively, on the Hun-
C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296 7
W (g H2O/g dry matter)
ter scale). The colorimeter was calibrated on a standard white tile (L* = 93.5, a* = 1.0, b* = 0.8) before each series of measurements. For each samples, at least five determinations were performed (coefficient of variation less than 3% for L*, less than 45% for a* and less than 48% for b*). Measurements were regularly performed on samples withdrawn during dehydration. The observer angle during colour analysis was 90. Sample weight were measured using a balance Gibertini (mod. Europe 500, Milan, Italy). Determinations were carried out in triplicate.
The kinetic constants reported in Table 1 show that drying speeds were in the order: belt drier > microwave oven > air cabinet, for all the considered samples. In particular, the time necessary to obtain complete dehydration (absolute moisture <0.11 g H2O/g dry matter) of the unblanched cubed potatoes (Fig. 1) amounted to 195 min for the belt drier, 210 min for the microwave oven and 330 min for the air cabinet. For the unblanched samples, Fig. 2 reports the dehydration speed as a function of the absolute moisture. The initial absolute moisture amounted to about 3.95 g H2O/g dry matter corresponding to 78.5% of water content. The graphical representation allows visualization of the
Microwave oven
Belt drier
5 4 3 2
0 0 15 30 45 60 75 90 105120135150165180195 210225240255270285300315330345360375390405420
Time (min)
Fig. 1. Decrease in absolute moisture (g H2O/g dry matter) in unblanced samples under different drying conditions as a function of the dehydration time. 0.16 Air cabinet Microwave oven
0.14
Belt drier.
dW/dt (g H2O/g dry matter*min)
3.1. Drying speed
Air cabinet
6
1
Kinetic constants (k) and correlation coefficients (r) were calculated from the linear regression on linear portions of the curves using Excel 2002 software (Microsoft Corporation, USA).
3. Results and discussion
291
0.12 0.1 0.08 0.06 0.04 0.02 0 0
0.5
1
1.5 2 2.5 W (g H2O/g dry matter)
3
3.5
4
Fig. 2. Dehydration speed of unblanced samples under different drying conditions as a function of the absolute moisture.
drying speed independently by the time of treatment. During the first phase of drying (characterized by high moisture level), remarkable differences in dehydration
Table 1 Drying speed (k), expressed as g H2O/(g dry matter · min), correlation coefficient (r) and significance as a function of the degrees of freedom of the systems (p) for the combined blanching–drying treatments Drying methods
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet k r p
0.019 0.998 <0.001
0.021 0.998 <0.001
0.012 0.998 <0.001
0.021 0.999 <0.001
0.016 0.998 <0.001
Microwave oven k r p
0.043 0.998 <0.001
0.064 0.937 <0.01
0.021 0.993 <0.001
0.070 0.952 <0.001
0.045 0.988 <0.001
Belt drier k r p
0.117 0.974 <0.001
0.151 0.940 <0.01
0.078 0.962 <0.05
0.228 0.993 <0.001
0.185 0.999 <0.001
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speeds among the three drying methods were seen. At the end of process, drying speeds, expressed as g H2O/ (g dry matter · min), were similar for the belt drier and the microwave oven (about 0.02 g H2O/g of dry matter) whereas the process performed in the air cabinet was slower. From Fig. 2, it is well evident that drying speed in the air cabinet was constant over a large range of absolute moisture. For cubed potatoes blanched in sodium-chloride solution, all the kinetic constants were higher than the corresponding ones for to the unblanched samples (Table 1). This behaviour was probably due to softening by the blanching treatment which facilitated the water removal. In the case of the belt drier, the time necessary for complete dehydration was equivalent to those for the unblanched samples. However, dehydration in the air cabinet and in the microwave oven was longer (405 and 210 min, respectively, Table 2). Considering drying speed as a function of the absolute moisture (data not shown), the trends were similar to those illustrated for the unblanched potatoes. For samples blanched in the Glicosa–NaCl solution, kinetic constants (Table 1) were lower than the corresponding k for the unblanched samples and potatoes blanched in NaCl solution. This behaviour was probably due to the blanching in the hypertonic solution which caused enrichment in soluble solids and consequent reduction in water activity. Kinetic constants for drying of samples blanched by microwaves were generally higher than the corresponding k values for the unblanched potatoes and the cubes submitted to blanching by immersion in hot solutions. In particular, kinetic constants for the dehydration of samples blanched by microwaves in distilled water were higher than the k values calculated for dehydration of cubed potatoes blanched by microwaves in a hypertonic (Glicosa–sodium-chloride) solution. Also in this case, these differences were due to the enrichment in soluble solids that increased the bound water. All the correlation coefficient values demonstrate the goodness of fit.
(i.e., an uncomplete dehydration). A good correlation is evident among the kinetic constants (Table 1) and drying times is evident for drying performed on the belt drier. Furthermore, dehydration performed by the belt drier on samples previously blanched by microwaves showed the greatest efficiency. However, dehydration performed by the air cabinet on the same types of samples was among the worst. This demonstrates that the efficiency of the dehydration systems in terms of water removal can compensate for the unfavourable effects deriving from the presence of moistening substances in the blanching solution. 3.3. Colour Quality of potato cubes was evaluated during drying through colour measurements. The average results of the colour measurements performed on the raw potatoes were the following: L* 52.55, a* 1.87, b* 15.84. Table 3 is a summary of the speeds of colour changes and reports the kinetic constants calculated from the trends in L*, a* and b* values during dehydration for all the considered samples. For unblanched potatoes, those dried by microwave oven browned faster than the others. The slowest browned cubes were those dried on the belt drier. In fact, as is evident from data reported in Table 3, L* kinetic constants were negative, indicating a decrease in luminosity whereas a* and b* increased to indicate an increase in the red and yellow value, respectively. The increase in a* and b* values are indices of Maillard reaction (Morales & van Boekel, 1998; Carabasa-Giribet & Ibarz-Ribas, 2000). The better results obtained by the belt drier can be explained in the following way: the potato cubes were brought faster than the others to temperatures unfavourable to the activity of enzyme responsible for browning; furthermore, these temperatures were not high enough to induce nonenzymatic browning (Maillard reaction). The colour of cubes blanched in the sodium-chloride solution did not undergo great changes during drying process (Table 3) with the exception of the b* value which increased (higher yellow value) probably due to the loss of water and the consequent colour concentration. It could be taken as an index of the start of the Maillard reaction (Morales & van Boekel, 1998; Carabasa-Giribet & Ibarz-Ribas, 2000).
3.2. Drying time Table 2 reports the drying time required to reduce the absolute moisture value below 0.25 g H2O/g dry matter
Table 2 Dehydration times for the combined blanching–drying treatments: dehydratation was prolonged until the samples reached an absolute moisture content equal to 0.25 g H2O/g dry matter Drying time (min)
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet Microwave oven Belt drier
255 145 105
>405 210 105
255 180 180
>360 240 85
325 240 75
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Table 3 Speed of colour changes (k), expressed as colour parameter per min and correlation coefficient (r) for the combined blanching–drying treatments Drying methods
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
L*
L*
L*
L*
L*
a*
b*
a*
b*
0.010 0.286
0.005 0.619
0.052 0.887
a*
b*
0.021 0.334
0.008 0.711
0.046 0.585
a*
b*
0.010 0.286
0.005 0.619
0.052 0.876
a*
b*
0.021 0.311
0.004 0.400
0.046 0.698
Air cabinet k r
0.032 0.489
0.015 0.844
1 · 10 2 · 10
Microwave oven k r
0.118 0.777
0.024 0.828
0.030 0.612
0.032 0.371
0.023 0.868
0.043 0.588
0.053 0.542
0.025 0.886
0.006 0.065
0.032 0.371
0.023 0.868
0.043 0.588
0.040 0.212
0.011 0.707
0.054 0.737
Belt drier k r
0.002 0.028
0.011 0.767
0.009 0.272
0.031 0.467
0.023 0.887
0.069 0.721
0.027 0.472
0.022 0.852
0.018 0.248
0.031 0.467
0.023 0.887
0.069 0.721
0.071 0.560
0.030 0.977
0.093 0.714
5 7
In the case of samples blanched in the Glicosa–NaCl solution, those dried by microwave oven browned faster than the others (greater loss of luminosity and higher red value, Table 3) probably due to the effects of microwaves on a higher solute concentration. In samples blanched by microwaves (independently by the kind of blanching solution), kinetic constants for the b* index showed the greatest values and indicate a beginning of the Maillard reaction. For all the considered samples, low correlation coefficients were low. These low values could be explained in terms of a very little dependence of the colour parameters from the dehydration time. 3.4. Rehydration After dehydration, samples were submitted to rehydration trials to check their capability to rehydrate in liquids of different nature. Rehydration in hot liquids was carried out for only 10 min to avoid cooking the potato. For the same reason, the temperature of the liquids was not kept at 80 C for all the time but decreased as a result of both the addition of potato cubes and the lack of heating. From Figs. 3–7, which report the rehydration curves of all the samples, it is evident that potato cubes absorbed more liquid if rehydrated in distilled water rather than in vegetable stock. This behaviour was probably due to the slower diffusion in the inner potato cubes of liquid containing soluble solids and a fat fraction. Furthermore, the behaviour of samples during reconstitution was different depending on the drying method to which they where submitted. Fig. 3 shows the increase in weight of unblanched samples during rehydration. Potato cubes dehydrated in the air cabinet and by microwaves had a slower rehydration than samples dehydrated on the belt drier. The
Fig. 3. Increase in weight (g/100 g dry matter) in unblanched samples as a function of the rehydration time in hot liquids.
latter absorbed a quantity of liquid amounting to about 100% of their weight during the first minute of immersion and, by the end of the rehydration, they had absorbed the highest quantity of liquid compared to the other samples. These differences were amplified in the case of samples blanched in the sodium-chloride solution (Fig. 4). Potato cubes dehydrated on the belt drier acquired after 10 min a quantity of liquid (water or vegetable stock) amounting to about 200% of their dry matter whereas samples dehydrated in the air cabinet and by microwaves absorbed liquid amounting 50% and 100% of their dry matter, respectively.
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Fig. 4. Increase in weight (g/100 g dry matter) in samples traditionally blanched in the NaCl solution as a function of the rehydration time in hot liquids.
Fig. 5. Increase in weight (g/100 g dry matter) in samples traditionally blanched in the glicosa–Nacl solution as a function of the rehydration time in hot liquids.
Samples blanched in the Glicosa–sodium-chloride solution showed slower rehydration kinetics and acquired the smallest quantity of liquid (Fig. 5). This behaviour was probably due to the acquisition of
Fig. 6. Increase in weight (g/100 g dry matter) in samples blanched by microwaves in distilled water as a function of the rehydration time in hot liquids.
Fig. 7. Increase in weight (g/100 g dry matter) in samples blanched by microwaves in the NaCl solution as a function of the rehydration time in hot liquids.
soluble solids during the blanching, which resulted in a decrease in porosity. The effect of the blanching treatment minimized the differences due to the drying methods.
C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296
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Table 4 Rehydration time in distilled water at room temperature for the combined blanching–drying treatments Rehydration time (min)
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet Microwave oven Belt drier
70 >120 >120
>120 >120 >120
95 >120 85
>120 >120 90
>120 >120 38
Fig. 6 shows the rehydration curves of samples blanched by microwaves in distilled water. In this case, microwaves induced a modification of the starch granules whose capacity to absorb liquid during rehydration was increased, according to the results reported by Sikora, Tomasik, and Pielichowski (1997). This effect was particularly evident in samples dehydrated on the belt drier which absorbed liquids amounting to about 300% of their dry matter. Among samples blanched by microwaves in distilled water, those showing the worse behaviour during rehydration were those dehydrated by microwaves. This can be explained considering that these samples were treated (blanched and de-hydrated) exclusively by microwaves which determined the complete starch gelatinization and, consequently, the minor water holding capacity during rehydration. Potato cubes blanched by microwaves in the sodiumchloride solution showed a behaviour similar to those blanched by microwaves in distilled water during rehydration (Fig. 7) even if the effects of microwaves were reduced by the presence of salt. Table 4 reports the time of rehydration in distilled water at room temperature necessary to bring samples to their natural water content. From this point of view, samples showing the best results were those blanched by microwaves in the sodium-chloride solution and successively dehydrated on the belt drier. The other samples needed double or triple rehydration times.
4. Conclusions Dehydration performed by the air cabinet was the slowest and did not yield, within the first phase of rehydration at least, a satisfying water holding capacity. Dehydration by microwaves gave a good drying speed coupled to a good water holding capacity and colour retention. Nevertheless, a blanching-drying process totally carried out by microwaves would be too expensive. Dehydration performed on the belt drier was the most effective both for speed and water absorption during rehydration, independently of the kind of blanching.
In terms of process speed and quality (colour and rehydration capability) of the obtained samples, the best combinations were by the blanching by microwaves coupled to dehydration on the belt drier.
References Bizzarri, G., Andreotti, R., & Massini, R. (1981). Enzyme inactivation in spinach and popato: catalase, peroxidase and phenolase. Industria Conserve, 56(2), 97–102. Carabasa-Giribet, M., & Ibarz-Ribas, A. (2000). Kinetics of color development in aqueous glucose systems at high temperatures. Journal of Food Engineering, 44, 181–189. Chen, S. C., Collins, J. L., McCarty, I. E., & Johnston, M. R. (1971). Blanching of white potatoes by microwave energy followed by boiling water. Journal of Food Science, 36(5), 742–743. Collins, J. L., & McCarty, I. E. (1969). Comparison of microwave energy with boiling water for blanching whole potatoes. Food Technology, 23(3), 337–340. Kidmose, U., & Martens, H. J. (1999). Changes in texture, microstructure and nutritional quality of carrot slices during blanching and freezing. Journal of the Science of Food and Agriculture, 79(12), 1747–1753. Krokida, M. K., Kiranoudis, C. T., & Maroulis, Z. B. (1999). Viscoelastic behaviour of dehydrated products during rehydration. Journal of Food Engineering, 40(4), 269–277. Krokida, M. K., & Maroulis, Z. B. (2000). The effect of drying methods on viscoelastic behaviour of dehydrated fruits and vegetables. International Journal of Food Science and Technology, 35(4), 391–400. Krokida, M. K., & Maroulis, Z. B. (2001). Structural properties of dehydrated products during re-hydration. International Journal of Food Science and Technology, 36(5), 529–538. Krokida, M. K., Maroulis, Z. B., & Saravacos, G. D. (2001). The effect of the method of drying on the colour of dehydrated products. International Journal of Food Science and Technology, 36(1), 53–59. Mapson, L. W., Swain, T., & Tomalin, A. W. (1963). Influence of variety, cultural conditions and temperature of storage on enzymic browning of potato tubers. Journal of the Science of Food and Agriculture, 13, 673. Morales, F. J., & van Boekel, M. A. J. S. (1998). A study on advanced Maillard reaction in heated casein/sugar solutions: colour formation. International Dairy Journal, 8, 907–915. Pinsent, B. R. W. (1962). Peroxidase regeneration and its effect on quality in frozen peas and thawed peas. Journal of Food Science, 27, 120. Ponne, C. T., Van Remmen, H. H. J., & Bartels, P. V. (1991). Application of electromagnetic energy in processing of whole potatoes. Voedingsmiddelentechnologie, 24(21), 44–46.
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Ramaswamy, H., & Van de Voort, F. R. (1990). Microwave application in food processing. Canadian. Institute of Food Science and Technology Journal, 23(1), 17–23. Severini, C., De Pilli, T., Baiano, A., Mastrocola, D., & Massini, R. (2001). Study on preventing enzymatic browning of potato by microwave blanching. Sciences des Aliments, 21(2), 149–160.
Sikora, M., Tomasik, P., & Pielichowski, K. (1997). Chemical starch modification in the field of microwaves III. Reaction of potato starch with carboxyamides and carboxylic esters. Polish Journal of Food and Nutrition Sciences, 6/47(3), 25–30. Smith, O., (1977). Potatoes: production, storing, processing. Avi Publishing (Eds.), Westpoty, Conn., USA.
Combined treatments of blanching and dehydration: study on potato cubes Carla Severini, Antonietta Baiano *, Teresa De Pilli, Barbara F. Carbone, A. Derossi Faculty of Agriculture, Department of Food Science, University of Foggia, Via Napoli 25, 71100 Foggia, Italy Received 23 January 2004; accepted 31 May 2004
Abstract Samples of Solanum tuberosum var. Primura were submitted to combined treatments of blanching and dehydration. Blanching was alternatively performed in hot distilled water, hot sugary-saline solution, by microwaves in distilled water or by microwaves in saline solution. Drying was alternatively carried out in an air cabinet, a microwave oven or a belt drier. In terms of process speed, colour retention and water absorption capacity, the best results were obtained combining microwave blanching with dehydration on the belt drier. In particular, dehydration on the belt drier levelled eventual negative effects determined by the blanching treatments. 2004 Elsevier Ltd. All rights reserved. Keywords: Blanching; Colour; Dehydration; Potato; Rehydration
1. Introduction In the production of canned, frozen and dehydrated vegetables, the main purpose of a blanching treatment is the inactivation of enzymes such as polyphenoloxidases, responsible for browning (Mapson, Swain, & Tomalin, 1963; Collins & McCarty, 1969), peroxidase, catalase and phenolase which leads to the development of off-flavours (Pinsent, 1962; Bizzarri, Andreotti, & Massini, 1981). In particular, thanks to their heat resistance or regeneration capacity, peroxidase, catalase and phenolase are considered as indices of the heat treatment efficacy. Among them, peroxidase is the most heat resistant. Smith (1977) found that it is necessary to performed a blanching treatment at 100 C for 3 min, to obtain its complete inactivation in cubed potatoes destined for dehydration. Bizzarri et al. (1981) obtained the complete and irreversible inactivation of peroxidase by blanching potatoes for 4 min at 97 C.
*
Corresponding author. Tel.: +39 881 589242; fax: +39 881 740211. E-mail address: [email protected] (A. Baiano).
0260-8774/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2004.05.045
Blanching can be performed by exposing vegetables to hot water (the most common method), hot and boiling solutions containing acids and/or salts, by steam (Kidmose & Martens, 1999) or by microwaving product dipped in water or solutions (Chen, Collins, McCarty, & Johnston, 1971; Ramaswamy & Van de Voort, 1990; Ponne, Van Remmen, & Bartels, 1991; Severini, De Pilli, Baiano, Mastrocola, & Massini, 2001) for several seconds or minutes. The effectiveness of a drying process depends on different factors: method of heat transfer, continuity or discontinuity of the process, direction of the heating fluids with respect to the product, pressure (atmospheric, low, deep vacuum). Dehydration can be performed by using different kinds of equipment: air cabinet, belt drier, tunnel drier, fluidized bed, spray drier, drum drier, foam drier, freeze-drier, microwave oven. An interesting drying method is represented by osmotic dehydration. Krokida, Maroulis, and Saravacos (2001) investigated the effects of different drying methods on the colour of the obtained products. They found that colour characteristics are significantly affected by the drying methods and that the
290
C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296
changes in redness (a*) and yellowness (b*) follow a first order kinetic model. In particular, air-, vacuum and microwave drying caused extensive browning in fruit and vegetables with a decrease in L* value and an increase in a* and b* values whereas freeze-drying improved colour characteristics. Osmotically pre-treated samples were preserved from browning to a certain measure. Drying methods also affect the behaviour of dehydrated product during rehydration. Krokida, Kiranoudis, and Maroulis (1999) and Krokida and Maroulis (2000, 2001) studied the viscoelastic behaviour of dehydrated products during rehydration. They found that dehydrated foods do not keep their viscoelastic behaviour after rehydration due to structural damages that occur during drying. Freeze-dried products showed the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pre-treatment helped freeze-dried materials to keep their elastic nature probably thanks to the solid gain. Air and vacuum dried foods showed the smallest hysteresis tendency, keeping their viscoelastic characteristics during rehydration close to those of dried materials. The purpose of this work was to study the effects of different combined systems of blanching and dehydration on dehydration speed, colour characteristics and the behaviour during rehydration of cubed potatoes.
2. Materials and methods
By immersion in a corn syrup solution diluted to 70 Brix with distilled water and containing sodium-chloride (3%, w/w) at 100 C for 5 min. The undiluted corn syrup (commercial name ‘‘Glicosa’’, supplied by Roquette Italia, Cassano Spinola, Italy) had the following characteristics and chemical composition: refractometer grade 80.6 Brix, pH value 5.1 at 20 C, 30% dextrose, 46% maltose, 10% trysaccharides, 14% polysaccharides. By microwaves in a domestic oven (De Longhi, Milan, Italy) upon immersion in distilled water at 850 W for 5 min. By microwaves in a domestic oven (De Longhi, Milan, Italy) upon immersion in a sodium-chloride solution (3%, w/w) at 850 W for 5 min. The product–solution ratio was always 1:5 (w/w). After blanching, the samples were cooled in tap water, drained and wiped with blotting paper. Unblanched potato cubes were kept as controls. 2.4. Dehydration Blanched and unblanched potato samples were submitted to three different methods of dehydration: in an air cabinet (I.S.C.O., Milan, Italy) at 100 C; in a microwave oven (De Longhi, Milan, Italy) at 85 W; in a monolayer on a concurrent belt drier (pilot plant, Sandvik Process Systems, Milan, Italy) at 100 C.
2.1. Raw material Trials were performed on potato tubers (Solanum tuberosum var. Primura) purchased at a local market. This Italian variety, characterized by an early ripening time, is the most commonly used in food industry thanks to the regular shape and the good resistance to cooking of its big tubers. 2.2. Sample preparation After grading, tubers were washed under running water, wiped with blotting paper, hand-peeled and cut into cubes (1 cm side). 2.3. Blanching Blanching treatments were performed in four different ways found as the optimal blanching conditions (in terms of texture retention and enzyme inactivation) in a previous screening: By immersion in a sodium-chloride solution (3%, w/ w) at 98 C for 5 min.
During dehydration, samples were withdrawn at regular intervals (15 min) until the samples showed constant weights. 2.5. Rehydration The liquids used for rehydration trials were distilled water and a vegetable stock obtained by dissolving 22 g of stock cubes in distilled water. Exactly weighed potato cubes were dipped in the hot (initial temperature 80 C) rehydration liquids (1:5 w/v ratio) for 0.5, 1, 2.5, 5, 7 and 10 min, recovered, drained and weighed. Rehydration tests were also performed at room temperature in water for 120 min with regular withdrawing. 2.6. Analyses Colour analysis: a tristimulus colorimeter (Chromameter-2 Reflectance, Minolta, Osaka, Japan) equipped with a CR 300 measuring head was used. Colour was expressed as L*, a* and b* (luminosity, red value and yellow value, respectively, on the Hun-
C. Severini et al. / Journal of Food Engineering 68 (2005) 289–296 7
W (g H2O/g dry matter)
ter scale). The colorimeter was calibrated on a standard white tile (L* = 93.5, a* = 1.0, b* = 0.8) before each series of measurements. For each samples, at least five determinations were performed (coefficient of variation less than 3% for L*, less than 45% for a* and less than 48% for b*). Measurements were regularly performed on samples withdrawn during dehydration. The observer angle during colour analysis was 90. Sample weight were measured using a balance Gibertini (mod. Europe 500, Milan, Italy). Determinations were carried out in triplicate.
The kinetic constants reported in Table 1 show that drying speeds were in the order: belt drier > microwave oven > air cabinet, for all the considered samples. In particular, the time necessary to obtain complete dehydration (absolute moisture <0.11 g H2O/g dry matter) of the unblanched cubed potatoes (Fig. 1) amounted to 195 min for the belt drier, 210 min for the microwave oven and 330 min for the air cabinet. For the unblanched samples, Fig. 2 reports the dehydration speed as a function of the absolute moisture. The initial absolute moisture amounted to about 3.95 g H2O/g dry matter corresponding to 78.5% of water content. The graphical representation allows visualization of the
Microwave oven
Belt drier
5 4 3 2
0 0 15 30 45 60 75 90 105120135150165180195 210225240255270285300315330345360375390405420
Time (min)
Fig. 1. Decrease in absolute moisture (g H2O/g dry matter) in unblanced samples under different drying conditions as a function of the dehydration time. 0.16 Air cabinet Microwave oven
0.14
Belt drier.
dW/dt (g H2O/g dry matter*min)
3.1. Drying speed
Air cabinet
6
1
Kinetic constants (k) and correlation coefficients (r) were calculated from the linear regression on linear portions of the curves using Excel 2002 software (Microsoft Corporation, USA).
3. Results and discussion
291
0.12 0.1 0.08 0.06 0.04 0.02 0 0
0.5
1
1.5 2 2.5 W (g H2O/g dry matter)
3
3.5
4
Fig. 2. Dehydration speed of unblanced samples under different drying conditions as a function of the absolute moisture.
drying speed independently by the time of treatment. During the first phase of drying (characterized by high moisture level), remarkable differences in dehydration
Table 1 Drying speed (k), expressed as g H2O/(g dry matter · min), correlation coefficient (r) and significance as a function of the degrees of freedom of the systems (p) for the combined blanching–drying treatments Drying methods
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet k r p
0.019 0.998 <0.001
0.021 0.998 <0.001
0.012 0.998 <0.001
0.021 0.999 <0.001
0.016 0.998 <0.001
Microwave oven k r p
0.043 0.998 <0.001
0.064 0.937 <0.01
0.021 0.993 <0.001
0.070 0.952 <0.001
0.045 0.988 <0.001
Belt drier k r p
0.117 0.974 <0.001
0.151 0.940 <0.01
0.078 0.962 <0.05
0.228 0.993 <0.001
0.185 0.999 <0.001
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speeds among the three drying methods were seen. At the end of process, drying speeds, expressed as g H2O/ (g dry matter · min), were similar for the belt drier and the microwave oven (about 0.02 g H2O/g of dry matter) whereas the process performed in the air cabinet was slower. From Fig. 2, it is well evident that drying speed in the air cabinet was constant over a large range of absolute moisture. For cubed potatoes blanched in sodium-chloride solution, all the kinetic constants were higher than the corresponding ones for to the unblanched samples (Table 1). This behaviour was probably due to softening by the blanching treatment which facilitated the water removal. In the case of the belt drier, the time necessary for complete dehydration was equivalent to those for the unblanched samples. However, dehydration in the air cabinet and in the microwave oven was longer (405 and 210 min, respectively, Table 2). Considering drying speed as a function of the absolute moisture (data not shown), the trends were similar to those illustrated for the unblanched potatoes. For samples blanched in the Glicosa–NaCl solution, kinetic constants (Table 1) were lower than the corresponding k for the unblanched samples and potatoes blanched in NaCl solution. This behaviour was probably due to the blanching in the hypertonic solution which caused enrichment in soluble solids and consequent reduction in water activity. Kinetic constants for drying of samples blanched by microwaves were generally higher than the corresponding k values for the unblanched potatoes and the cubes submitted to blanching by immersion in hot solutions. In particular, kinetic constants for the dehydration of samples blanched by microwaves in distilled water were higher than the k values calculated for dehydration of cubed potatoes blanched by microwaves in a hypertonic (Glicosa–sodium-chloride) solution. Also in this case, these differences were due to the enrichment in soluble solids that increased the bound water. All the correlation coefficient values demonstrate the goodness of fit.
(i.e., an uncomplete dehydration). A good correlation is evident among the kinetic constants (Table 1) and drying times is evident for drying performed on the belt drier. Furthermore, dehydration performed by the belt drier on samples previously blanched by microwaves showed the greatest efficiency. However, dehydration performed by the air cabinet on the same types of samples was among the worst. This demonstrates that the efficiency of the dehydration systems in terms of water removal can compensate for the unfavourable effects deriving from the presence of moistening substances in the blanching solution. 3.3. Colour Quality of potato cubes was evaluated during drying through colour measurements. The average results of the colour measurements performed on the raw potatoes were the following: L* 52.55, a* 1.87, b* 15.84. Table 3 is a summary of the speeds of colour changes and reports the kinetic constants calculated from the trends in L*, a* and b* values during dehydration for all the considered samples. For unblanched potatoes, those dried by microwave oven browned faster than the others. The slowest browned cubes were those dried on the belt drier. In fact, as is evident from data reported in Table 3, L* kinetic constants were negative, indicating a decrease in luminosity whereas a* and b* increased to indicate an increase in the red and yellow value, respectively. The increase in a* and b* values are indices of Maillard reaction (Morales & van Boekel, 1998; Carabasa-Giribet & Ibarz-Ribas, 2000). The better results obtained by the belt drier can be explained in the following way: the potato cubes were brought faster than the others to temperatures unfavourable to the activity of enzyme responsible for browning; furthermore, these temperatures were not high enough to induce nonenzymatic browning (Maillard reaction). The colour of cubes blanched in the sodium-chloride solution did not undergo great changes during drying process (Table 3) with the exception of the b* value which increased (higher yellow value) probably due to the loss of water and the consequent colour concentration. It could be taken as an index of the start of the Maillard reaction (Morales & van Boekel, 1998; Carabasa-Giribet & Ibarz-Ribas, 2000).
3.2. Drying time Table 2 reports the drying time required to reduce the absolute moisture value below 0.25 g H2O/g dry matter
Table 2 Dehydration times for the combined blanching–drying treatments: dehydratation was prolonged until the samples reached an absolute moisture content equal to 0.25 g H2O/g dry matter Drying time (min)
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet Microwave oven Belt drier
255 145 105
>405 210 105
255 180 180
>360 240 85
325 240 75
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293
Table 3 Speed of colour changes (k), expressed as colour parameter per min and correlation coefficient (r) for the combined blanching–drying treatments Drying methods
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
L*
L*
L*
L*
L*
a*
b*
a*
b*
0.010 0.286
0.005 0.619
0.052 0.887
a*
b*
0.021 0.334
0.008 0.711
0.046 0.585
a*
b*
0.010 0.286
0.005 0.619
0.052 0.876
a*
b*
0.021 0.311
0.004 0.400
0.046 0.698
Air cabinet k r
0.032 0.489
0.015 0.844
1 · 10 2 · 10
Microwave oven k r
0.118 0.777
0.024 0.828
0.030 0.612
0.032 0.371
0.023 0.868
0.043 0.588
0.053 0.542
0.025 0.886
0.006 0.065
0.032 0.371
0.023 0.868
0.043 0.588
0.040 0.212
0.011 0.707
0.054 0.737
Belt drier k r
0.002 0.028
0.011 0.767
0.009 0.272
0.031 0.467
0.023 0.887
0.069 0.721
0.027 0.472
0.022 0.852
0.018 0.248
0.031 0.467
0.023 0.887
0.069 0.721
0.071 0.560
0.030 0.977
0.093 0.714
5 7
In the case of samples blanched in the Glicosa–NaCl solution, those dried by microwave oven browned faster than the others (greater loss of luminosity and higher red value, Table 3) probably due to the effects of microwaves on a higher solute concentration. In samples blanched by microwaves (independently by the kind of blanching solution), kinetic constants for the b* index showed the greatest values and indicate a beginning of the Maillard reaction. For all the considered samples, low correlation coefficients were low. These low values could be explained in terms of a very little dependence of the colour parameters from the dehydration time. 3.4. Rehydration After dehydration, samples were submitted to rehydration trials to check their capability to rehydrate in liquids of different nature. Rehydration in hot liquids was carried out for only 10 min to avoid cooking the potato. For the same reason, the temperature of the liquids was not kept at 80 C for all the time but decreased as a result of both the addition of potato cubes and the lack of heating. From Figs. 3–7, which report the rehydration curves of all the samples, it is evident that potato cubes absorbed more liquid if rehydrated in distilled water rather than in vegetable stock. This behaviour was probably due to the slower diffusion in the inner potato cubes of liquid containing soluble solids and a fat fraction. Furthermore, the behaviour of samples during reconstitution was different depending on the drying method to which they where submitted. Fig. 3 shows the increase in weight of unblanched samples during rehydration. Potato cubes dehydrated in the air cabinet and by microwaves had a slower rehydration than samples dehydrated on the belt drier. The
Fig. 3. Increase in weight (g/100 g dry matter) in unblanched samples as a function of the rehydration time in hot liquids.
latter absorbed a quantity of liquid amounting to about 100% of their weight during the first minute of immersion and, by the end of the rehydration, they had absorbed the highest quantity of liquid compared to the other samples. These differences were amplified in the case of samples blanched in the sodium-chloride solution (Fig. 4). Potato cubes dehydrated on the belt drier acquired after 10 min a quantity of liquid (water or vegetable stock) amounting to about 200% of their dry matter whereas samples dehydrated in the air cabinet and by microwaves absorbed liquid amounting 50% and 100% of their dry matter, respectively.
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Fig. 4. Increase in weight (g/100 g dry matter) in samples traditionally blanched in the NaCl solution as a function of the rehydration time in hot liquids.
Fig. 5. Increase in weight (g/100 g dry matter) in samples traditionally blanched in the glicosa–Nacl solution as a function of the rehydration time in hot liquids.
Samples blanched in the Glicosa–sodium-chloride solution showed slower rehydration kinetics and acquired the smallest quantity of liquid (Fig. 5). This behaviour was probably due to the acquisition of
Fig. 6. Increase in weight (g/100 g dry matter) in samples blanched by microwaves in distilled water as a function of the rehydration time in hot liquids.
Fig. 7. Increase in weight (g/100 g dry matter) in samples blanched by microwaves in the NaCl solution as a function of the rehydration time in hot liquids.
soluble solids during the blanching, which resulted in a decrease in porosity. The effect of the blanching treatment minimized the differences due to the drying methods.
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Table 4 Rehydration time in distilled water at room temperature for the combined blanching–drying treatments Rehydration time (min)
Unblanched cubed potatoes
Potatoes traditionally blanched in the NaCl solution
Potatoes traditionally blanched in the Glicosa–NaCl solution
Potatoes blanched by microwaves in distilled water
Potatoes blanched by microwaves in the NaCl solution
Air cabinet Microwave oven Belt drier
70 >120 >120
>120 >120 >120
95 >120 85
>120 >120 90
>120 >120 38
Fig. 6 shows the rehydration curves of samples blanched by microwaves in distilled water. In this case, microwaves induced a modification of the starch granules whose capacity to absorb liquid during rehydration was increased, according to the results reported by Sikora, Tomasik, and Pielichowski (1997). This effect was particularly evident in samples dehydrated on the belt drier which absorbed liquids amounting to about 300% of their dry matter. Among samples blanched by microwaves in distilled water, those showing the worse behaviour during rehydration were those dehydrated by microwaves. This can be explained considering that these samples were treated (blanched and de-hydrated) exclusively by microwaves which determined the complete starch gelatinization and, consequently, the minor water holding capacity during rehydration. Potato cubes blanched by microwaves in the sodiumchloride solution showed a behaviour similar to those blanched by microwaves in distilled water during rehydration (Fig. 7) even if the effects of microwaves were reduced by the presence of salt. Table 4 reports the time of rehydration in distilled water at room temperature necessary to bring samples to their natural water content. From this point of view, samples showing the best results were those blanched by microwaves in the sodium-chloride solution and successively dehydrated on the belt drier. The other samples needed double or triple rehydration times.
4. Conclusions Dehydration performed by the air cabinet was the slowest and did not yield, within the first phase of rehydration at least, a satisfying water holding capacity. Dehydration by microwaves gave a good drying speed coupled to a good water holding capacity and colour retention. Nevertheless, a blanching-drying process totally carried out by microwaves would be too expensive. Dehydration performed on the belt drier was the most effective both for speed and water absorption during rehydration, independently of the kind of blanching.
In terms of process speed and quality (colour and rehydration capability) of the obtained samples, the best combinations were by the blanching by microwaves coupled to dehydration on the belt drier.
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