infrared onions
Blackwell Science, LtdOxford, UKJFPPJournal of Food Processing and Preservation0145-8892Copyright 2005 by Food & Nutrition Press, Inc., Trumbull, Connecticut.292132150Original Article IR AND
HOT-AIR DRYING OF ONIONS D.G. PRAVEEN KUMAR
ET AL.
INFRARED AND HOT-AIR DRYING OF ONIONS D.G. PRAVEEN KUMAR1, H. UMESH HEBBAR1,3, D. SUKUMAR2 and M.N. RAMESH1 1
2
Department of Food Engineering
Department of Lipid Science and Traditional Foods Central Food Technological Research Institute Mysore, 570 020 India Accepted for Publication May 02, 2005
ABSTRACT The combination of infrared (IR) and hot-air drying of onion slices was explored, and the effects of processing conditions such as drying temperature, slice thickness, air temperature and velocity on onion slice characteristics were studied. The onion slice quality was evaluated on the basis of the color and the pyruvic acid content, an index of flavor. Drying of thin slices of onion (2 mm) at low temperature (60C) with a moderate air velocity (2 m/s) and air temperature (40C) retained greater flavor and color. An empiric equation developed to correlate the drying process variables and the onion slice moisture with the drying time provided a good fit (R2 = 0.92). Similar equations developed to correlate the drying process variables and the drying time with the pyruvic acid content provided an excellent fit (R2 = 0.96), while the equations fit for the total color change of onion slices were satisfactory (R2 = 0.86). Combination drying resulted in shorter drying process time and in better onion slice quality as compared to IR and hot-air drying applied individually.
INTRODUCTION Onions (Allium cepa Linnaeus) are widely grown and are one of the most popular vegetables in the world. The demand for dried onions is increasing in recent decades. The European Union, one of the major consumers of dried onions, meets its demand by importing onions from the United States, Egypt, India and Hungary (Lewicki et al. 1995). To meet quality demands, many new cultivars of onions suitable for dehydration are being grown. The best onions 3
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Journal of Food Processing and Preservation 29 (2005) 132–150. All Rights Reserved. © Copyright 2005, Blackwell Publishing
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for drying exhibit high pungency, high total solids and minimal discoloration during processing (Jones and Mann 1963). India produced nearly 4.9 million tonnes of onions throughout 2001 and 2002 and exported around 7000 tonnes of dehydrated onion products (APEDA Export Statement 2002). Dried onion products are produced in several forms: flaked, minced, chopped and powdered. Dried onions are used as flavor additives in wide varieties of food formulations such as comminuted meats, sauces, soups, salad dressings, pickles and pickle relishes. The technique of drying is probably the oldest method of food preservation practiced by mankind for the extension of food shelf life. The use of artificial drying to preserve agricultural commodities is expanding, creating a need for more rapid drying techniques and methods that reduce the large amount of energy required in drying processes. New and innovative techniques that increase drying rates and enhance dried onion quality are receiving considerable attention (Mongpraneet et al. 2002). The major quality problems faced during onion drying are loss of flavor, discoloration and poor rehydration characteristics of the dried onions. Onion flavor and color are generally perceived as important quality attributes. Quality changes during the drying process are influenced by drying temperatures. The volatile compounds responsible for the aroma and flavor of onions exhibit low boiling points and, accordingly, are often lost during hightemperature drying. Maillard browning reactions induced by the drying process decrease nutritional value, changes the color and flavor and induces textural changes (Adam et al. 2000). Several quality standards for dried onions were developed over time; the official standards of the American Dehydrated Onion and Garlic Association (ADOGA) are considered the primary standard. Hot-air drying is the most commonly employed commercial technique for drying vegetables and fruits, but the hot-air drying process remains largely an art (Mazza and LeMaguer 1980). In industrialized countries, onions are dried using high-temperature commercial dryers such as conveyor belt driers or fluidized bed driers. Prepared onions are uniformly sliced to the required thickness, and the onion slices are dried in a thin layer at a temperature near 60C to a final moisture content less than 9%. However, the major disadvantage associated with hot-air drying is the long drying time even at temperatures near 60C, resulting in the degradation of sliced onion quality. Extensive research was carried out on onion drying by conventional hot-air methods (Sarsavadia et al. 1999). The thin-layer drying rates of brined onion slices were experimentally determined at selected drying temperatures, air velocities and relative humidities. Elustondo et al. (1996) developed a simple model for the dehydration of onion pieces. The kinetics of the nonenzymatic Maillard browning of onion slices during isothermal heating was reported by Rapusas and Driscoll (1995), and the temperature-dependent browning of onion slices
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was successfully modeled as a zero-order reaction. Singh and Kumar (1984) also studied the loss of pungency and the discoloration in selected cultivars of onions during hot-air drying. Infrared (IR) irradiation heating offers many advantages over conventional hot-air drying. When IR irradiation is used to heat or to dry moist vegetables or fruits, the radiation impinges on the exposed vegetable or fruit surfaces and penetrates, and the energy of radiation is converted into heat (Ginzburg 1969). The depth of penetration depends on the composition and structure of the vegetables and fruits and on the wavelengths of the IR irradiation. When a food is exposed to the irradiation, the food is heated intensely, and the temperature gradient throughout the food is reduced within a short period. Further, by the application of intermittent IR irradiation, wherein the periods of heating are followed by cooling, the intense displacement of moisture from the core toward the surface can be achieved. The displacement of moisture results in increased rates of heat transfer compared to conventional drying, and the vegetables and fruits are more uniformly heated resulting in improved quality characteristics (Hebbar and Ramesh 2001). IR processing was attempted in baking, roasting, thermal preservation (blanching, pasteurization and sterilization) and drying of foods (Sandu 1986). Combinations of electromagnetic irradiation and hot air for drying are more efficient than irradiation or hot-air heating alone, presumably providing a synergistic effect. The energy conserved and the quality improvements of barley, which were observed after combined far-IR irradiation and hot-air convection drying, were reported by Afzal et al. (1999). Combinationmode drying reduced the total energy requirements by nearly 245% when compared with hot-air drying at 70C. Drying of garlic, which is another herb that exhibits quality characteristics similar to onions, was studied using combined microwave and hot-air heat sources by Sharma and Prasad (2001). The effects of drying with far-IR radiation under vacuum conditions on the quality of Welsh onions were explored by Mongpraneet et al. (2002). The present study discusses the influence of selected drying conditions such as drying temperature, slice thickness, air temperature and air velocity during combined IR and hot-air drying on the drying characteristics and on the quality of sliced onions.
MATERIALS AND METHODS The Bellary cultivar of onions available in a local market was selected for experimentation. The raw onions had an initial moisture content of 86– 88% (Wb). Onion preparation included manual trimming and peeling followed by slicing of the onions with a domestic slicer.
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BLOWER
HEATER
TEMPERATURE SENSOR
900 mm
TEMPERATURE SENSOR
CONVEYOR DRYING CHAMBERS IR HEATERS
5500 mm
FIG. 1. COMBINED INFRARED AND HOT-AIR DRYING SYSTEM
A combined IR and hot-air heating system, developed in Central Food Technological Research Institute (CFTRI) in Mysore, India (Hebbar and Ramesh 2001), was used to carry out the experiments. A schematic diagram of the heating system is presented in Fig. 1. The equipment designed for continuous operation is fitted with mid-IR (2.4–3.0 mm) heat sources on either side of a wire mesh conveyor. Through flow, hot air is provided for convective heating. Experimental Design The experiments were carried out in batch mode, and 2.0 kg of sliced onions (25-mm diameter) were used for each experiment. The slices were spread uniformly in a single layer (monolayer) over the wire mesh conveyor.
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The combined mode experiments were carried out at three drying temperatures (60, 70 or 80C), slice thicknesses (2, 4 or 6 mm), air temperatures (30, 40 or 50C) and air velocities (0.8, 1.4 and 2.0 m/s) (Table 1). The drying and the hot-air temperatures were controlled with thermostats. Air velocity was controlled by regulating air flow at the blower inlet. During each experiment, one of the parameters was varied, keeping other conditions the same. The onion slices were dried to a moisture content of 7–8% (Wb). The convective and the IR drying of the onion slices (2-mm thick) were also carried out at 60C independently. Onion slices were removed at regular intervals for moisture analysis. The withdrawn onion slices were analyzed in duplicate, and mean values were reported. The moisture contents of the raw and the dried onion slices were determined with an oven method (Ranganna 1977) and expressed as percentage on a wet weight basis. The pyruvic acid content, an index of the flavor strength of onions, was estimated with a spectrophotometric method (Schwimmer and Wetson 1961) and reported as mmol/g. The color of the dehydrated onion slices was determined using a Hunter color measurement system (Laboratory Scan XE, C Illuminant, 2∞view angle, Hunter Laboratory, Reston, VA, USA). The L, a and b color difference values, representing the differences in the three color dimensions from the standard color values representing the onion slices, were utilized to calculate the total color change (dE). The browning index (BI), indicating the purity of the brown color of the onion slices, was calculated using Eqs. (1) and (2) (Maskan 2001): 100( x - 0.31) 0.17 a + 1.75 L x= 5.645 L + a - 3.012b BI =
(1) (2)
where L = whiteness or brightness/darkness; a = redness/greenness; b = yellowness/blueness. Small values of dE, BI and x indicate acceptable color quality. Rehydration moisture was estimated by reconstituting the dried onion slices in boiling water (Maskan 2001). The rehydrated moisture content was reported as percentage on a wet weight basis. Statistical Analyses The results were analyzed with analysis of variance (ANOVA) software. Regression equations were developed from experimental results. Correlation coefficients were calculated from predicted and experimental drying process and from onion slice quality variables. Significance of P £ 0.05 or P £ 0.65 in onion slice thickness was noted.
60 70 80 60 60 60 60 60 60 60 60 60
1 2 3 4 5 6 7 8 9 10 11 12
2 2 2 2* 4** 6** 2 2 2 2 2 2
Slice thickness (mm)*
†
*SD-0.5; **SD-0.65. To dry the product 7–8% moisture.
Drying temperature (C)
S. No.
40 40 40 40 40 40 30 40 50 40 40 40
Air temperature (C) 2 2 2 2 2 2 2 2 2 2 1.4 0.8
Air velocity (m/s) 220 160 140 220 280 320 340 220 240 220 260 320
Drying time† (min) 16.95 16.70 15.67 16.95 15.27 14.07 15.54 16.95 9.06 16.95 18.29 15.46
Pyruvic acid content (mmol/g) 20.63 22.58 22.65 20.63 25.67 29.46 25.03 20.63 24.25 20.63 23.78 21.50
Total color change
TABLE 1. QUALITY OF ONION SLICES DRIED AT SELECTED PROCESSING CONDITIONS OF COMBINATION DRYING
13.74 20.56 20.70 13.74 14.30 21.94 16.06 13.74 15.30 13.70 12.15 11.80
Browning index IR AND HOT-AIR DRYING OF ONIONS 137
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D.G. PRAVEEN KUMAR ET AL.
RESULTS AND DISCUSSION Processing Temperatures and Drying The moisture curves for the drying of onion slices are presented in Fig. 2. The drying characteristics of onion slices were significantly influenced by the drying temperature. The increase in drying temperatures resulted in increased rates of heat and mass transfer, leading to a reduction in drying times. A reduction of 36.7% in drying time was observed when the drying temperature was increased from 60C to 80C. Similar trends were observed by Mazza and LeMaguer (1980) and Sarsavadia et al. (1999) during the convection drying of onion slices. Faster drying rates were also observed by Mongpraneet et al. (2002) during the far-IR onion drying under vacuum conditions. The pyruvic acid contents of the onion slices (Table 1) decreased with increases in drying temperature. Similar observations were reported by Adam et al. (2000) and Sharma and Prasad (2001). The loss of pyruvic acid may be attributed to the damage to the cell structure of the onion slices and to subsequent losses of allinase at elevated temperatures. Compared to fresh onion slices, the pyruvic acid content in the dried onion slices decreased by 100 90
60C 70C 80C
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
220
Time (min) FIG. 2. MOISTURE OF ONION SLICES AT SELECTED DRYING TEMPERATURES Air temperature, 40C; slice thickness, 2 mm; air velocity, 2 m/s.
240
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22–25%, resulting in what may be considered an acceptable concentration, considering even greater losses during hot-air drying. Drying 2-mm-thick onion slices at temperatures of 80C resulted in 65% loss in the pungent pyruvic acid component (Adam et al. 2000). The color of the onion slices, expressed as dE, and the BI, are presented in Table 1. The dEs and the BIs verify the observed browning of the onion slices as the drying temperature is increased. Sugars in the dehydrated onion play a major role in the temperature-dependent nonenzymatic browning of onion slices (Singh and Kumar 1984). Considering flavor and color as the two important quality criteria of dried onion slices, the drying temperature of 60C was the best among the experimental temperatures selected, and a drying temperature of 60C was maintained for the remaining experiments. Mazza and LeMaguer (1980) and Adam et al. (2000) also reported that drying temperatures of approximately 60C were favorable to the retention of the flavor and of the color of dried onion slices. Slice Thickness and Drying A variation in slice thickness altered the drying time (Fig. 3). Changes in the quality parameters of the selected thicknesses of the onion slices are 100 90 6 mm
Moisture content (% Wb)
80
4 mm
70
2 mm
60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
Time (min)
FIG. 3. MOISTURE OF ONION SLICES WITH THICKNESS OF 2 mm, DRYING TEMPERATURE OF 60C, AIR TEMPERATURE OF 40C AND AIR VELOCITY OF 2 m/s
360
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D.G. PRAVEEN KUMAR ET AL.
provided in Table 1. The time required for drying increased by nearly 31% when the onion slice thickness was increased by 4 mm. The extent of IR penetration varies with the physicochemical nature of the food. Increases in thickness reduce the penetration of the IR radiation and thereby decreases the rate of mass transfer. Hence, the selection of the most appropriate thickness for efficient IR drying is a critical factor. In the optimization of slice thickness for IR drying, the drying time, energy costs and onion slice quality must be kept in view. Onion slices with 2-mm thickness resulted in the quickest drying. Greater losses of pyruvic acid (9.91%) that were observed as the slice thicknesses increased were attributed to longer drying times that were necessary to accommodate the increased thickness. Increase in slice thickness also increased the dE and the BI, again attributed to increased drying time with thicker onion slices. Many reports (Prabhanjan et al. 1995; Maskan 2001) report the effect of drying times and temperatures on the flavor and color of dried onion products. The drying of the thin onion slices (2 mm) resulted in better quality dried onion slices, based on color and on pyruvic acid content, in relatively shorter drying times. Adam et al. (2000) observed that during the convection drying of onion slices, the maintenance of thin onion slices resulted in better dried onion slice quality. Additional experiments were carried out maintaining an onion slice thickness of 2 mm. Air Temperature and Drying Increased drying temperatures reduced the drying time (Fig. 4). Drying with air at 30C required considerably longer time (340 min) to reduce the moisture content to the desired 7–8% moisture (Table 1). Although the IR drying temperatures were maintained at 60C, the warm air flow at 30C cooled the surface of the onion slices quickly, leading to reduced drying efficiency. An increase in air temperature by 10C reduced the drying time by 35%. Ginzburg (1969) also observed a decrease in drying times with increases in air temperature during the combination drying of slow-drying foods. However, when the drying temperatures were increased to 50C, a significant reduction in drying time was not observed. An air temperature of 40C slightly improved the retention of pyruvic acid compared to the retention of pyruvic acid in onion slices dried at 30C. The greater retention of pyruvic acid may be attributed to the reduced drying time at the elevated temperatures. However, when drying temperatures were further increased to 50C, the pyruvic acid content in the onion slices was reduced drastically to 9.06 mmol/g, which was attributed to the increase in the drying temperature.
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100
30C 40C 50C
90
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
Time (min) FIG. 4. MOISTURE OF ONION SLICES AT SELECTED AIR TEMPERATURES Drying temperature, 60C; slice thickness, 2 mm; air velocity, 2 m/s.
The variation in the colors of dried onion slices values are related to the combination of drying temperature and drying time. The lower drying temperature (30C) requiring longer drying time and the higher drying temperature (50C) requiring smaller drying time exhibited similar color changes. Drying temperatures of 40C resulted in onion slices of more desirable color than drying temperatures of 30 or 50C. Combination-mode drying with moderate air temperatures of 40C resulted in the most acceptable dried onion slices on the basis of color changes and pyruvic acid contents. Air temperatures of 40C were selected for additional experiments. Air Velocity and Drying The moisture curves (Fig. 5) illustrate that the drying characteristics of onion slices are influenced by air velocity. When the air velocity was increased from 0.8 m/s to 1.4 m/s or to 2.0 m/s, the drying time was reduced (Table 1) as a result of greater rates of mass transfer. The drying times were reduced
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100
0.8 m/s 90
1.4 m/s 2.0 m/s
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
Time (min)
FIG. 5. MOISTURE OF ONION SLICES AT SELECTED AIR VELOCITY Processing temperature, 60C; slice thickness, 2 mm; air temperature, 40C.
by 19% and by 31% when the onion slices were dried at 1.4-m/s and 2.0-m/ s air flow velocity, respectively. Air flow at smaller velocities may not be effective in penetrating the onion slices or in evaporating adequate moisture from the onion slice surfaces. Increasing the air velocity may evaporate more moisture from the onion slice surfaces, resulting in faster drying rates. However, very high air velocities may result in a negative effect resulting from the evaporative cooling of the onion slice surfaces. The evaporative cooling effect resulting from excessive air flow velocities was also reported by Sharma and Prasad (2001) during the combined microwave and hot-air drying of garlic cloves. Mazza and LeMaguer (1980) also reported that increasing the air flow velocity beyond a certain velocity resulted in increased drying times. The pyruvic acid contents of the dried onion slices varied with air velocity. Although a direct relationship between peruvic acid content and drying air velocity was not observed, the retention of pyruvic acid may be related to the variability in drying times. Although marginal increases in pyruvic acid content were observed when the drying air velocity was reduced from 2.0 m/ s to 1.4 m/s, the pyruvic acid content decreased to 15.46 mmol/g on further
IR AND HOT-AIR DRYING OF ONIONS
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reduction in drying air velocity to 0.8 m/s. Marginal decreases in browning indices were noticed with the reduction in drying air velocities. The variability in the browning indices was attributed to the variability in the drying times. In the present study, an air velocity of 1.4–2.0 m/s provided the most desirable dried onion slices. Faster-drying air velocities resulted in the slight fluidization of the partially dried onion slices, which may be desirable during IR drying, because greater fluidization exposes greater surface areas of the drying onion slices to IR irradiation. The air velocities can also be optimized on the basis of the evaporative cooling effect, energy input and onion slice fluidization at selected stages of combination drying. Model Development Moisture contents (88–7%) and drying process variables, namely, drying temperature (60, 70 or 80C), onion slice thickness (2, 4 or 6 mm), air temperature (30, 40 or 50C) and air velocity (0.8, 1.4 or 2.0 m/s), were correlated with drying times. A multivariate equation (Eq. 3) was developed using partial least squares regression to predict the drying time at given moisture contents: Drying time = 600.66 - 3.46x1 + 6.65x2 - 2.33x3 - 22.26x4 - 3.02x5
(3)
where x1, x2, x3, x4 and x5 are drying temperature, slice thickness, air temperature, air velocity and onion slice moisture content, respectively. The predicted drying times gave a very good fit (R2 = 0.92) with the plot of experimental variables. Similarly, the dependent variable, drying time (140–340 min), was correlated with the drying process variables and with the dried onion slice quality (pyruvic acid and total color change). The predicted pyruvic acid contents and the total color changes were estimated using regression Eqs. (4) and (5), which were developed from the experimental data. The parity plot presented in Figs. 6 and 7 illustrates the degree of the fit. The details of the ANOVA are provided in Table 2. The correlation coefficient of the predicted and the experimental pyruvic acid contents of dried onion slices was excellent (R2 = 0.96), whereas the correlation coefficient of the predicted and the experimental total color changes was acceptable (R2 = 0.86). Y1 = 71.187 - 0.392x1 + 1.231x2 - 0.133x3 - 5.528x4 - 0.076x5
(4)
2
Y2 = -28.2771 + 0.29785x1 + 0.79911x + 0.33414x3 - 2.945x4 + 0.05x5 (5) where Y1 = pyruvic acid content; Y2 = total color change or dE; x1–x4 are the process variables and x5 is the dependent variable, drying time. Thus, the empiric regression Eqs. (3)–(5) may be used to predict the drying times and to estimate the dried onion slice quality in terms of the pyruvic acid and the total color change under known drying process conditions.
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20 18 16 Predicted values
14 12 10 8 6 4 2 0 0
5
10 Experimental values
15
20
FIG. 6. PYRUVIC ACID CONTENT OF DRIED ONION SLICES
Comparison of Hot-Air and IR Drying The combined drying process (hot air and IR) for onion slices was compared with individual conventional hot-air or IR drying processes for onion slices. The drying conditions adopted for the three modes of operation are presented in Table 3. Figure 8 provides typical moisture curves for the three drying processes. The time required for drying onion slices was substantially shorter with IR and with combined IR and hot-air drying compared to only hot-air drying. A reduction of 30% in drying time was observed with IR drying of onion slices, whereas drying time was nearly 36% shorter for combined IR and hot-air drying of onion slices. The greater IR heat intensity, IR heat penetrability and the synergistic effect of IR and hot-air drying resulted in shorter drying times. Short drying times were also reported by Afzal et al. (1999) during combined far-IR and hot-air drying of barley. Combined microwave and hot-air drying resulted in a reduction in drying times by a factor of 4 for mushrooms and by a factor of 2 for apples compared to hot-air drying alone (Funebo and Ohlsson 1998).
IR AND HOT-AIR DRYING OF ONIONS
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35 30
Predicted values
25 20 15 10 5 0 0
5
10
15
20
25
30
35
Experimental values FIG. 7. TOTAL COLOR CHANGE IN DRIED ONION SLICES
The combined drying process of dried onion slices retained greater quantities of pyruvic acid compared to individual IR or hot-air drying of onion slices. The IR dried onion slices exhibited the smallest retention of pyruvic acid directly related to the exposure of the onion slices to high-intensity IR radiation for a considerable time. The total color change of the dried onion slices was minimal following drying with the combined drying process, whereas IR drying resulted in substantial color changes which were again related to the exposure to high-intensity IR irradiation. Rehydration Characteristics The quality of the dried onion slices was also assessed after the rehydration of the dried onion slices. The rehydration of the dried onion slices is dependent on the extent of the structural damage to the onion slices during the drying processes. The rehydration moisture content of dried onion slices at selected time intervals following the three drying processes is presented in
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Table 2. REGRESSION STATISTICS AND ANOVA VALUES FOR PYRUVIC ACID AND FOR TOTAL COLOR CHANGES OF DRIED ONION SLICES Regression statistics Y1 (Pyruvic acid content)
Y2 (Total color change)
0.98 0.96 0.92 0.67 12
0.93 0.86 0.75 1.33 12
df
Sum of squares
Mean square
F
Significance F
Y1 (Pyruvic acid content) Regression 5 Residual 6 Total 11
58.29 2.65 60.94
11.65 0.44 –
26.33 – –
0.00051 – –
Y2 (Total color change) Regression 5 Residual 6 Total 11
69.25 10.74 79.99
13.85 1.79 –
7.73 – –
0.01355 – –
Multiple R R Square Adjusted R square Standard error Observations Analysis of variance
TABLE 3. QUALITY OF ONION SLICES DRIED UNDER SELECTED DRYING PROCESSES Mode of drying
Drying Slice Air Air Drying Pyruvic Total Browning temperature thickness temperature velocity time acid color index (C) (mm) (C) (m/s) (min) content change (mmol/g)
Combined 60 Hot air 60 IR 60
2 2 2
40 60 –
2 2 –
220 340 280
16.95 10.96 9.83
20.63 26.60 30.62
13.74 18.99 15.85
Fig. 9. The rehydration moisture of the dried onion slices following the combined drying process was greater after any selected time interval compared to the individual IR or hot-air drying processes. The dried onion slices dried with the combined drying process attained maximum moisture contents of 77.4% (Wb) after 9 min. The moisture content of the dried onion slices was nearly 2–7% more than the onion slices dried with IR irradiation or hot air. The textural superiority of the rehydrated onion slices dried with the combined
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100 90
Hot air IR Combined
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
400
Time (min) FIG. 8. MOISTURE OF ONION SLICES DURING DRYING PROCESSES Drying temperature, 60C; slice thickness, 2 mm; air temperature, 40C; air velocity, 2 m/s.
drying process was observed when compared to the textures of the rehydrated onion rings dried with IR irradiation or hot air. In a review by Sakai and Hanzawa (1994), the rehydration of Welsh onions dried with far-IR irradiation under vacuum was greater than the rehydration of Welsh onions dried with hot air. CONCLUSIONS Drying process conditions exhibited a significant effect on the drying characteristics and on the quality of dried onion slices during a combination IR irradiation and hot-air drying process. Drying of thin (2 mm) slices of onion at 60C with a convective flow of 2 m/s and a moderate air temperature of 40C retained greater pungency and color. The combined IR irradiation and hot-air drying of onion slices resulted in a rapid drying process and improved the quality of the onion slices dried with IR irradiation or hot air alone. Empirical equations developed to predict the drying times and the quality of dried onion slices gave a good fit. The drying process combining IR irradiation
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90
Moisture content (% Wb)
80 70 60 50 40
Combined IR Hot air
30 20 10 0 0
3
6
9
12
15
18
Time (min) FIG. 9. REHYDRATION MOISTURE CONTENT OF ONION SLICES DRIED WITH SELECTED DRYING PROCESSES
and hot air provides a potential alternative to the hot-air drying of onions. Detailed studies, particularly the determination of energy consumption and the drying economics during the combined IR irradiation and hot-air drying process are needed before a future scale-up of the drying process. ACKNOWLEDGMENTS The authors thank Dr V. Prakash, Director, CFTRI and Dr K.S.M.S. Ragahvarao, Head, Food Engineering, CFTRI, for their kind support and encouragement. The authors wish to thank Dr B.R. Lokesh, Head, Lipid Science and Traditional Foods for his help in product analysis. REFERENCES ADAM, E., MUHLBAUER, W., ESPER, A., WOLF, W. and SPIESS, W. 2000. Quality changes of onion (Allium Cepa L.) as affected by the drying process. Nahrung 44, 32–37.
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SCHWIMMER, S. and WETSON, W.J. 1961. Enzymatic development of pyruvic acid in onions as a measure of pungency. J. Agric. Food. Chem. 9, 301–304. SHARMA, G.P. and PRASAD, S. 2001. Drying of garlic (Allium Sativium) cloves by microwave hot air combination. J. Food Eng. 50, 99–105. SINGH, B. and KUMAR, B.L.M. 1984. Dehydration of onion varieties. Indian Food Packer 67–74.
HOT-AIR DRYING OF ONIONS D.G. PRAVEEN KUMAR
ET AL.
INFRARED AND HOT-AIR DRYING OF ONIONS D.G. PRAVEEN KUMAR1, H. UMESH HEBBAR1,3, D. SUKUMAR2 and M.N. RAMESH1 1
2
Department of Food Engineering
Department of Lipid Science and Traditional Foods Central Food Technological Research Institute Mysore, 570 020 India Accepted for Publication May 02, 2005
ABSTRACT The combination of infrared (IR) and hot-air drying of onion slices was explored, and the effects of processing conditions such as drying temperature, slice thickness, air temperature and velocity on onion slice characteristics were studied. The onion slice quality was evaluated on the basis of the color and the pyruvic acid content, an index of flavor. Drying of thin slices of onion (2 mm) at low temperature (60C) with a moderate air velocity (2 m/s) and air temperature (40C) retained greater flavor and color. An empiric equation developed to correlate the drying process variables and the onion slice moisture with the drying time provided a good fit (R2 = 0.92). Similar equations developed to correlate the drying process variables and the drying time with the pyruvic acid content provided an excellent fit (R2 = 0.96), while the equations fit for the total color change of onion slices were satisfactory (R2 = 0.86). Combination drying resulted in shorter drying process time and in better onion slice quality as compared to IR and hot-air drying applied individually.
INTRODUCTION Onions (Allium cepa Linnaeus) are widely grown and are one of the most popular vegetables in the world. The demand for dried onions is increasing in recent decades. The European Union, one of the major consumers of dried onions, meets its demand by importing onions from the United States, Egypt, India and Hungary (Lewicki et al. 1995). To meet quality demands, many new cultivars of onions suitable for dehydration are being grown. The best onions 3
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132
+091-0821-2513910;
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Journal of Food Processing and Preservation 29 (2005) 132–150. All Rights Reserved. © Copyright 2005, Blackwell Publishing
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for drying exhibit high pungency, high total solids and minimal discoloration during processing (Jones and Mann 1963). India produced nearly 4.9 million tonnes of onions throughout 2001 and 2002 and exported around 7000 tonnes of dehydrated onion products (APEDA Export Statement 2002). Dried onion products are produced in several forms: flaked, minced, chopped and powdered. Dried onions are used as flavor additives in wide varieties of food formulations such as comminuted meats, sauces, soups, salad dressings, pickles and pickle relishes. The technique of drying is probably the oldest method of food preservation practiced by mankind for the extension of food shelf life. The use of artificial drying to preserve agricultural commodities is expanding, creating a need for more rapid drying techniques and methods that reduce the large amount of energy required in drying processes. New and innovative techniques that increase drying rates and enhance dried onion quality are receiving considerable attention (Mongpraneet et al. 2002). The major quality problems faced during onion drying are loss of flavor, discoloration and poor rehydration characteristics of the dried onions. Onion flavor and color are generally perceived as important quality attributes. Quality changes during the drying process are influenced by drying temperatures. The volatile compounds responsible for the aroma and flavor of onions exhibit low boiling points and, accordingly, are often lost during hightemperature drying. Maillard browning reactions induced by the drying process decrease nutritional value, changes the color and flavor and induces textural changes (Adam et al. 2000). Several quality standards for dried onions were developed over time; the official standards of the American Dehydrated Onion and Garlic Association (ADOGA) are considered the primary standard. Hot-air drying is the most commonly employed commercial technique for drying vegetables and fruits, but the hot-air drying process remains largely an art (Mazza and LeMaguer 1980). In industrialized countries, onions are dried using high-temperature commercial dryers such as conveyor belt driers or fluidized bed driers. Prepared onions are uniformly sliced to the required thickness, and the onion slices are dried in a thin layer at a temperature near 60C to a final moisture content less than 9%. However, the major disadvantage associated with hot-air drying is the long drying time even at temperatures near 60C, resulting in the degradation of sliced onion quality. Extensive research was carried out on onion drying by conventional hot-air methods (Sarsavadia et al. 1999). The thin-layer drying rates of brined onion slices were experimentally determined at selected drying temperatures, air velocities and relative humidities. Elustondo et al. (1996) developed a simple model for the dehydration of onion pieces. The kinetics of the nonenzymatic Maillard browning of onion slices during isothermal heating was reported by Rapusas and Driscoll (1995), and the temperature-dependent browning of onion slices
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was successfully modeled as a zero-order reaction. Singh and Kumar (1984) also studied the loss of pungency and the discoloration in selected cultivars of onions during hot-air drying. Infrared (IR) irradiation heating offers many advantages over conventional hot-air drying. When IR irradiation is used to heat or to dry moist vegetables or fruits, the radiation impinges on the exposed vegetable or fruit surfaces and penetrates, and the energy of radiation is converted into heat (Ginzburg 1969). The depth of penetration depends on the composition and structure of the vegetables and fruits and on the wavelengths of the IR irradiation. When a food is exposed to the irradiation, the food is heated intensely, and the temperature gradient throughout the food is reduced within a short period. Further, by the application of intermittent IR irradiation, wherein the periods of heating are followed by cooling, the intense displacement of moisture from the core toward the surface can be achieved. The displacement of moisture results in increased rates of heat transfer compared to conventional drying, and the vegetables and fruits are more uniformly heated resulting in improved quality characteristics (Hebbar and Ramesh 2001). IR processing was attempted in baking, roasting, thermal preservation (blanching, pasteurization and sterilization) and drying of foods (Sandu 1986). Combinations of electromagnetic irradiation and hot air for drying are more efficient than irradiation or hot-air heating alone, presumably providing a synergistic effect. The energy conserved and the quality improvements of barley, which were observed after combined far-IR irradiation and hot-air convection drying, were reported by Afzal et al. (1999). Combinationmode drying reduced the total energy requirements by nearly 245% when compared with hot-air drying at 70C. Drying of garlic, which is another herb that exhibits quality characteristics similar to onions, was studied using combined microwave and hot-air heat sources by Sharma and Prasad (2001). The effects of drying with far-IR radiation under vacuum conditions on the quality of Welsh onions were explored by Mongpraneet et al. (2002). The present study discusses the influence of selected drying conditions such as drying temperature, slice thickness, air temperature and air velocity during combined IR and hot-air drying on the drying characteristics and on the quality of sliced onions.
MATERIALS AND METHODS The Bellary cultivar of onions available in a local market was selected for experimentation. The raw onions had an initial moisture content of 86– 88% (Wb). Onion preparation included manual trimming and peeling followed by slicing of the onions with a domestic slicer.
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BLOWER
HEATER
TEMPERATURE SENSOR
900 mm
TEMPERATURE SENSOR
CONVEYOR DRYING CHAMBERS IR HEATERS
5500 mm
FIG. 1. COMBINED INFRARED AND HOT-AIR DRYING SYSTEM
A combined IR and hot-air heating system, developed in Central Food Technological Research Institute (CFTRI) in Mysore, India (Hebbar and Ramesh 2001), was used to carry out the experiments. A schematic diagram of the heating system is presented in Fig. 1. The equipment designed for continuous operation is fitted with mid-IR (2.4–3.0 mm) heat sources on either side of a wire mesh conveyor. Through flow, hot air is provided for convective heating. Experimental Design The experiments were carried out in batch mode, and 2.0 kg of sliced onions (25-mm diameter) were used for each experiment. The slices were spread uniformly in a single layer (monolayer) over the wire mesh conveyor.
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The combined mode experiments were carried out at three drying temperatures (60, 70 or 80C), slice thicknesses (2, 4 or 6 mm), air temperatures (30, 40 or 50C) and air velocities (0.8, 1.4 and 2.0 m/s) (Table 1). The drying and the hot-air temperatures were controlled with thermostats. Air velocity was controlled by regulating air flow at the blower inlet. During each experiment, one of the parameters was varied, keeping other conditions the same. The onion slices were dried to a moisture content of 7–8% (Wb). The convective and the IR drying of the onion slices (2-mm thick) were also carried out at 60C independently. Onion slices were removed at regular intervals for moisture analysis. The withdrawn onion slices were analyzed in duplicate, and mean values were reported. The moisture contents of the raw and the dried onion slices were determined with an oven method (Ranganna 1977) and expressed as percentage on a wet weight basis. The pyruvic acid content, an index of the flavor strength of onions, was estimated with a spectrophotometric method (Schwimmer and Wetson 1961) and reported as mmol/g. The color of the dehydrated onion slices was determined using a Hunter color measurement system (Laboratory Scan XE, C Illuminant, 2∞view angle, Hunter Laboratory, Reston, VA, USA). The L, a and b color difference values, representing the differences in the three color dimensions from the standard color values representing the onion slices, were utilized to calculate the total color change (dE). The browning index (BI), indicating the purity of the brown color of the onion slices, was calculated using Eqs. (1) and (2) (Maskan 2001): 100( x - 0.31) 0.17 a + 1.75 L x= 5.645 L + a - 3.012b BI =
(1) (2)
where L = whiteness or brightness/darkness; a = redness/greenness; b = yellowness/blueness. Small values of dE, BI and x indicate acceptable color quality. Rehydration moisture was estimated by reconstituting the dried onion slices in boiling water (Maskan 2001). The rehydrated moisture content was reported as percentage on a wet weight basis. Statistical Analyses The results were analyzed with analysis of variance (ANOVA) software. Regression equations were developed from experimental results. Correlation coefficients were calculated from predicted and experimental drying process and from onion slice quality variables. Significance of P £ 0.05 or P £ 0.65 in onion slice thickness was noted.
60 70 80 60 60 60 60 60 60 60 60 60
1 2 3 4 5 6 7 8 9 10 11 12
2 2 2 2* 4** 6** 2 2 2 2 2 2
Slice thickness (mm)*
†
*SD-0.5; **SD-0.65. To dry the product 7–8% moisture.
Drying temperature (C)
S. No.
40 40 40 40 40 40 30 40 50 40 40 40
Air temperature (C) 2 2 2 2 2 2 2 2 2 2 1.4 0.8
Air velocity (m/s) 220 160 140 220 280 320 340 220 240 220 260 320
Drying time† (min) 16.95 16.70 15.67 16.95 15.27 14.07 15.54 16.95 9.06 16.95 18.29 15.46
Pyruvic acid content (mmol/g) 20.63 22.58 22.65 20.63 25.67 29.46 25.03 20.63 24.25 20.63 23.78 21.50
Total color change
TABLE 1. QUALITY OF ONION SLICES DRIED AT SELECTED PROCESSING CONDITIONS OF COMBINATION DRYING
13.74 20.56 20.70 13.74 14.30 21.94 16.06 13.74 15.30 13.70 12.15 11.80
Browning index IR AND HOT-AIR DRYING OF ONIONS 137
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RESULTS AND DISCUSSION Processing Temperatures and Drying The moisture curves for the drying of onion slices are presented in Fig. 2. The drying characteristics of onion slices were significantly influenced by the drying temperature. The increase in drying temperatures resulted in increased rates of heat and mass transfer, leading to a reduction in drying times. A reduction of 36.7% in drying time was observed when the drying temperature was increased from 60C to 80C. Similar trends were observed by Mazza and LeMaguer (1980) and Sarsavadia et al. (1999) during the convection drying of onion slices. Faster drying rates were also observed by Mongpraneet et al. (2002) during the far-IR onion drying under vacuum conditions. The pyruvic acid contents of the onion slices (Table 1) decreased with increases in drying temperature. Similar observations were reported by Adam et al. (2000) and Sharma and Prasad (2001). The loss of pyruvic acid may be attributed to the damage to the cell structure of the onion slices and to subsequent losses of allinase at elevated temperatures. Compared to fresh onion slices, the pyruvic acid content in the dried onion slices decreased by 100 90
60C 70C 80C
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
20
40
60
80
100
120
140
160
180
200
220
Time (min) FIG. 2. MOISTURE OF ONION SLICES AT SELECTED DRYING TEMPERATURES Air temperature, 40C; slice thickness, 2 mm; air velocity, 2 m/s.
240
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22–25%, resulting in what may be considered an acceptable concentration, considering even greater losses during hot-air drying. Drying 2-mm-thick onion slices at temperatures of 80C resulted in 65% loss in the pungent pyruvic acid component (Adam et al. 2000). The color of the onion slices, expressed as dE, and the BI, are presented in Table 1. The dEs and the BIs verify the observed browning of the onion slices as the drying temperature is increased. Sugars in the dehydrated onion play a major role in the temperature-dependent nonenzymatic browning of onion slices (Singh and Kumar 1984). Considering flavor and color as the two important quality criteria of dried onion slices, the drying temperature of 60C was the best among the experimental temperatures selected, and a drying temperature of 60C was maintained for the remaining experiments. Mazza and LeMaguer (1980) and Adam et al. (2000) also reported that drying temperatures of approximately 60C were favorable to the retention of the flavor and of the color of dried onion slices. Slice Thickness and Drying A variation in slice thickness altered the drying time (Fig. 3). Changes in the quality parameters of the selected thicknesses of the onion slices are 100 90 6 mm
Moisture content (% Wb)
80
4 mm
70
2 mm
60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
Time (min)
FIG. 3. MOISTURE OF ONION SLICES WITH THICKNESS OF 2 mm, DRYING TEMPERATURE OF 60C, AIR TEMPERATURE OF 40C AND AIR VELOCITY OF 2 m/s
360
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D.G. PRAVEEN KUMAR ET AL.
provided in Table 1. The time required for drying increased by nearly 31% when the onion slice thickness was increased by 4 mm. The extent of IR penetration varies with the physicochemical nature of the food. Increases in thickness reduce the penetration of the IR radiation and thereby decreases the rate of mass transfer. Hence, the selection of the most appropriate thickness for efficient IR drying is a critical factor. In the optimization of slice thickness for IR drying, the drying time, energy costs and onion slice quality must be kept in view. Onion slices with 2-mm thickness resulted in the quickest drying. Greater losses of pyruvic acid (9.91%) that were observed as the slice thicknesses increased were attributed to longer drying times that were necessary to accommodate the increased thickness. Increase in slice thickness also increased the dE and the BI, again attributed to increased drying time with thicker onion slices. Many reports (Prabhanjan et al. 1995; Maskan 2001) report the effect of drying times and temperatures on the flavor and color of dried onion products. The drying of the thin onion slices (2 mm) resulted in better quality dried onion slices, based on color and on pyruvic acid content, in relatively shorter drying times. Adam et al. (2000) observed that during the convection drying of onion slices, the maintenance of thin onion slices resulted in better dried onion slice quality. Additional experiments were carried out maintaining an onion slice thickness of 2 mm. Air Temperature and Drying Increased drying temperatures reduced the drying time (Fig. 4). Drying with air at 30C required considerably longer time (340 min) to reduce the moisture content to the desired 7–8% moisture (Table 1). Although the IR drying temperatures were maintained at 60C, the warm air flow at 30C cooled the surface of the onion slices quickly, leading to reduced drying efficiency. An increase in air temperature by 10C reduced the drying time by 35%. Ginzburg (1969) also observed a decrease in drying times with increases in air temperature during the combination drying of slow-drying foods. However, when the drying temperatures were increased to 50C, a significant reduction in drying time was not observed. An air temperature of 40C slightly improved the retention of pyruvic acid compared to the retention of pyruvic acid in onion slices dried at 30C. The greater retention of pyruvic acid may be attributed to the reduced drying time at the elevated temperatures. However, when drying temperatures were further increased to 50C, the pyruvic acid content in the onion slices was reduced drastically to 9.06 mmol/g, which was attributed to the increase in the drying temperature.
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100
30C 40C 50C
90
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
Time (min) FIG. 4. MOISTURE OF ONION SLICES AT SELECTED AIR TEMPERATURES Drying temperature, 60C; slice thickness, 2 mm; air velocity, 2 m/s.
The variation in the colors of dried onion slices values are related to the combination of drying temperature and drying time. The lower drying temperature (30C) requiring longer drying time and the higher drying temperature (50C) requiring smaller drying time exhibited similar color changes. Drying temperatures of 40C resulted in onion slices of more desirable color than drying temperatures of 30 or 50C. Combination-mode drying with moderate air temperatures of 40C resulted in the most acceptable dried onion slices on the basis of color changes and pyruvic acid contents. Air temperatures of 40C were selected for additional experiments. Air Velocity and Drying The moisture curves (Fig. 5) illustrate that the drying characteristics of onion slices are influenced by air velocity. When the air velocity was increased from 0.8 m/s to 1.4 m/s or to 2.0 m/s, the drying time was reduced (Table 1) as a result of greater rates of mass transfer. The drying times were reduced
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100
0.8 m/s 90
1.4 m/s 2.0 m/s
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
Time (min)
FIG. 5. MOISTURE OF ONION SLICES AT SELECTED AIR VELOCITY Processing temperature, 60C; slice thickness, 2 mm; air temperature, 40C.
by 19% and by 31% when the onion slices were dried at 1.4-m/s and 2.0-m/ s air flow velocity, respectively. Air flow at smaller velocities may not be effective in penetrating the onion slices or in evaporating adequate moisture from the onion slice surfaces. Increasing the air velocity may evaporate more moisture from the onion slice surfaces, resulting in faster drying rates. However, very high air velocities may result in a negative effect resulting from the evaporative cooling of the onion slice surfaces. The evaporative cooling effect resulting from excessive air flow velocities was also reported by Sharma and Prasad (2001) during the combined microwave and hot-air drying of garlic cloves. Mazza and LeMaguer (1980) also reported that increasing the air flow velocity beyond a certain velocity resulted in increased drying times. The pyruvic acid contents of the dried onion slices varied with air velocity. Although a direct relationship between peruvic acid content and drying air velocity was not observed, the retention of pyruvic acid may be related to the variability in drying times. Although marginal increases in pyruvic acid content were observed when the drying air velocity was reduced from 2.0 m/ s to 1.4 m/s, the pyruvic acid content decreased to 15.46 mmol/g on further
IR AND HOT-AIR DRYING OF ONIONS
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reduction in drying air velocity to 0.8 m/s. Marginal decreases in browning indices were noticed with the reduction in drying air velocities. The variability in the browning indices was attributed to the variability in the drying times. In the present study, an air velocity of 1.4–2.0 m/s provided the most desirable dried onion slices. Faster-drying air velocities resulted in the slight fluidization of the partially dried onion slices, which may be desirable during IR drying, because greater fluidization exposes greater surface areas of the drying onion slices to IR irradiation. The air velocities can also be optimized on the basis of the evaporative cooling effect, energy input and onion slice fluidization at selected stages of combination drying. Model Development Moisture contents (88–7%) and drying process variables, namely, drying temperature (60, 70 or 80C), onion slice thickness (2, 4 or 6 mm), air temperature (30, 40 or 50C) and air velocity (0.8, 1.4 or 2.0 m/s), were correlated with drying times. A multivariate equation (Eq. 3) was developed using partial least squares regression to predict the drying time at given moisture contents: Drying time = 600.66 - 3.46x1 + 6.65x2 - 2.33x3 - 22.26x4 - 3.02x5
(3)
where x1, x2, x3, x4 and x5 are drying temperature, slice thickness, air temperature, air velocity and onion slice moisture content, respectively. The predicted drying times gave a very good fit (R2 = 0.92) with the plot of experimental variables. Similarly, the dependent variable, drying time (140–340 min), was correlated with the drying process variables and with the dried onion slice quality (pyruvic acid and total color change). The predicted pyruvic acid contents and the total color changes were estimated using regression Eqs. (4) and (5), which were developed from the experimental data. The parity plot presented in Figs. 6 and 7 illustrates the degree of the fit. The details of the ANOVA are provided in Table 2. The correlation coefficient of the predicted and the experimental pyruvic acid contents of dried onion slices was excellent (R2 = 0.96), whereas the correlation coefficient of the predicted and the experimental total color changes was acceptable (R2 = 0.86). Y1 = 71.187 - 0.392x1 + 1.231x2 - 0.133x3 - 5.528x4 - 0.076x5
(4)
2
Y2 = -28.2771 + 0.29785x1 + 0.79911x + 0.33414x3 - 2.945x4 + 0.05x5 (5) where Y1 = pyruvic acid content; Y2 = total color change or dE; x1–x4 are the process variables and x5 is the dependent variable, drying time. Thus, the empiric regression Eqs. (3)–(5) may be used to predict the drying times and to estimate the dried onion slice quality in terms of the pyruvic acid and the total color change under known drying process conditions.
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20 18 16 Predicted values
14 12 10 8 6 4 2 0 0
5
10 Experimental values
15
20
FIG. 6. PYRUVIC ACID CONTENT OF DRIED ONION SLICES
Comparison of Hot-Air and IR Drying The combined drying process (hot air and IR) for onion slices was compared with individual conventional hot-air or IR drying processes for onion slices. The drying conditions adopted for the three modes of operation are presented in Table 3. Figure 8 provides typical moisture curves for the three drying processes. The time required for drying onion slices was substantially shorter with IR and with combined IR and hot-air drying compared to only hot-air drying. A reduction of 30% in drying time was observed with IR drying of onion slices, whereas drying time was nearly 36% shorter for combined IR and hot-air drying of onion slices. The greater IR heat intensity, IR heat penetrability and the synergistic effect of IR and hot-air drying resulted in shorter drying times. Short drying times were also reported by Afzal et al. (1999) during combined far-IR and hot-air drying of barley. Combined microwave and hot-air drying resulted in a reduction in drying times by a factor of 4 for mushrooms and by a factor of 2 for apples compared to hot-air drying alone (Funebo and Ohlsson 1998).
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35 30
Predicted values
25 20 15 10 5 0 0
5
10
15
20
25
30
35
Experimental values FIG. 7. TOTAL COLOR CHANGE IN DRIED ONION SLICES
The combined drying process of dried onion slices retained greater quantities of pyruvic acid compared to individual IR or hot-air drying of onion slices. The IR dried onion slices exhibited the smallest retention of pyruvic acid directly related to the exposure of the onion slices to high-intensity IR radiation for a considerable time. The total color change of the dried onion slices was minimal following drying with the combined drying process, whereas IR drying resulted in substantial color changes which were again related to the exposure to high-intensity IR irradiation. Rehydration Characteristics The quality of the dried onion slices was also assessed after the rehydration of the dried onion slices. The rehydration of the dried onion slices is dependent on the extent of the structural damage to the onion slices during the drying processes. The rehydration moisture content of dried onion slices at selected time intervals following the three drying processes is presented in
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Table 2. REGRESSION STATISTICS AND ANOVA VALUES FOR PYRUVIC ACID AND FOR TOTAL COLOR CHANGES OF DRIED ONION SLICES Regression statistics Y1 (Pyruvic acid content)
Y2 (Total color change)
0.98 0.96 0.92 0.67 12
0.93 0.86 0.75 1.33 12
df
Sum of squares
Mean square
F
Significance F
Y1 (Pyruvic acid content) Regression 5 Residual 6 Total 11
58.29 2.65 60.94
11.65 0.44 –
26.33 – –
0.00051 – –
Y2 (Total color change) Regression 5 Residual 6 Total 11
69.25 10.74 79.99
13.85 1.79 –
7.73 – –
0.01355 – –
Multiple R R Square Adjusted R square Standard error Observations Analysis of variance
TABLE 3. QUALITY OF ONION SLICES DRIED UNDER SELECTED DRYING PROCESSES Mode of drying
Drying Slice Air Air Drying Pyruvic Total Browning temperature thickness temperature velocity time acid color index (C) (mm) (C) (m/s) (min) content change (mmol/g)
Combined 60 Hot air 60 IR 60
2 2 2
40 60 –
2 2 –
220 340 280
16.95 10.96 9.83
20.63 26.60 30.62
13.74 18.99 15.85
Fig. 9. The rehydration moisture of the dried onion slices following the combined drying process was greater after any selected time interval compared to the individual IR or hot-air drying processes. The dried onion slices dried with the combined drying process attained maximum moisture contents of 77.4% (Wb) after 9 min. The moisture content of the dried onion slices was nearly 2–7% more than the onion slices dried with IR irradiation or hot air. The textural superiority of the rehydrated onion slices dried with the combined
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100 90
Hot air IR Combined
Moisture content (% Wb)
80 70 60 50 40 30 20 10 0 0
40
80
120
160
200
240
280
320
360
400
Time (min) FIG. 8. MOISTURE OF ONION SLICES DURING DRYING PROCESSES Drying temperature, 60C; slice thickness, 2 mm; air temperature, 40C; air velocity, 2 m/s.
drying process was observed when compared to the textures of the rehydrated onion rings dried with IR irradiation or hot air. In a review by Sakai and Hanzawa (1994), the rehydration of Welsh onions dried with far-IR irradiation under vacuum was greater than the rehydration of Welsh onions dried with hot air. CONCLUSIONS Drying process conditions exhibited a significant effect on the drying characteristics and on the quality of dried onion slices during a combination IR irradiation and hot-air drying process. Drying of thin (2 mm) slices of onion at 60C with a convective flow of 2 m/s and a moderate air temperature of 40C retained greater pungency and color. The combined IR irradiation and hot-air drying of onion slices resulted in a rapid drying process and improved the quality of the onion slices dried with IR irradiation or hot air alone. Empirical equations developed to predict the drying times and the quality of dried onion slices gave a good fit. The drying process combining IR irradiation
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90
Moisture content (% Wb)
80 70 60 50 40
Combined IR Hot air
30 20 10 0 0
3
6
9
12
15
18
Time (min) FIG. 9. REHYDRATION MOISTURE CONTENT OF ONION SLICES DRIED WITH SELECTED DRYING PROCESSES
and hot air provides a potential alternative to the hot-air drying of onions. Detailed studies, particularly the determination of energy consumption and the drying economics during the combined IR irradiation and hot-air drying process are needed before a future scale-up of the drying process. ACKNOWLEDGMENTS The authors thank Dr V. Prakash, Director, CFTRI and Dr K.S.M.S. Ragahvarao, Head, Food Engineering, CFTRI, for their kind support and encouragement. The authors wish to thank Dr B.R. Lokesh, Head, Lipid Science and Traditional Foods for his help in product analysis. REFERENCES ADAM, E., MUHLBAUER, W., ESPER, A., WOLF, W. and SPIESS, W. 2000. Quality changes of onion (Allium Cepa L.) as affected by the drying process. Nahrung 44, 32–37.
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