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Preparation of dry honey by microwave–vacuum drying
Journal of Food Engineering 84 (2008) 582–590 www.elsevier.com/locate/jfoodeng
Preparation of dry honey by microwave–vacuum drying Zheng-Wei Cui a,*, Li-Juan Sun a, Wei Chen b, Da-Wen Sun c,* a
Food Engineering & Machinery Group, School of Mechanical Engineering, Jiangnan University, Wuxi, Jiangsu 214122, PR China b School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122, PR China c Department of Biosystems Engineering, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland Received 15 November 2006; received in revised form 26 June 2007; accepted 29 June 2007 Available online 20 July 2007
Abstract Microwave–vacuum (MWV) drying was investigated as a potential method for obtaining high-quality dried honey. Liquid honey was heated and dehydrated in a MWV dryer to a moisture content less than 2.5% within about 10 min. The drying curves and the temperature changes of samples were tested during MWV drying at a different of microwave power, vacuum pressure levels and sample thicknesses. Fructose, glucose, maltose and sucrose contents in the liquid and dry honey were determined by high-performance liquid chromatography (HPLC). The volatiles in the liquid and dry honey were concentrated by solid-phase microextraction (SPME), separated and identified by gas chromatography–mass spectrometry (GC–MS). A sample thickness of less than 8 mm and a vacuum pressure of 30 mbar were identified as the better parameters for the MWV drying. The core temperatures of the sample were about the same as the surface temperatures, the temperature changes were from 30 to 50 °C with higher dehydration rates while no darkening of the honey took place during MWV drying. There were no significant changes on the contents of fructose, glucose, maltose and sucrose in the honey after MWV drying. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of the used liquid honey and the content of alcohols and the esters changed slightly. The acids decreased markedly whereas the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Microwave–vacuum drying; Honey; Dry honey; Fructose; Flavors
1. Introduction Honey is a sweet viscous yellowish liquid with tempting flavors, which is elaborated by the honeybee from the nectar of plants. It contains fructose and glucose (60–85%) as the predominant monosaccharides, maltose and sucrose (7–10%) as the most important disaccharides, melezitose as the main trisaccharide and other low molecular weight oligosaccharides (Doner, 1977; Doner & Hicks, 1982; Lazaridou, Biliaderis, Bacandritsos, & Sabatini, 2004). Beside those, antioxidants (such as pinocembrin, pinobanksin, chrysin and galagin), acids (primarily gluconic acid), pro*
Corresponding authors. Tel./fax: +86 510 85912082 (Z.-W. Cui). E-mail addresses: [email protected] (Z.-W. Cui), [email protected] (D.-W. Sun). URLs: www.ucd.ie/refrig, www.ucd.ie/sun (D.-W. Sun). 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.06.027
tein, minerals, flavonoids, vitamins and enzymes among others are also found in honey (Bouseta, Scheirman, & Collin, 1996; Sabatier, Amiot, Tacchini, & Aubert, 1992; Wang, Gheldof, & Engeseth, 2004). Therefore, honey is often eaten as a hygienic food that is good for health. Most honeys are supersaturated solutions of fructose and glucose with low pH (3.4–6.1), which have a tendency to crystallize spontaneously at room temperature, making them less appealing to the consumer. Moreover, in many cases, crystallization of honey results in increased moisture of the liquid phase which can allow naturally occurring yeast cells to multiply causing fermentation of the product (Doner, 1977). It also causes metal containers to corrode easily. All these characteristics lead to the inconvenience for storage and transportation of honey. Honey, in its liquid and natural state, presents significant handling problems in mass production operations or
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Nomenclature Cp m ni N0 Qabs Q s SE t DT
specific heat capacity of sample (kJ/kg K) mass of sample (kg) number of replicates of the experimental point i number of experimental points for each experiment energy absorbed by sample per unit time (W) microwave power output in magnetrons (W) standard experimental error at experiment point i total standard experimental error microwave drying time (s) temperature rise in sample (°C)
consumption due to its viscosity and stickiness. There is a strong and constant consumer demand for dried honey that is convenient to be consumed or used in the food industry. The available dry honey products are derived from pure liquid honey, which have been dried to low moisture (not more than 2.5%). The dry honeys can be divided into two groups. One group is the honey that is solidified into blocks or flakes by crystallizing the components able to crystallize in lower moisture contents. This group of honey is usually consumed as honey candy. In this case evaporation in a vacuum has been carried out (Taizo, 1994). The other group is the honey powders comprising 50–70% honey with or without other sweeter solids such as high fructose corn syrup or glucose syrup and other processing aids and/or ingredients. This kind of commercial dried honey product, such as ‘‘ADM Honi-Bake Dry Honey Powder”, ‘‘ADM Honi-Bake 705 Honey Powder” available from ADM (Archer Daniels Midland) Company with its headquarter in Decatur (Illinois, USA) are formulated and processed to be free-flowing. As the honey powder has very low moisture content, it can be directly added into dry mixes, seasonings or dry coatings, be easily blended with other dry ingredients maintaining a full honey flavor, and be used in commercial technologies or other areas where process and product constraints previously prevented the use of liquid honey (Ferriola & Stone, 1998). Other advantages of honey powder may include: convenience, free-flow, ease of handing and weighing, reduced storage space, ease of cleaning and sanitary aspects. For preparation of the honey powder, special drying processes such as spray drying, tunnel drying and drum drying are usually used (White, 1978). In fact, various concerns arise during these processes due to the higher viscosity of honey. Hitherto, the preparation of dry honey in large scale is still a challenging issue and the output of dry honey in the world is very limited. MWV drying has been investigated as a potential method for obtaining high-quality dried foodstuffs, including fruits, vegetables and grains (Cui, Xu, & Sun, 2003, 2004a; Drouzas & Schubert, 1996; Kaensup, Chutima, &
T(i,j) T ðiÞ M(i,j) M ðiÞ
moisture content at experiment point i and at replicate j (°C) mean moisture content at experiment point i (°C) moisture content at experiment point i and at replicate j (% w.b.) mean moisture content at experiment point i(% w.b.)
Subscripts i experiment point j replicate
Wongwises, 2002; Yongsawatdigul & Gunasekaran, 1996b; Yousif, Durance, Scaman, & Girard, 2000). It combines the advantages of both vacuum drying and microwave drying with higher drying rates, lower temperatures (25–50 °C) and more uniform energy efficient compared to other drying methods (Decareau, 1985; Durance & Wang, 2002; Yongsawatdigul & Gunasekaran, 1996a). It dissipates energy throughout a product, and is able to automatically level any moisture variation within it. Specific product features such as aroma and flavor components, and color are conserved. The objectives of this study were (1) to optimize the operating parameters of microwave–vacuum drying of honey and (2) to evaluate the quality of dry honey by the current drying methods. 2. Materials and methods 2.1. Samples Liquid acacia honey samples were provided by Nanjing Lao Shan Honey Co. Ltd., Nanjing, PR China. It was already processed through separation from the comb by centrifugal force, gravity, straining or other means. It was minimally processed and can be approximately regarded as botanic origin. 2.2. Drying equipment A lab scale MWV dryer in which the materials to be dried can be rotated in the cavity was developed and described in detail elsewhere (Cui et al., 2003). The rotation speed of the turntable is 5 rpm. In our MWV dryer, the magnetron and voltage changer can be sufficiently cooled by two fans with higher power to ensure the consistent power output of magnetron. The Sliding vane rotary vacuum pump (Model: 4XZ, Suction Capacity: 4 L/s, Wuxi Vacuum Pump Works, Jiangsu, China) is used to pump the air in the cavity to desired vacuum pressure within 50 s.
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2.3. Microwave power output measurement In this study, the measurement of the MWV dryer power output was determined calorimetrically, which was to measure the change of temperature of a known mass of water (1000 g) for a known period of time. The increase in temperature of water per unit time could be given by Qabs ¼ Q ¼
mC p DT 4187 DT ¼ t t
ð1Þ
where Q = microwave power output in magnetrons (W), Qabs = energy absorbed by sample per unit time (W), m = mass of sample (kg), Cp = specific heat of sample (J/ kg K), DT = temperature rise in sample (°C), t = microwave heating time (s). The standard procedure described by Schiffmamn (1987) was used to determine the power output. In the current study, the power output for full power and 80% full power were 330.0 ± 1.5 W and 290.0 ± 2.3 W, respectively. 2.4. Drying experiments and procedure 2.4.1. Microwave–vacuum drying experiments For preparation of dry honey, it is important to control the drying temperature and drying time. Over-higher temperature and long drying time will damage the nutrition, color and flavors of dry honey. In order to investigate the drying curves and the temperature distribution and changes of samples during MWV drying, different microwave powers, vacuum pressure and sample thickness were concerned. The drying experiments methods are described as below: MWV drying liquid honey in different thickness (6, 8, 10 mm) to moisture content less than 3% to exam the temperature gradient along the thickness throughout the drying process (microwave output power, 330 W; vacuum pressure, 30 mbar); MWV drying liquid honey at the different vacuum pressure (30 mbar, 50 mbar) and microwave output power (330 W, 290 W) to moisture content less than 3% to study the drying curves and the temperature changes of samples throughout the drying process (the thickness of sample, 8 mm).
2.4.2. Experimental procedure The initial moisture content of the honey was 20.83% (wet basis), which was measured according to the vacuum oven method (AOAC, 1995). During drying, the sample was spread to a thickness of 8, 12 and 16 mm respectively, in a cylinder dish made of tetrafluoroethylene with the diameter of 155 mm and rotated with the turntable and then the appropriate experimental conditions (vac-
uum and microwave power) were imposed. For each experiment, the vacuum was interrupted and the sample was taken out and its core and surface temperatures were measured at three locations along radius using an automatic check rig with 16 K-type thermocouple probes (Model XMD-16, thermocouple: Platinum–Rhodium 10–Platinum, accuracy = ±0.25%T, diameter of probe in ball = 1 mm, response time 6 10 s, Shanghai Automatic Instrument Co. Ltd., Shanghai, China), then weighed by electronic balance (Model MP2000D, accuracy = ±0.01 g, Shanghai Electronic Balance Instrument Co. Ltd., Shanghai, China) every 2 min and watered the probes. The sample was dried until the moisture content was less than 2.5% (wet basis). All the measurements were taken within 0.5 min. The moisture of the dried sample at the end of every drying period was calculated according to the loss of weight and the value of the initial moisture content. Compared to the evaporation heat, the sensible heat lost due to the above interruption was small and could be neglected (continuous drying experiment on similar weight was conducted to examine the effect of this interruption during drying on weight loss and it is found the effect was negligible). Each experiment was done in triplicate. The MWV drying experiments were carried out for two levels of microwave power (330.0 W and 290.0 W) and two levels of vacuum pressure (30 mbar and 50 mbar). The lower power levels were obtained from a magnetron that was cycled between on and off. The experimental data points and the process conditions are presented in Figs. 1 and 2. In these figures, Measurements were taken at each experimental data point and the averages and standard errors (s) for each experimental data point are reported and the total standard errors (SE) is also calculated and shown. The equations for calculating s and SE are given below vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 3ni u 1 X 2 s¼t ð2Þ ðT ðjÞ T ðjÞ Þ 3ni 1 i¼1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ni 1 X 2 s¼ ð3Þ ðM ðjÞ M ðjÞ Þ ni 1 i¼1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3ni N0 P P 1 ðT ði;jÞ T ðiÞ Þ2 3ni 1 SE ¼
SE ¼
i¼1
j¼1
N0 s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ni N0 P P 2 1 ðM ði;jÞ M ðiÞ Þ ni 1 i¼1
j¼1
N0
ð4Þ
ð5Þ
where ni is the number of replicates of the experimental point i, N0 is the number of experimental points for each experiment, T(i,j) and M(i,j) are the temperature and moisture content at experiment point i respectively at replicate j, T ðiÞ and M ðiÞ are the mean temperature and moisture content at experiment point i, respectively.
Z.-W. Cui et al. / Journal of Food Engineering 84 (2008) 582–590 60
25 Temperature (T, 330 W) Temperature (T, 290 W) Moisture content (M, 330 W) Moisture content (M, 290 W)
20 15
Tco=Ts + 2.6 οC
40
SE(Ts)=0.95 SE(Tco)=0.93 SE(M)=0.42
10
30
5
20
50
Temperature (οC)
Moisture content, M
50
Moisture content (% w.b.)
Temperature (οC )
Core temperature, Tco
2
4
6
15
20
8
Time (min) 60
10 ο
Tco=Ts+ 5.6 C
5
SE(Ts)=1.53 SE(M)=0.38
4
6
8
15
40
10
5
0
2
4
40
10 Tco=Ts+ 9.2 οC SE(Ts)=1.46
5
SE(Tco)=1.06 SE(M)=0.41
20
0 6
8
Moisture content (% w.b.)
15
6
8
0
Time (min)
Moisture content (% w.b.)
Temperature (οC )
50
4
0
20
SE(T, 330W)=1.03 SE(T, 290W)=1.15 SE(M, 330W)=0.49 SE(M, 290W)=0.45
25
20
Moisture content, M
2
Temperature (T, 330 W) Temperature (T, 290 W) Moisture content (M, 330 W) Moisture content (M, 290 W)
25
Core temperature, Tco
0
8
50
20
Surface Temperature, Ts
30
6
0
Time (min)
60
4
30
SE(Tco)=1.62
2
2
60
Temperature (οC)
15
0
0
25
Moisture content (% w.b.)
Temperature (οC )
20
Moisture content, M
40
20
5
25
Core temperature, Tco
30
10
Time (min)
Surface Temperature, Ts
50
SE(T,300W)=0.94 SE(T,290W)=1.28 SE(M,330W)=0.52 SE(M,290W)=0.46
40
30
0 0
20
Moisture content (% w.b.)
25
Surface Temperature, Ts
Moisture content (% w.b.)
60
585
Moisture content (330 W, 30 mbar)
20
Moisture content (330 W, 50 mbar)
15
Moisture content (290 W, 30 mbar)
10
Moisture content (290 W, 50 mbar)
5 0
0
2
4
6
8
Time (min)
Time (min) Fig. 1. Temperature changes at core and surface points for three different sample thickness: microwave output power = 330 W, vacuum pressure = 30 mbar (a) Initial sample weight = 186.4 g, initial sample thickness = 8 mm; (b) initial sample weight = 234.5 g, initial sample thickness = 12 mm; (c) initial sample weight = 281.2 g, initial sample thickness = 16 mm.
2.5. Color measurement Color was measured by a colorimeter (WSC-S system, Shanghai Precision Instrument Co. Ltd., Shanghai, China). The cylindrical plastic dishes (58 mm in diameter and 15 mm in depth) containing the same quantity of liquid honey and dried honey (rehydrated to original moisture content) samples were placed at the light port (50 mm in diameter), respectively. Each sample was measured for color values three times. The instrument was initially cali-
Fig. 2. Temperature and moisture content changes during drying in samples at the microwave output power 330.0 W and 290.0 W: initial sample weight = 187.1 g, initial sample thickness = 8 mm. (a) P = 30 mbar and (b) P = 50 mbar.
brated with a white standard plate (L* = 96.22, a* = 6.11, b* = 15.05). The information given by L, a and b is generally expressed as the total color of the samples, with L representing the brightness or dullness, a for redness to greenness and b for yellowness to blueness. 2.6. Determinations of the main sugar compositions in liquid and dry honey High-performance liquid chromatography (HPLC) analysis was achieved by reference to the method of GB/ T 2491-2004 (2004, National Standard of PR China). The
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HPLC equipment was composed of a solvent delivery system (Waters 600, Waters Co., Milford, MA, USA), work station (M32, Waters Co.), and a refractive index detector (Waters 2410). Two milliliters of liquid honey was added into a 100 mL volumetric flask and diluted to the volume. Then, 15 lL dilute honey was injected into HPLC column (Water Co.). For analysis of the glucose and fructose, the Waters Sugar-PakTM-I column (6.5 300 mm) was used with water (0.4 mL/min) as the mobile phase at 85 °C. Sucrose and maltose were separated on a Waters–NH2 column (4.6 250 mm) with 80% acetonitrile solution (1 mL/ min) as mobile phase at temperature of 30 °C. They were detected using a refractive index detector system. The contents of the sugars in honey were calculated based on the ratio of integrated peak areas using commercial glucose, fructose, sucrose and maltose (Sigma–Aldrich, St. Loius, MO, USA) as the standard compounds. For the determination of main sugar composition in the dry honey, 20 g dry honey was rehydrated to its original moisture content. After stirring for about half an hour, it was completely dissolved followed by the rest of the procedure being similar to that for the liquid honey. The sugar analysis for each sample was replicated twice. 2.7. Determinations of the flavor of liquid and dry honey Headspace–solid-phase microextraction–gas chromatography–mass spectrometry (HS–SPME–GC–MS) is widely used to determine volatile organic compounds in fruits, vegetables, botanic lipin and honey (Josep & Francesc, 2003; Ngassoum, Jirovetz, & Buchbauer, 2001; Rosa, Consuel, Rosa, & Jose, 2002; Song, Fan, & Beaudry, 1998). Headspace–solid-phase microextraction (HS– SPME) was used as the sample preparation technique before the determination of the volatile organic compounds by GC–MS. Ten milliliters of diluted liquid honey and the reconstituted dry honey samples were introduced into a 15 mL headspace vial with a magneto-stirrer. Extraction was carried out at 60 °C and an equilibration time of 1 h. After extraction, the samples were introduced into the GC injection liner and desorbed at 250 °C for 2 min. Volatile composition analysis i.e. SPME desorption analysis was carried out on a Finnigan Trace MS (GC– MS). One microliters (1 lL) of the samples were injected into the chromatographic system and the volatile compounds were separated using a 30 m 25 mm PEG-20 m column with film thickness 0.25 lm. Helium, at a flow rate of 0.8 mL/min, was used as the carrier with a spilt ratio of 10:1. The following programmed temperature was applied: initial temperature of the column at 40 °C was held for 3 min and then increased to 120 °C at a heating rate of 4 °C/min. From this point the temperature was increased to 230 °C at a heating rate of 10 °C/min and kept at 230 °C for 8 min. The temperature of vaporizing chamber was 250 °C. The mass spectrograph was operated in electron-impact (EI) mode. The emission current was 200 lA and ionization voltage was 70 eV. The ion source tempera-
ture and the interface temperature were 200 °C and 250 °C, respectively while the detector voltage was 350 V. 3. Results and discussion 3.1. Influence of sample thickness on temperature distribution Fig. 1 shows that the core temperature of the sample is 2.6 °C (average value) higher than that of the surface temperature for samples with 8 mm thickness, indicating that a less internal or external mass transfer resistance exists for liquid honey with thickness less than 8 mm during MWV drying. The temperature gradient develops as the sample thickness increases to more than 8 mm, and the thicker the sample is, the greater the temperature gradient will develop. For high-viscosity liquid honey, the controlling resistance and moisture transport are strongly related to the sample thickness and the drying periods and the drying process can be described by models in one dimension for samples in which the thickness is normally much less than the other two dimensions. If sample thickness is less than 8 mm then a homogeneous temperature distribution in the sample will take place, with almost no temperature gradient or pressure gradient generated across the sample due to very little internal mass transfer resistance. This internal mass transfer resistance will become the controlling factor when the sample is more than 8 mm thick. For thicker samples, although external mass transfer resistance due to vacuum does not apply, the temperature gradient or pressure gradient will develop along the dimension of thickness as confirmed by Koumoutsakos, Avramidis, and Hatzikiriakos (2001a, 2001b). In order to avoid the over-higher temperature which probably make the product darker in the center of samples. Usually, the thickness, 8 mm, of liquid honey during MWV drying is the better choice. 3.2. Drying curves and temperature changes during drying of honey In MWV drying, electromagnetic energy is directly converted into the kinetic energy of the water molecules, thus generating heat within the product, and energy transport is not affected by conductivity barriers, especially in high-viscosity materials (Cui, Xu, Sun, & Chen, 2006). The amount of heat generated depends on the strength of the electromagnetic field and the dielectric properties of the material being heated. The energy absorbed by the material initiates moisture evaporation, which increases the internal pressure and drives the moisture from the interior to the surface. An absolute pressure of 20–70 mbar, usually applied during MWV drying, corresponds to a water evaporation temperature and, consequently, product temperature, of approximately 23–45 °C. The saturation temperatures of water are 28.6 °C and 37.5 °C at the vacuum pressure of 30 mbar and 50 mbar, respectively. Fig. 2a and b shows that the temperatures of honey being dried are very close to the saturation
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temperature of water corresponding to the used vacuum at the beginning of the drying period when much water need to be evaporated. In the later drying stages, a little amount of water is available, and the energy needed for moisture vaporization is much less than the thermal energy converted from the microwave power, resulting in the temperature of sample being higher than the saturation temperature of water. The lower the vacuum pressure, the lower of drying temperature, but the electrical discharge will be easily caused in the cavity if the vacuum pressure is less than 20 mbar. Therefore, the vacuum pressure in the range of 25–30 mbar is a better choice. It can also be seen from Fig. 2a and b that the more the microwave power density is, the higher the drying rate is. In the later drying stages, the temperature of the sample with little moisture available will rise rapidly if the microwave power is not properly supplied. Therefore, sophisticated process control will be needed to obtain high evaporation rates at gentle conditions with minimized deterioration of temperature-sensitive compounds in the materials being dried and/or to conserve the specific product features such as aroma, flavor components and color. Fig. 2c shows that the effect of vacuum pressure on the drying curves. The drying curves of 50 mbar are almost the same with those of 30 mbar. It is because of the drying rate is related to the value of latent heat of evaporation of water at vacuum pressure (Cui, Xu, & Sun, 2004b), while the value of latent heat of evaporation of water at 30 mbar is a little different from that of 50 mbar. In order to reserve the flavors that are easily volatilized by higher temperature and lower pressure, the microwave output power (density) should be carefully chosen so that the total drying time is shorter as possible and drying temperature is mild. 3.3. Changes in product color The L value is designated by the Hunter Colorimeter to measure the degree of whiteness and blackness in the product. Together with the amount of the yellowness measured (b), an interpretation on the degree of browning can be interpreted. Table 1 shows that the values of L and b of liquid honey are not significantly different from those of the rehydrated dry honey, indicating that the product did not become darker as the honey was being dried by MWV drying and little or no Maillard browning occurred.
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much of the physical nature of honey, its viscosity, hygroscopicity, granulation properties and energy values (Crane, 1975). In nearly all honeys, fructose predominates in the form of levulose. However, some few varieties contain a higher percentage of glucose in the form of dextrose. These two sugars together account for 85–95% of the honey carbohydrates. More complex sugars (oligosaccharides) constitute the remainder except for a trace amount of polysaccharides. Other dissacharides have been identified including maltose, isomaltose, nigerose, turanose, maltulose, kojibiose, leucrose, neotrehalose, gentiobiose and laminaribiose. Theoretically, Maillard browning reactions would occur during the period of the drying of liquid honey when the existence of sugars (such as fructose), amino acids and proteins in honey are heated together. The end result is a product with a brown color and a decrease of the sugars. However from Table 2, it is calculated that the contents of fructose and glucose increased by 1.71% and 2.45%, respectively. Moreover, the content of maltose and sucrose decreased by 8.60% and 4.07%, respectively. As no darkening took place after the MWV drying of honey, it can be deduced that the decreases of maltose and sucrose did not result from the Maillard browning reaction and are probably the effects of invertase in the honey on sucrose and maltose. The mild temperature (30–50 °C) and microwave radiation enhances the activity of invertase with the result that sucrose and maltose are converted into fructose and glucose during MWV drying (which is desirable because of the eat of metabolism of the monosaccharides). 3.5. Retention of volatile aromatic components in honey Honey varies highly in color, flavor, moisture content and sugar composition. These attributes depend on the climate, the floral type and of course, individual bee keeping practices. Perhaps the most attractive feature of honey is its characteristic flavor. Moreover, the volatile aromatic materials are the most important components for its flavor. Therefore the maximum retention of volatile aromas in honey is very important for the quality of dry honey. HS–SPME–GC–MS was applied to examine the volatile aroma characteristics in the original liquid and the dry honey. Figs. 3 and 4 show that over 100 compounds were separated in 30 m 25 mm PEG-20m column. The acids (29.33%), alcohols (45.5%), aldehydes (10.98%) and esters
3.4. Changes of main sugar composition The largest portion of the dry matter in the honey consists of sugars. In general, the sugars are responsible for
Table 2 Monosaccharides and disaccharides values in the liquid and the dry honey and changes following drying
Table 1 Mean color values (±standard error) of the liquid and the dry honey
Liquid honey Dry honey (p < 0.05).
L
a
b
37.10 ± 0.51 33.03 ± 0.36
5.82 ± 0.47 7.77 ± 0.40
127.1 ± 0.64 134.8 ± 0.71
Fructose Glucose Maltose Sucrose
Liquid honey (mg/mL)
Dry honey (mg/mL)
Loss or increase (%)
399.5 ± 1.10 325.90 ± 1.23 18.48 ± 0.36 2.70 ± 0.18
406.35 ± 1.40 333.90 ± 1.15 16.89 ± 0.41 2.59 ± 0.24
+1.71 +2.45 8.60 4.07
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Fig. 3. Flavor of liquid honey.
Fig. 4. Flavor of dry honey (microwave output = 330.0 W, initial sample weight = 187.0 g, initial sample thickness = 8 mm, vacuum pressure = 30 mbar).
Table 3 Volatile alcohols in the liquid and the dry honey Composition
Ethanol 1-Butanol, 3-methyl 1-propanol, 2-methyl 1-Octanol 2,3-Butanediol 1,2-Propanediol Linalool oxide 2-Furanme thanol Benzyl alcohol Phenylethyl alcohol Benzenemethanol, 4-methoxy Isosorbide
Liquid honey
Dry honey
Peak area (A)
% area (calculated in-house)
Peak area (A0 )
% Area (calculated in-house)
85 978 956 2 778 334
26.21 0.85
793 672 16 878 345 1 123 775 1 502 454 1 486 461 13 651 701 21 240 007 1 582 676 2 229 046
0.24 5.15 0.34 0.46 0.45 4.16 6.48 0.48 0.68
72 789 688 725 256 7 645 033 4 966 969 26 518 754 1 952 041 3 278 006 5 425 205 13 883 699 5 682 176
20.28 0.20 2.13 1.38 7.39 0.54 0.91 1.51 3.87 1.58
A = Peak area of composition in liquid honey. A0 = Peak area of composition in dry honey. 0 a Composition loss or increase ¼ AA A 100%.
Composition loss or increasea (%)
15.34 73.90 525.82 57.12 73.70 118.18 264.97 1.70 73.25
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(1.86%) made up the bulk of the identified compounds in liquid honey. The main volatile alcohols, acids, aldehydes, esters and other volatiles in the liquid and dry honey (relative percentage > 0.2) are listed in Tables 3–7. From Table 4, it is clear that the kinds and total content of the acids decreased markedly, while that the contents of the alde-
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hydes and the ketones increased remarkably in the dry honey after MWV drying as shown in Table 5 and Table 7. Moreover, the alcohols and the esters are changed slightly in the dry honey as shown in Table 3 and Table 6. Upon drying, increases in alcohols, aldehydes and ketones suggested that macromolecules such as esters and
Table 4 Volatile acids in the liquid and the dry honey Composition
Acetic acid Propanoic acid Butyric acid Butanoic acid Caproic acid Octanoic acid Nonanoic acid Geranic acid Benzoic acid
Liquid honey
Dry honey
Peak area (A)
% Area (calculated in-house)
Peak area (A0 )
% Area (calculated in-house)
82 142 486 1 316 909 1 494 240 1 739 945 1 059 862 1 419 443 1 041 518 4 679 341 1 325 304
25.04 0.40 0.46 0.53 0.32 0.43 0.32 1.43 0.40
11 613 572
3.23
960 299
0.27
804 567
0.22
Composition loss or increase (%) 85.86 100 100 100 9.40 100 100 82.81 100
Table 5 Volatile aldehydes in the liquid and the dry honey Composition
Liquid honey Peak area (A)
Butyraldehyde Hexanal Nonaldehyde Furfural Benzaldehyde Benzeneacetaldehyde Benzaldehyde, 4-methosy-
1 249 220 14 402 354 2 728 870 16 616 539 1 010 415
Dry honey % Area (calculated in-house)
0.38 4.39 0.83 5.07 0.31
Peak area (A0 )
% Area (calculated in-house)
2 725 319 1 260 924
0.76 0.35
13 504 987 7 980 983 46 801 943
3.76 2.22 13.04
Composition loss or increase (%)
100 6.23 192.46 181.66 100
Table 6 Volatile Esters in the liquid and the dry honey Composition
ethyl acetate Ethyl lactate 2(3H)-Furanone, dihydro-(cas) 2-Hydroxy-2-cyclohexane 3, 7- Dimethyl - Octanoic acid, ethyl ester 1,2-Benzenedicarboxylic acid, dibutyl ester
Liquid Honey
Dry Honey
Peak Area (A)
%Area (calculated in-house)
Peak Area (A0 )
%Area (calculated in-house)
1854121 1307860 1499050 827475 753486
0.56 0.40 0.46 0.25 0.23
2146364 1251991 1307860
0.60 0.35 0.47
695294
0.21
Composition loss or increase, %
15.76 4.27 12.75 100 100 100
Table 7 Other volatile compositions in liquid honey and dry honey Composition
2-Butanone, 3-hydroxy Pyridinetrimethyl 2-Propanone, 1-hydroxy Acetophenone 4,5-Dimethyl-2-formylfuran 2-Furancarbixaldehyde, 5-(hydroxymethyl)-
Liquid Honey
Dry Honey
Peak Area (A)
%Area (calculated in-house)
Peak Area (A0 )
%Area (calculated in-house)
2533595
0.77
1808997
0.55
1153737 817725
0.35 0.25
6065465 1438491 2146364 23123726 719245 856596
1.69 0.40 0.60 6.43 0.20 0.24
Composition loss or increase, %
139.40 18.65 37.66 4.75
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acids probably decomposed into low molecular alcohols, aldehydes and ketones caused by the heating, microwave radiation and the native enzymes. The decrease in alcohol is clear in Table 3 possibly due to its lower boiling point. The contents of the cooking aroma such as furfural, pyridine and furan in the liquid and dry honey had very limited and little change, it also indicted that no Maillard browning reaction occurred during the MWV drying. Although some changes of the volatile aromatic components and content in the honey treated with MWV drying were difficult to avoid, the flavors did not change much due to short heating/radiation time in our MWV drying process. 4. Conclusions This study has shown that a thickness less than 8 mm and a vacuum pressure of 30 mbar were the better parameters for the MWV drying of honey. The core temperatures of the sample were about the same as its surface temperatures, and the temperature changes were from 30 to 50 °C with higher dehydration rate during MWV drying when the dryer was done at those parameters. The color of dry honey is not significantly different from that of the liquid honey, and the contents of main sugars (fructose, glucose, maltose and sucrose) are slighted changed. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of current used liquid honey, and the content of alcohols and the esters changed slightly, the acids decreased markedly while the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying. Acknowledgement This work is supported by National Natural Science Foundation of China (20436020). References AOAC (1995). Official methods of analysis of AOAC International (16th ed.). Washington, USA: Association of Official Analytical Chemists. Bouseta, A., Scheirman, V., & Collin, S. (1996). Flavor and free amino acid composition of lavender and eucalyptus honeys. Journal of Food Science, 61(4), 683–687, 694. Crane, E. (1975). In E. Crane (Ed.), Honey: A comprehensive survey (pp. 160–200). London: Heinemann. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2003). Dehydration of garlic slices by combined microwave–vacuum and air drying. Drying Technology, 21(7), 1173–1185. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004a). Effect of microwave– vacuum drying on the carotenoids retention of carrot slices and chlorophyll retention of Chinese chive leaves. Drying Technology, 22(3), 561–574. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004b). Microwave–vacuum drying kinetics of carrot slices. Journal of Food Engineering, 65(2), 157–164. Cui, Z. W., Xu, S. Y., Sun, D. W., & Chen, W. (2006). Dehydration of concentrated Ganoderma lucidum extraction by combined microwave– vacuum and conventional vacuum drying. Drying Technology, 24(5), 595–599.
Decareau, R. V. (1985). Microwave in the food processing industry. New York: Academic Press. Doner, L. W. (1977). The sugars of honey – A review. Journal of the Science of Food Agricultural, 28, 443–456. Doner, L. W., & Hicks, K. B. (1982). Lactose and the sugars of honey and maple: Reactions, properties, and analysis. In D. R. Lineback & G. E. Inglett (Eds.), Food carbohydrates (pp. 74–112). West Port, CT: AVI Publishing Company. Drouzas, A. E., & Schubert, H. (1996). Microwave application in vacuum drying of fruits. Journal of Food Engineering, 28, 203–209. Durance, T. D., & Wang, J. H. (2002). Energy consumption, density, and rehydration rate of vacuum microwave- and hot-air convectiondehydrated tomatoes. Journal of Food Science, 67(6), 2212–2216. Ferriola, D., & Stone, M. (1998). Sweetener effects on flaked millet breakfast cereals. Journal of Food Science, 63(4), 726–729. GB/T 2491-2004. Isomaltooligosccharide. National Development and Reform Commission Press. Beijing, PR China. Josep, S. B., & Francesc, V. C. (2003). Flavor index and aroma profiles of fresh and processed honeys. Journal of the Science of Food and Agriculture, 83, 275–282. Kaensup, W., Chutima, S., & Wongwises, S. (2002). Experimental study on drying of chilli in a combined microwave–vacuum-rotary drum dryer. Drying Technology, 20(10), 2067–2079. Koumoutsakos, A., Avramidis, S., & Hatzikiriakos, S. G. (2001a). Radio frequency vacuum drying of wood-I mathematical model. Drying Technology, 19(1), 65–84. Koumoutsakos, A., Avramidis, S., & Hatzikiriakos, S. G. (2001b). Radio frequency vacuum drying of wood-II experimental model evaluation. Drying Technology, 19(1), 85–98. Lazaridou, A., Biliaderis, C. G., Bacandritsos, N., & Sabatini, A. G. (2004). Composition, thermal and rheological behaviour of selected Greek honeys. Journal of Food Engineering, 64, 9–21. Ngassoum, M., Jirovetz, L., & Buchbauer, G. (2001). SPME/GC/MS analysis of headspace aroma compounds of the Cameroonian fruit Tetrapleura tetraptera (Thonn.) Taub. European Food Research and Technology, 213(1), 18–21. Rosa, A. P., Consuel, S. B., Rosa, M. C., & Jose, L. T. (2002). Analysis of volatiles from Spanish honey by solid-phase microextraction and gas chromatography- mass spectrometry. Journal of Agricultural and Food Chemistry, 50, 2633–2637. Sabatier, S., Amiot, M. J., Tacchini, M., & Aubert, S. (1992). Identification of flavonoids in sunflower honey. Journal of Food Science, 57(3), 773–774, 777. Schiffmamn, R. F. (1987). Microwave and dielectric drying. In A. S. Mujunmdar (Ed.), Handbook of industrial drying (pp. 345–372). New York: Marcel Dekker. Song, J., Fan, L., & Beaudry, R. M. (1998). Application of solid-phase microextraction and gas chromatography/times-of-flight mass spectrometry for rapid analysis of flavor volatiles in tomato and strawberry fruits. Journal of Agricultural and Food Chemistry, 46(9), 3721–3726. Taizo, K. (1994). Process for production solid honey. US Patent 5,356,650. Wang, X. H., Gheldof, N., & Engeseth, N. J. (2004). Effect of processing and storage on antioxidant capacity of honey. Journal of Food Science, 69(2), C96–C101. White, J. W. Jr., (1978). Honey. In C. O. Chichester, E. M. Mark, & G. F. Stewart (Eds.), Advances in Food Research (pp. 287–374). New York: Academic Press. Yongsawatdigul, J., & Gunasekaran, S. (1996a). Pulsed microwave– vacuum drying of cranberries, part I. Energy use and efficiency. Journal of Food Processing and Preservation, 20(2), 121–143. Yongsawatdigul, J., & Gunasekaran, S. (1996b). Pulsed Microwave– vacuum drying of cranberries, part II. quality evaluation. Journal of Food Processing and Preservation, 20(3), 145–156. Yousif, A. N., Durance, T. D., Scaman, C. H., & Girard, B. (2000). Headspace volatiles and physical characteristics of vacuum–microwave, air, and freeze-dried oregano (Lippia berlandieri Schauer). Journal of Food Science, 65(6), 926–930.
Preparation of dry honey by microwave–vacuum drying Zheng-Wei Cui a,*, Li-Juan Sun a, Wei Chen b, Da-Wen Sun c,* a
Food Engineering & Machinery Group, School of Mechanical Engineering, Jiangnan University, Wuxi, Jiangsu 214122, PR China b School of Food Science and Technology, Jiangnan University, Wuxi, Jiangsu 214122, PR China c Department of Biosystems Engineering, University College Dublin, National University of Ireland, Earlsfort Terrace, Dublin 2, Ireland Received 15 November 2006; received in revised form 26 June 2007; accepted 29 June 2007 Available online 20 July 2007
Abstract Microwave–vacuum (MWV) drying was investigated as a potential method for obtaining high-quality dried honey. Liquid honey was heated and dehydrated in a MWV dryer to a moisture content less than 2.5% within about 10 min. The drying curves and the temperature changes of samples were tested during MWV drying at a different of microwave power, vacuum pressure levels and sample thicknesses. Fructose, glucose, maltose and sucrose contents in the liquid and dry honey were determined by high-performance liquid chromatography (HPLC). The volatiles in the liquid and dry honey were concentrated by solid-phase microextraction (SPME), separated and identified by gas chromatography–mass spectrometry (GC–MS). A sample thickness of less than 8 mm and a vacuum pressure of 30 mbar were identified as the better parameters for the MWV drying. The core temperatures of the sample were about the same as the surface temperatures, the temperature changes were from 30 to 50 °C with higher dehydration rates while no darkening of the honey took place during MWV drying. There were no significant changes on the contents of fructose, glucose, maltose and sucrose in the honey after MWV drying. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of the used liquid honey and the content of alcohols and the esters changed slightly. The acids decreased markedly whereas the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Microwave–vacuum drying; Honey; Dry honey; Fructose; Flavors
1. Introduction Honey is a sweet viscous yellowish liquid with tempting flavors, which is elaborated by the honeybee from the nectar of plants. It contains fructose and glucose (60–85%) as the predominant monosaccharides, maltose and sucrose (7–10%) as the most important disaccharides, melezitose as the main trisaccharide and other low molecular weight oligosaccharides (Doner, 1977; Doner & Hicks, 1982; Lazaridou, Biliaderis, Bacandritsos, & Sabatini, 2004). Beside those, antioxidants (such as pinocembrin, pinobanksin, chrysin and galagin), acids (primarily gluconic acid), pro*
Corresponding authors. Tel./fax: +86 510 85912082 (Z.-W. Cui). E-mail addresses: [email protected] (Z.-W. Cui), [email protected] (D.-W. Sun). URLs: www.ucd.ie/refrig, www.ucd.ie/sun (D.-W. Sun). 0260-8774/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2007.06.027
tein, minerals, flavonoids, vitamins and enzymes among others are also found in honey (Bouseta, Scheirman, & Collin, 1996; Sabatier, Amiot, Tacchini, & Aubert, 1992; Wang, Gheldof, & Engeseth, 2004). Therefore, honey is often eaten as a hygienic food that is good for health. Most honeys are supersaturated solutions of fructose and glucose with low pH (3.4–6.1), which have a tendency to crystallize spontaneously at room temperature, making them less appealing to the consumer. Moreover, in many cases, crystallization of honey results in increased moisture of the liquid phase which can allow naturally occurring yeast cells to multiply causing fermentation of the product (Doner, 1977). It also causes metal containers to corrode easily. All these characteristics lead to the inconvenience for storage and transportation of honey. Honey, in its liquid and natural state, presents significant handling problems in mass production operations or
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583
Nomenclature Cp m ni N0 Qabs Q s SE t DT
specific heat capacity of sample (kJ/kg K) mass of sample (kg) number of replicates of the experimental point i number of experimental points for each experiment energy absorbed by sample per unit time (W) microwave power output in magnetrons (W) standard experimental error at experiment point i total standard experimental error microwave drying time (s) temperature rise in sample (°C)
consumption due to its viscosity and stickiness. There is a strong and constant consumer demand for dried honey that is convenient to be consumed or used in the food industry. The available dry honey products are derived from pure liquid honey, which have been dried to low moisture (not more than 2.5%). The dry honeys can be divided into two groups. One group is the honey that is solidified into blocks or flakes by crystallizing the components able to crystallize in lower moisture contents. This group of honey is usually consumed as honey candy. In this case evaporation in a vacuum has been carried out (Taizo, 1994). The other group is the honey powders comprising 50–70% honey with or without other sweeter solids such as high fructose corn syrup or glucose syrup and other processing aids and/or ingredients. This kind of commercial dried honey product, such as ‘‘ADM Honi-Bake Dry Honey Powder”, ‘‘ADM Honi-Bake 705 Honey Powder” available from ADM (Archer Daniels Midland) Company with its headquarter in Decatur (Illinois, USA) are formulated and processed to be free-flowing. As the honey powder has very low moisture content, it can be directly added into dry mixes, seasonings or dry coatings, be easily blended with other dry ingredients maintaining a full honey flavor, and be used in commercial technologies or other areas where process and product constraints previously prevented the use of liquid honey (Ferriola & Stone, 1998). Other advantages of honey powder may include: convenience, free-flow, ease of handing and weighing, reduced storage space, ease of cleaning and sanitary aspects. For preparation of the honey powder, special drying processes such as spray drying, tunnel drying and drum drying are usually used (White, 1978). In fact, various concerns arise during these processes due to the higher viscosity of honey. Hitherto, the preparation of dry honey in large scale is still a challenging issue and the output of dry honey in the world is very limited. MWV drying has been investigated as a potential method for obtaining high-quality dried foodstuffs, including fruits, vegetables and grains (Cui, Xu, & Sun, 2003, 2004a; Drouzas & Schubert, 1996; Kaensup, Chutima, &
T(i,j) T ðiÞ M(i,j) M ðiÞ
moisture content at experiment point i and at replicate j (°C) mean moisture content at experiment point i (°C) moisture content at experiment point i and at replicate j (% w.b.) mean moisture content at experiment point i(% w.b.)
Subscripts i experiment point j replicate
Wongwises, 2002; Yongsawatdigul & Gunasekaran, 1996b; Yousif, Durance, Scaman, & Girard, 2000). It combines the advantages of both vacuum drying and microwave drying with higher drying rates, lower temperatures (25–50 °C) and more uniform energy efficient compared to other drying methods (Decareau, 1985; Durance & Wang, 2002; Yongsawatdigul & Gunasekaran, 1996a). It dissipates energy throughout a product, and is able to automatically level any moisture variation within it. Specific product features such as aroma and flavor components, and color are conserved. The objectives of this study were (1) to optimize the operating parameters of microwave–vacuum drying of honey and (2) to evaluate the quality of dry honey by the current drying methods. 2. Materials and methods 2.1. Samples Liquid acacia honey samples were provided by Nanjing Lao Shan Honey Co. Ltd., Nanjing, PR China. It was already processed through separation from the comb by centrifugal force, gravity, straining or other means. It was minimally processed and can be approximately regarded as botanic origin. 2.2. Drying equipment A lab scale MWV dryer in which the materials to be dried can be rotated in the cavity was developed and described in detail elsewhere (Cui et al., 2003). The rotation speed of the turntable is 5 rpm. In our MWV dryer, the magnetron and voltage changer can be sufficiently cooled by two fans with higher power to ensure the consistent power output of magnetron. The Sliding vane rotary vacuum pump (Model: 4XZ, Suction Capacity: 4 L/s, Wuxi Vacuum Pump Works, Jiangsu, China) is used to pump the air in the cavity to desired vacuum pressure within 50 s.
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2.3. Microwave power output measurement In this study, the measurement of the MWV dryer power output was determined calorimetrically, which was to measure the change of temperature of a known mass of water (1000 g) for a known period of time. The increase in temperature of water per unit time could be given by Qabs ¼ Q ¼
mC p DT 4187 DT ¼ t t
ð1Þ
where Q = microwave power output in magnetrons (W), Qabs = energy absorbed by sample per unit time (W), m = mass of sample (kg), Cp = specific heat of sample (J/ kg K), DT = temperature rise in sample (°C), t = microwave heating time (s). The standard procedure described by Schiffmamn (1987) was used to determine the power output. In the current study, the power output for full power and 80% full power were 330.0 ± 1.5 W and 290.0 ± 2.3 W, respectively. 2.4. Drying experiments and procedure 2.4.1. Microwave–vacuum drying experiments For preparation of dry honey, it is important to control the drying temperature and drying time. Over-higher temperature and long drying time will damage the nutrition, color and flavors of dry honey. In order to investigate the drying curves and the temperature distribution and changes of samples during MWV drying, different microwave powers, vacuum pressure and sample thickness were concerned. The drying experiments methods are described as below: MWV drying liquid honey in different thickness (6, 8, 10 mm) to moisture content less than 3% to exam the temperature gradient along the thickness throughout the drying process (microwave output power, 330 W; vacuum pressure, 30 mbar); MWV drying liquid honey at the different vacuum pressure (30 mbar, 50 mbar) and microwave output power (330 W, 290 W) to moisture content less than 3% to study the drying curves and the temperature changes of samples throughout the drying process (the thickness of sample, 8 mm).
2.4.2. Experimental procedure The initial moisture content of the honey was 20.83% (wet basis), which was measured according to the vacuum oven method (AOAC, 1995). During drying, the sample was spread to a thickness of 8, 12 and 16 mm respectively, in a cylinder dish made of tetrafluoroethylene with the diameter of 155 mm and rotated with the turntable and then the appropriate experimental conditions (vac-
uum and microwave power) were imposed. For each experiment, the vacuum was interrupted and the sample was taken out and its core and surface temperatures were measured at three locations along radius using an automatic check rig with 16 K-type thermocouple probes (Model XMD-16, thermocouple: Platinum–Rhodium 10–Platinum, accuracy = ±0.25%T, diameter of probe in ball = 1 mm, response time 6 10 s, Shanghai Automatic Instrument Co. Ltd., Shanghai, China), then weighed by electronic balance (Model MP2000D, accuracy = ±0.01 g, Shanghai Electronic Balance Instrument Co. Ltd., Shanghai, China) every 2 min and watered the probes. The sample was dried until the moisture content was less than 2.5% (wet basis). All the measurements were taken within 0.5 min. The moisture of the dried sample at the end of every drying period was calculated according to the loss of weight and the value of the initial moisture content. Compared to the evaporation heat, the sensible heat lost due to the above interruption was small and could be neglected (continuous drying experiment on similar weight was conducted to examine the effect of this interruption during drying on weight loss and it is found the effect was negligible). Each experiment was done in triplicate. The MWV drying experiments were carried out for two levels of microwave power (330.0 W and 290.0 W) and two levels of vacuum pressure (30 mbar and 50 mbar). The lower power levels were obtained from a magnetron that was cycled between on and off. The experimental data points and the process conditions are presented in Figs. 1 and 2. In these figures, Measurements were taken at each experimental data point and the averages and standard errors (s) for each experimental data point are reported and the total standard errors (SE) is also calculated and shown. The equations for calculating s and SE are given below vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u 3ni u 1 X 2 s¼t ð2Þ ðT ðjÞ T ðjÞ Þ 3ni 1 i¼1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ni 1 X 2 s¼ ð3Þ ðM ðjÞ M ðjÞ Þ ni 1 i¼1 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3ni N0 P P 1 ðT ði;jÞ T ðiÞ Þ2 3ni 1 SE ¼
SE ¼
i¼1
j¼1
N0 s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ni N0 P P 2 1 ðM ði;jÞ M ðiÞ Þ ni 1 i¼1
j¼1
N0
ð4Þ
ð5Þ
where ni is the number of replicates of the experimental point i, N0 is the number of experimental points for each experiment, T(i,j) and M(i,j) are the temperature and moisture content at experiment point i respectively at replicate j, T ðiÞ and M ðiÞ are the mean temperature and moisture content at experiment point i, respectively.
Z.-W. Cui et al. / Journal of Food Engineering 84 (2008) 582–590 60
25 Temperature (T, 330 W) Temperature (T, 290 W) Moisture content (M, 330 W) Moisture content (M, 290 W)
20 15
Tco=Ts + 2.6 οC
40
SE(Ts)=0.95 SE(Tco)=0.93 SE(M)=0.42
10
30
5
20
50
Temperature (οC)
Moisture content, M
50
Moisture content (% w.b.)
Temperature (οC )
Core temperature, Tco
2
4
6
15
20
8
Time (min) 60
10 ο
Tco=Ts+ 5.6 C
5
SE(Ts)=1.53 SE(M)=0.38
4
6
8
15
40
10
5
0
2
4
40
10 Tco=Ts+ 9.2 οC SE(Ts)=1.46
5
SE(Tco)=1.06 SE(M)=0.41
20
0 6
8
Moisture content (% w.b.)
15
6
8
0
Time (min)
Moisture content (% w.b.)
Temperature (οC )
50
4
0
20
SE(T, 330W)=1.03 SE(T, 290W)=1.15 SE(M, 330W)=0.49 SE(M, 290W)=0.45
25
20
Moisture content, M
2
Temperature (T, 330 W) Temperature (T, 290 W) Moisture content (M, 330 W) Moisture content (M, 290 W)
25
Core temperature, Tco
0
8
50
20
Surface Temperature, Ts
30
6
0
Time (min)
60
4
30
SE(Tco)=1.62
2
2
60
Temperature (οC)
15
0
0
25
Moisture content (% w.b.)
Temperature (οC )
20
Moisture content, M
40
20
5
25
Core temperature, Tco
30
10
Time (min)
Surface Temperature, Ts
50
SE(T,300W)=0.94 SE(T,290W)=1.28 SE(M,330W)=0.52 SE(M,290W)=0.46
40
30
0 0
20
Moisture content (% w.b.)
25
Surface Temperature, Ts
Moisture content (% w.b.)
60
585
Moisture content (330 W, 30 mbar)
20
Moisture content (330 W, 50 mbar)
15
Moisture content (290 W, 30 mbar)
10
Moisture content (290 W, 50 mbar)
5 0
0
2
4
6
8
Time (min)
Time (min) Fig. 1. Temperature changes at core and surface points for three different sample thickness: microwave output power = 330 W, vacuum pressure = 30 mbar (a) Initial sample weight = 186.4 g, initial sample thickness = 8 mm; (b) initial sample weight = 234.5 g, initial sample thickness = 12 mm; (c) initial sample weight = 281.2 g, initial sample thickness = 16 mm.
2.5. Color measurement Color was measured by a colorimeter (WSC-S system, Shanghai Precision Instrument Co. Ltd., Shanghai, China). The cylindrical plastic dishes (58 mm in diameter and 15 mm in depth) containing the same quantity of liquid honey and dried honey (rehydrated to original moisture content) samples were placed at the light port (50 mm in diameter), respectively. Each sample was measured for color values three times. The instrument was initially cali-
Fig. 2. Temperature and moisture content changes during drying in samples at the microwave output power 330.0 W and 290.0 W: initial sample weight = 187.1 g, initial sample thickness = 8 mm. (a) P = 30 mbar and (b) P = 50 mbar.
brated with a white standard plate (L* = 96.22, a* = 6.11, b* = 15.05). The information given by L, a and b is generally expressed as the total color of the samples, with L representing the brightness or dullness, a for redness to greenness and b for yellowness to blueness. 2.6. Determinations of the main sugar compositions in liquid and dry honey High-performance liquid chromatography (HPLC) analysis was achieved by reference to the method of GB/ T 2491-2004 (2004, National Standard of PR China). The
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HPLC equipment was composed of a solvent delivery system (Waters 600, Waters Co., Milford, MA, USA), work station (M32, Waters Co.), and a refractive index detector (Waters 2410). Two milliliters of liquid honey was added into a 100 mL volumetric flask and diluted to the volume. Then, 15 lL dilute honey was injected into HPLC column (Water Co.). For analysis of the glucose and fructose, the Waters Sugar-PakTM-I column (6.5 300 mm) was used with water (0.4 mL/min) as the mobile phase at 85 °C. Sucrose and maltose were separated on a Waters–NH2 column (4.6 250 mm) with 80% acetonitrile solution (1 mL/ min) as mobile phase at temperature of 30 °C. They were detected using a refractive index detector system. The contents of the sugars in honey were calculated based on the ratio of integrated peak areas using commercial glucose, fructose, sucrose and maltose (Sigma–Aldrich, St. Loius, MO, USA) as the standard compounds. For the determination of main sugar composition in the dry honey, 20 g dry honey was rehydrated to its original moisture content. After stirring for about half an hour, it was completely dissolved followed by the rest of the procedure being similar to that for the liquid honey. The sugar analysis for each sample was replicated twice. 2.7. Determinations of the flavor of liquid and dry honey Headspace–solid-phase microextraction–gas chromatography–mass spectrometry (HS–SPME–GC–MS) is widely used to determine volatile organic compounds in fruits, vegetables, botanic lipin and honey (Josep & Francesc, 2003; Ngassoum, Jirovetz, & Buchbauer, 2001; Rosa, Consuel, Rosa, & Jose, 2002; Song, Fan, & Beaudry, 1998). Headspace–solid-phase microextraction (HS– SPME) was used as the sample preparation technique before the determination of the volatile organic compounds by GC–MS. Ten milliliters of diluted liquid honey and the reconstituted dry honey samples were introduced into a 15 mL headspace vial with a magneto-stirrer. Extraction was carried out at 60 °C and an equilibration time of 1 h. After extraction, the samples were introduced into the GC injection liner and desorbed at 250 °C for 2 min. Volatile composition analysis i.e. SPME desorption analysis was carried out on a Finnigan Trace MS (GC– MS). One microliters (1 lL) of the samples were injected into the chromatographic system and the volatile compounds were separated using a 30 m 25 mm PEG-20 m column with film thickness 0.25 lm. Helium, at a flow rate of 0.8 mL/min, was used as the carrier with a spilt ratio of 10:1. The following programmed temperature was applied: initial temperature of the column at 40 °C was held for 3 min and then increased to 120 °C at a heating rate of 4 °C/min. From this point the temperature was increased to 230 °C at a heating rate of 10 °C/min and kept at 230 °C for 8 min. The temperature of vaporizing chamber was 250 °C. The mass spectrograph was operated in electron-impact (EI) mode. The emission current was 200 lA and ionization voltage was 70 eV. The ion source tempera-
ture and the interface temperature were 200 °C and 250 °C, respectively while the detector voltage was 350 V. 3. Results and discussion 3.1. Influence of sample thickness on temperature distribution Fig. 1 shows that the core temperature of the sample is 2.6 °C (average value) higher than that of the surface temperature for samples with 8 mm thickness, indicating that a less internal or external mass transfer resistance exists for liquid honey with thickness less than 8 mm during MWV drying. The temperature gradient develops as the sample thickness increases to more than 8 mm, and the thicker the sample is, the greater the temperature gradient will develop. For high-viscosity liquid honey, the controlling resistance and moisture transport are strongly related to the sample thickness and the drying periods and the drying process can be described by models in one dimension for samples in which the thickness is normally much less than the other two dimensions. If sample thickness is less than 8 mm then a homogeneous temperature distribution in the sample will take place, with almost no temperature gradient or pressure gradient generated across the sample due to very little internal mass transfer resistance. This internal mass transfer resistance will become the controlling factor when the sample is more than 8 mm thick. For thicker samples, although external mass transfer resistance due to vacuum does not apply, the temperature gradient or pressure gradient will develop along the dimension of thickness as confirmed by Koumoutsakos, Avramidis, and Hatzikiriakos (2001a, 2001b). In order to avoid the over-higher temperature which probably make the product darker in the center of samples. Usually, the thickness, 8 mm, of liquid honey during MWV drying is the better choice. 3.2. Drying curves and temperature changes during drying of honey In MWV drying, electromagnetic energy is directly converted into the kinetic energy of the water molecules, thus generating heat within the product, and energy transport is not affected by conductivity barriers, especially in high-viscosity materials (Cui, Xu, Sun, & Chen, 2006). The amount of heat generated depends on the strength of the electromagnetic field and the dielectric properties of the material being heated. The energy absorbed by the material initiates moisture evaporation, which increases the internal pressure and drives the moisture from the interior to the surface. An absolute pressure of 20–70 mbar, usually applied during MWV drying, corresponds to a water evaporation temperature and, consequently, product temperature, of approximately 23–45 °C. The saturation temperatures of water are 28.6 °C and 37.5 °C at the vacuum pressure of 30 mbar and 50 mbar, respectively. Fig. 2a and b shows that the temperatures of honey being dried are very close to the saturation
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temperature of water corresponding to the used vacuum at the beginning of the drying period when much water need to be evaporated. In the later drying stages, a little amount of water is available, and the energy needed for moisture vaporization is much less than the thermal energy converted from the microwave power, resulting in the temperature of sample being higher than the saturation temperature of water. The lower the vacuum pressure, the lower of drying temperature, but the electrical discharge will be easily caused in the cavity if the vacuum pressure is less than 20 mbar. Therefore, the vacuum pressure in the range of 25–30 mbar is a better choice. It can also be seen from Fig. 2a and b that the more the microwave power density is, the higher the drying rate is. In the later drying stages, the temperature of the sample with little moisture available will rise rapidly if the microwave power is not properly supplied. Therefore, sophisticated process control will be needed to obtain high evaporation rates at gentle conditions with minimized deterioration of temperature-sensitive compounds in the materials being dried and/or to conserve the specific product features such as aroma, flavor components and color. Fig. 2c shows that the effect of vacuum pressure on the drying curves. The drying curves of 50 mbar are almost the same with those of 30 mbar. It is because of the drying rate is related to the value of latent heat of evaporation of water at vacuum pressure (Cui, Xu, & Sun, 2004b), while the value of latent heat of evaporation of water at 30 mbar is a little different from that of 50 mbar. In order to reserve the flavors that are easily volatilized by higher temperature and lower pressure, the microwave output power (density) should be carefully chosen so that the total drying time is shorter as possible and drying temperature is mild. 3.3. Changes in product color The L value is designated by the Hunter Colorimeter to measure the degree of whiteness and blackness in the product. Together with the amount of the yellowness measured (b), an interpretation on the degree of browning can be interpreted. Table 1 shows that the values of L and b of liquid honey are not significantly different from those of the rehydrated dry honey, indicating that the product did not become darker as the honey was being dried by MWV drying and little or no Maillard browning occurred.
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much of the physical nature of honey, its viscosity, hygroscopicity, granulation properties and energy values (Crane, 1975). In nearly all honeys, fructose predominates in the form of levulose. However, some few varieties contain a higher percentage of glucose in the form of dextrose. These two sugars together account for 85–95% of the honey carbohydrates. More complex sugars (oligosaccharides) constitute the remainder except for a trace amount of polysaccharides. Other dissacharides have been identified including maltose, isomaltose, nigerose, turanose, maltulose, kojibiose, leucrose, neotrehalose, gentiobiose and laminaribiose. Theoretically, Maillard browning reactions would occur during the period of the drying of liquid honey when the existence of sugars (such as fructose), amino acids and proteins in honey are heated together. The end result is a product with a brown color and a decrease of the sugars. However from Table 2, it is calculated that the contents of fructose and glucose increased by 1.71% and 2.45%, respectively. Moreover, the content of maltose and sucrose decreased by 8.60% and 4.07%, respectively. As no darkening took place after the MWV drying of honey, it can be deduced that the decreases of maltose and sucrose did not result from the Maillard browning reaction and are probably the effects of invertase in the honey on sucrose and maltose. The mild temperature (30–50 °C) and microwave radiation enhances the activity of invertase with the result that sucrose and maltose are converted into fructose and glucose during MWV drying (which is desirable because of the eat of metabolism of the monosaccharides). 3.5. Retention of volatile aromatic components in honey Honey varies highly in color, flavor, moisture content and sugar composition. These attributes depend on the climate, the floral type and of course, individual bee keeping practices. Perhaps the most attractive feature of honey is its characteristic flavor. Moreover, the volatile aromatic materials are the most important components for its flavor. Therefore the maximum retention of volatile aromas in honey is very important for the quality of dry honey. HS–SPME–GC–MS was applied to examine the volatile aroma characteristics in the original liquid and the dry honey. Figs. 3 and 4 show that over 100 compounds were separated in 30 m 25 mm PEG-20m column. The acids (29.33%), alcohols (45.5%), aldehydes (10.98%) and esters
3.4. Changes of main sugar composition The largest portion of the dry matter in the honey consists of sugars. In general, the sugars are responsible for
Table 2 Monosaccharides and disaccharides values in the liquid and the dry honey and changes following drying
Table 1 Mean color values (±standard error) of the liquid and the dry honey
Liquid honey Dry honey (p < 0.05).
L
a
b
37.10 ± 0.51 33.03 ± 0.36
5.82 ± 0.47 7.77 ± 0.40
127.1 ± 0.64 134.8 ± 0.71
Fructose Glucose Maltose Sucrose
Liquid honey (mg/mL)
Dry honey (mg/mL)
Loss or increase (%)
399.5 ± 1.10 325.90 ± 1.23 18.48 ± 0.36 2.70 ± 0.18
406.35 ± 1.40 333.90 ± 1.15 16.89 ± 0.41 2.59 ± 0.24
+1.71 +2.45 8.60 4.07
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Fig. 3. Flavor of liquid honey.
Fig. 4. Flavor of dry honey (microwave output = 330.0 W, initial sample weight = 187.0 g, initial sample thickness = 8 mm, vacuum pressure = 30 mbar).
Table 3 Volatile alcohols in the liquid and the dry honey Composition
Ethanol 1-Butanol, 3-methyl 1-propanol, 2-methyl 1-Octanol 2,3-Butanediol 1,2-Propanediol Linalool oxide 2-Furanme thanol Benzyl alcohol Phenylethyl alcohol Benzenemethanol, 4-methoxy Isosorbide
Liquid honey
Dry honey
Peak area (A)
% area (calculated in-house)
Peak area (A0 )
% Area (calculated in-house)
85 978 956 2 778 334
26.21 0.85
793 672 16 878 345 1 123 775 1 502 454 1 486 461 13 651 701 21 240 007 1 582 676 2 229 046
0.24 5.15 0.34 0.46 0.45 4.16 6.48 0.48 0.68
72 789 688 725 256 7 645 033 4 966 969 26 518 754 1 952 041 3 278 006 5 425 205 13 883 699 5 682 176
20.28 0.20 2.13 1.38 7.39 0.54 0.91 1.51 3.87 1.58
A = Peak area of composition in liquid honey. A0 = Peak area of composition in dry honey. 0 a Composition loss or increase ¼ AA A 100%.
Composition loss or increasea (%)
15.34 73.90 525.82 57.12 73.70 118.18 264.97 1.70 73.25
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(1.86%) made up the bulk of the identified compounds in liquid honey. The main volatile alcohols, acids, aldehydes, esters and other volatiles in the liquid and dry honey (relative percentage > 0.2) are listed in Tables 3–7. From Table 4, it is clear that the kinds and total content of the acids decreased markedly, while that the contents of the alde-
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hydes and the ketones increased remarkably in the dry honey after MWV drying as shown in Table 5 and Table 7. Moreover, the alcohols and the esters are changed slightly in the dry honey as shown in Table 3 and Table 6. Upon drying, increases in alcohols, aldehydes and ketones suggested that macromolecules such as esters and
Table 4 Volatile acids in the liquid and the dry honey Composition
Acetic acid Propanoic acid Butyric acid Butanoic acid Caproic acid Octanoic acid Nonanoic acid Geranic acid Benzoic acid
Liquid honey
Dry honey
Peak area (A)
% Area (calculated in-house)
Peak area (A0 )
% Area (calculated in-house)
82 142 486 1 316 909 1 494 240 1 739 945 1 059 862 1 419 443 1 041 518 4 679 341 1 325 304
25.04 0.40 0.46 0.53 0.32 0.43 0.32 1.43 0.40
11 613 572
3.23
960 299
0.27
804 567
0.22
Composition loss or increase (%) 85.86 100 100 100 9.40 100 100 82.81 100
Table 5 Volatile aldehydes in the liquid and the dry honey Composition
Liquid honey Peak area (A)
Butyraldehyde Hexanal Nonaldehyde Furfural Benzaldehyde Benzeneacetaldehyde Benzaldehyde, 4-methosy-
1 249 220 14 402 354 2 728 870 16 616 539 1 010 415
Dry honey % Area (calculated in-house)
0.38 4.39 0.83 5.07 0.31
Peak area (A0 )
% Area (calculated in-house)
2 725 319 1 260 924
0.76 0.35
13 504 987 7 980 983 46 801 943
3.76 2.22 13.04
Composition loss or increase (%)
100 6.23 192.46 181.66 100
Table 6 Volatile Esters in the liquid and the dry honey Composition
ethyl acetate Ethyl lactate 2(3H)-Furanone, dihydro-(cas) 2-Hydroxy-2-cyclohexane 3, 7- Dimethyl - Octanoic acid, ethyl ester 1,2-Benzenedicarboxylic acid, dibutyl ester
Liquid Honey
Dry Honey
Peak Area (A)
%Area (calculated in-house)
Peak Area (A0 )
%Area (calculated in-house)
1854121 1307860 1499050 827475 753486
0.56 0.40 0.46 0.25 0.23
2146364 1251991 1307860
0.60 0.35 0.47
695294
0.21
Composition loss or increase, %
15.76 4.27 12.75 100 100 100
Table 7 Other volatile compositions in liquid honey and dry honey Composition
2-Butanone, 3-hydroxy Pyridinetrimethyl 2-Propanone, 1-hydroxy Acetophenone 4,5-Dimethyl-2-formylfuran 2-Furancarbixaldehyde, 5-(hydroxymethyl)-
Liquid Honey
Dry Honey
Peak Area (A)
%Area (calculated in-house)
Peak Area (A0 )
%Area (calculated in-house)
2533595
0.77
1808997
0.55
1153737 817725
0.35 0.25
6065465 1438491 2146364 23123726 719245 856596
1.69 0.40 0.60 6.43 0.20 0.24
Composition loss or increase, %
139.40 18.65 37.66 4.75
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acids probably decomposed into low molecular alcohols, aldehydes and ketones caused by the heating, microwave radiation and the native enzymes. The decrease in alcohol is clear in Table 3 possibly due to its lower boiling point. The contents of the cooking aroma such as furfural, pyridine and furan in the liquid and dry honey had very limited and little change, it also indicted that no Maillard browning reaction occurred during the MWV drying. Although some changes of the volatile aromatic components and content in the honey treated with MWV drying were difficult to avoid, the flavors did not change much due to short heating/radiation time in our MWV drying process. 4. Conclusions This study has shown that a thickness less than 8 mm and a vacuum pressure of 30 mbar were the better parameters for the MWV drying of honey. The core temperatures of the sample were about the same as its surface temperatures, and the temperature changes were from 30 to 50 °C with higher dehydration rate during MWV drying when the dryer was done at those parameters. The color of dry honey is not significantly different from that of the liquid honey, and the contents of main sugars (fructose, glucose, maltose and sucrose) are slighted changed. The volatile acids, alcohols, aldehydes and esters made up the bulk of the identified aroma compounds of current used liquid honey, and the content of alcohols and the esters changed slightly, the acids decreased markedly while the aldehydes and the ketones increased remarkably in the honey dehydrated by MWV drying. Acknowledgement This work is supported by National Natural Science Foundation of China (20436020). References AOAC (1995). Official methods of analysis of AOAC International (16th ed.). Washington, USA: Association of Official Analytical Chemists. Bouseta, A., Scheirman, V., & Collin, S. (1996). Flavor and free amino acid composition of lavender and eucalyptus honeys. Journal of Food Science, 61(4), 683–687, 694. Crane, E. (1975). In E. Crane (Ed.), Honey: A comprehensive survey (pp. 160–200). London: Heinemann. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2003). Dehydration of garlic slices by combined microwave–vacuum and air drying. Drying Technology, 21(7), 1173–1185. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004a). Effect of microwave– vacuum drying on the carotenoids retention of carrot slices and chlorophyll retention of Chinese chive leaves. Drying Technology, 22(3), 561–574. Cui, Z. W., Xu, S. Y., & Sun, D. W. (2004b). Microwave–vacuum drying kinetics of carrot slices. Journal of Food Engineering, 65(2), 157–164. Cui, Z. W., Xu, S. Y., Sun, D. W., & Chen, W. (2006). Dehydration of concentrated Ganoderma lucidum extraction by combined microwave– vacuum and conventional vacuum drying. Drying Technology, 24(5), 595–599.
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