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Improving quality of macadamia nut (Macadamia integrifolia) through the use of hybrid drying process
Journal of Food Engineering 93 (2009) 348–353
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Improving quality of macadamia nut (Macadamia integrifolia) through the use of hybrid drying process Chaleeda Borompichaichartkul a,*, Kanonrat Luengsode a, Ninnart Chinprahast a, Sakamon Devahastin b a b
Department of Food Technology, Faculty of Science, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand Department of Food Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha u-tid Road, Tungkru, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 20 November 2008 Received in revised form 29 January 2009 Accepted 29 January 2009 Available online 7 February 2009 Keywords: Color Drying kinetics Heat pump drying Hybrid drying Macadamia nut Peroxide value Rancidity Reducing sugar
a b s t r a c t Macadamia nut is rich in monounsaturated fatty acids and may reduce serum cholesterol when being included in human diet. On the negative side, high content of unsaturated fatty acids leads to oxidative reactions, which result in rancidity that decreases the quality of the nut. Drying is thus needed to reduce the nut moisture content and hence alleviate the above-mentioned problem. This research was conducted to determine a suitable strategy to reduce the moisture content of fresh macadamia nut to a safe level of 1–2% (d.b.). Hybrid drying process, namely, the use of heat pump drying (40 °C) followed by hot air drying (50–70 °C) was chosen to investigate its feasibility to dry macadamia nut in a relatively fast and economic fashion. The effects of the intermediate moisture content (IMC) (8–11% (d.b.)), which is the moisture content at the end of the first-stage heat pump drying, as well as the drying temperature on the drying kinetics and quality of the nut were determined. The quality of dried macadamia nut was assessed in terms of the moisture content, water activity (aw), color, reducing sugar content and peroxide value. The results showed that heat pump drying followed by hot air drying with IMC of 11.1% (d.b.) gave shortest drying time (15–45.5 h). The temperature and moisture content of the nut prior to the secondstage drying had significant effects on the peroxide value (p 6 0.05); higher drying temperatures led to higher peroxide values. However, the interaction between the drying temperature and IMC had no significant effect on the internal and external total color changes as well as on the amount of reducing sugar (p > 0.05). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Macadamia tree (Macadamia integrifolia) belongs to the botanical family Proteaceae. Macadamia nut is rich in monounsaturated fatty acids, with oleic acid, which is claimed to be a potent inhibitor of fatty acid and cholesterol synthesis (Francesco et al., 2007), contributing to more than 70% of the total fatty acids. This unsaturated fatty acid can help decrease cholesterol and triglyceride levels, thus lowering the risk of heart disease (Grag et al., 2003; Salmolin and Grosvenor, 2000). The world production of macadamia nut is located in Australia (40%), USA (24%), South Africa (15%), South and Central America (12%) and others (9%) (Australia Macadamia Society, 2008). In Thailand, macadamia plantation is located mainly in the northern region. Since macadamia nut contains high amount of oleic acid, which is good for health, the price of macadamia nut is always high. The price of macadamia nut generally increases substantially when the nut is processed into candy, confectionery, nut butter and oil.
* Corresponding author. Tel.: +66 2 218 5534; fax: +66 2 254 4314. E-mail address: [email protected] (C. Borompichaichartkul). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.01.035
Macadamia nut quality depends significantly on its moisture content as well as its water activity. Dominguez et al. (2007) reported that when the moisture content is in the range of 1.2– 1.6% (d.b.), which corresponds to aw of 0.36–0.44, macadamia nut has higher stability against lipid oxidation, changes in color and changes in texture. As macadamia nut contains high amount of unsaturated fatty acids it is prone to hydrolytic and oxidative rancidity when it contains high level of free moisture (Woodroof, 1979). Oxygen is also an important factor that triggers lipid oxidation in high-oil nut (Kaijser et al., 2000). In terms of the changes of the nut color browning is likely to occur when high moisture nut is dried at higher-temperature due mostly to non-enzymatic reaction (Maillard reaction). Cavaletto (1981) suggested that macadamia nut should not be dried at temperatures higher than 40 °C when the nut moisture content is higher than 8.7–11.1% (d.b.) to avoid high concentration of reducing sugar in the middle of the kernel, which could result further in center-browning of the kernel after drying. A high-quality macadamia nut product is characterized by low rancidity (peroxide value lower than 30 meq (milliequivalent) O2/kg) and light color (TISI, 2006). Freshly harvested macadamia nut typically has moisture content up to 81.8% (d.b.) in husk and 33.3% (d.b.) in nut-in-shell
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(NIS) (Chu et al., 1953; Kowitz et al., 1998). Therefore, dehusking must be done within 24 h to prevent heating of the nut from its respiration, metabolic and enzymic activities, which can cause deterioration of the nut quality (Moltzau and Ripperton, 1939). Drying is another most critical step in macadamia nut processing; the nut generally needs to be dried to 1.5% (d.b.) to increase the kernel recovery during cracking and to prevent flavor deterioration during storage (Weinert, 1993). Several methods of drying have been employed to dry macadamia nut such as in-bin drying (Kowitz, 2004), hot air drying (Prichavudhi and Yamamoto, 1965), combination of in-bin and hot air drying (Mason and McConachie, 1994) and heat pump drying (Kowitz, 2004). A major drawback of these methods is the long total drying time, resulting in a slow reduction of moisture content. The longer exposure to oxygen during drying, the more oxidation is likely to occur at high level of moisture content. Hence, rancidity is promoted. A typical industrial drying process employed for macadamia nut requires a very long total drying cycle, extending even to a period of more than one month (Silva et al., 2005). The first-stage pre-drying on-farm, which reduces macadamia nut moisture content down to around 8.7–11.1% (d.b.), takes three to four weeks. After that, up to six days (144 h) may be needed for the second-stage drying, which generally involves hot air drying starting at 40 °C and finishing at 60 °C; the kernel moisture content is reduced to 1.5% (d.b.) (Silva et al., 2005). To speed up the drying process combined hot air and microwave drying was attempted. In this case, hot air drying was used to dry fresh macadamia nut down to 11.1% (d.b.); microwave apparatus was then used to dry the nut further to the moisture content of 1.5% (d.b.) (Silva et al., 2005). Although the combined technique was faster, requiring only 4.5–5.5 h to dry macadamia, it required prohibitively high investment and operating costs. Another interesting option to speed up the drying process and, at the same time, enhance the quality of many food products is the use of heat pump drying (Teeboonma et al., 2003; Sunthonvit, 2005). A wide range of drying conditions, typically from 20 °C to 100 °C (with auxiliary heating), is feasible. Heat pump drying has main advantages of energy recovery from the exhaust and the ability to control temperature and humidity independently. Latent heat of vaporization is extracted from the exhaust moist air by vapor condensation at an evaporator coil (Fig. 1, No. 4), and the recovered energy is delivered as sensible heat to the drying
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air stream passing through a condenser coil (Fig. 1, No. 5) (Islam and Mujumdar, 2008). Hence, there is a potential of using a heat pump dryer for drying macadamia nut. Many studies on heat pump drying of fruits and vegetables have indeed shown that it is an attractive option to preserve the quality of many food products _ including nuts, e.g., hazelnut (Ilhan and Mustafa, 2008), stone fruit (Sunthonvit, 2005), macadamia nut (Kowitz, 2004), as well as papaya and mango (Teeboonma et al., 2003). However, using only heat pump drying may add significantly the electricity cost to the production cost as the dryer depends mainly on electric power. More appropriate technique for drying macadamia nut should not only contribute to time saving but also preserves the natural quality of the nut. Using only one drying method may not be highly beneficial for these purposes. On the other hand, use of combined or hybrid drying process has ability to combine advantages of each drying technique. Combination of different drying processes usually offers unique advantages that single drying process can not achieve (Kudra and Mujumdar, 2002). For example, innovative combination of superheated steam drying and heat pump drying has proven to improve the quality of dried shrimp significantly (Namsanguan et al., 2004). The purpose of this research was to determine a suitable postharvest treatment of macadamia nut-in-shell using heat pump drying followed by hot air drying at different temperatures. The effects of the intermediate moisture content (IMC) and drying temperature on the drying kinetics and quality of the nut were determined. The quality of dried macadamia nut was assessed in terms of the moisture content, water activity (aw), color, reducing sugar content and peroxide value.
2. Materials and methods 2.1. Raw materials Macadamia nut (M. integrifolia) harvested in 2006/2007 harvesting season from Chiang Mai province in the northern part of Thailand was used in this study. The nut was dehusked within 24 h after being harvested; it was then stored with shell in nylon sacks prior to being transported to Chulalongkorn University in Bangkok. The unshelled nut was stored at 18 °C until the time of experiment.
Fig. 1. A schematic diagram of the laboratory dryer: 1-blower; 2-heater chamber; 3-drying chamber; 4-evaporator; 5-condensing unit; 6-valve; 7-valve; 8-cover; and 9-sample tray.
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2.2. Experimental set-up and drying experiments A laboratory heat pump dryer located at the King Mongkut’s University of Technology Thonburi, which could be operated in multi-mode, i.e., hot air drying and heat pump drying, is shown in Fig. 1. The dryer consists of a drying chamber with dimensions of 0.52 0.52 0.52 m. A wire-screen tray with dimensions of 0.28 0.33 m was placed in a perpendicular direction to the airflow. The drying temperature was controlled by a proportionalintegral-differential (PID) controller with an accuracy of ±1 °C while the air velocity (over a cross section of 0.52 0.52 m) was fixed at 0.3 m/s in all experiments. The power of the condenser and evaporator of the heat pump dryer was 3.5 and 2.0 kW, respectively. The first-stage heat pump drying was performed at the temperature of 40 °C and the relative humidity of 20% in order to decrease the nut-in-shell (NIS) moisture content from an initial value of 20– 22% (d.b.) to the IMC of either 8.7 or 11.1% (d.b.). Instantly after the first-stage drying, the process was switched to hot air drying; different temperatures (50, 60 and 70 °C) were used for hot air drying. The drying sample was weighed continuously until the final moisture content reached 1.5–2.7% (d.b.) as suggested by the Australian Macadamia Society (2008). For the conventional industrial method fresh NIS was subjected to hot air drying at 35 °C for 7 days; this was followed by drying at 40 °C for 7 days and 60 °C for another 3 days using a tray dryer. Dried nut from each drying method was kept in a vacuum sealed laminated aluminum packet and stored at 18 °C until further analysis. 2.3. Physical quality analysis Moisture content of nut-in-shell was determined following the method suggested by Wall and Gentry (2006) with some modification. Ten grams of the sample was ground and placed in a vacuum oven at 70 °C, 90 mbar for 24 h. Each measurement was done in triplicate and the average percentage of moisture content on a dry weight basis (g/100 g) was reported. Water activity measurement was performed using AquaLab water activity meter (AquaLink 3.0, Pullman, WA). Calibration of the water activity meter was done using distilled water to obtain aw in the range of 1.00 ± 0.003. Color Flex (HunterLab 45°/0°, Reston, VA) colorimeter was used to measure the external and internal colors of fresh, rewetted and dried kernels to determine the changes of color, in terms of L*, a* and b*, with the rewetting condition, drying condition and storage time. The total color difference (DE) was also calculated according to Eq. (1). Twenty kernels were used for each color analysis.
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 :
boiled in boiling water for 10 min. The sample was then cooled down and 10 mL cool distilled water was added. The absorbance of the sample was then measured at 540 nm. The absorbance obtained was compared with the standard invert sugar curve. The measurement was done in triplicate. Reducing sugar of the nut from each condition was compared with the initial value before drying; the results were expressed as the percentage of change. 2.5. Peroxide value analysis Macadamia nut oil was extracted from 30 g ground nut sample by Soxhlet extraction equipment (Soxtherm, Gerhardt, Königswinter, Germany) using petroleum ether as a solvent for 4 h. Oil was concentrated by eliminating excess solvent with a rotary evaporator (Eyela, Miyagi, Japan) at 45 °C for 45 min. The peroxide value (PV) was then determined following the AOCS method Cd8-53 (1998). The PV of macadamia nut oil from each condition was compared with the initial value of oil from raw macadamia nut before drying and expressed as the percentage of change. The experiment was performed in triplicate. 3. Results and discussion 3.1. Drying kinetic of macadamia Figs. 2 and 3 show the drying curves of macadamia nut subjected to first-stage heat pump drying at 40 °C. The moisture content of the nut was reduced from an initial moisture content of 20–21% (d.b.) to either 8.7 or 11.1% (d.b.). This was followed by second-stage hot air drying at 50, 60 and 70 °C to further reduce the moisture content from the first-stage values to 1.5–2.7% (d.b.). The drying time of the nut to the IMC of 8.7 and 11.1% (d.b.) during heat pump drying was 12.4 and 7.1 h, respectively (Table 1). Nut with an IMC of 11.11% (d.b.) required less drying time during heat pump drying and was subjected to second-stage drying before the nut with an IMC of 8.7% (d.b.). This led to shorter total drying time for nut with an IMC of 11.1% (d.b.) in both stages compared with the nut with an IMC of 8.7% (d.b.). This is because the drying temperature in the second-stage was higher, resulting in a higher drying rate. On the other hand, a higher IMC would lead to a longer drying time if the temperature was reduced in the second-stage drying (Namsanguan et al., 2004). Increasing of the drying temperature expectedly promoted drying rate in all cases. Macadamia nut dried at 70 °C reached its equilibrium moisture content faster comparing to the drying temperatures of 50 °C and 60 °C.
ð1Þ
The calibration of the colorimeter was done using a white calibration sheet to obtain the standard values of X = 78.89 ± 0.3, Y = 83.78 ± 0.3, Z = 87.74 ± 0.3. 2.4. Reducing sugar analysis The amount of reducing sugar, calculated on weight basis (g/ 100 g), was determined following the DNSA (dinitrosalicylic acid) method (AOAC, 1995). Defatted macadamia kernels (2.5 g) was extracted with 100 mL of 80% (v/v) ethanol mixed with 100 mL water. The sample was heated for 25 min at 80–85 °C in a water bath with occasional stirring. The sample was then cooled down and filtered (Whatman No. 1 paper) into 100 mL volumetric flask; the volume was then adjusted with 80% (v/v) ethanol to 100 mL. One milliliter of the sample was taken and mixed with 1 mL of DNSA in a test tube. The mixed sample-DNSA was shaken and
25
HA50 IMC8.7
Moisture content (%d.b.)
350
HA60 IMC8.7 20
HA70 IMC8.7
15
10
5
0 0
10
20
30
40
50
60
Time (h) Fig. 2. Drying curves of macadamia nuts with intermediate moisture content of 8.7% (d.b.). HA: hot air, IMC: intermediate moisture content.
351
25
25
20
20
external color
d internal color
15
HA60 IMC11.1 HA70 IMC11.1
10
bc cd
HA50 IMC11.1
Kernel ΔE
Moisture content (% d.b.)
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abc
abc
abc
abc
15
ab
a
a a
ab 10
5
5
0
0 0
10
20
30
40
50
Time(h) Fig. 3. Drying curves of macadamia nuts with intermediate moisture content of 11.1% (d.b.). HA: hot air, IMC: intermediate moisture content.
HA50
HA60
HA70
HA50
HA60
HA70
IMC8.7
IMC8.7
IMC8.7
IMC11.1
IMC11.1
IMC11.1
Drying condition Fig. 4. Total color difference (DE) of dried external and internal kernels. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
3.2. Quality of macadamia nut Moisture content and water activity of macadamia kernel after drying at each condition are shown in Table 1. It was observed that lower drying temperature in the second-stage resulted in lower NIS moisture content and aw. At higher drying temperature it might be possible that water was removed rapidly resulting in surface hardening and blockage of the rest of water in the kernel to move out. Therefore, the nut moisture content remained high. The change of the internal kernel color was more pronounced than the change of the external color (Fig. 4). This is because the moisture content in the internal kernel was higher than that at the surface. Therefore, water dissolved sugar toward the inside of the kernel; non-enzymatic browning reaction could thus occur more intensively. It is noted that brown pigments are the result of the reaction between reducing sugars and amines in the kernel; the protein content of macadamia nut was around 7.8–9.2 g per 100 g kernel (Weinert, 1993). As drying temperature increased more change of color was observed (increase of DE, see Fig. 4). At an IMC of 8.7% (d.b.) the changes of internal and external colors were not significantly different at any drying temperature. However, at an IMC of 11.1% (d.b.) DE sharply increased when the temperature increased from 60 °C to 70 °C. At an IMC of 11.1% (d.b.) water molecules were more mobile than at an IMC of 8.7% (d.b.). Elevation of temperature and higher availability of water promoted browning reactions, resulting in marked changes of color. An increase in the temperature could promote hydrolysis of sucrose in the kernel to glucose and fructose, resulting in more intensive browning of the nut (Wall and Gentry, 2006). Therefore, to avoid internal browning it is recommended that the drying temperature in the second-stage be maintained at not more than 60 °C. However, when the IMC was 11.1% (d.b.) hot air temperature of 50 °C led to the least change of the internal overall color. For the nut that had an IMC of 11.1% (d.b.) its residence time (7.1 ± 0.8 h)
in the first-stage heat pump drying was shorter than the nut with an IMC of 8.7% (d.b.) (12.4 ± 1.2 h). Browning was then observed at a much lower extent. The result of DE from each drying condition corresponded to the amount of change of reducing sugar that remained in the kernel (Fig. 5). When the second-stage hot air drying temperature was low more reducing sugar remained because more reducing sugar was used in the Maillard reaction to form brown pigments during higher-temperature drying. From Fig. 6 it was observed that change of the peroxide value in the nut was the highest (>600%) when the IMC of 11.1% (d.b.) and the second-stage drying temperature of 70 °C were used. It was observed that when using an IMC of 8.7% (d.b.) macadamia nut could be dried at any temperature (50–70 °C) with not much difference in lipid quality. On the other hand, when using an IMC of 11.1% (d.b.) there was a limit to dry the nut at temperature only up to 60 °C; otherwise, the nut lipid quality would be significantly altered as lipid oxidation could be highly accelerated when the temperature and moisture content increased (Cavaletto et al., 1966). At an IMC of 8.7% (d.b.) free moisture content was lower than in the case of IMC of 11.1% (d.b.); oxidation slowly occurred leading to lower formation of peroxides. However, the changes of the peroxide values of the nuts with an IMC of 11.1% (d.b.) after second-stage drying at 50–60 °C were lower than those of the nuts with an IMC of 8.7% (d.b.). This is due to shorter residence time of the nuts with an IMC of 11.1% (d.b.) during heat pump drying. Lipid oxidation involves three-stage series of reactions including initiation, propagation and termination (Frankel, 2005). Once extremely reactive free radicals from fatty acid molecules are produced during the initiation step they would react with oxygen to form peroxyl radicals (ROO). These peroxyl radicals in turn react with unsaturated lipids to form hydroperoxides (ROOH), which could be detected and
Table 1 Drying time and properties of macadamia nut subjected to heat pump drying at 40 °C with different intermediate moisture contents and second-stage drying temperatures. First-stage drying/intermediate moisture content (% d.b.)
Second-stage hot air drying temperature (°C)
Final NIS moisture content (% d.b.)
Final aw
PV (meq O2/ kg oil)
Drying time in heat pump dryer (h)
Total drying time (h)
HP40/IMC8.7
50 60 70
1.91 ± 0.39 3.53 ± 0.39 2.73 ± 0.36
0.58 ± 0.02 0.69 ± 0.01 0.68 ± 0.02
4.79 ± 0.01 5.75 ± 0.22 5.39 ± 0.11
12.4 ± 1.2
52.3 28.5 22.7
HP40/IMC11.1
50 60 70
1.96 ± 0.27 2.50 ± 0.11 3.20 ± 0.52
0.60 ± 0.01 0.63 ± 0.02 0.69 ± 0.02
4.00 ± 0.01 4.27 ± 0.06 13.58 ± 0.07
7.1 ± 0.8
45.5 22.8 15.0
HP: heat pump, IMC: intermediate moisture content.
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Fig. 5. Percentage of change in reducing sugar (DR) of dried nut compared with raw nut at different drying conditions. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
Peroxide value ΔP (%)
700
f
600 500 400 300
e c
200
d
b
a
100 0 HA50 IMC8.7 HA60 IMC8.7 HA70 IMC8.7
HA50 IMC11.1
HA60 IMC11.1
HA70 IMC11.1
Drying condition Fig. 6. Percentage of change in peroxide value (DP) of dried nut compared with raw nut at different drying conditions. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
measured as peroxide value. More production of hydroperoxides implies higher level of auto-oxidation. Nevertheless, hydroperoxides are not stable and can decompose further under accelerated condition (temperature > 100 °C, the presence of metal and expose to light) (Frankel, 2005). The decomposition of lipid hydroperoxides produces carbonyl compounds, alcohols and hydrocarbons. Therefore, more formation of hydroperoxides would give a fair prediction of rancidity of the product, which contains high unsaturated fat content. The chain reaction can continue until free
radicals reach sufficiently high concentrations; they tend to react together during the termination step. Therefore, after drying, macadamia nut should have as low value of peroxide as possible. This implies that the level of primary stage oxidation would be low. Lipid quality of the nut could thus be maintained for further processing and during storage. However, since the peroxide value from each drying condition (Table 1) was within the standard limits (<30 meq O2/kg oil, TISI (2006)), the drying process was acceptable. Based on the quality observation above it could be suggested that the best condition for drying macadamia nut is the use of heat pump drying at 40 °C to reduce the initial moisture content of the nut to 11.1% (d.b.). This should be followed by second-stage hot air drying at 50 °C until the moisture content reaches 1–2% (d.b.). This would give the best quality of the nut, both in terms of color and lipid quality. The results from the best aforementioned conditions were also compared with those from the conventional industrial method, i.e., hot air drying at 35 °C for 7 days followed by drying at 40 °C for 7 days and 60 °C for another 3 days (Australian Macadamia Society, 2008). The quality observations are displayed in Table 2. The overall quality observations showed that hybrid drying process, i.e., the use of heat pump drying in combination with hot air drying, helped maintain nut quality, especially in terms of the prevention of lipid deterioration, better than the conventional industrial method. The total drying time was shorter and color appearances of both external and internal kernels were also more satisfactory. Internal L* value of dried macadamia kernel from the hybrid drying method was significantly higher than that of the commercial product. Internal color of dried macadamia kernel is indeed one of the quality indices to grade dried macadamia nut; brown-center nut is often discarded or classified as low-grade nut (Weinert, 1993). Although the commercial product was subjected to lower drying temperature during the first-stage drying (35 °C, 7 days) than in the case of the hybrid drying method (40 °C, 7 h), the color quality of the kernel was inferior. This is because of the long drying time at high moisture content (20–21% (d.b.)) leading to an increase in non-enzymatic browning. This could be confirmed by the amount of the remaining reducing sugar; the commercial dried nut had significantly lower amount of reducing sugar. The similar results were also observed in the cases of a*, b* and DE. Finally, it was observed that the peroxide value of the dried nut from the hybrid drying process was significantly lower than that of the commercial product, i.e., the figures being at half. Mason and Wills (2000) indeed emphasized that drying is an important step to be considered to prevent any hydrolytic rancidity or mould development after harvesting of macadamia nut.
Table 2 Quality of dried macadamia nuts by industrial method and from combined heat pump and hot air drying. Parameter
Commercial product
Combined heat pump (40 °C, IMC 11.1% (d.b)) and hot air dried (50 °C) product
Moisture content NIS (% d.b.) aw Color L*
1.03 ± 0.06a 0.29 ± 0.00a 68.31 ± 7.60a 68.51 ± 9.69a 4.20 ± 3.40b 3.98 ± 4.35b 27.94 ± 2.39b 25.99 ± 3.70b 7.05 ± 3.87b 11.97 ± 7.22b 0.15 ± 0.00a 11.24 ± 1.46b 17 days
1.47 ± 0.08b 0.34 ± 0.01b 70.14 ± 3.98a 73.62 ± 3.63b 1.74 ± 1.68a 1.56 ± 0.31a 26.86 ± 2.10a 22.12 ± 0.71a 4.32 ± 1.07a 4.10 ± 1.18a 0.21 ± 0.00b 5.67 ± 0.58a 45.5 h
a* b*
DE Reducing sugar (g/100 g) Peroxide value (meq O2/kg oil) Total drying time
External Internal External Internal External Internal External Internal
IMC: intermediate moisture content. Values are the mean of three replicates ± standard deviation. Different superscripts in the same line mean that the values are significant different (p 6 0.05).
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4. Conclusion A means to improve the quality of macadamia nut-in-shell was investigated in this study. The total drying time (of both stages) was in the range of 15–52 h with the shortest drying time observed in the case of the IMC of 11.1% (d.b.) and the use of the secondstage hot air drying temperature of 70 °C (15 h). The longest drying time was noted when the IMC was 8.7% (d.b.) and the second-stage hot air drying temperature was 50 °C (52 h). It was found that the drying temperature and moisture content of macadamia nut prior to the second-stage hot air drying had significant effects on the percentage of DP (p 6 0.05); higher drying temperatures led to higher change of peroxide value. The nut IMC of 8.7% (d.b.) led to higher percentage of DP at every drying condition except at 70 °C. However, the interaction between the drying temperature and IMC had no significant effect on the internal and external DE as well as the reducing sugar content (p > 0.05). Nevertheless, internal DE increased more significantly than the external DE when the second-stage hot air drying temperature was elevated. Based on the comparison between the commercially available product and the product obtained from the present hybrid drying process it could be concluded that the suitable drying conditions for macadamia nut-in-shell are the use of heat pump drying at 40 °C to decrease the moisture content to 11.1% d.b.; this should be followed by second-stage hot air drying at 50 °C until the moisture content was reduced down to 1–2% (d.b.). Combined heat pump and hot air drying shows benefit of time saving and natural quality preservation of macadamia nut. Acknowledgements The authors express their sincere appreciation to the Commission on Higher Education and the Thailand Research Fund (TRF) for supporting this study financially. Our appreciation also goes to the School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi for its support with the heat pump dryer. References AOAC, 1995. Official Methods of Analysis, 16th ed. Association of Official Analytical Chemists, Washington, DC. AOCS, 1998. Official Methods and Recommended Practices, fifth ed. American Oil Chemists’ Society Press, Champaign. Australian Macadamia Society, 2008. The Australian Macadamia Nut Industry.
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Cavaletto, C.G., 1981. Quality evaluation of macadamia nuts. Quality of Foods and Beverages: Chemistry and Technology, vol. 2. Academic Press, New York, pp. 71–82. Cavaletto, C.G., Dela Cruz, A., Ross, E., Yamamoto, H.Y., 1966. Factors affecting macadamia nut stability. 1. Raw kernel. Food Technology 20, 108–110. Chu, A.C., King, G.S., Sherman, G.D., 1953. Macadamia storage studies. Hawaii Agricultural Experiment Station Progress Notes, No. 90. Dominguez, I.L., Azuara, E., Vernon-Carter, E.J., Beristain, C.I., 2007. Thermodynamic analysis of effect of water activity on the stability of macadamia nut. Journal of Food Engineering 81, 566–571. Francesco, N., Luisa, S., Serafina, S., Gabriele, G., 2007. Oleic acid is a potent inhibitor of fatty acid and cholesterol synthesis in C6 glioma cells. Journal of Lipid Research 48, 1966–1975. Frankel, E.N., 2005. Lipid Oxidation, second ed. The Oily Press, Bridgwater. Grag, M.L., Rudra, P., Blake, R., Wills, R., 2003. Macadamia nut consumption lowers plasma cholesterol levels in hypercholesterolemic men. Journal of Nutrition 133, 1060–1063. _Ilhan, C., Mustafa, A., 2008. Modeling of a hazelnut dryer assisted heat pump by using artificial neural networks. Journal of Food Engineering 85, 841–854. Islam, Md.R., Mujumdar, A.S., 2008. Heat pump-assisted drying. In: Chen, X.D., Mujumdar, A.S. (Eds.), Drying Technologies in Food Processing. WileyBlackwell, West Sussex, pp. 190–224. Kaijser, A., Dutta, P., Savage, G., 2000. Oxidative stability and lipid composition of macadamia nuts grown in New Zealand. Food Chemistry 71, 67–70. Kowitz, T.J., 2004. Drying macadamia nut-in-shell on-farm. Doctoral Dissertation, School of Land and Food Sciences, University of Queensland, St. Lucia, Australia. Kowitz, T.J., Mason, R.L., Yong, G., 1998. Ambient air temperature and relative humidity can limit the on-farm drying of macadamia nut-in-shell. Australian Macadamia Society News Bulletin 25, 67–72. Kudra, T., Mujumdar, A.S., 2002. Advanced Drying Technologies. Marcel Dekker, New York. Mason, R.L., McConachie, I., 1994. A hard nut to crack. Food Australia 46, 466–471. Mason, R.L., Wills, R.B.H., 2000. Macadamia nut quality research: the proceeding challenge. Food Australia 52, 416–419. Moltzau, R.H., Ripperton, J.C., 1939. Processing of macadamia. Hawaii Agricultural Experiment Station. University of Hawaii, Bulletin No. 83. Namsanguan, Y., Tia, W., Devahastin, S., Soponronnarit, S., 2004. Drying kinetics and quality of shrimp undergoing different two-stage drying processes. Drying Technology 22, 759–778. Prichavudhi, K., Yamamoto, H.Y., 1965. Effect of drying temperature on chemical composition and quality of macadamia nuts. Food Technology 19, 121–128. Salmolin, L.A., Grosvenor, M.B., 2000. Nutrition Science and Application, third ed. Saunders College Publishing, Virginia. Silva, F.A., Marsaioli, A.J., Maximo, G.J., Silva, M.A.A.P., Goncalves, L.A.G., 2005. Microwave assisted drying of macadamia nuts. Journal of Food Engineering 77, 550–558. Sunthonvit, N., 2005. Effects of drying strategies on volatile compounds in stone fruit. Doctoral Dissertation, Food Science and Technology. University of New South Wales, Sydney, Australia. Teeboonma, U., Tiansuwan, J., Soponronnarit, S., 2003. Optimisation of heat pump fruit dryers. Journal of Food Engineering 59, 369–377. TISI, 2006. Thai Industrial Standards. Thai Industrial Standards Institute, Thailand, No. 1145/2549. Wall, M.M., Gentry, T.S., 2006. Carbohydrate composition and colour development during drying and roasting of macadamia nuts (Macadamia integrifolia). LWT – Food Science and Technology 40, 587–593. Weinert, A.G.I., 1993. Macadamia nut processing. The Southern African Macadamia Growers Association Yearbook. (July, 2008). Woodroof, J.G., 1979. Tree Nuts: Production, Processing, Products, second ed. AVI Publisher, Westport.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Improving quality of macadamia nut (Macadamia integrifolia) through the use of hybrid drying process Chaleeda Borompichaichartkul a,*, Kanonrat Luengsode a, Ninnart Chinprahast a, Sakamon Devahastin b a b
Department of Food Technology, Faculty of Science, Chulalongkorn University, Phayathai Road, Patumwan, Bangkok 10330, Thailand Department of Food Engineering, Faculty of Engineering, King Mongkut’s University of Technology Thonburi, 126 Pracha u-tid Road, Tungkru, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 20 November 2008 Received in revised form 29 January 2009 Accepted 29 January 2009 Available online 7 February 2009 Keywords: Color Drying kinetics Heat pump drying Hybrid drying Macadamia nut Peroxide value Rancidity Reducing sugar
a b s t r a c t Macadamia nut is rich in monounsaturated fatty acids and may reduce serum cholesterol when being included in human diet. On the negative side, high content of unsaturated fatty acids leads to oxidative reactions, which result in rancidity that decreases the quality of the nut. Drying is thus needed to reduce the nut moisture content and hence alleviate the above-mentioned problem. This research was conducted to determine a suitable strategy to reduce the moisture content of fresh macadamia nut to a safe level of 1–2% (d.b.). Hybrid drying process, namely, the use of heat pump drying (40 °C) followed by hot air drying (50–70 °C) was chosen to investigate its feasibility to dry macadamia nut in a relatively fast and economic fashion. The effects of the intermediate moisture content (IMC) (8–11% (d.b.)), which is the moisture content at the end of the first-stage heat pump drying, as well as the drying temperature on the drying kinetics and quality of the nut were determined. The quality of dried macadamia nut was assessed in terms of the moisture content, water activity (aw), color, reducing sugar content and peroxide value. The results showed that heat pump drying followed by hot air drying with IMC of 11.1% (d.b.) gave shortest drying time (15–45.5 h). The temperature and moisture content of the nut prior to the secondstage drying had significant effects on the peroxide value (p 6 0.05); higher drying temperatures led to higher peroxide values. However, the interaction between the drying temperature and IMC had no significant effect on the internal and external total color changes as well as on the amount of reducing sugar (p > 0.05). Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Macadamia tree (Macadamia integrifolia) belongs to the botanical family Proteaceae. Macadamia nut is rich in monounsaturated fatty acids, with oleic acid, which is claimed to be a potent inhibitor of fatty acid and cholesterol synthesis (Francesco et al., 2007), contributing to more than 70% of the total fatty acids. This unsaturated fatty acid can help decrease cholesterol and triglyceride levels, thus lowering the risk of heart disease (Grag et al., 2003; Salmolin and Grosvenor, 2000). The world production of macadamia nut is located in Australia (40%), USA (24%), South Africa (15%), South and Central America (12%) and others (9%) (Australia Macadamia Society, 2008). In Thailand, macadamia plantation is located mainly in the northern region. Since macadamia nut contains high amount of oleic acid, which is good for health, the price of macadamia nut is always high. The price of macadamia nut generally increases substantially when the nut is processed into candy, confectionery, nut butter and oil.
* Corresponding author. Tel.: +66 2 218 5534; fax: +66 2 254 4314. E-mail address: [email protected] (C. Borompichaichartkul). 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.01.035
Macadamia nut quality depends significantly on its moisture content as well as its water activity. Dominguez et al. (2007) reported that when the moisture content is in the range of 1.2– 1.6% (d.b.), which corresponds to aw of 0.36–0.44, macadamia nut has higher stability against lipid oxidation, changes in color and changes in texture. As macadamia nut contains high amount of unsaturated fatty acids it is prone to hydrolytic and oxidative rancidity when it contains high level of free moisture (Woodroof, 1979). Oxygen is also an important factor that triggers lipid oxidation in high-oil nut (Kaijser et al., 2000). In terms of the changes of the nut color browning is likely to occur when high moisture nut is dried at higher-temperature due mostly to non-enzymatic reaction (Maillard reaction). Cavaletto (1981) suggested that macadamia nut should not be dried at temperatures higher than 40 °C when the nut moisture content is higher than 8.7–11.1% (d.b.) to avoid high concentration of reducing sugar in the middle of the kernel, which could result further in center-browning of the kernel after drying. A high-quality macadamia nut product is characterized by low rancidity (peroxide value lower than 30 meq (milliequivalent) O2/kg) and light color (TISI, 2006). Freshly harvested macadamia nut typically has moisture content up to 81.8% (d.b.) in husk and 33.3% (d.b.) in nut-in-shell
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(NIS) (Chu et al., 1953; Kowitz et al., 1998). Therefore, dehusking must be done within 24 h to prevent heating of the nut from its respiration, metabolic and enzymic activities, which can cause deterioration of the nut quality (Moltzau and Ripperton, 1939). Drying is another most critical step in macadamia nut processing; the nut generally needs to be dried to 1.5% (d.b.) to increase the kernel recovery during cracking and to prevent flavor deterioration during storage (Weinert, 1993). Several methods of drying have been employed to dry macadamia nut such as in-bin drying (Kowitz, 2004), hot air drying (Prichavudhi and Yamamoto, 1965), combination of in-bin and hot air drying (Mason and McConachie, 1994) and heat pump drying (Kowitz, 2004). A major drawback of these methods is the long total drying time, resulting in a slow reduction of moisture content. The longer exposure to oxygen during drying, the more oxidation is likely to occur at high level of moisture content. Hence, rancidity is promoted. A typical industrial drying process employed for macadamia nut requires a very long total drying cycle, extending even to a period of more than one month (Silva et al., 2005). The first-stage pre-drying on-farm, which reduces macadamia nut moisture content down to around 8.7–11.1% (d.b.), takes three to four weeks. After that, up to six days (144 h) may be needed for the second-stage drying, which generally involves hot air drying starting at 40 °C and finishing at 60 °C; the kernel moisture content is reduced to 1.5% (d.b.) (Silva et al., 2005). To speed up the drying process combined hot air and microwave drying was attempted. In this case, hot air drying was used to dry fresh macadamia nut down to 11.1% (d.b.); microwave apparatus was then used to dry the nut further to the moisture content of 1.5% (d.b.) (Silva et al., 2005). Although the combined technique was faster, requiring only 4.5–5.5 h to dry macadamia, it required prohibitively high investment and operating costs. Another interesting option to speed up the drying process and, at the same time, enhance the quality of many food products is the use of heat pump drying (Teeboonma et al., 2003; Sunthonvit, 2005). A wide range of drying conditions, typically from 20 °C to 100 °C (with auxiliary heating), is feasible. Heat pump drying has main advantages of energy recovery from the exhaust and the ability to control temperature and humidity independently. Latent heat of vaporization is extracted from the exhaust moist air by vapor condensation at an evaporator coil (Fig. 1, No. 4), and the recovered energy is delivered as sensible heat to the drying
349
air stream passing through a condenser coil (Fig. 1, No. 5) (Islam and Mujumdar, 2008). Hence, there is a potential of using a heat pump dryer for drying macadamia nut. Many studies on heat pump drying of fruits and vegetables have indeed shown that it is an attractive option to preserve the quality of many food products _ including nuts, e.g., hazelnut (Ilhan and Mustafa, 2008), stone fruit (Sunthonvit, 2005), macadamia nut (Kowitz, 2004), as well as papaya and mango (Teeboonma et al., 2003). However, using only heat pump drying may add significantly the electricity cost to the production cost as the dryer depends mainly on electric power. More appropriate technique for drying macadamia nut should not only contribute to time saving but also preserves the natural quality of the nut. Using only one drying method may not be highly beneficial for these purposes. On the other hand, use of combined or hybrid drying process has ability to combine advantages of each drying technique. Combination of different drying processes usually offers unique advantages that single drying process can not achieve (Kudra and Mujumdar, 2002). For example, innovative combination of superheated steam drying and heat pump drying has proven to improve the quality of dried shrimp significantly (Namsanguan et al., 2004). The purpose of this research was to determine a suitable postharvest treatment of macadamia nut-in-shell using heat pump drying followed by hot air drying at different temperatures. The effects of the intermediate moisture content (IMC) and drying temperature on the drying kinetics and quality of the nut were determined. The quality of dried macadamia nut was assessed in terms of the moisture content, water activity (aw), color, reducing sugar content and peroxide value.
2. Materials and methods 2.1. Raw materials Macadamia nut (M. integrifolia) harvested in 2006/2007 harvesting season from Chiang Mai province in the northern part of Thailand was used in this study. The nut was dehusked within 24 h after being harvested; it was then stored with shell in nylon sacks prior to being transported to Chulalongkorn University in Bangkok. The unshelled nut was stored at 18 °C until the time of experiment.
Fig. 1. A schematic diagram of the laboratory dryer: 1-blower; 2-heater chamber; 3-drying chamber; 4-evaporator; 5-condensing unit; 6-valve; 7-valve; 8-cover; and 9-sample tray.
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2.2. Experimental set-up and drying experiments A laboratory heat pump dryer located at the King Mongkut’s University of Technology Thonburi, which could be operated in multi-mode, i.e., hot air drying and heat pump drying, is shown in Fig. 1. The dryer consists of a drying chamber with dimensions of 0.52 0.52 0.52 m. A wire-screen tray with dimensions of 0.28 0.33 m was placed in a perpendicular direction to the airflow. The drying temperature was controlled by a proportionalintegral-differential (PID) controller with an accuracy of ±1 °C while the air velocity (over a cross section of 0.52 0.52 m) was fixed at 0.3 m/s in all experiments. The power of the condenser and evaporator of the heat pump dryer was 3.5 and 2.0 kW, respectively. The first-stage heat pump drying was performed at the temperature of 40 °C and the relative humidity of 20% in order to decrease the nut-in-shell (NIS) moisture content from an initial value of 20– 22% (d.b.) to the IMC of either 8.7 or 11.1% (d.b.). Instantly after the first-stage drying, the process was switched to hot air drying; different temperatures (50, 60 and 70 °C) were used for hot air drying. The drying sample was weighed continuously until the final moisture content reached 1.5–2.7% (d.b.) as suggested by the Australian Macadamia Society (2008). For the conventional industrial method fresh NIS was subjected to hot air drying at 35 °C for 7 days; this was followed by drying at 40 °C for 7 days and 60 °C for another 3 days using a tray dryer. Dried nut from each drying method was kept in a vacuum sealed laminated aluminum packet and stored at 18 °C until further analysis. 2.3. Physical quality analysis Moisture content of nut-in-shell was determined following the method suggested by Wall and Gentry (2006) with some modification. Ten grams of the sample was ground and placed in a vacuum oven at 70 °C, 90 mbar for 24 h. Each measurement was done in triplicate and the average percentage of moisture content on a dry weight basis (g/100 g) was reported. Water activity measurement was performed using AquaLab water activity meter (AquaLink 3.0, Pullman, WA). Calibration of the water activity meter was done using distilled water to obtain aw in the range of 1.00 ± 0.003. Color Flex (HunterLab 45°/0°, Reston, VA) colorimeter was used to measure the external and internal colors of fresh, rewetted and dried kernels to determine the changes of color, in terms of L*, a* and b*, with the rewetting condition, drying condition and storage time. The total color difference (DE) was also calculated according to Eq. (1). Twenty kernels were used for each color analysis.
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðDL Þ2 þ ðDa Þ2 þ ðDb Þ2 :
boiled in boiling water for 10 min. The sample was then cooled down and 10 mL cool distilled water was added. The absorbance of the sample was then measured at 540 nm. The absorbance obtained was compared with the standard invert sugar curve. The measurement was done in triplicate. Reducing sugar of the nut from each condition was compared with the initial value before drying; the results were expressed as the percentage of change. 2.5. Peroxide value analysis Macadamia nut oil was extracted from 30 g ground nut sample by Soxhlet extraction equipment (Soxtherm, Gerhardt, Königswinter, Germany) using petroleum ether as a solvent for 4 h. Oil was concentrated by eliminating excess solvent with a rotary evaporator (Eyela, Miyagi, Japan) at 45 °C for 45 min. The peroxide value (PV) was then determined following the AOCS method Cd8-53 (1998). The PV of macadamia nut oil from each condition was compared with the initial value of oil from raw macadamia nut before drying and expressed as the percentage of change. The experiment was performed in triplicate. 3. Results and discussion 3.1. Drying kinetic of macadamia Figs. 2 and 3 show the drying curves of macadamia nut subjected to first-stage heat pump drying at 40 °C. The moisture content of the nut was reduced from an initial moisture content of 20–21% (d.b.) to either 8.7 or 11.1% (d.b.). This was followed by second-stage hot air drying at 50, 60 and 70 °C to further reduce the moisture content from the first-stage values to 1.5–2.7% (d.b.). The drying time of the nut to the IMC of 8.7 and 11.1% (d.b.) during heat pump drying was 12.4 and 7.1 h, respectively (Table 1). Nut with an IMC of 11.11% (d.b.) required less drying time during heat pump drying and was subjected to second-stage drying before the nut with an IMC of 8.7% (d.b.). This led to shorter total drying time for nut with an IMC of 11.1% (d.b.) in both stages compared with the nut with an IMC of 8.7% (d.b.). This is because the drying temperature in the second-stage was higher, resulting in a higher drying rate. On the other hand, a higher IMC would lead to a longer drying time if the temperature was reduced in the second-stage drying (Namsanguan et al., 2004). Increasing of the drying temperature expectedly promoted drying rate in all cases. Macadamia nut dried at 70 °C reached its equilibrium moisture content faster comparing to the drying temperatures of 50 °C and 60 °C.
ð1Þ
The calibration of the colorimeter was done using a white calibration sheet to obtain the standard values of X = 78.89 ± 0.3, Y = 83.78 ± 0.3, Z = 87.74 ± 0.3. 2.4. Reducing sugar analysis The amount of reducing sugar, calculated on weight basis (g/ 100 g), was determined following the DNSA (dinitrosalicylic acid) method (AOAC, 1995). Defatted macadamia kernels (2.5 g) was extracted with 100 mL of 80% (v/v) ethanol mixed with 100 mL water. The sample was heated for 25 min at 80–85 °C in a water bath with occasional stirring. The sample was then cooled down and filtered (Whatman No. 1 paper) into 100 mL volumetric flask; the volume was then adjusted with 80% (v/v) ethanol to 100 mL. One milliliter of the sample was taken and mixed with 1 mL of DNSA in a test tube. The mixed sample-DNSA was shaken and
25
HA50 IMC8.7
Moisture content (%d.b.)
350
HA60 IMC8.7 20
HA70 IMC8.7
15
10
5
0 0
10
20
30
40
50
60
Time (h) Fig. 2. Drying curves of macadamia nuts with intermediate moisture content of 8.7% (d.b.). HA: hot air, IMC: intermediate moisture content.
351
25
25
20
20
external color
d internal color
15
HA60 IMC11.1 HA70 IMC11.1
10
bc cd
HA50 IMC11.1
Kernel ΔE
Moisture content (% d.b.)
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abc
abc
abc
abc
15
ab
a
a a
ab 10
5
5
0
0 0
10
20
30
40
50
Time(h) Fig. 3. Drying curves of macadamia nuts with intermediate moisture content of 11.1% (d.b.). HA: hot air, IMC: intermediate moisture content.
HA50
HA60
HA70
HA50
HA60
HA70
IMC8.7
IMC8.7
IMC8.7
IMC11.1
IMC11.1
IMC11.1
Drying condition Fig. 4. Total color difference (DE) of dried external and internal kernels. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
3.2. Quality of macadamia nut Moisture content and water activity of macadamia kernel after drying at each condition are shown in Table 1. It was observed that lower drying temperature in the second-stage resulted in lower NIS moisture content and aw. At higher drying temperature it might be possible that water was removed rapidly resulting in surface hardening and blockage of the rest of water in the kernel to move out. Therefore, the nut moisture content remained high. The change of the internal kernel color was more pronounced than the change of the external color (Fig. 4). This is because the moisture content in the internal kernel was higher than that at the surface. Therefore, water dissolved sugar toward the inside of the kernel; non-enzymatic browning reaction could thus occur more intensively. It is noted that brown pigments are the result of the reaction between reducing sugars and amines in the kernel; the protein content of macadamia nut was around 7.8–9.2 g per 100 g kernel (Weinert, 1993). As drying temperature increased more change of color was observed (increase of DE, see Fig. 4). At an IMC of 8.7% (d.b.) the changes of internal and external colors were not significantly different at any drying temperature. However, at an IMC of 11.1% (d.b.) DE sharply increased when the temperature increased from 60 °C to 70 °C. At an IMC of 11.1% (d.b.) water molecules were more mobile than at an IMC of 8.7% (d.b.). Elevation of temperature and higher availability of water promoted browning reactions, resulting in marked changes of color. An increase in the temperature could promote hydrolysis of sucrose in the kernel to glucose and fructose, resulting in more intensive browning of the nut (Wall and Gentry, 2006). Therefore, to avoid internal browning it is recommended that the drying temperature in the second-stage be maintained at not more than 60 °C. However, when the IMC was 11.1% (d.b.) hot air temperature of 50 °C led to the least change of the internal overall color. For the nut that had an IMC of 11.1% (d.b.) its residence time (7.1 ± 0.8 h)
in the first-stage heat pump drying was shorter than the nut with an IMC of 8.7% (d.b.) (12.4 ± 1.2 h). Browning was then observed at a much lower extent. The result of DE from each drying condition corresponded to the amount of change of reducing sugar that remained in the kernel (Fig. 5). When the second-stage hot air drying temperature was low more reducing sugar remained because more reducing sugar was used in the Maillard reaction to form brown pigments during higher-temperature drying. From Fig. 6 it was observed that change of the peroxide value in the nut was the highest (>600%) when the IMC of 11.1% (d.b.) and the second-stage drying temperature of 70 °C were used. It was observed that when using an IMC of 8.7% (d.b.) macadamia nut could be dried at any temperature (50–70 °C) with not much difference in lipid quality. On the other hand, when using an IMC of 11.1% (d.b.) there was a limit to dry the nut at temperature only up to 60 °C; otherwise, the nut lipid quality would be significantly altered as lipid oxidation could be highly accelerated when the temperature and moisture content increased (Cavaletto et al., 1966). At an IMC of 8.7% (d.b.) free moisture content was lower than in the case of IMC of 11.1% (d.b.); oxidation slowly occurred leading to lower formation of peroxides. However, the changes of the peroxide values of the nuts with an IMC of 11.1% (d.b.) after second-stage drying at 50–60 °C were lower than those of the nuts with an IMC of 8.7% (d.b.). This is due to shorter residence time of the nuts with an IMC of 11.1% (d.b.) during heat pump drying. Lipid oxidation involves three-stage series of reactions including initiation, propagation and termination (Frankel, 2005). Once extremely reactive free radicals from fatty acid molecules are produced during the initiation step they would react with oxygen to form peroxyl radicals (ROO). These peroxyl radicals in turn react with unsaturated lipids to form hydroperoxides (ROOH), which could be detected and
Table 1 Drying time and properties of macadamia nut subjected to heat pump drying at 40 °C with different intermediate moisture contents and second-stage drying temperatures. First-stage drying/intermediate moisture content (% d.b.)
Second-stage hot air drying temperature (°C)
Final NIS moisture content (% d.b.)
Final aw
PV (meq O2/ kg oil)
Drying time in heat pump dryer (h)
Total drying time (h)
HP40/IMC8.7
50 60 70
1.91 ± 0.39 3.53 ± 0.39 2.73 ± 0.36
0.58 ± 0.02 0.69 ± 0.01 0.68 ± 0.02
4.79 ± 0.01 5.75 ± 0.22 5.39 ± 0.11
12.4 ± 1.2
52.3 28.5 22.7
HP40/IMC11.1
50 60 70
1.96 ± 0.27 2.50 ± 0.11 3.20 ± 0.52
0.60 ± 0.01 0.63 ± 0.02 0.69 ± 0.02
4.00 ± 0.01 4.27 ± 0.06 13.58 ± 0.07
7.1 ± 0.8
45.5 22.8 15.0
HP: heat pump, IMC: intermediate moisture content.
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Fig. 5. Percentage of change in reducing sugar (DR) of dried nut compared with raw nut at different drying conditions. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
Peroxide value ΔP (%)
700
f
600 500 400 300
e c
200
d
b
a
100 0 HA50 IMC8.7 HA60 IMC8.7 HA70 IMC8.7
HA50 IMC11.1
HA60 IMC11.1
HA70 IMC11.1
Drying condition Fig. 6. Percentage of change in peroxide value (DP) of dried nut compared with raw nut at different drying conditions. HA: hot air, IMC: intermediate moisture content. Different superscripts on each bar mean that the values are significant different (p 6 0.05).
measured as peroxide value. More production of hydroperoxides implies higher level of auto-oxidation. Nevertheless, hydroperoxides are not stable and can decompose further under accelerated condition (temperature > 100 °C, the presence of metal and expose to light) (Frankel, 2005). The decomposition of lipid hydroperoxides produces carbonyl compounds, alcohols and hydrocarbons. Therefore, more formation of hydroperoxides would give a fair prediction of rancidity of the product, which contains high unsaturated fat content. The chain reaction can continue until free
radicals reach sufficiently high concentrations; they tend to react together during the termination step. Therefore, after drying, macadamia nut should have as low value of peroxide as possible. This implies that the level of primary stage oxidation would be low. Lipid quality of the nut could thus be maintained for further processing and during storage. However, since the peroxide value from each drying condition (Table 1) was within the standard limits (<30 meq O2/kg oil, TISI (2006)), the drying process was acceptable. Based on the quality observation above it could be suggested that the best condition for drying macadamia nut is the use of heat pump drying at 40 °C to reduce the initial moisture content of the nut to 11.1% (d.b.). This should be followed by second-stage hot air drying at 50 °C until the moisture content reaches 1–2% (d.b.). This would give the best quality of the nut, both in terms of color and lipid quality. The results from the best aforementioned conditions were also compared with those from the conventional industrial method, i.e., hot air drying at 35 °C for 7 days followed by drying at 40 °C for 7 days and 60 °C for another 3 days (Australian Macadamia Society, 2008). The quality observations are displayed in Table 2. The overall quality observations showed that hybrid drying process, i.e., the use of heat pump drying in combination with hot air drying, helped maintain nut quality, especially in terms of the prevention of lipid deterioration, better than the conventional industrial method. The total drying time was shorter and color appearances of both external and internal kernels were also more satisfactory. Internal L* value of dried macadamia kernel from the hybrid drying method was significantly higher than that of the commercial product. Internal color of dried macadamia kernel is indeed one of the quality indices to grade dried macadamia nut; brown-center nut is often discarded or classified as low-grade nut (Weinert, 1993). Although the commercial product was subjected to lower drying temperature during the first-stage drying (35 °C, 7 days) than in the case of the hybrid drying method (40 °C, 7 h), the color quality of the kernel was inferior. This is because of the long drying time at high moisture content (20–21% (d.b.)) leading to an increase in non-enzymatic browning. This could be confirmed by the amount of the remaining reducing sugar; the commercial dried nut had significantly lower amount of reducing sugar. The similar results were also observed in the cases of a*, b* and DE. Finally, it was observed that the peroxide value of the dried nut from the hybrid drying process was significantly lower than that of the commercial product, i.e., the figures being at half. Mason and Wills (2000) indeed emphasized that drying is an important step to be considered to prevent any hydrolytic rancidity or mould development after harvesting of macadamia nut.
Table 2 Quality of dried macadamia nuts by industrial method and from combined heat pump and hot air drying. Parameter
Commercial product
Combined heat pump (40 °C, IMC 11.1% (d.b)) and hot air dried (50 °C) product
Moisture content NIS (% d.b.) aw Color L*
1.03 ± 0.06a 0.29 ± 0.00a 68.31 ± 7.60a 68.51 ± 9.69a 4.20 ± 3.40b 3.98 ± 4.35b 27.94 ± 2.39b 25.99 ± 3.70b 7.05 ± 3.87b 11.97 ± 7.22b 0.15 ± 0.00a 11.24 ± 1.46b 17 days
1.47 ± 0.08b 0.34 ± 0.01b 70.14 ± 3.98a 73.62 ± 3.63b 1.74 ± 1.68a 1.56 ± 0.31a 26.86 ± 2.10a 22.12 ± 0.71a 4.32 ± 1.07a 4.10 ± 1.18a 0.21 ± 0.00b 5.67 ± 0.58a 45.5 h
a* b*
DE Reducing sugar (g/100 g) Peroxide value (meq O2/kg oil) Total drying time
External Internal External Internal External Internal External Internal
IMC: intermediate moisture content. Values are the mean of three replicates ± standard deviation. Different superscripts in the same line mean that the values are significant different (p 6 0.05).
C. Borompichaichartkul et al. / Journal of Food Engineering 93 (2009) 348–353
4. Conclusion A means to improve the quality of macadamia nut-in-shell was investigated in this study. The total drying time (of both stages) was in the range of 15–52 h with the shortest drying time observed in the case of the IMC of 11.1% (d.b.) and the use of the secondstage hot air drying temperature of 70 °C (15 h). The longest drying time was noted when the IMC was 8.7% (d.b.) and the second-stage hot air drying temperature was 50 °C (52 h). It was found that the drying temperature and moisture content of macadamia nut prior to the second-stage hot air drying had significant effects on the percentage of DP (p 6 0.05); higher drying temperatures led to higher change of peroxide value. The nut IMC of 8.7% (d.b.) led to higher percentage of DP at every drying condition except at 70 °C. However, the interaction between the drying temperature and IMC had no significant effect on the internal and external DE as well as the reducing sugar content (p > 0.05). Nevertheless, internal DE increased more significantly than the external DE when the second-stage hot air drying temperature was elevated. Based on the comparison between the commercially available product and the product obtained from the present hybrid drying process it could be concluded that the suitable drying conditions for macadamia nut-in-shell are the use of heat pump drying at 40 °C to decrease the moisture content to 11.1% d.b.; this should be followed by second-stage hot air drying at 50 °C until the moisture content was reduced down to 1–2% (d.b.). Combined heat pump and hot air drying shows benefit of time saving and natural quality preservation of macadamia nut. Acknowledgements The authors express their sincere appreciation to the Commission on Higher Education and the Thailand Research Fund (TRF) for supporting this study financially. Our appreciation also goes to the School of Energy, Environment and Materials, King Mongkut’s University of Technology Thonburi for its support with the heat pump dryer. References AOAC, 1995. Official Methods of Analysis, 16th ed. Association of Official Analytical Chemists, Washington, DC. AOCS, 1998. Official Methods and Recommended Practices, fifth ed. American Oil Chemists’ Society Press, Champaign. Australian Macadamia Society, 2008. The Australian Macadamia Nut Industry.
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