J. ugric. Engng Rex (1989) 43, 103-111

A Macadamia Nut Curing System for Improving Kernel Recovery

TUNG LIANG*; SUHAS K. MEHRA; M. A. KHAN

The outcome of macadamia nut kernel recovery operations depends on the nut moisture prior to cracking for kernel extraction. The batch driers used throughout the macadamia industry cannot efficiently cure macadamia nuts to the desired moisture content owing to lack of an easy to use moisture sensor and a mechanism to release the layer of nuts which has reached the desired moisture content.

The temperature gradient in a batch drier column of macadamia nuts was found to be closely correlated with the moisture content of the nuts in the column as long as the velocity, relative humidity and temperature of the drying air were kept constant. The temperature gradient between a given thickness of nut layer was found to provide a convenient means for monitoring macadamia nut moisture without disturbing the nut column. An innovative gate capable of releasing the layer of nuts immediately above the gate without excessively disturbing the nut column was also developed. The temperature gradient based moisture sensor, coupled with the innovate gate, transformed a batch drier into a device capable of curing nuts to the desired moisture content for higher kernel recovery.

1. Introduction

Liang’ reported excessive kernel chips and low kernel recovery in a macadamia nut processing plant and suggested several changes to improve the nut cracking operation for higher kernel recovery. One of those changes was to cure in-shell macadamia nuts to a uniform kernel moisture content of 3% d.b. prior to cracking. Tang et aL2 reported improved kernel recovery by subjecting in-shell macadamia nuts to flash drying, which achieved low shell moisture content while the kernel moisture content was 6% d.b. prior to flash drying. The flash drying process eventually brought the nut kernels to 3% m.c.d.b. Both findings suggested a need for curing macadamia nuts to a predetermined moisture content for higher kernel recovery during cracking.

Most macadamia nut processors use batch column driers of circular or rectangular cross-section. A moisture gradient exists in these systems until equilibrium moisture content corresponding to the drying air is reached. The nut layer that the curing air passes through first always reaches the equilibrium moisture content first. The nut layer that the curing air passes through last also reaches the equilibrium moisture content last. These layers will be referred to as the first and the last layer hereafter. In order to reduce the curing time and to increase the curing capacity of nut driers, curing air at the maximum allowable temperature of 49°C (Prichavudhi and Yamamoto3) has been used by most Hawaiian processors. Depending on the amount of moisture in the air, nuts are dried to less than 1.5% d.b. A device capable of monitoring the moisture content of the first layer of nuts could help to avoid overdrying by signalling the time at which the layer of nuts first cured should be removed from further drying. A mechanism for removing the first layer of nuts immediately after these nuts reach the desired moisture content is also needed. The mechanism should allow the rest of the nut column to move without mixing towards the void vacated by the first layer of nuts. The batch systems would then become

* Department of Agricultural Engineering, University of Hawaii at Manoa, Honolulu, HI 96 822, USA

Received 8 July 1988; accepted in revised form 12 March 1989

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0021-8634/89/060103 + 09 %03.00/O @ 1989 The British Society for Research in Agricultural Engineering

104 A MACADAMIA NUT CURING SYSTEM

a semi-continuous drying system as fresh nuts enter the drying column at the last layer and eventually leave the drier by passing through the first layer space. A curing system such as this was expected to minimize nut overdrying and at the same time to increase the throughput of the drier.

2. Moisture monitoring

As curing air flows through a batch drier, the macadamia nuts in the drying column lose their moisture by the process of evaporation, which in turn cools the curing air. The amount of evaporation responsible for cooling the curing air depends on the available moisture of nuts for evaporation and the curing air condition. Usually, nuts closer to the source of drying air lose moisture sooner than those farther away. The air temperature decreases along the air stream flow direction. This temperature gradient gradually disappears as the drying front moves from first to last layer. Positioning two temperature probes at A and B (Fig. 1) was thought to be a possible approach to monitor the nut moisture content when a constant quality drying air is used. In other words, the difference between the temperature measured at A and B should indicate how much moisture remains in the layer of nuts between the two points. In theory, a quantifiable correlation between the temperature difference and the absolute moisture of macadamia nuts should exist. The question remaining to be answered was whether the relationship was precise enough for the purpose of monitoring the nut moisture content in the A to B layer.

2.1. The experimental curing system

For evaluating the moisture monitoring approach, an experimental closed-loop semi-continuous curing system (Fig. 1) was constructed. The cross-section of the curing column was O-4 m by O-4 m and 1.2 m high. This column could hold approximately 110 kg of in-shell macadamia nuts. At the bottom of this column, a wire gate was installed to facilitate the release of properly cured macadamia nuts without excessive mixing of nuts remaining in the drying column. At 0.15 m and 0.43 m from the wire gate were located

Probe for air flow role

\,Flow regulottng ton / Nut Inlet

8 Temperature probes

A Gate I (cable gate)

Gate 2 (nut outlet)

Weight balance By-pass chamber

Fig. 1. Schematic drawing of the curing system

T. LIANG ET AL. 105

probes “A” and “B” respectively. These probes were employed to measure the difference in the temperature between the two locations. For collecting samples for nut kernel moisture content determination, three locations 1,2 and 3 were selected at 0.14, 0.3 and 0.42 m respectively from the wire gate. Nuts were loaded at the top of the column by using a conveyor belt.

A 0.57 kW blower was used to force conditioned air into the column. A small proportion of the air was passed into a bypass chamber, in which a weight balance was placed. Nuts similar to those initially placed inside the column were placed on this weight balance. The change of weight recorded on the balance provided the nut drying rate which is used as a guide for determining when to take nuts out of the column for moisture determination. Air, after passing this chamber, was directed back into the column. Exhaust air from the column was treated depending upon its condition, either by the humidifier or the dehumidifier. Both the humidifier and the dehumidifier were in- dividually regulated by a humidistat. The treated air was then returned to the blower for recirculation.

2.2. Experiment

Variables believed to be relevant to the evaluation of the proposed moisture monitoring method were as follows.

(a) Curing air temperature. Air temperature was measured at the blower outlet by using a Hewlett Packard Module #17 502 A, having a resolution of fl”C. All experiments were conducted in the temperature range 37.8 to 62°C.

(b) Curing air relative humidity (Rh). Relative humidity of the curing air was calculated on the basis of dry and wet bulb temperatures measured at the blower outlet. These temperatures were also measured by using the Hewlett Packard Module #17502A. The dry and the wet bulb temperatures were then used as input to the psychometric models proposed by Wilhelm4 for computing relative humidity.

(c) Curing air velocity. The air passing through the drying column was adjusted to 6 to 7 m/s for most experiments. A Weathermeasure Corporation E141-A hot wire anemo- meter with an accuracy of f3% was used for air velocity measurement.

(d) Moisture content. The oven method was used for the determination of kernel moisture content. Samples of five nuts each were collected from each of the three locations (1,2, and 3). These nuts were cracked and the sample weights were recorded to the fourth decimal. Nuts were then kept in the oven at 70°C for 96 h. Again, the samples were weighed to the fourth decimal place. Moisture content was calculated from the expression:

where

MC= WV- Wd K

W, = Initial weight of the nuts. W, = Weight after keeping the nuts in the oven for 96 h.

(e) Temperature difference (AT). The difference in temperatures between A and B locations, AT, was measured using thermistors by Omega Engineering Inc., having an accuracy of f 1°C.

Four curing experiments were conducted for various combinations of curing air temperature, relative humidity and air flow rate. Each experiment, on average, lasted 3-4 days and approximately 20 sets of data were collected. The temperature difference, AT and nut kernel moisture content, MC, were fitted by the following equation:

MC=b,,+b,log,(AT+b,)

106 A MACADAMIA NUT CURING SYSTEM

0123456709

Temperature difference (AT), “C

Fig. 2. Kernel moisture content versus temperature difference. MC = 4.21 log, (AT + 1 .O) with R2 = 0.98 while curing air temperature = 46”C, velocity = 6-6 m/s and relative humidity = 76%

II-

IO-

9-

: a-

z - 7- 73

a

OL I I I I I I I I I 01234567091

Temperature difference (AT), “C

Fig. 3. Kernel moisture content versus temperature difference. MC = 2.8 log, (AT + l-0) with R2 = 0.99 while curing air temperature = 42”C, velocity = 6-l m/s and relative humidity = 82%

T. LlANG ET AL. 107

ob I I I I I I I I I 01234567091

Temperature dlfference (AT), 'C

Fig. 4. Kernel moisture content versus temperature difference. MC =4.1 log, (AT + 1-O) with R2 = 0.99 while curing air temperature = 54”C, velocity = 7-l m/s and relative humidity = 71%

01 I I I I I I I I I I I I 01234567091 6 7 0

Temperoture difference (AT), OC

Fig. 5. Kernel moisture content versus temperature difference. MC = 2.2log, (AT + 1 .O) with R2 = O-94 while curing air temperature = 62”c, velociv = 15m/s and relative humidity = 65%

108 A MACADAMIA NUT CURING SYSTEM

The equation was in fact forced to pass through the origin, that is, b,, was set equal to zero. The best bz parameter for all four experiments was found to be 1. The data points and the fitted equation are shown in Figs 2-5.

2.3. Results The standard errors of the parameter b1 for the four experiments were O-04, O-13, 0.13

and 0.14. Since the equation will be used to determine when the nut will be cured to 3% moisture content which is known to be the best condition for kernel extraction, it is important to estimate the random error of b, on nut moisture prediction. Corresponding to the moisture content of 3-O%, the AT values are approximately l-04, 1.92, 1.08 and 2.91”C (Figs 2-5). For AT = 2.91”C or log, (2.91 + 1.0) = 1.36 and twice the standard error for 6, of 0.28 (2 x 0*14), the equation will generate a moisture prediction error equal to 0.28 x 1.36% = 0.38%. Based on past cracking or kernel extraction experience, this error is tolerable. This error was probably caused by the experimental curing system using only on-off controls to maintain its temperature and humidity. The system, nevertheless, could maintain the curing air temperature and humidity within a tolerable range. Figs 6 and 7 show the fluctuation of these two parameters of the curing air measured at the outlet of the blower. These fluctuations can be reduced if more sophisticated controls are used. The error analysis did not support the extra investment. Nevertheless, AT of the first layer is highly correlated to the moisture content of the macadamia nut kernel in the same layer. Therefore, the equation can be used to monitor the kernel moisture by simply measuring AT. However, the attempt to incorporate curing air parameters such as air velocity, temperature and relative humidity into a model was not successful. The equation should be calibrated for the condition of the air used and it is expected that calibration will be necessary for each installation.

15L I I I I I I I 0 IO 20 30 40 5C 60 70 13O+idd&

Time, mm

Fig. 6. Curing air temperature at air velocity 8m/s

T. LIANG ET AL. 109

50; I I I I I I I I I I , 1 0 IO 20 30 40 50 60 70 80 90 100 I IO 120

Fig. 7. Curing air relative humidity at air velocity 84s

Macadamia nut processors use constant temperature air for curing. The air’s relative humidity fluctuates. In order to use the relationship between MC and AT for indirectly measuring MC, the air’s relative humidity must be controlled approximately constant. The requirement on humidity fluctuation is not stringent and can be met rather easily (Fig. 7). The application of MC and AT equation for MC estimation requires the equation be calibrated for the selected air condition. However, for reducing nut curing time and minimizing monitoring as macadamia nuts approach the desired MC, it is preferable to complete the curing process with two different types of air. A high temperature and low relative humidity air should be used first to remove most of the water in the nut and the process should be finished by using air which produces nuts with nut moisture content equal to the desired MC. In either case, there is no need to keep a whole series of calibrated MC and AT equations.

3. Nut gate

The task of releasing the first layer of nuts is different from that of emptying the entire batch-drier column. It is essential to avoid excessive movement among nuts between layers so that none of the nuts in the first layer space will be left behind. Mixing of nuts between other layers should also be avoided to prevent mixing nuts of different moisture content. In fact, it is desirable to slide the remaining nut column as a whole toward the space vacated by the discharge of the first layer of nuts. Fresh nuts are then used to fill up the void in the last layer space. The gate designed for achieving this purpose must be able to be closed without crushing nuts.

For a vertical drying column such as shown in Fig. 1, the friction between the stationary walls of the drier column and the nuts being lowered is generally greater than that between adjacent nuts. Therefore, nuts in the centre of the drying column move through the gate before the nuts located closer to the wall. The moving front assumes a convex shape. The only way to avoid discharging incompletely cured nuts is to stop discharging when the centre portion of the cured nuts is emptied. Many nuts already cured and situated away from the centre will remain in the drying column and occupy useful drying space. Therefore, a gate capable of avoiding this convex front is essential.

Several designs were tried without success. The mechanical opening and closing of a passage usually involves sliding or rotating a rigid piece either away or towards another. A closure based on these principles has a propensity to crush and damage nuts. An

110 A MACADAMIA NUT CURING SYSTEM

innovative approach to gate opening and closing was therefore developed. Thin wire ropes strung parallel to each other at a spacing smaller than the smallest nut were tried to see if these would support a column of nuts. The wire ropes were tensioned according to column weight. A mechanism was provided to release the tension of two adjacent wires while wires on either side of these two released wires were kept under tension. In other words, one-third of the wires remained under tension and two-thirds were released to create openings which allowed nuts to drop through the wire gate uniformly across the entire nut column cross-section. The column was divided equally into three sections of which each was filled with nuts sprayed white, red and brown. The experimental gate was opened and closed many times. Mixing of nuts with different

Fig. 8. (Top) Gate closed; (Bottom) Gate opened

T. LIANG ET AL. 111

colours was minimal. The borderline between red and white nuts as well as red and brown nuts did not exhibit any significant curvature. The gate seemed to be capable of accomplishing its design objective. The wire gate is shown in Fig. 8. The cross-section of this particular gate was 419.1 mm square. It may be necessary to add rigid support members if a larger column cross-section gate is needed.

4. Conclusion

The importance of controlling the moisture content of in-shell macadamia nuts for kernel extraction has been known for a long time. Using conventional macadamia nut driers to achieve the desired moisture content had not been successful due to lack of moisture monitoring and a suitable nut discharging mechanism. The temperature gradient along a drying column was shown to be a suitable indirect parameter for monitoring macadamia nut moisture content for known drying air conditions. A wire gate capable of releasing the nuts that reached the desired moisture content made it possible to design a drying or curing system for control of macadamia nut moisture content. The temperature probes and the gate can be added to existing batch drying systems.

References

’ Liang, T. A new processing system for maximizing macadamia nut kernel recovery. Transactions of the ASAE 1977, m(3): 438-443

* Tang, G. P.; Snobar, B.; Liang, T. Flash drying macadamia nuts for improved kernel extraction. Transactions of the ASAE 1982, 25(6): 1733-1735

3 Prichavudhi, K.; Yamamoto, H. K. Effect of drying temperature on chemical composition and quality of macadamia nuts. M.Sc. Thesis. University of Hawaii, Honolulu, 1965, pp. 28

’ Wilhelm, L. R. Numerical calculations of psychrometric properties in SI units. Transactions of the ASAE 1976, 19(2): 318-321, 325