Abstract

Moisture content / air humidity isotherms were gene- rated for oil palm kernels from three major producing areas of Costa Rica. The assays comprised a moisture range from 4 to 15% w.b. and two temperatures (25 and 32 °C). A protimeter was used to measure humidity and an oven method (12 hours / 120 °C) for kernel moisture determinations. Isotherms for the two temperatures tested at each location showed only minor differences, as did isotherms from the three locations. A polynomial model (three terms) best fit the experimental data, and was used to draw a general moisture/humidity equilibrium curve for oil palm kernels. A chart derived from this model is recommended to be used locally as a guide for choosing the conditions for proper drying and storing of this type of kernel.

Introduction

Water vapor pressure from dry agricultural products varies with the nature of the material and its moisture content. When the product's vapor pressure (VP) is equal to the surrounding atmosphere's VP, its moisture is known as equilibrium moisture content (EMC) and the air's relative humidity (RH), as equilibrium relative humidity (ERH).

Different products even under the same temperature and RHshow different CMCvalues. These variations in the EMC values is mainly due to differences in chemical composition, handling history and the accuracy of the measuring techniques.

Moisture content and air humidity differences are found among varieties of the same botanical species, and even within kernels of a single lot of grains, depending on their physiological stage when harvested and postharvest handling (Brooker, Bakker-Arkema and Hall, 1981). For example, when some grains reach a temperature of 60 °C or higher during artificial drying, the EMC of products like corn are altered by 0.5% to 1%, as a result of small chemical changes (Tuite and Foster, cited by Foster, 1982).

Oil seeds like oil palm kernels have a much smaller EMC than cereals. This is basically the result of their high oil content, since all water is in the hydrophilic portion (Booth, 1984) wich is proportionaly less here.

There is a lack of information on the moisture/ humidity equilibrium of several agricultural products. In Gough and Lippiatt's (1977) study, there is only one reference to oil palm kernels, Somade (1955). In this study, kernels from Nigeria with 5.6% moisture reached equilibrium with ambient air at 68.6% RH after a drying (desorption) process. When equilibrium was reached by adsorpting water from air, also at 68.6% RH, similar kernels had an EMC of 4.9%. In these tests, ambient temperature ranged from 24.5 to 31 °C. From these results, the existence of a histereatic effect in oil palm kernels was clear.

This research was conducted with the main objective of generating moisture content / air humidity isotherms for oil palm kernels produced in Costa Rica.

Materials and Methods

Sample origin and preparation

Samples undertaken from three extraction mills in Costa Rica. One is situated on the South Pacific Coast (Coto), and two on the Central Pacific Coasts (Quepos: Naranjo and Palo Seco). Samples were collected from the mills right after the kernels had undergone all processing stages but drying. Initial moisture content in the samples was over 20% wet base (WB). The samples were sent to the Grains and Seeds Research Center (CIGRAS, Agronomy Department) of the University of Costa Rica, for cleaning (removing of fibers, endocarp fragments, kernel fragments and any other contaminant).

Lots of kernels of 10 kg each were dried at 60 to 80 °C, in a convection oven to simulate normal operating conditions during mill processing. Every 15 to 30 min, 400 g subsamples were taken, which moisture content ranged from 15% to 4%. The first samples obtained reached lower temperatures than those collected later. Once in the laboratory, the samples were stored at 10 ± 2 °C in hermetically closed plastic bags.

Moisture content determination

Kernel samples of 50 g each were dried at 120 °C for 12 hours in a mechanical convection oven, using three replicates per sample.

In preliminary trials, no significant differences in moisture values were obtained when ground or whole material from a single sample was used. Therefore, all moisture determinations were performed without grinding.

Equilibrium relative humidity determination

A protimeter (dew point meter) was used to determine the equilibrium relative humidity (ERH). This meter reads two properties of air surrounding its sensors: the dry bulb temperature and the temperature at which dew forms on the surface of an electrically-cooled solid-gold mirror.

The protimeter was calibrated against ambient of known temperature and relative humidity (R.H. controlled by saturated salts). The temperature and dew point readings of the protimeter were used to calculate humidity values with ASAE psychometric equations (ASAE, 1984). Then, these values were regressed to those of the saturated salts. Finally, the resulting equation was used to prepare a calibration chart, eventhough deviations with R.H. values given by the protimeter were small.

To determine the ERH, kernel samples of 125 g were placed in 230 ml glass jars; then, the sensor of the protimeter, tightly fitted to a hole in the center of a # 12 rubber stopper, was placed over the jar mouth and hermetically sealed. Two hours later, the readings of the meter were recorded. All tests were conducted inside temperature-controlled chambers, where samples had been kept for at least 48 hours, in similar jars sealed with unperforated stoppers. Readings were obtained for two replicates of each sample.

Two sets of data were obtained, one for each of the two temperatures tested (25 °C and 32 °C), which represent the average ambient temperatures at which oil palm kernels are commonly stored at the three sampling sites (mills).

Results and Discussions

Moisture humidity isotherms

The equilibrium relationship between the material moisture content and the atmospheric RH at a given temperature can be plotted as a curve, commonly known as isotherm (Brooker, Bakker-Arkema and Hall, 1981).

The isotherms for 25 and 30 °C obtained for oil palm kernels produced in Coto, Naranjo and Palo Seco, are shown in Fig. 1 , Fig 2 and Fig. 3 , respectively. Fig. 4 shows isotherms for the pooled data from the three sites combined, and Fig. 5 shows a general moisture/humidity curve for all data combined (three sites and two temperatures).

Regression analysis

To find the best equation to fit the EMC and ERH data, five different regression models were examined: four polynomial and one exponential equation. The analysis showed that a third degree polynomial model had the highest regression coefficient. Therefore, the best estimations of moisture content (with the smallest deviations), at the given relative humidity range, were obtained with this model.

Parameters used to compare the models were the determination coefficient (R²) and the standard error of the estimates (s.e.), which in this case represent the deviation in the moisture content (in wet base) estimates obtained with the model. An additional criterion used was the distribution of residuals.

Cubic and cuadratic polynomial models showed similar R² and s.e., but the distribution of residuals was better in the cubic model. Thus, it was concluded that this model gave the best description of the relationship between the relative humidity and moisture content in oil palm kernels.

Similar comparisons between models were made for data from each site and the two temperatures tested. In these comparisons it was observed that the exponential model overestimated EMC of kernels at relative humidities from 70 to 80%, and underestimated EMC at R.H. from 80 to 95%.

The curves obtained with cubic models were S shaped, which agreed with the Brunauer classification, i.e. that isotherms for grains belong to the type II, called sigmoid isotherms (Henderson and Perry, 1976).

Regression and determination coefficients for the best fit equations for the data in the 35% to 95% RH range are shown in table 1 . These equations have the following general form:

EMC=a+b*ERH+c*ERH 2 +d*ERH 3                  (1)

where:

EMC = Equilibrium moisture content , % w.b.
ERH = Equilibrium relative humidity, %
a, b, c, d = regression coefficients.

The main purpose was to study changes in moisture/humidity relationship at two temperatures which are to the average low and high at which oil palm kernels are stored in Costa Rica.

Brooker, Bakker-Arkema and Hall (1981) indicated that the EMC/ERH relationship of most agricultural products is affected by temperature. Such phenomenon is not clearly observed for oil palm kernels (Fig. 4), probably because of the small temperature differential under study.

Considering this last result and also the fact that no significant differences were observed among EMC/ERH ratio of kernels produced in the three sites, a single general equation was calculated for the totality of the data collected, which could be useful under local conditions. As in previous cases, the cubic model best fitted the experimental data (Fig. 5). The equation had a R2 of 0.954 and a standard error of the estimates of 0.55%.

EMC= -116,823 +5,037 * ERH -6,927X10 -2 *ERH 2 +3,241X10 -4 *ERH 3      (2)

Equilibrium moisture content and its applications for kernel drying and storage

The EMC is used to determine whether a product could gain or lose moisture under certain ambient conditions (temperature and relative humidity), since the product would always tend to reach equilibrium with the surrounding atmosphere.

The main objective for drying a product like oil palm kernels is to reduce its moisture to a level in equilibrium with an atmosphere whose relative humidity is below a level considered safe for storage. It is widely accepted that fungal activity is negligible at ERH below 60%; therefore, considering the results, oil palm kernels produced in Costa Rica with a 6% moisture content could be considered dry and ready for long-term storage, (six months to a year).

When the storage period is shorter, it is possible to store kernels at a slightly higher moisture content without risking fungal damage. Regarding this possibility, Foster (1982) indicates that grain fungi grow very slowly at relative humidities below 70%. Therefore, it can be derived from Fig. 5 that it might be risky to store oil palm kernels with moisture levels of 7.5% or higher, as the ERH would be over 70% R.H. ( Table 2 ).

Finally, the results of this work show important differences from those published by Somade (1955), who obtained much lower EMC values for oil palm kernels. The largest difference was about 1.8% for the 75% R.H. data provided by Somade and the corresponding data of this experiment ( Table 2 ). Most likely, the observed differences in EMC values for both experiments originated in the different genetic compositions of kernels used by Somade and those used in this experiment, as well as the postharvest handling of the materials. Another aspect to consider in trying to explain these differences is the moisture content and relative humidity determination techniques used, which were different in both studies.

Unfortunately, the discrepancies between the studies are larger wihin an important range (60-80% R.H.); the relative humidity at which the three locations studied remain most of the time. Overdrying of kernels could have been the end result, had Somade,s results been used to derive guidelines for the postharvest handling of Costa Rican oil palm kernels. This is one example of many, of the importance of generating information with local materials in the process of adapting known technologies to local conditions.

Regarding the possibility that laboratory drying conditions could have affected the material tested, specifically its properties to establish normal MC/RH equilibrium, no evidence of discontinuity was observed in data produced. This can be graphically determined in Figs. 1, 2 and 3. Therefore, any physical or chemical changes that might have occurred during drying had no significant effects on the hydrophilic properties of oil palm kernels.

References

AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS. 1984. ASAE Standards. 31 ed. St. Joseph, MI., The Society. p.38-40.

BOOTH, G. 1984. Post-harvest handling and storage. Part III: Pulses, Oilseeds, and Nuts. Agribusiness Worldwide (May/ June):19-38.

BROOKER, D.B.; BAKKER-ARKEMA, F.W.; HALL, C.W. 1981. Drying Cereal Grains. 4 ed. Westport, Conn., The AVI Publishing Co. p. 69-87.

FOSTER, G.H. 1982. Drying cereal grains. In Storage of Cereal Grains and their Products. Ed. by C.M. Christensen. St. Paul, Minn., American Association of Cereal Chemists. p.86.

GOUGH, M.C.; LIPPIATT, G.A. 1977. Moisture humidity equilibria of tropical stored produce. Part II. Oilseeds. Tropical Stored Products Information 34:49-61.

HENDERSON, S.M.; PERRY, R.L. 1976. Agricultural Process Engineering. 3 ed. Wesport, Conn., The AVI Publishing Co.

SOMADE, B. 1955. Moisture equilibrium of palm kernels. Journal of Science, Food and Agriculture 6:425-427.