CONTENT IN MACADAMIA¹

Ionel Rosenthal², Uzi Merin², Gita Popel², Solange Bernstein² and Amnon Kadman³

Reprinted from CMS 1985

INTRODUCTION

During our studies of the technological parameters of Macadamia nut (Kadman et al., 1984; Rosenthal et al., 1984; Rosenthal et al., 1985 a), we frequently had to determine the oil content of the nut. Oil content is usually determined by the Soxhlet extraction method (Horwitz, 1975), in spite of the long time it requires.

In the dairy industry there is great importance to the determination of fat in dairy products. Fast and accurate test methods were developed for this purpose. Two, basically similar, methods are in use: the Gerber and the Babcock methods (Atherton & Newlander. 1977). The Gerber method is the standard one in most European countries, whereas the Babcock is more common in many states of the U.S.A., although the Gerber method is also employed. Both methods are based on the digestion of all milk components, except oil, by concentrated sulfuric acid and the separation of the oil from the aqueous phase by centrifugation in special glassware, called butyrometers. In addition to fluid milk products, the Gerber test can be used for the assay of fat in cream, concentrated milk, ice cream, cheeses and sausages (Pearson, 1970), provided that special both-ends-open butyrometers are employed. The very similar Babcock method, with various digesting reagents, has been applied satisfactorily to fish products (Horwitz, 1975).

We report the use of a modified Gerber test for the determination of oil in Macadamia nuts. The speed, simplicity and low expense of operation of the Gerber method could lead to the emergence as an accurate and convenient assay (Rosenthal et al., 1985b).

MATERIALS AND METHODS

The butyrometers used were for cheese (Van Gulik type), graduated from 0 to 40%. A standard Gerber centrifuge, with 12 positions provided with a heating element and a 3O-min. timer, was used (K. Schneider & Co. Ltd., Zurich). A thermostatically controlled water bath (65 ± 2 C) served for holding the butyrometers until digestion was complete. For digestion, 70% sulfuric acid was used (m/m; D²⁰ =1.615 g/ml).

Samples of Macadamia nuts were fine-grated. Then 1.5 g was weighed into the glass beaker, which is attached to the large stopper and inserted into the butyrometer. The butyrometers were shaken gently to loosen the sample in order to avoid lumps. Then 12 ml of acid was added through the narrow opening and the butyrometers were immersed vertically in the water bath for 60 min., or until complete digestion of the nuts. The butyrometers were shaken every 10-15 min. to accelerate digestion. When the liquid mass appeared homogeneous, the butyrometers were filled with acid to the 30% graduation mark and plugged with the small stopper. Then they were inserted in the heated centrifuge and spun for 15-20 min., followed by placement in the heated water bath for 3-5 mm. The height of the oil column was read off the graduated neck and the reading obtained at the bottom was subtracted from that obtained at the top of the oil column (lower meniscus). The reading should preferably be done by moving the large stopper slightly in or out, to bring the bottom of the oil column, with a minimum of movement, to a graduation mark. The reading, multiplied by a calibration factor of 2.0, gives the oil content, as expressed in grams of oil per 100 g of sample.

RESULTS AND DISCUSSION

In order to obtain a fast and easy means of measuring the oil content in Macadamia according to the proposed method, we checked various acid concentrations and digesting temperatures. Not every combination of acid and digestion temperature was suitable for this nut. Acid concentration higher than 70% and digestion temperatures of 40-80 C resulted in complete burning of the sample including the oil. On the other hand, acid concentrations of 40-65%, and very high temperatures (up to 90 C) did not digest all the pulp and oil separation was incomplete. After running a large number of samples, the conditions described in the Materials and Methods section were attained. It should be noted that, at the given conditions, a longer digestion time does not alter the final oil reading. The time stated was set, as the minimum required for complete digestion. In some cases when the nuts have a high oil content, this period might have to be extended.

Over 90 samples from different picking times, with oil levels of 70-80%, were checked. In parallel, the same samples were tested for oil content by the Soxhlet extraction method.

Statistical analysis of the regression correlation between the standard Soxhlet method and the new method was performed and the results are summarized in Table 1. The results of the new proposed method correlate well at the 99% level with the other test, as can be seen from the r values obtained: Soxhlet vs. Gerber, r = 0.928

Table 1. Linear regression of oil determination according to the Soxhiet extraction and Gerber methods.

LITERATURE CITED

1. Atherton, H. V. and Newlander, J. A. 1977. Chemistry and Testing of Dairy Products. Ch. 4,4th edn. Avi Publ. Co., Inc., Westport, CT.
2. Horwitz, W. (Ed.) 1975. Analytical Chemists. Official Methods of Analysis of the Association of Official 12th edn. p. 311. Association of Official Analytical Chemists, Washington, DC.
3. Kadman, A., Merin, U., Basker, D. and Rosenthal, I. 1984. Some characteristics of Macadamia nuts of the "Yonik" variety. YB. Calif. Macadamia Soc., 29: 142-143.
4. Pearson, D. 1970. The Chemical Analysis of Foods. 6th edn. p. 14, J. and A. Churchill, London.
5. Rosenthal, I., Merin, U., Basker, D. and Kadman, A. 1984. A study of Macadamia nuts of the "Yonik" variety. .J. Fd Quality, 7:67-73.
6. Rosenthal, I., Merin, U. and Kadman, A. 1985a. Comparison of some properties of Macadamia nuts of the "Yonik" and "Beaumont" cultivars. Lebensm.-Wiss. u.-Technol., 18 (in press).
7. Rosenthal, I., Merin, U. Popel, G. and Bernstein, S. 1985b. A general approach to fat determination in vegetable foods. J. Ass. off anal Chem., 68 (in press).