Reprint From CMS Yearbook 1987

SUMMARY

The effect of drying temperature from ambient to 160 F on sugars, fat, and roasting quality of Macadamia nuts, Macadamia integrifolia, was investigated. Fresh high-moisture Macadamia nuts dried at elevated temperatures were high in reducing sugars and low in roasting quality. Nuts dried at high temperatures developed dark-brown centers when roasted. The reducing sugar was higher in the brown center than in the light-colored outer layer. Temperature sensitivity was related to moisture content. Undesirable enzymatic activity under high-moisture high-temperature conditions was indicated as a possible mechanism for brown centering. Initial drying at ambient or 100 F lessened undesirable high-temperature effects. The results indicate that reducing and total sugar content decreased during low-temperature initial drying. However, no changes in fat content were observed.

INTRODUCTION

The Macadamia industry in Hawaii is in early stages of accelerated growth. With this growth, new problems have appeared and it has become increasingly important to refine old technology, and to develop a new understanding of physiological and chemical changes occurring during processing.

Processing of Macadamia nuts was pioneered and developed by Moltzau and Ripperton (1939). Briefly, Macadamia nuts, which have fallen on the ground are harvested and then mechanically husked. The husked nuts are then dried and, when the moisture in the kernels has been reduced to approximately 3-5% moisture, the nuts are cracked. After an extensive separation and sorting operation, the kernels are further dried to approximately 1-1 1/2 % moisture and roasted in coconut oil. After roasting, excess oil is removed, adhesive and salt are added, and the nuts are vacuum packed.

In practice, there are considerable variations in the processing procedure just described. For example, drying times may vary from days to weeks, drying temperatures may differ as much as 40 F, and drying to 1-1 1/2 % moisture may be done completely in-shell. The variability of processing methods is in part due to lack of information on chemical and biochemical effects of Macadamia drying. Moltzau and Ripperton (1939) used air drying by circulating ambient air, sun drying by exposure to direct rays of the sun, and artificial drying by heated air. Of particular interest was the observation that even the highest temperature employed (172 F) had no apparent undesirable effect on the quality of the kernel. However, problems were encountered by industry when Macadamia nuts were dried at elevated temperatures. Those dried at high temperatures often developed an undesirable dark brown center when roasted. Furthermore, there is some popular belief that fat is synthesized from sugars during slow drying or curing". It was of interest, therefore, to investigate the effects of drying conditions on the chemical composition and quality of Macadamia nuts.

MATERIALS AND METHODS

Used in this study was the smooth-shell Macadamia, Macadamia integrifolia, Kakea 508 (Hamilton and Fukunaga, 1959), from the University of Hawaii’s Kona Branch Station. To obtain freshly fallen nuts of relatively uniform high moisture content, only nuts with green husks were harvested. After harvest they were husked immediately in a rubber-tire husker (Hamilton and Fukunaga, 1959) and then shipped in sacks by airfreight to Honolulu. The nuts were in transit approximately 2 days. Upon arrival at the University of Hawaii the nuts were dried in shell in forced-draft ovens or in a constant-temperature room with air directed over the nuts by a fan.

Approximately 50 lb. of in shell Macadamia were used for each temperature treatment. Raw Macadamia kernels were sampled in one of two ways for chemical analysis. Since cracking high moisture nuts by machine was ineffective, fresh nuts or partially dried nuts were individually cracked with a vice. Approximately 50 nuts were cracked, and about 25 plump, smooth-based, and light-colored nuts, which were presumably fully mature nuts were selected for chemical analysis. In the case of dried nuts, which could be effectively cracked in the cracking machine, approximately 8 lb. were machine cracked, giving approximately 3 lb. of kernels, and about 25 fully mature nuts were selected for chemical analysis.

The samples for chemical analysis were chopped in a Davison-Kennedy peanut slicer. Moisture content was determined by drying 5-g portions of chopped nuts overnight to constant weight under high vacuum and at 70 C.

The dried samples from the moisture determinations were used for fat determination. The samples were extracted with anhydrous peroxide-free ethyl in a Goldfisch extractor for 6 hr. and then ground to a fine powder and re-extracted for an additional 6 hr. At the end of the extraction the ether in the extracted oil was evaporated and the oil dried to constant weight under vacuum and at 70 C. The percent ether-extractable material was calculated on the dry-weight basis.

Sugars in the extracted residue were extracted and purified by AOAC method (1960), and the reducing sugar content determined by the Nelson method (1944) both before and after inversion with HCI.

Roasting quality of the Macadamia nuts from the various treatments was determined on kernels dried to approximately 1.5-% moisture. The kernels were roasted for 15 min. in coconut oil (mp 76 F) at 260 F, and excess oil was removed by centrifuging in a basket centrifuge. They were then separated into 4 grades: 1) nuts of smooth texture and light color; 2) nuts with color defects, that is, uneven browning or off-colored nuts; 3) brown-centered nuts; 4) dark, shriveled nuts. The weight percent was then determined.

RESULTS AND DISCUSSION

The effects of drying temperatures on the quality of Macadamia nuts. The observation of brown centering of Macadamia nuts has not been reported previously. In order to determine the magnitude of the problem, freshly fallen Macadamia nuts were dried 4 days at ambient air or 125, 140, and 160 F, and then transferred to 125 F for 3 more days. The latter 3 days of drying were to equilibrate the samples to approximately equal moisture levels prior to roasting and evaluation. After the nuts were dried, a portion of the raw kernels was analyzed for moisture and sugar content and the remainder of kernels roasted and evaluated. The initial moisture content of the fresh Macadamia nuts was 28%. Table 1 shows the moisture of the samples after the initial 4 days of drying at different temperatures and after equilibration at 125 F. Kernels from the lower 3 drying temperatures were light-colored and indistinguishable. Kernels from 160 F treatment, in contrast, were browned to varying degrees and obviously damaged. Table 2 shows the sugar content of the fresh nut and after drying at the different temperatures. The reducing sugar content of the nuts dried at different temperature shows a clear trend. Nuts dried at higher temperatures have a higher percentage of reducing sugar then nuts dried at lower temperatures. A trend was not clear in the case of nonreducing sugar content.

As shown in Table 3, it is obvious that the quality of the finished product was affected by drying temperature. As determined after roasting, almost 70% light-colored and highly acceptable nuts were obtained from nuts dried in ambient air or 125 F, while not acceptable nuts were obtained at 140 or 160 F. The phenomenon of brown centering was clearly demonstrated in this experiment. With higher-temperature drying the zone of brown centering increased so that in the 160 F sample only a very narrow zone was the light, characteristic color of roasted Macadamia nut. These results suggested that high-temperature drying increased reducing sugars in the nuts and that the resulting nuts of high reducing sugar content browned when roasted. The fact the kernels dried at 140 F were indistinguishable when raw from kernels dried at lower temperatures but completely unacceptable when roasted, is particularly significant and points out that quality determinations on the basis of roasted rather than raw kernels provide a better indication of quality.

The effect of high temperatures on nuts of varying moisture levels. Preliminary results indicated that the effects of high-temperature drying were influenced by moisture level of the nuts. In other words, low-moisture nuts appeared to tolerate high-temperature drying better than high-moisture nuts. An experiment was therefore conducted to determine the moisture level drying temperature relationship and to determine the moisture level at which high-temperature drying would not affect roasting quality. Approximately 70 lb. of fresh Macadamia nuts, 28% moisture content, were used in this experiment. Seven-pound samples were dried initially in ambient air, and, after 1, 2, 4, and 6 days, 7-lb. samples were transferred to 125 and 140 F for 4 days of final drying. After the final drying, a portion of the kernels were analyzed for sugar and other-extractable content and the remainder roasted and graded. The results are shown in Fig. 1-4. In these figures the chemical and quality determinations are plotted against the number of days of initial drying (top scale, nonlinear) and the corresponding moisture content (bottom scale, linear).

As shown in Fig. 1, initial drying at ambient temperature prior to final drying at either 125 or at 140 F increased the yield of acceptable kernels. The relationship of undesirable high-temperature effects and moisture content of the kernels is clearly evident. Yield of acceptable nuts (75%) was highest from initial drying at ambient temperature to approximately 6.5% moisture and final drying at 125 F.

Fig. 2 shows that the effect of high temperature in producing brown centering was dependent on the moisture content. For example, no brown-centered nuts were obtained when kernels of 8% moisture were dried at 125 F, but when the same nuts were dried at 149 F, 30% brown-centered nuts were obtained. At 140 F drying a small amount of brown centering was observed even in nuts dried initially to 6.5% moisture content.

Figs. 3 and 4 show the reducing sugar and total sugar content of the raw Macadamia nuts. The high reducing sugar content of nuts dried at 140 F is consistent with results obtained earlier. Macadamia nuts dried initially to at least 1 5% before drying at 125 F had the lowest reducing sugar content. With 140 F drying, initial drying to 8% moisture level was necessary to produce a minimal reducing sugar content. It must be emphasized that the chemical analyses for sugar were done on raw Macadamia nuts. There were no obvious outward differences among the various samples. Thus, the chemical data represent the over-all chemical composition of the raw Macadamia kernels and include potential brown-centering nuts as well as acceptable nuts. In spite of this the direct relationship between reducing sugar content and roasting quality is again apparent. Fig. 4 shows the total sugar content of the nuts from the various treatments. Although the results in this case were less consistent than in the case of the reducing sugar determination, generally the total sugar content was lower in samples initially dried for longer times. These results suggest that there was a reduction in both total sugar and reducing sugar content during drying.

The ether-extractable content was the same (77.97 ± O.32%) in nuts dried directly at elevated temperatures, or dried initially in ambient air before final drying at 125 or 140 F. Drying conditions, such as high temperature and high moisture of nuts, which affected sugar content and roasting quality did not affect oil content. It is reasonable to conclude that no detectable synthesis of fat occurs during extended initial drying or "curing" at ambient temperatures.

The experiment was repeated at 100 F instead of ambient air to lower the moisture content of the nuts prior to drying at 125 or 149 F. The qualitative and quantitative relationships between moisture content and the effects of high temperature on the chemical composition and roasting quality were similar to those in the previous experiment. An important difference, however, was the rate of drying. In ambient-air drying, 6 days were required to reduce moisture content to 6.5%. At 100 F, however, the moisture content was reduced to 4 % in only 2 days. The results indicate that the initial drying of fresh high-moisture Macadamia can be accomplished rapidly and without undesirable effects at 100 F.

Sugar content of brown-centered nuts. In order to determine the significance of reducing sugars in browning of nuts, brown-centered samples accumulated from previous experiments were carefully separated into two parts: the outer layer, with light color; and the brown-centered portion. The sugar content of both parts was determined. Analysis of variance showed that the 0.058% average reducing sugar content of the brown-center was significantly higher (1 % confidence level) than the 0.032% average reducing sugar content of the outer, light-colored layer. This pointed out in another way the relationship between reducing sugar content and brown centering was obtained.

We have presented evidence, which indicates that, the total and reducing sugar content of freshly fallen Macadamia nuts decreased during low-temperature drying. We have shown that: 1) the Macadamia nuts with low reducing sugar content have the best roasting quality; 2) the brown centers of Macadamia nuts, damaged by high-temperature drying, contain more reducing sugars then the light outer layer; and 3) temperature sensitivity of Macadamia was moisture dependent. An interpretation consistent with these findings is that the effect of drying temperature on the physiological activity of Macadamia. At low temperatures, there is a general decrease in sugar concentration but at high temperatures the reducing sugar concentration increases because of enzymatic inversion of nonreducing sugars to reducing sugars. Brown-centering or localized browning is probably the net result of both moisture gradient in the kernels and increased enzymatic activity in the moist center.

To test the enzymatic nature of brown-centering, freshly fallen husked nuts were steam blanched for 8 min. to a center temperature of 210 F, dried at 125 F for 12 days, roasted, and compared with a similarly treated unblanched control. Brown-centered nuts were observed in the controls but not in blanched samples. This result, although not establishing the detailed mechanism supports the enzymatic mechanism for brown centering. The blanched samples, however, roasted slightly darker than nuts dried initially at low temperatures. Darker roasting of the blanched samples is in accord with our hypothesis that the effect of low-temperature drying in decreasing the reducing sugar concentration of freshly-fallen nuts is also enzymatic. Because blanched nuts tend to roast slightly darker than nuts dried initially at low temperatures and because the effect of blanching on storage stability has not been established, we do not presently recommend blanching followed by high-temperature drying for commercial application.

It is clear from this study that the drying of Macadamia nuts requires attention to moisture-temperature relationships. Ambient and 100 F drying temperatures may be used at any moisture level, but 125 and 140 F should be used only after the moisture content is reduced to approximately 8 and 6%, respectively.

AOAC. 1960. Official Methods of Analysis. 9th ed. Assoc. Offic. Agr. Chemists. Hamilton, R. A., and E. T. Fukunaga. 1959. Growing Macadamia nuts in Hawaii. Hawaii Agr. Expt. Sta. Bull. 121.

Nelson, N. 1944. A photometric adaptation of the Somogyi method for the determination of glucose. J. Biol. Chem. 153, 378.