The variability in shelf life of Australian grown macadamia kernel has become a noticeable and unacceptable quality problem within the macadamia industry. In this study, the quality and shelf life of cultivated macadamia kernels were evaluated, with a particular emphasis on the kernel composition, growing location and cultivar.
Previous research has shown that properly dried and packaged macadamias may take over 16 months before kernel deterioration is noticeable (Cavaletto et al., 1966). However, macadamia kernels can begin deteriorating at a dramatically increased rate for a variety of reasons, including environmental and processing factors and poor packaging (Anon, 2000a). Accelerated storage trials offer one solution to investigate the rates of kernel degradation. In this project a number of accelerated shelf life trials were conducted to study factors affecting macadamia shelf life. Accelerated shelf life trials have previously been conducted on macadamias (Cavaletto et al, 1966; Dela Cruz et al, 1966; Wootton, 1990; Mason et al., 1998); however most of these studies have sort to examine the effects of moisture content on kernel degradation. As this project aims to examine factors affecting the storage stability of dried macadamia kernels (less than 1.5 g l00g-1moisture), preliminary investigations were required to devise the accelerated storage conditions.
An initial accelerated trial was conducted by storing macadamia kernels at 30°C in atmospherically sealed lacquered cans for 8 weeks. This preliminary trial showed that 30°C was an adequate temperature for conducting accelerated storage investigations on macadamia kernels.
To determine the role oxygen concentration plays in promoting kernel oxidation in the storage system (atmospherically sealed lacquered cans), a separate accelerated storage trial was conducted. Kernels were stored at 40°C for 14 weeks with oxygen concentration measured weekly. The storage temperature was increased so as to promote an even greater level of oxidative deterioration, compared to the initial trial where the storage temperature was 30°C. The rate of oxygen consumption by kernels decreased as storage time increased, with the oxygen utilisation rate different between the two cultivars examined (namely, Hawaiian Agriculture Experiment Station (HAES) 344 and Hidden Valley A16). Cultivar A16 kernels absorbed greater quantities of oxygen compared to cultivar HAES 344 kernels. However, after 14 weeks at 40°C, sufficient oxygen remained in the cans such that the concentration of oxygen in the sealed system was not a limiting factor for any further oxidation reactions to proceed.
It was the intention of this project to examine whether growing location, cultivar and kernel oil content influenced macadamia quality and shelf life. Therefore, the composition and shelf life was examined from macadamias grown in 1998 and 1999 from specified Australian locations and cultivars.
In 1998, the composition and shelf life of macadamias originating from 3 locations (Lismore, Gympie and Bundaberg) across the typical Australian macadamia growing region was examined. Standard analytical and food analysis techniques, such as gas chromatography (GC) and high performance liquid chromatography (HPLC), were used to determine total lipids, sugars, protein, vitamin E and fatty acid profiles in kernels (cultivar HAES 344). The average kernel composition was found to significantly vary between these growing locations. The Lismore site produced macadamias with the largest compositional differences compared with the other two Queensland sites examined (Gympie and Bundaberg), indicating a possible environmental effect influencing the physiological development of kernels.
Kernel samples were oil graded using differing specific gravity (SG) solutions applying the established relationship between kernel oil content and kernel SG (Ripperton et al., 1938; Mason, 1982). A strong inverse relationship was found between the oil and sucrose concentration in macadamia kernels. An increase in the level of polyunsaturated fatty acids occurs as total kernel oil content increases which suggests that higher oil content kernels are more susceptible to oxidation.
To determine the effects of growing location and kernel oil content on the shelf life and eating quality, an accelerated storage trial was conducted on kernels graded into different oil content grades. Shelf life was measured by determining peroxide values, free fatty acid values, headspace GC of sample volatiles and sensory rancidity.
Minimal rancidity occurred in kernel s stored for 10 weeks at 30°C in aerobically sealed lacquered cans. Nevertheless, headspace gas chromatography mass spectroscopy (GC/MS) of these stored macadamias indicated that oxidation proceeded at a more rapid rate in kernel s with the highest oil contents (>78 g l00g-1). The compound hexanal was identified as a possible indicator as to the extent of oxidation in macadamia kernel s.
In addition to the effects of growing location influencing kernel shelf life, anecdotal evidence from the Australian macadamia industry indicates that kernel shelf life varies according to cultivar and between seasons (Anon, 2000a). To investigate these factors, the composition and shelf life of cultivar HAES 344 macadamias harvested from two of the same sites (Lismore and Bundaberg) was again assessed in the subsequent year (1999). Further, the composition and shelf life of cultivar A16 was tested from kernels grown at a Lismore site. It was revealed that the compositional trends found between the two growing locations (Lismore and Bundaberg, HAES 344) were similar to those trends identified in the previous season. The Lismore site produced a greater proportion of low oil content kernels compared to the Bundaberg site. Hence, the average sucrose concentration was slightly higher in kernels grown at the Lismore site compared with those from Bundaberg. Generally, the fatty acid profile for cultivar HAES 344 kernels was similar between seasons.
The A16 cultivar was found to contain lower sucrose quantities compared with cultivar HAES 344 kernels. In addition, the A16 cultivar contained far greater levels of linoleic acid. The reduced sucrose levels may contribute to the poorer eating quality of A16 kernels, while the increased linoleic acid concentration is likely to promote oxidation and therefore reduce A16 shelf life.
As only minimal oxidation occurred in the 1998 accelerated storage conditions, a revised accelerated storage trial was conducted to examine the effect of growing location, cultivar and kernel oil content on the shelf life of macadamia kernels from the 1999 harvest. The graded kernel samples were stored for 24 weeks at 37°C with samples taken every 4 weeks. The rate of oxidation was influenced by all three treatments (growing location, cultivar and oil content). kernels from the Lismore site for both cultivars (HAES 344 and A16) oxidised and developed rancidity significantly (P<0.05) faster than Bundaberg grown kernels of cultivar HAES 344. The degradation of free fatty acids was the most likely cause of hexanal production in kernels from the Lismore site. The rate of rancidity development increased as kernel oil content increased. However, this effect was mostly observable in cultivar HAES 344 graded kernels from the Bundaberg site, which highlights the different degradation mechanisms involved in the deterioration of macadamia kernels from the two locations (Lismore and Bundaberg). Measuring hexanal proved to be a more accurate method for determining the level of rancidity in macadamia kernels compared to the traditional peroxide value technique. Therefore, future shelf life studies would best utilise hexanal as a rancidity marker, for routine rancidity determinations.
Given that this investigation highlighted many areas for further work in order to optimise the shelf life of macadamias, a standardised set of conditions for an accelerated storage procedure for possible implementation by nut processors was provided for the interim. These conditions involve storing kernels in aerobically sealed containers at 37°C for no longer than 12 weeks, or until they reach a free fatty acid level of 0.150 g l00g-1 as oleic acid. If the kernels develop greater than this level of free fatty acids prior to 12 weeks, they will have a dramatically reduced shelf life.
Overall, this study highlighted the need for a more standardised approach to macadamia grower and processor procedures in to order produce macadamia with an optimum shelf life. Processors should seriously consider oil grading kernels so as to gain a greater control over variable eating quality and shelf life. More importantly, growers and processors should investigate methods of processing cultivars (or groups of cultivars) separately so as to maintain a standardised eating quality. Further work is required to investigate the possibility of combining similar cultivars. Also, it is strongly advised that macadamias from different growing regions be kept separate during processing as growing location was found to have the most profound influence on shelf life.