David A. Walton

Quality changes in macadamia kernel between harvest and farm‐gate

BACKGROUND Macadamia integrifolia, Macadamia tetraphylla and their hybrids are cultivated for their edible kernels. After harvest, nuts-in-shell are partially dried on-farm and sorted to eliminate poor-quality kernels before consignment to a processor. During these operations, kernel quality may be lost. In this study, macadamia nuts-in-shell were sampled at five points of an on-farm postharvest handling chain from dehusking to the final storage silo to assess quality loss prior to consignment. Shoulder damage, weight of pieces and unsound kernel were assessed for raw kernels, and colour, mottled colour and surface damage for roasted kernels. RESULTS Shoulder damage, weight of pieces and unsound kernel for raw kernels increased significantly between the dehusker and the final silo. Roasted kernels displayed a significant increase in dark colour, mottled colour and surface damage during on-farm handling. CONCLUSION Significant loss of macadamia kernel quality occurred on a commercial farm during sorting and storage of nuts-in-shell before nuts were consigned to a processor. Nuts-in-shell should be dried as quickly as possible and on-farm handling minimised to maintain optimum kernel quality.

Maintaining high moisture content of macadamia nuts-in-shell during storage induces brown centres in raw kernels.

BACKGROUND Kernel brown centres in macadamia are a defect causing internal discolouration of kernels. This study investigates the effect on the incidence of brown centres in raw kernel after maintaining high moisture content in macadamia nuts-in-shell stored at temperatures of 30°C, 35°C, 40°C and 45°C. RESULTS Brown centres of raw kernel increased with nuts-in-shell storage time and temperature when high moisture content was maintained by sealing in polyethylene bags. Almost all kernels developed the defect when kept at high moisture content for 5 days at 45°C, and 44% developed brown centres after only 2 days of storage at high moisture content at 45°C. This contrasted with only 0.76% when stored for 2 days at 45°C but allowed to dry in open-mesh bags. At storage temperatures below 45°C, there were fewer brown centres, but there were still significant differences between those stored at high moisture content and those allowed to dry (P < 0.05). CONCLUSION Maintenance of high moisture content during macadamia nuts-in-shell storage increases the incidence of brown centres in raw kernels and the defect increases with time and temperature. On-farm nuts-in-shell drying and storage practices should rapidly remove moisture to reduce losses. Ideally, nuts-in-shell should not be stored at high moisture content on-farm at temperatures over 30°C.

Effects of roasting on kernel peroxide value, free fatty acid, fatty acid composition and crude protein content

Roasting nuts may alter their chemical composition leading to changes in their health benefits. However, the presence of testa may alleviate the negative effects of thermal treatments. Hence, this study aimed to explore the effects of roasting on kernel chemical quality and colour development of Canarium indicum and examine to what extent testa would protect kernels against damage from roasting. Roasting decreased peroxide value but increased free fatty acid, probably due to increased cell destruction and lack of enzyme inactivation, respectively. Protein content of kernels significantly decreased after roasting compared to raw kernels. However, testa-on kernels contained significantly higher protein content compared to testa-off kernels. Whilst colour development and mottling were observed in temperatures beyond 120°C, roasting did not alter fatty acid compositions of kernels. The mild roasting and presence of testa in kernels can be used to enhance health benefits of kernels.

Designing food and habitat trees for urban koalas: graft compatibility, survival and height of tall eucalypt species grafted onto shorter rootstocks

The Corymbia and Eucalyptus species eaten by koalas are generally large trees, but these are often unpopular with urban landowners and councils because of the dangers of limbs falling from a great height. We aimed to develop shorter koala food and habitat trees for urban areas by heterografting tall eucalypt species onto rootstocks of shorter species and comparing their survival and growth with homografted trees and control ungrafted trees. In total, 12 of 14 interspecific scion/rootstock combinations were grafted successfully in the nursery but graft compatibility and field survival depended on taxonomic relatedness. The six interspecific combinations that had multiple surviving trees at 5 years after planting were all between species within the same taxonomic section or between a species and its own interspecific hybrid. Almost all trees died from grafts between species in different taxonomic sections. In most cases, the height of surviving interspecific grafted trees did not differ from control intraspecific grafted trees or from ungrafted trees of their scion species. Grafting elicited a ‘thrive or not survive’ response that diminished its usefulness for producing shorter trees. However, one combination, E. moluccana/E. behriana, had field survival of 40% and reduced height (4.0 m vs 9.9 m). These could be valuable habitat trees for koalas and other fauna in urban areas.

Ultrastructure and anatomy of Macadamia (Proteaceae) kernels

Macadamia integrifolia Maiden & Betche and M. tetraphylla L.A.S.Johnson, and their hybrids, are cultivated for their edible kernels (mature embryos). Some kernels separate into halves (cotyledons) during post-harvest handling, and embryo cuticular characteristics may influence breakage. Some kernels have a gap between cotyledons before nut cracking, and this anatomical feature may be related to breakage. Kernels from cultivars producing high or low frequency of breakage were examined by transmission electron microscopy and scanning electron microscopy to identify differences in inner-cuticular wax. Whole seeds of each cultivar were opened without cracking to measure gaps between cotyledons and determine relationships between gaps and embryo breakage. Kernels from cultivars with low frequency of breakage had inner-epicuticular wax layers that were of the sculptured crust type whereas those of cultivars with high frequency of breakage were mainly of a wax-film type. Gap width and length of kernels separate at nut opening were more than twice those for intact kernels. There was a negative relationship between whole-kernel percentage and gap width between cotyledons.

A non-destructive determination of peroxide values, total nitrogen and mineral nutrients in an edible tree nut using hyperspectral imaging

Nuts are nutritionally valuable for a healthy diet but can be prone to rancidity due to their high unsaturated fat content. Nutrient content of nuts is an important component of their health benefits but measuring both rancidity and nutrient content of nuts is laborious, tedious and expensive. Hyperspectral imaging has been used to predict chemical composition of plant parts. This technique has the potential to rapidly predict chemical composition of nuts, including rancidity. Hence, this study explored to what extent hyperspectral imaging (400–1000 nm) could predict chemical components of Canarium indicum nuts. Partial least squares regression (PLSR) models were developed to predict kernel rancidity using peroxide value (PV) for two different batches of kernels, and macro- and micronutrients of kernels using the spectra of the samples obtained from hyperspectral images. The models provided acceptable prediction abilities with strong coefficients of determination (R2) and ratios of prediction to deviation (RPD) of the test set for PV, first batch (R2 = 0.72; RPD = 1.66), PV, second batch (R2 = 0.81; RPD = 2.30), total nitrogen (R2 = 0.80; RPD = 1.58), iron (R2 = 0.75; RPD = 1.46), potassium (R2 = 0.51; RPD = 0.94), magnesium (R2 = 0.81; RPD = 2.04), manganese (R2 = 0.71; RPD = 1.84), sulphur (R2 = 0.76; RPD = 1.84) and zinc (R2 = 0.62; RPD = 1.37) using selected wavelengths. This study indicated that visible-near infrared (VNIR) hyperspectral imaging has the potential to be used for prediction of chemical components of C. indicum nuts without the need for destructive analysis. This technique has potential to be used to predict chemical components in other nuts.

Designing food and habitat trees for urban koalas: identifying short ecotypes of Corymbia intermedia

The eucalypt trees eaten by koalas are generally tall, but urban landholders prefer to plant shorter trees that pose less danger of limbs falling from a great height or damaging powerlines. Our aim was to develop shorter eucalypt trees to provide food and shelter for koalas and other fauna in urban areas. We identified short ecotypes of Corymbia intermedia (R.T.Baker) K.D.Hill & L.A.S.Johnson growing naturally on exposed coastal headlands, and tested whether their seedlings were shorter than the seedlings of nearby tall ecotypes when planted in cultivation. Trees raised from the short ecotypes were 22–43% shorter than trees raised from the tall ecotypes, being around 5–7 m tall rather than 8–12 m tall after 8 years. This demonstrated that there was a genetic basis for the short stature of C. intermedia trees on coastal headlands. These shorter C. intermedia trees could be valuable food and habitat trees for urban koalas and other fauna.

Comparison of three pollination methods for Eucalyptus argophloia, a small-flowered eucalypt

ContextMost studies assess pollination success at capsule maturity, and studies of pre-zygotic processes are often lacking.AimsThis study investigates the suitability of controlled pollination for a potential forestry plantation species, Eucalyptus argophloia, by examining pre- and post-zygotic pollination success.MethodsPollen tube development, capsule set and seed set are compared following three-stop pollination, artificially induced protogyny (AIP), AIP unpollinated and open pollination. The fecundity of stored pollen was compared with that of fresh pollen.ResultsThree-stop pollination, AIP and open pollination had similar numbers of pollen tubes, but AIP unpollinated had none. Open pollination produced significantly more capsules and total number of seeds than the other treatments. There were significantly more seeds per retained capsule for the open pollination and three-stop pollination treatments than for the AIP and AIP unpollinated pollination treatments. There were no significant differences relative to the age of pollen.ConclusionsPre-zygotic success in terms of pollen tubes was similar for open-pollinated, three stop and AIP, but was not reflected in post-zygotic success when the open pollination and three-stop method produced significantly more seeds per retained capsule than the AIP treatments and open pollination yielded more seeds. Capsule set and total seed set for open pollination, and fewer capsules in controlled pollinations, may reflect physical damage to buds because of the small E. argophloia flowers. Suitable alternative breeding strategies other than controlled pollinations are discussed for this species.

Pollination methods, stigma receptivity and pollen tube growth in Eucalyptus argophloia

Abstract Eucalyptus argophloia Blakely (Western white gum) has shown potential as a commercial forestry timber species in marginal environments of north-eastern Australia. We measured early pollination success in Eucalyptus argophloia to compare pollination methods, determine the timing of stigma receptivity and compare fresh and stored pollen. Early pollination success was measured by counting pollen tubes in the style of E. argophloia 12 days after pollination. We compared the early pollination success of 1) Artificially Induced Protogyny (AIP), one-stop and three-stop methods of pollination; 2) flowers pollinated at 2 day intervals between 2 days before and 6 days after anthesis and 3) fresh pollen and pollen that had been stored for 9 months. Our results show significantly more pollen tubes from unpollinated AIP and AIP treatments than either the one-stop pollination or three-stop pollination treatments. This indicates that self-pollination occurs in the unpollinated AIP treatment. There was very little pollen tube growth in the one-stop method indicating that the three-stop method is the most suitable for this species. Stigma receptivity in E. argophloia commenced six days after anthesis and no pollen tube growth was observed prior to this. Fresh pollen resulted in pollen tube growth in the style whereas the stored pollen resulted in a total absence of pollen tube growth. We recommend that breeding programs incorporating E. argophloia as a female parent use the three-stop pollination method, and controlled pollination be carried out at least six days after anthesis using fresh pollen.