Submitted to: International Food Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2006
Publication Date: 1/24/2007
Citation: Wall, M.M., Gentry, T.S. 2007. Carbohydrate composition and color development during drying and roasting of macadamia nuts (macadamia integrifolia). LWT-Food Science and Technology 40:587-593. Interpretive Summary: The macadamia nut crop has a $40 million farm value in Hawaii that is increased substantially by processing into snack nuts, candies, confections, nut butters, and oils. Macadamia nuts have a unique delicate flavor, crunchy texture, and high oil content (75%). Cream-color kernels have higher quality and value than brown kernels. Changes in sugar composition during processing of four macadamia nut cultivars were quantified to determine whether variability in sugar content contributes to differences in kernel browning. Also, immature kernels were compared to mature kernels for color quality and sugar content to determine whether the presence of immature kernels would impact the final color of roasted kernels. Variability in sugar composition in fresh, mature nuts had a minimal impact on browning when drying and roasting conditions were well-controlled. However, immature kernels had higher levels of sugars, and more internal and external browning after roasting, than mature kernels. The presence of immature nuts can contribute to kernel browning during commercial processing, and cultivars may differ in the percentage and composition of immature nuts at harvest.
Technical Abstract: Research was conducted to determine whether variability in sugar content contributes to differences in kernel browning for macadamia nuts (Macadamia integrifolia) during processing. At harvest, total sugar content of fresh macadamia kernels varied from 2.9 to 5.6 g/100 g dry weight basis (db), and the average moisture content ranged from 15.6 to 23.6 g/100 g fresh weight. Cultivars differed in kernel sucrose content, but not reducing sugar content. Reducing sugars decreased during drying, and kernel centers darkened slightly. An incremental drying process limited sucrose hydrolysis, minimizing the amount of glucose and fructose available for browning reactions. Therefore, the centers of roasted kernels were not darker than dried kernels. The variability in sugar composition in fresh kernels had a minimal impact on color quality when low-temperature drying and roasting at 125 °C were used. However, when roasted kernels received from a processor were separated based on color quality, kernels with internal or external browning had higher reducing sugar concentrations (0.24 to 0.27 g/100 g db) than cream-colored kernels (0.03 g/100 g db). Immature kernels had higher sucrose and reducing sugar contents and more browning than mature kernels. During commercial processing, optimal conditions may not be achieved and the presence of immature nuts can contribute to kernel browning.