Effect of different storage temperatures on the quality and metabolic changes of fresh macadamia nuts

【Objective】This study aimed to explore the effects of different storage temperatures on the quality and metabolic changes of fresh macadamia (Macadamia spp.) nuts, elucidating the mechanisms by which storage temperature influences the physiological metabolism of the nuts, thereby providing a theoretical basis for extending the shelf life of fresh macadamia nuts and maintaining its post- harvest quality.【Methods】In this experiment, fresh green- peel kernels (XQ) and shelled kernels (XK) were used as the main test materials, which were packaged in PE bags and stored under different temperatures (25 ℃, 4 ℃, -18 ℃), and divided into 6 groups: 25XQ, 4XQ, -18XQ, 25XK, 4XK, -18XK, and samples were taken after 0, 7, 14, 21, 28, 35, and 42 days of storage. The moisture content, protein content, crude fat content, color difference, energy metabolism level, polyphenol oxidase (PPO), peroxidase (POD), superoxide dismutase (SOD), catalase (CAT) enzyme activities, and malondialdehyde (MDA) content were determined. Experimental data were collected and collated by Microsoft Excel 2016, graphed with Origin 2022, and one- way analysis of variance (ANOVA) was performed with SPSS 27.0 software (P＜0.05).【Results】The results showed that 4 ℃ was the optimal temperature for preserving the moisture content, crude fat content, and protein content of fresh macadamia kernels. On day 0, the kernels exhibited a relatively high initial moisture content (approximately 23.43% ), with crude fat and protein contents of about 59.34% and 6.13%, respectively. When stored at room temperature (25 ℃ ), the crude fat content of 25XQ ranged from 58.80% to 61.73%, while its protein content varied between 6.04% and 6.38%. For 25XK, the crude fat content ranged from 59.34% to 72.15%, and the protein content ranged from 6.13% to 7.45%. In contrast, under 4 ℃ storage, the crude fat and protein contents ranged from 59.34% to 62.05% and 6.13% to 6.41%, respectively. Similarly, 4XK maintained crude fat content between 59.34% and 64.34%, and protein content between 6.13% and 6.65%. Meanwhile, 4 ℃ proved to be a more effective method for preserving the color of both XQ and XK kernels. The color difference (ΔE) of -18XQ was significantly higher than those of 25XQ and 4XQ (P＜ 0.05), indicating greater changes in appearance color under -18 ℃ compared with storage at 25 ℃ and 4 ℃. Similarly, the ΔE value of 25XK was significantly higher than those of 4XK and -18XK (P＜ 0.05). Storage at 4 ℃ enabled better preservation of the energy metabolism in both XQ and XK kernels. Compared with those stored at 25 ℃ and -18 ℃, the ATP content, ADP content, and energy charge of kernels kept at 4 ℃ consistently maintained at relatively higher levels. The treatment of storage at 4 ℃ and -18 ℃ reduced POD activity in macadamia, significantly inhibiting the degradation of hydroperoxides derived from unsaturated fatty acids and thereby decreasing the formation of off-flavors. Besides, 4 ℃ significantly reduced the PPO activity and delayed the browning of macadamias. Furthermore, 4 ℃ was more helpful to maintain the activity of SOD and CAT. The SOD and CAT activities of 4XQ and 4XK were significantly higher than those of 25XQ, 25XK, -18XQ and -18XK (P＜0.05). The synergistic action of SOD and CAT efficiently eliminated reactive oxygen species (ROS), reduced oxidative damage to the membrane system. Storage at 4 ℃ effectively delayed the rate of MDA generation by maintaining higher activities of SOD and CAT.【Conclusions】Storage at 4 ℃ could effectively maintain the moisture content of the macadamia, inhibit the increase of color difference values, and sustain a high energy charge state, thus ensuring the ATP-dependent repair mechanisms. Additionally, 4 ℃ could maintain quality by reducing the activities of PPO/POD, sustaining high activities of SOD/CAT, and ensuring a stable energy supply. Therefore, 4 ℃ seems to be the optimal storage temperature for fresh macadamia nuts.