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Abstract
Temperature and relative humidity (RH) influence post-harvest ripening, a crucial stage for quality promotion in some oil plants or fruits. Torreya grandis cv. Merrillii nuts, which are rich in unsaturated fatty acids (UFA), are easily affected by temperature and humidity, and they oxidize quickly during the post-harvest ripening stage, leading to the deterioration of nut quality. In this study, the main nutraceutical components, fatty acid composition, and related metabolic parameters of lipid rancidity under four treatments (20°C and 70% RH, T20-LH; 30°C and 70% RH, T30-LH; 20°C and 90% RH, T20-HH; 30°C and 90% RH, T30-HH) were measured. The post-harvest ripening process was advanced under HH treatments (T20-HH and T30-HH) compared to LH treatments (T20-LH and T30-LH) and was associated with a shorter time for the seed coat to turn dark black and a faster reduction in starch content. The amount of unsaturated fatty acids significantly increased under the T20-HH treatment, but significantly decreased under the T30-HH treatment from 12 to 16 d of ripening time. The acid value (AV) and lipase activity under the T30-HH treatment remained virtually constant from 12 to 16 d of ripening time, and this was accompanied by a dramatic increase in peroxide value (POV), lipoxygenase (LOX) activity, and relative expression of the LOX2 gene. Meanwhile, a significant positive correlation between LOX activity and POV, malondialdehyde (MDA) content, and O2⋅– content was observed. The results imply that the lower amount of oxidative rancidity induced by the T20-HH treatment is related to the LOX activity induced by down-regulation of the LOX2 gene during the late after-ripening stage. Therefore, the T20-HH treatment not only promoted the post-harvest process of T. grandis ‘Merrillii’ nuts but also delayed lipid oxidation, which was ultimately associated with better oil quality at the late after-ripening stage.
Highlights
The moisture content of kernels significantly decreased by 26.5% and 30.2% under the T20-LH and T30-LH treatments from 4 to 16 d of after-ripening time, respectively, and the moisture significantly decreased by 3.5% and 5.8% under the T20-HH and T30-HH treatments, respectively
The cell wall of interior seed coat was intact on 0 d, and it degraded with increasing after-ripening time (Figures 3A–D)
Our results showed that the peroxide value (POV) and LOX activity under the T30-HH treatment increased significantly by 14.9% and 32.7% from 12 to 16 d of afterripening time, and this was accompanied by a significant positive correlation between the LOX activity and POV, indicating that primary oxidation rancidity induced by LOX activity might be the main reason for unsaturated fatty acids (UFA) degradation under the T30-HH treatment during the late after-ripening stage
Summary
T. grandis nuts are popular in China because of their good flavor, nutrientrich properties and high oil content (approximately 42–54%). Unsaturated fatty acids (UFAs) account for the vast majority (approximately 80%) of the total oil content and are mainly composed of oleic acid (C18:1), linoleic acid (C18:2), and sciadonic acid (C20:3), making T. grandis unique among other nuts (Ni and Shi, 2014; Wu et al, 2018). The demand for T. grandis seeds has been increasing; this species is being cultivated in various locations in China (Chen et al, 2008). The cultivation area of T. grandis has rapidly expanded, increasing approximately 11.5-fold from 2000 to 2012 (He et al, 2013). T. grandis nuts are different from other tree nuts, such as hickory and walnut, in that they need a post-harvest ripening stage for oil accumulation and favorable fatty acid composition before roasting (Ye et al, 2017)
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