Ginger (cultivar UG1, also known as “white variety”) rhizomes were harvested at their peak maturity and thoroughly washed under running water to remove any foreign materials. Thereafter, the rhizomes were manually sorted to remove all pest-infested ones and divided into four batches to await storage treatment.

All the reagents used for the laboratory tests were of analytical grade, manufactured and mainly produced by Merck KGaA Company in Germany. The tests were conducted in triplicate and the mean values documented accordingly. The recovery rates for the Certified Reference Materials (CRMs) fell within the range of 91 and 103%.

The atherogenicity index (AI), thrombogenicity index (TI), and the ratio of hypocholesterolemic to hypercholesterolemic (HH) of the fatty acids, were computed from Equations 2, 3 and 4. These indices are used to evaluate the health benefits of the oil in relation to cardiovascular and heart diseases (Aremu et al. 2023; Rabiej-Kozioł et al., 2023).

The SPSS (version 20.0) and Matlab (version 20.0) were used to carry out the statistical analysis and modeling of the experimental results. The effects of rhizome storage temperature on physiochemical parameters, vitamin C, phytochemical properties, fatty acid composition and dielectric parameters were determined by one-way analysis of variance; while the means were separated into significance levels of 0.05, using the Duncan’s Multiple Range Test (DMRT)

The results of the vitamin C contents and physiochemical properties of the various GO batches are presented in Table 1. It was seen that apart from the moisture content, the heat treatment had a significant effect on the remaining GO physiochemical parameters investigated. Notably, the GO SV ranged from 164.07 to 216.81 mg KOH/g, acid value varied from 3.79 to 4.16 mg KOH/g, FFA content ranged from 1.90 to 2.08 mg KOH/g, peroxide value varied from 10.40 to 13.90 meq O₂/kg, MC value ranged from 0.73 to 0.75%, iodine value varied from 82.37 to 101.13 gI₂/100 g, and the KV ranged from 122.00 to 164.00 mm²/s. The high AV and FFA recorded in the GO is similar to the observation by Aremu (2023) and Prasad and Tyagi (2015), who stated that ginger oil contains significant amounts of organic acids. FFA, PV, and AV are crucial physiochemical factors which affect the phytochemical properties of oils (Abdiani et al., 2024). However, the results show that the GO iodine value decreased as the AV, PV and FFA in the oil underwent a continuous increase. Elevated FFA, PV and AV contents are indicators of poor oil quality as they signify that the oil is highly susceptible to oxidation and rancidity (Esfarjani et al., 2019). Oils with high PV are linked to cardiovascular diseases, cancers, and obesity, reducing their suitability for dietary use.

Remarkably, the rhizomes stored under T1 produced the GO with the lowest concentrations of SV, AV, FFA, PV and KV; while rhizomes stored under T2 produced GO with the highest concentration of SV, AV, FFA, PV and KV. Likewise, the rhizomes kept at lower temperature (T1) yielded oil with highest IV. This is an indication that the elevated temperature accelerated the process of rancidity and the degradation of glycerides within the ginger rhizomes’ oil cells. High temperatures facilitated the UFAs oxidation, leading to the formation of peroxides and other oxidation by-products. These substances further degraded into smaller molecules such as free fatty acids, thereby altering the oil’s acidity and electrical properties (Tian et al., 2023; Abdiani et al., 2024). Iodine, saponification, peroxide, and acid values are essential parameters used to assess oil quality. The SV and AV offer reliable insights into the molecular weight and the carboxylic acid groups of fatty acids present in oil. The IV and SV indicate the level of unsaturation in oil, while the PV reflects the degree of oxidative deterioration. Additionally, the AV and IV are commonly used to predict the electrical and fuel properties of oils.

With respect to the vitamin C content in the oil, it was observed that the vitamin C levels in the Natural, T1, T2 and T3 oil samples were 72.58, 86.99, 76.15 and 63.34 mg/L, respectively. It was noted that the storage temperature had a significant impact on the GO’s vitamin C level, with the highest and lowest vitamin C contents recorded in the rhizomes stored at 20 ^oC and 40 ^oC, respectively (Table 1). Vitamin C is a heat and UV radiation sensitive water-soluble vitamin; hence, the higher temperature could be linked to the depleted vitamin C concentration in the oil produced from Natural and T4 storage conditions. High temperatures facilitate the oxidation of vitamin C to dehydroascorbic acid and other inactive substances (Abdiani et al., 2024). Also, the lower AV, PV and FFA values recorded in the T1 oil specimen can be linked to the higher vitamin C content documented in the T1 oil, as those physiochemical parameters have the ability to degrade and oxidize natural vitamins and antioxidants. Vitamin C is a major antioxidant which plays a critical role in defending the body against oxidative stress by scavenging reactive oxygen species (Kim et al., 2017).

Table 2 presents the results of the GO phytochemical analysis. It was observed that the concentration of phytochemical compounds depreciated as the environmental temperature rose. Generally, the oil produced from rhizomes kept at 20 ^oC (T1) tended to have the highest percentage of phytochemical compounds, while from rhizomes stored at 40 ^oC (T3) produced GO with the lowest percentage of phytochemical compounds. According to Mao et al. (2019), heat treatment and storage duration can alter the phytochemical compositions of ginger rhizomes’ essential oil.

Interestingly, 25 out of the 26 phytochemical compounds analyzed in ginger oil are phenolic compounds with potential antioxidant properties, highlighting the remarkable antioxidant potential of ginger oil. It was noted that Lunamarin was the only non-phenolic and non-antioxidant compound detected in the ginger oil. Also, it was noted that catechin, artemetin, epicatechin, quercetin, vanillic acid and myricetin were the predominant compounds present in the GO, regardless of the treatment applied. Remarkably, while heat treatment leads to a reduction in essential phenolic compounds, it effectively detoxifies the lunamarin content in the oil by accelerating its hydrolysis, thereby reducing the oil’s cyanogenic potential.

The total phenolic compound (THC) contents in the ginger oil ranged from 78.91 to 102.56 mg/kg, which is within the THC range reported for cottonseed oil (Zhao et al., 2021). The THC level in the GO was greater than the THC values documented for walnut (Juglans regia L.) oil, palm kernel oil, rice (Oryza sativa) bran oil, and maize (Zea mays) oil (Mikolajczak et al., 2021); but was lower than the amount reported for pomegranate (Punica granatum) seed oil (Drinic et al., 2020) and grape (Vitis vinifera L.) seed oil (Mikolajczak et al., 2021). The wide variation in THC observed in the investigated oils could be attributed to the total number of quantified phenolic compounds, prevailing climatic conditions, as well as the processing, storage, and testing methods used (Tian et al., 2023; Uguru et al., 2023). Phenolics are essential natural compounds that help slow the oxidative degradation of oils, thereby preserving their nutritional values, functional properties and electrical properties.

The fatty acids profile results of the GO are presented in Table 3. It was observed that the considerable initial amount of SFA tended to increase as the storage temperature increased. The Natural, T1, T2 and T3 GO sample SFA values were 41.91, 36.37, 39.13 and 45.47 ppm, respectively. Palmitic, oleic and linoleic acids were the most prominent SFA, MUFA and PUFA in the oil, respectively. Apart from palmitic acid, other notable SFA in the GO were caprylic, capric, lauric and myristic acids; while other PUFA present in the GO in appreciable amounts were linolenic, arachidonic, osbond, and docosahexaenoic acids. The substantial presence of linoleic, oleic and palmitic acids found in the current study’s GO closely aligns with the findings reported by Ramadan (2020) and Oforma et al. (2019). Additionally, The SFA quantity recorded in this research waslower than those documented for palm oil (Choudhary and Grover, 2019) and coconut oil (Dorni et al., 2018), but greater than the results reported for pumpkin, sunflower and pomegranate seed oil (Drinic et al., 2020; Khalili Tilami and Kouřimská, 2022).

It was also observed that amount of lauric acid in the GO was lower than the value reported for coconut oil (Dorni et al., 2018). Lauric acid is an UFA known for its strong antimicrobial and anti-inflammatory properties, and it may also support cardiovascular and skin health. The palmitic acid value of the GO was lower than the result recorded for palm oil (Choudhary and Grover, 2019). Palmitic acid is a significant SFA which is crucial for ATP production and possesses antimicrobial properties. However, its toxicity is linked to an increased risk of cardiovascular disease. Linoleic acid, a key essential omega-6 fatty acid abundantly found in GO, has the potential to exhibit anti-inflammatory properties, lower blood LDL levels, and boost the body’s immune system (Tian et al., 2023).

Furthermore, it was noted that the ginger oil’s AI, TI and HH values ranged from 0.54 to 0.75, 0.38 to 0.55, and 1.81 to 2.47, respectively. Although the ginger oil TI and AI values increased with rising storage temperatures, their values remained below 1. Furthermore, the HH values were greater than 1.0, regardless of the treatment applied. The results of this research GO’s AI and TI values were less than the values (AI ~0.91, TI ~0.80) recorded by Aremu et al. (2023) for ginger oil; but were higher than the values (AI ~0.04, TI ~0.23) for cold-pressed sunflower oil (Rabiej-Kozioł et al., 2023). Similarly, the HH values recorded in this study were lower than those documented for cold-pressed pumpkin oil (6.47) and sunflower oils (13.23) by Rabiej-Kozioł et al. (2023).

Atherogenicity index, TI, and HI are nutritional indices used to evaluate the suitability of oils for human consumption, as their values are used to determine the possibility of oil causing health hazards, such as blood clots and thrombus conditions. From a nutritional perspective, low values of AI and TI are preferred in the human diet. Additionally, oils with higher HH values are more beneficial for human health (Aremu et al., 2023). Remarkably, though the SFA value of the oil was relatively high, the higher PUFA: SFA and HH values (˃1), coupled with lower AT and TI values (<1), are indications that GO is beneficial for heart health and preferred in the human diet. Oils with lower PUFA: SFA and HH values (<1), and elevated AT and TI values (˃1) are linked to increased risk of heart disease, through increased cholesterol levels (Rokosik et al., 2020; Khalili Tilami et al, 2022).

The results of the GO dielectric constant are presented in Figure 1. It was observed that the ɛʹ of the natural condition, T1, T2 and T3 were 3.152, 3.052, 3.128 and 3.258, respectively, which is an indication that the heat treatment had a substantial effect on the electrical properties of the oil. Interestingly, the ɛʹ values of the GO were comparable to those reported by Elmosalami et al. (2022) for olive and corn oils. The increase in the GO’s ɛʹ values with rising storage temperatures can be attributed to the oxidation of the oil components in the ginger rhizomes, resulting in oil with higher AV, FFA and PV parameters of the oil (Table 1). This process is accompanied by increased acidity of the oil, which consequently leads to a rise in the dielectric constant (Šegatin et al., 2020). It was observed that the oil’s ε′ value tended to increased unevenly as the SFAs content in the oil increased. A high acid value is an indication of oil degradation through the hydrolysis and oxidation of the unsaturated fatty acids (Table 3). This generates more saturated fatty acid (a more polar compound) which produce more dielectric constant charges within the oil (Inoue et al., 2002).

The GO ɛʹ values recorded in this study were higher than the values reported for safflower and sunflower oils, but smaller when compared to the documented corn oil ɛʹ values (Lizhi et al., 2008). The higher ɛʹ value of T3 GO will enhance the oil’s capacity to minimize electrical discharges, thereby upholding the reliability of the electrical equipment in which it is used. A high ɛʹ generally increases the insulating and electrical energy storage competences of oil, further improving its effectiveness in high voltage and capacitance applications (Lim et al., 2023). The dielectric constant value is dependent on the acids and structural compositions, moisture level, viscosity and temperature of the oil (Lizhi et al., 2008). According to Elmosalami et al. (2022), in addition to their relevance in the electrical sector, dielectric properties are essential for assessing the nutritional and pharmaceutical qualities of edible oils.