3.1. Properties of Oleogels
Changes in the composition of fatty acids are indicative of oxidative stability and the nutritional attributes of oils and fats. Oils constituting more unsaturated fatty acids than saturated fatty acids are easily oxidized as the unsaturated fatty acid components have elevated amounts of double bonds, which decrease oxidative stability [
28].
The fatty acid composition of the oleogel oil is shown in
Table 1. The hemp oil (control) and oleogels contained similar polyunsaturated fatty acids (PUFAs), saturated fatty acids (SFAs), and monounsaturated fatty acids (MUFAs). In quantitative terms, α-linolenic acid (C18:3) and linoleic acid (C18:2) were the main fatty acids present in the hemp oil and the oleogels (approximately 93%). The initial saturated fatty acid (SFA) content in hemp oil was low (9.22%), with monounsaturated fatty acids (MUFAs) having a content level of 10.28% and polyunsaturated fatty acids (PUFAs) reaching a content level of 80.47%. The SFA and MUFA contents decreased during the storage period in all samples. The PUFA concentrations increased in all samples from day 1 until the end of the storage period. The Omega 6/3 ratio of the hemp-seed-oil samples decreased slightly during the storage period.
The addition of candelilla and rice bran waxes to hemp seed oil may be a feasible and pragmatic approach to protect oil from oxidation during storage. Moreover, using natural waxes may provide a novel opportunity for producers and manufacturers to preserve the unsaturated fatty acids in oils. This approach can be further utilized to develop oleogels derived from candelilla and rice bran waxes to ensure the preservation of vegetable oils.
The oxidative stability results of the hemp seed oil and oleogels are presented in
Table 2. The oxidative stability index (OSI), expressed as the induction period, shows the relative resistance of fats and oils to oxidation. The induction period (IP) value for the hemp-oil sample (control) was 3.10 h. The results revealed that hemp oil exhibited a shorter (
p < 0.05) induction period than the oleogels. A similar tendency was determined for an oleogel made from soybean oil [
29]. This indicates that natural waxes have a positive effect on preventing the oxidation of hemp oil.
The oleogels with rice wax (O-R5 and O-R7) exhibited significantly higher induction period values than those produced with candelilla wax (approximately 8%). However, it was observed that increasing the concentration of waxes in the oleogel shortens the induction period in both oleogel matrices.
These results are in agreement with the tendency determined for rapeseed oil, in which the stability of the oleogel depends on the properties and fatty acid composition of the oleogelator used [
30].
Among different natural waxes, candelilla wax and rice bran waxes have been found to possess powerful gelling capabilities [
31] which can decrease oxidation rates. This fact is beneficial to the nutritional quality of the oleogels, considering the high content of unsaturated fatty acids and the oxidation rate of the oil. Changes in the chemical stability of hemp oil and oleogels during storage at room temperature for 50 days are presented in
Table 2.
A higher peroxide value indicates lower oxidative stability. The peroxide value of hemp oil gradually increased during storage; however, a significant decrease was observed at the end of the storage period. The same pattern of change was observed for the peroxide values of the oleogels. The peroxide value of hemp oil increased at a faster rate during storage and reached a higher value than that of the oleogels.
A high temperature (100 °C) was used during oleogel preparation, which could have a negative effect on the oxidation of the oleogels. For this reason, oleogel samples sometimes exhibit higher peroxide values than fresh oil, as has been found for canola oil (Lim, 2017). However, this tendency has not been observed for flaxseed-oil-based oleogels [
32]. An analysis of the oleogels prepared in our study did not reveal significant differences in the peroxide values of the hemp oil and oleogels produced with the hemp oil.
The results suggested that, as the oleogels became harder their peroxide value lowered. This situation may arise because of the restriction of oil mobility and because migration via organogelation was effective in retarding oil oxidation during storage [
33].
In our study, the oleogels revealed similar oxidation patterns to those of the control hemp-oil samples observed over the storage period, thus confirming the effects of several oxidation products provided by the mixture of waxes and hemp oil mixes. The peroxide values of all oleogel samples on day 20 were lower than that of the control hemp oil sample. The peroxide values of all samples reached their peaks at 25 °C on day 33.
Our findings are congruent with the results of other research groups, revealing a slower increase in peroxide value in oleogels prepared using rice bran wax and candelilla wax than in control oil [
34]. On the other hand, the olive-oil-candelilla-wax oleogel demonstrated a higher peroxide value than stack oil at 20 °C [
35]. The reasons for such a variation in peroxide value and rate of change could be attributed to the differences in experimental conditions such as sample preparation, proportion of surface area to the bulk of samples, storage temperature of samples, parameters of technological process, etc.
Candelilla and rice bran waxes demonstrated significant potential in delaying the time taken to attain peak peroxide value for the first 33 days of storage when compared to hemp oil. Plant compounds reduce lipid oxidation because of their radical scavenging capacity. Phenolic structures may delay the onset of oxidation due to the decline of hydroperoxides. Several phenolic compounds have distinct capabilities for postponing lipid oxidation. These different effects are traditionally explained by the diversity in their structure [
36].
Free fatty acids (FFA) are formed by the hydrolysis of ester bonds in triglycerides by certain lipases. Our results showed that the acidity values (AV) and FFA content of the hemp-oil samples increased at the end of their storage period. Notably, the AV and FFA values of hemp oil were significantly higher than those of the oleogels.
There was a steady rise in the FFA content of the oleogel samples as well as in the control oil sample by day 33. The FFA content of the hemp-oil samples was higher than that of the oleogels.
On day 1, the acidity values of the oleogels were similar to those of the hemp-seed-oil samples. However, on day 33, the acidity of the hemp-oil samples reached 3.44 KOH/g oil, while for oleogels this value was significantly lower (
Table 2). When compared to the control oil samples, the acidity values increased slightly until their peak point during storage. At the end of the storage period, the acidity values of rice-bran-wax oleogel samples were calculated to be 1.86 KOH/g oil for sample O-R5 and 2.00 KOH/g oil for sample O-R7. The acidity values measured for candelilla-wax oleogel samples were 1.96 KOH/g oil for sample O-C3 and 2.10 KOH/g oil for sample O-C7 mg KOH/g oil.
The high concentration of wax in hemp oil provides a higher degree of firmness, denser crystal networks, and higher melting points for oleogels’ physical properties [
31]. To understand the effect of wax concentration on oil preservation, oleogels produced with two different concentrations of hemp oil were analyzed. The contents of candelilla and rice bran waxes did not affect the initial peroxide values. After six days of storage at 25 °C, the oleogel sample OR5 showed a lower PV than the remaining oleogel samples. Our results indicate that higher wax concentrations do not provide enhanced protection from oxidation of hemp oil.
The results of the conjugated diene (CD) value estimation showed that the oleogels exhibited a similar behavior (
Table 2) to that of hemp oil.
The study results also indicate that increasing the concentration of rice bran wax may have a reverse effect on the oxidation protection of hemp oil. This may be attributed to the pro-oxidant effect of the rice bran wax. Interestingly, the peroxide values of the oleogel samples prepared on day six did not demonstrate significant pro-oxidant action, perhaps because of the shorter incubation/storage period, which was insufficient to display the desired effects. Similarly, sample OC3 showed a longer induction period (p < 0.05) at 3.65 ± 0.02 h when compared to sample OC7 with an incubation time of 3.45 h. This supports our conclusion that a higher rice-bran-wax content may have a pro-oxidant effect on hemp seed oil.