The long‐term storage of cold‐pressed oil from roasted rapeseed: Effects on antioxidant activity and levels of canolol and tocopherols

Comparison of chemical compositions and aroma characteristics of walnut oil prepared by different roasting processes

Walnut oil is an edible oil with high nutritional value, and the roasting process influences its quality and flavor. This study aimed to investigate the effects of roasting on the fatty acid composition, bioactive compounds (tocopherols, polyphenols, and phytosterols), and antioxidant capacity of walnut oil. Additionally, the aroma compounds and sensory characteristics were evaluated to comprehensively assess the variations in walnut oil after roasting. Roasting resulted in no notable impact on the fatty acid composition of walnut oil but increased the content of tocopherols and polyphenols in walnut oil, increasing its antioxidant capacity. Heavy roasting (160°C/20 min) reduced the phytosterol content in walnut oil by 2.3%. In total, 146 volatile compounds were detected in both cold-pressed and roasted walnut oil using headspace solid-phase microextraction-gas chromatography-mass spectrometry, and 32 key aroma compounds were identified. Aromatic aldehydes, aliphatic aldehydes, and heterocyclic compounds significantly contributed to fragrant walnut oil. Furthermore, the principal component analysis based on quality characteristics and sensory evaluation indicated that moderate roasting (130°C/20 min, 130°C/30 min, and 160°C/10 min) provided walnut oil with a sweet, nutty, and roasted aroma, as well as high levels of linoleic acid, phytosterols, and γ-tocopherol. Although heavy roasting (160°C/15 min and 160°C/20 min) enhanced the antioxidant capacities of walnut oils due to high levels of polyphenols, the oils exhibited an unpleasant burnt aroma. This study showed that roasting promoted the quality and flavor of walnut oil, and moderate conditions endowed walnut oil with a characteristic-rich flavor while maintaining excellent quality.

Effect of enzymatic Maillard reaction conditions on physicochemical properties, nutrition, fatty acids composition and key aroma compounds of fragrant rapeseed oil

BACKGROUND

A new enzymatic hydrolysis-based process inspired by the Maillard reaction can produce strong flavored, high-value rapeseed oil that meets safety requirements. In the present study, the effect of reaction time (10-30 min) and temperature (130-160 °C) on the physicochemical properties, nutritional status, fatty acids composition and key aroma compounds of fragrant rapeseed oil (FRO) was investigated.

RESULTS

An increasing reaction time and temperature substantially decreased the total tocopherol, polyphenol and sterol contents of FRO, but increased benzo[a]pyrene content, as well as the acid and peroxide values, which did not exceed the European Union legislation limit. Among the volatile components, 2,5-dimethyl was the main substance contributing to the barbecue flavor of FRO. The 150 °C for 30 min reaction conditions produced a FRO with a strong, fragrant flavor, with high total tocopherol (560.15 mg kg-1 ), polyphenol (6.82 mg kg-1 ) and sterol (790.65 mg kg-1 ) contents; acceptable acid (1.60 mg g-1 ) and peroxide values (4.78 mg g-1 ); and low benzo[a]pyrene (1.39 mg g-1 ) content. These were the optimal conditions for the enzymatic Maillard reaction, according to the principal component analysis. Furthermore, hierarchical cluster analysis showed that reaction temperature had a stronger effect on FRO than reaction time.

CONCLUSION

The optimal enzymatic Maillard reaction conditions for the production of FRO are heating at 150 °C for 30 min. These findings provide new foundations for better understanding the composition and flavor profile of FRO, toward guiding its industrial production. © 2023 Society of Chemical Industry.

Roasting treatments affect oil extraction rate, fatty acids, oxidative stability, antioxidant activity, and flavor of walnut oil

Introduction

The quality of pressed walnut oil can be improved by moderate roasting treatment.

Methods

This study compared physicochemical characteristics and antioxidant ability of walnut oils pressed from differently roasted pretreated walnuts, analyzed the correlation among these indicators by using Pearson correlation coefficient and correlation coefficient heatmap, and evaluated the volatile organic compounds (VOCs) of walnut oil under optimal pretreatment roasting conditions using headspace-gas chromatography-ion mobility spectrometry (HS-GC-IMS).

Results

Hierarchical cluster analysis (HCA) and principal component analysis (PCA) were able to remarkably distinguish walnut oil produced by different roasting processes. In addition, correlation analysis showed that there was a significant impact among indicators. There were 73 VOCs were identified in the optimum roasted treated walnut oil, consisting of 30 aldehydes, 13 alcohols, 11 ketones, 10 esters, 5 acids, 2 oxygen-containing heterocycles, 1 nitrogen-containing heterocycle and 1 other compound. GC-IMS results showed that aldehydes contributed significantly to the volatile flavor profile of walnut oil, especially (E)-2-heptenal, (E)-2-pentenal and hexenal.

Discussion

The properties of walnut oil based on varying roasting pretreatment of walnut kernels were significantly differentiated. Roasting at 120°C for 20 min is a suitable pretreatment roasting condition for pressing walnut oil. Roasting at 120°C for 20 min is a suitable pretreatment roasting condition for pressing walnut oil.

Comprehensive two-dimensional gas chromatography-time of flight mass spectrometry as a tool for tracking roasting-induced changes in the volatilome of cold-pressed rapeseed oil

The aim of this study was to track changes in the volatilome of cold-pressed oil and press cakes obtained from roasted seeds and to combine it with the profile of non-volatile metabolites in a single study, in order to understand pathways of volatile organic compound (VOC) formation caused by thermal processing. Comprehensive two-dimensional gas chromatography-time of flight mass spectrometry was used for the analysis of VOCs in cold-pressed oils and corresponding press cakes obtained after roasting of seeds at 140 and 180 °C prior to pressing. Contents of primary metabolites (amino acids, saccharides, fatty acids) as well as selected secondary metabolites (glucosinolates, polyphenols) were determined, as many of them serve as precursors to volatile compounds formed especially in thermal reactions. After roasting, the formation of Maillard reaction products increased, which corresponded to the reduction of free amino acids and monosaccharides. Moreover, levels of the products of thermal oxidation of fatty acids, such as aldehydes and ketones, increased with the increasing temperature of roasting, although no significant changes were noted for fatty acids. Among sulphur-containing compounds, contents of the products and intermediates of methionine Strecker degradation increased significantly with the increasing temperature of roasting. Degradation of glucosinolates to nitriles occurred after thermal treatment. The results of this study confirmed that seed roasting before cold pressing has a significant effect on the volatiles, but also indicated roasting-induced changes in non-volatile metabolites of oil and press cake. Such an approach helps to understand metabolic changes occurring during rapeseed processing in cold-pressed oil production.

Microwave pre-treatment of canola seeds and flaked seeds for increased hot expeller oil yield

Microwave (MW) pre-treatment of canola seeds or flaked seeds was found to be a superior alternative to the conventional thermal pre-treatment (steam). Flaked seeds were "cooked" (heat-treated) with steam or using microwave treatments in the temperature range of 62-130 °C prior to expeller pressing. Microwave cooking at 100 °C resulted in the highest increase in the pressed oil yield, which is an increase of 3.7% (w/w) on a pressed oil basis or 9.0% (oil in seed basis) compared with steam cooking. Whole canola seeds conditioning was conducted with microwaves or steam, in the temperature range of 40-75 °C, followed by microwave or steam cooking at 100 °C to evaluate the effect of MW treatment during conditioning on the expeller oil yield. The use of a continuous microwave process for combined conditioning of whole seeds at 55 °C and subsequent cooking of flaked seeds at 100 °C resulted in a 4.0% increase in expeller oil yield, compared with steam conditioning and cooking. The influence of dry basis (db %) moisture contents of 5%, 11.5%, and 16.5% on oil yield after steam or MW treatments of seeds and flaked seeds was also studied. The moisture content of 11.5% (db %) yielded the highest net oil yield for both MW and steam at best conditioning and cooking temperatures of 55 °C and 100 °C, respectively. No significant impact of MW cooking was seen on oil quality compared with conventional steam cooking.

Comparative Analysis of Rapeseed Oils Prepared by Three Different Methods

Flavoured rapeseed oils prepared using traditional technologies (oils A and B) and a fragrant rapeseed oil obtained using an enzymatic Maillard reaction (oil C) were analysed to show that oil C featured basic indicators and a fatty acid composition similar to those of traditional oils while exhibiting a higher comprehensive sensory evaluation score. Volatile component, odour activity value (OAV), and relative odour activity value (ROAV) analyses revealed that oil C had an elevated content of pyrazines (20.83%) and aldehydes (38.15%), which resulted in stronger charred and caramel flavours. The aroma of oil C was directly impacted by 3-methylbutyraldehyde (OAV > 1) and was modified by 3-methylthiopropionaldehyde and nonanal (RAOV > 1 in both cases). Thus, the developed technology was found to be well suited for the production of novel and safe fragrant rapeseed oil.

Preparation of rapeseed oil with superhigh canolol content and superior quality characteristics by steam explosion pretreatment technology

In this study, rapeseed was pretreated by steam explosion pretreatment technology and subsequently pressed to prepare rapeseed oil. GC, UPLC, and HPLC techniques were employed to analyze the quality characteristics of the rapeseed oil, including the canolol content and other quality characteristics. Additionally, the effect of steam explosion pretreatment technology on the canolol content of rapeseed oil was studied and the formation mechanism of canolol elucidated. The results revealed that when the steam explosion pressure reached 1.0 MPa, the canolol content of the tested oil increased from 41.21 to 2,168.69 mg/kg (52.63-fold increase) and that sinapic acid played a significant role in the conversion of canolol. Thus, the sinapine was converted into the intermediate (sinapic acid) by hydrolysis, which in turn was transformed into canolol through decarboxylation. The instantaneous high-energy environment generated by steam explosion pretreatment could intensify the hydrolysis and decarboxylation reactions of sinapine and sinapinic acid, thereby significantly increasing the canolol content of the oil. To prove the superiority of steam explosion pretreatment, we compared it with other pretreatment technologies, including traditional high-temperature roasting and popular microwave pretreatment. The results revealed that rapeseed oil prepared by steam explosion pretreatment displayed the best quality characteristics. This study can be a reference for the preparation process of rapeseed oil with superhigh canolol content and superior quality characteristics using steam explosion pretreatment.

Seed-Roasting Process Affects Oxidative Stability of Cold-Pressed Oils

The oxidative stability of vegetable oils mainly depends on their fatty acid composition, their degree of unsaturation, and the presence of compounds with antioxidant activity. This paper reports on the effects of the process of roasting oil seeds, prior to pressing them, on the basic characteristics of the oils produced and their oxidative stability. The differential scanning calorimetry (DSC) technique was used to study the process of oxidation of the oil samples in an oxygen-flow cell. Chromatographic analysis revealed that roasting the seeds increased the levels of chlorophyll and β-carotene in all the cold-pressed oils. Similar results were observed for the oil's antioxidant activity, measured by the scavenging 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical method. Our results also indicated that roasting seeds prior to pressing them for oil had a positive effect on the oil's stability, as determined by the DSC method. This manifested in both the extension of oxidation induction time and the final oxidation time.

The effect of microwave pre-treatment of rapeseed on the degradation kinetics of lipophilic bioactive compounds of the oil during storage

This study examined the storage stability of tocochromanols and carotenoids in the oils prepared from microwave pre-treated (MV) rapeseeds (2-10 min, 800W) during storage at 20 °C for 12 months. In line with lipophilic antioxidant degradation throughout the storage period, changes in the antioxidant capacity of the oil were monitored. Microwaving significantly affected the concentration of lipophilic antioxidants in the oil. After 10 min of MV pre-treatment the highest content of total tocochromanols (76.64 mg/100g) was achieved, whereas a maximum carotenoid concentration (861.28 μg/100g) was obtained following 6 min seed MV pre-treatment. The degradation kinetics for the tocochromanols and carotenoids followed a zero-order kinetic model. From the kinetic analysis, it was shown that the degradation rate constant (k) of both tocochromanols and carotenoids decreased with longer seed exposure to MV radiation. The kinetics of antioxidant capacity degradation during the storage of oils followed a zero-order reaction. The rate of antioxidant capacity degradation in the control oil was higher (k=9.1 x 10-2 mmol TEAC/l/month) compared with oils prepared from MV pre-treated seeds (k=6.8-8.0 x 10-2 mmol TEAC/l/month).