Properties of Oil Extracted from Oat Grains before and after the Roasting Process Conducted under Different Conditions ^†

Abstract

Oat (Avena sativa L.) grains are an important source of protein, minerals and oil. They contain at least twice as much fat as other common cereals, such as wheat, barley or rye, and the fat contained in oat grains significantly contributes to their properties, including improved calorific value and nutritional quality. Heat treatment, particularly roasting, has long been considered the essential method of processing grains, as roasting effectively extends shelf life, provides a pleasant roasted flavor and improves taste. The paper covers the improving effect of the roasting process of the whole oat grain on the quality of the oil contained in the grains. A differential scanning calorimeter coupled with a high-pressure cell has been used to determine the oxidative stability of oil. A calorimetric bomb was applied to determine the whole grain calorific value and GC (Gas Chromatography) determined the fatty acid composition of oil.

Published: 14 October 2023

1. Introduction

Oat (Avena sativa L.) is an ancient cereal crop that has been cultivated at least since the time of Theophrastus (371–286 BC) [1]. This seed is widely cultivated around the world. The global production of oat increased by around 11% from 2019 to 2020, reaching more than 25 million metric tons in 2020. It has always been considered a valuable crop due to its nutritional properties [2].

Oat grains consist mainly of starch (39–55%), proteins (9–16%), lipids (2–18%) and dietary fiber (20–39%). Oats were not used as a source of edible oil because their amount in the seeds is quite small compared to oilseeds; however, they contain at least twice as much fat as other common cereals, such as wheat, barley or rye, and the fat contained in oat grains significantly contributes to its properties, including improved calorific value and nutritional quality. Therefore, the oil contained in oats has nutritional and technological potential [3].

Roasting is the process that causes substantial sensory improvement due to physical, chemical and structural changes occurring during seed processing. Roasting changes the texture, color, flavor and appearance of the seeds, and the resulting product has unique crunch and flavor characteristics compared to raw seeds. This process also modifies the profile of phenolic compounds, and in some cases, it improves their health benefit by enhancing their antioxidant capacity [4].

Whole grains of the Bingo oat variety were roasted in a laboratory oven. Roasting was carried out at 100 °C and 130 °C for 20 and 40 min. Then the oil was extracted from the oats using the Soxhlet method.

The aim of the current investigation is to establish and discuss the differences in the quality characteristics of oils obtained from oats before and after roasting in various conditions.

2. Methods

2.1. Fat Extraction from Oat

The oat was grounded before extraction. The fat was extracted according to the procedure depicted by Boselli et al. [5] and Dolatowska-Żebrowska et al. [6].

2.2. GC Analysis

The fatty acid composition of the fat extracted from the oat was determined as fatty acid methyl esters, according to the ISO method. The YL6100 (Young Lin Bldg., Anyang, Hogyedong, Republic of Korea) gas chromatograph equipped with a flame ionization detector and a BPX-70 capillary column (SGE Analytical Science, Milton Keynes, UK) was used. The determination was carried out according to the procedure described by Bryś et al. [7].

2.3. Oxidative Stability by Pressure Differential Scanning Calorimetry (PDSC) Method

A PDSC instrument (DSC Q20 TA Instruments, Newcastle, WA, USA) was used to determine the oxidative stability of oils. The experiment was conducted at a constant temperature of 120 °C under 1400 kPa of oxygen. The procedure for determining PDSC induction time was described by Symoniuk et al. [8].

2.4. Calorimetric Bomb Method

The calorific value of the whole grains has been determined by using the calorimetric bomb in correspondence with ISO standards 1928:2009 [9].

2.5. Acid Value Determination

Acid value (AV) was determined by titration with 0.1 M ethanolic potassium hydroxide in correspondence with ISO standards 660: 2009 [10].

2.6. Peroxide Value Determination

Peroxide values (PV) of oils were determined by the iodometric technique in correspondence with ISO standards 3960: 2007 [11].

3. Results and Discussion

Although several methods have been used to analyze and monitor lipid oxidation [12], oxidation reactions cannot be controlled by a single method due to their complexity. One of the methods used to determine oxidative stability was the PDSC. This method allows for determining the Oxidation Induction Time (OIT). The longer the OIT, the more oxidatively stable an oil sample is. Taking into account the obtained results (Figure 1), it can be concluded that the oil extracted from the oat that has been previously roasted is characterized by a higher OIT than the oil extracted from the unroasted oat grains. Oil extracted from oats roasted at lower temperatures and for a shorter time is more oxidatively stable than oil extracted from oats roasted at higher temperatures and for a longer time. This is graphically presented in Figure 1.

Considering the results regarding the calorific value of oats (Figure 2), it is concluded that heat treatment does not result in statistically significant differences in the calorific value of oat grain. A slight increase in calorific value, however statistically insignificant, was observed only for samples roasted at a lower temperature. In the studies by Valendes et al. [13], microwave torrefaction of oat hull was conducted to change its physicochemical properties. The results obtained showed that the heat treatment conducted at high temperatures increased the calorific values while decreasing mass yield.

Considering the composition of fatty acids in the investigated samples (

Table 1. Fatty acid composition present in both unroasted and roasted oat’s oils.

Fatty Acid	Unroasted	100 °C/20 min	100 °C/40 min	130 °C/20 min	130 °C/40 min
C14:0	0.23 ± 0.14	0.35 ± 0.01	0.55 ± 0.30	0.35 ± 0.01	0.40 ± 0.08
C16:0	17.73 ± 0.35	16.26 ± 0.08	19.04 ± 4.99	16.12 ± 0.35	16.29 ± 0.25
C16:1	0.240 ± 0.001	0.32 ± 0.01	0.38 ± 0.09	0.32 ± 0.02	0.34 ± 0.01
C18:0	1.89 ± 0.14	1.380 ± 0.001	1.28 ± 0.17	1.35 ± 0.03	1.39 ± 0.10
C18:1 n-9	42.10 ± 0.18	37.00 ± 0.02	35.95 ± 2.40	37.02 ± 0.14	37.41 ± 0.46
C18:2 n-6	34.62 ± 0.13	40.090 ± 0.007	38.29 ± 2.28	39.99 ± 0.10	38.68 ± 1.56
C18:3 n-3	1.20 ± 0.01	2.03 ± 0.01	1.84 ± 0.22	2.06 ± 0.01	1.94 ± 0.08
C20:0	0.31 ± 0.01	0.19 ± 0.01	0.23 ± 0.04	0.19 ± 0.04	0.32 ± 0.14
C20:1	1.68 ± 0.04	1.16 ± 0.02	1.16 ± 0.23	1.18 ± 0.64	1.51 ± 0.52
Others	1.080 ± 0.001	1.19 ± 0.07	1.26 ± 0.16	1.410 ± 0.001	1.70 ± 0.61

, Figure 3), it is concluded that unsaturated fatty acids dominate the fatty acid composition of oat oil. Among the monounsaturated acids, oleic acid is the most abundant (range of 36-37%), while among the polyunsaturated acids, linoleic acid is the most abundant (range of 38-40%). Oat oil does not contain a lot of saturated fatty acids. Among the saturated acids, palmitic acid is the most abundant. Results obtained by other scientists agree that oat oil is considered to be a good source of polyunsaturated essential fatty acids, and three fatty acids dominate in oats—oleic, linoleic and palmitic acids [14,15]. The proportion of unsaturated fatty acids (oleic, linoleic and linolenic) in oats is about 75% of all fatty acids [16].

It was observed that the roasting process did not significantly affect the composition of fatty acids.

The acid value (AV) is an indicator of the degree of oil hydrolysis. Considering the results obtained, it is concluded that, in general, the heat treatment causes increases in the acid value (Figure 4). The higher the roasting temperature, the higher the degree of hydrolysis of the oil from roasted oats. Other studies have also reported a rise in the AV of oil in the case of the walnut roasting process [17].

The peroxide value is an indicator of the content of primary oxidation products. Considering the obtained results (Figure 5), it is concluded that oil from grain processed at a high temperature (130 °C) has a higher peroxide value compared to oil from oat seeds roasted at 100 °C. It has also been observed that heat treatment generally increases the peroxide value. Another study reported that increasing the roasting time increased the peroxide value of sesame seed oil [18].

4. Conclusions

The conclusions drawn in the current investigation are based on the determination of oxidative induction time using a PDSC, acid values by titration, calorific value using calorimetric bomb and fatty acid compositions using Gas Chromatography. The oxidative stability of the oil increased after the roasting of oat grains. Oil from roasted oat grains is characterized by a lower acid and peroxide value than oil from grains that have not been thermally treated. The higher the temperature and the longer the roasting time, the worse the hydrolytic and oxidative quality of the oil. Roasting does not cause significant changes in the composition of fatty acids.