Coriander seed oil methyl esters as biodiesel fuel: Unique fatty acid composition and excellent oxidative stability

Abstract

Coriander (Coriandrum sativum L.) seed oil methyl esters were prepared and evaluated as an alternative biodiesel fuel and contained an unusual fatty acid hitherto unreported as the principle component in biodiesel fuels: petroselinic (6Z-octadecenoic; 68.5 wt%) acid. Most of the remaining fatty acid profile consisted of common 18 carbon constituents such as linoleic (9Z,12Z-octadeca-dienoic; 13.0 wt%), oleic (9Z-octadecenoic; 7.6 wt%) and stearic (octadecanoic; 3.1 wt%) acids. A standard transesterification procedure with methanol and sodium methoxide catalyst was used to provide C. sativum oil methyl esters (CSME). Acid-catalyzed pretreatment was necessary beforehand to reduce the acid value of the oil from 2.66 to 0.47 mg g⁻¹. The derived cetane number, kinematic viscosity, and oxidative stability (Rancimat method) of CSME was 53.3, 4.21 mm² s⁻¹ (40 °C), and 14.6 h (110 °C). The cold filter plugging and pour points were −15 °C and −19 °C, respectively. Other properties such as acid value, free and total glycerol content, iodine value, as well as sulfur and phosphorous contents were acceptable according to the biodiesel standards ASTM D6751 and EN 14214. Also reported are lubricity, heat of combustion, and Gardner color, along with a comparison of CSME to soybean oil methyl esters (SME). CSME exhibited higher oxidative stability, superior low temperature properties, and lower iodine value than SME. In summary, CSME has excellent fuel properties as a result of its unique fatty acid composition.

Introduction

Biodiesel is defined as the monoalkyl esters of long-chain fatty acids prepared from vegetable oils, animal fats, or other lipids [1], [2]. Advantages of biodiesel over conventional petroleum diesel fuel (petrodiesel) include derivation from renewable feedstocks, displacement of imported petroleum, superior lubricity and biodegradability, lower toxicity, essentially no sulfur content, higher flash point, and a reduction in most exhaust emissions. Disadvantages include inferior oxidative and storage stability, lower volumetric energy content, reduced low temperature operability, and higher oxides of nitrogen exhaust emissions [2], [3]. Biodiesel must be satisfactory according to accepted fuel standards (Table 1) such as ASTM D6751 [1] in the United States or the Committee for Standardization (CEN) standard EN 14214 [4] in Europe before combustion in diesel engines.

Feedstock availability for biodiesel production varies considerably according to geography and climate. Thus, rapeseed/canola oil is principally used in Europe, palm oil predominates in tropical countries, and soybean oil and animal fats are primarily used in the United States [2], [3]. However, the combined supply of these fats and oils is sufficient to displace only a small percentage of petrodiesel at current usage levels. Consequently, alternative feedstocks for biodiesel production have attracted considerable attention, as evidenced by recent reports on jatropha (Jatropha curcas L.) [5], wild mustard (Brassica juncea L.) [6], field pennycress (Thlaspi arvense L.) [7], moringa (Moringa oleifera L.) [8], and camelina (Camelina sativa L.) [9] oils, among numerous others [3].

Vegetable oils are generally composed of five common fatty acids (FA): palmitic (hexadecanoic), stearic (octadecanoic), oleic (9Z-octadecenoic), linoleic (9Z,12Z-octadeca-dienoic), and linolenic (9Z,12Z,15Z-octadecatrienoic) acids [3], [10]. Other FA such as erucic (13Z-docosenoic) acid may be found in plant oils from the Brassicaceae family, of which wild mustard [6] and field pennycress [7] are examples. Biodiesel prepared from vegetable oils containing a significant percentage of less common FA are largely unreported, with the exceptions of capric (decanoic) acid-containing cuphea (Cuphea viscosissima × C. lanceolata) [11] oil and lauric (dodecanoic) acid-containing coconut [12], palm kernel [12], and babassu [13] oils. Fuel properties of biodiesel are largely dependant on the FA composition of the lipid from which it was prepared [3], [10]. As a result, biodiesel fuels with different FA compositions have different fuel properties and may serve as models for other oils with similar FA profiles. Additionally, such oils may guide the genetic modification of existing oilseed crops for optimum biodiesel fuel properties [14]. A plant of commercial significance that contains a vegetable oil with an unusual FA profile is coriander.

Coriander (Coriandrum sativum L.), also known as cilantro, is an annual herb belonging to the Apiaceae family that is widely cultivated but is indigenous to southwestern Asia and North Africa [15]. All parts of the plant are edible with the fresh leaves and dried seeds most commonly used as culinary ingredients. The yield of C. sativum seeds is reported to be around 954 kg ha⁻¹ [16]. The seeds, which contain 26–29 wt% vegetable oil [17], are also used in perfumery, cosmetic, and medicinal applications [18]. The primary FA constituent in C. sativum oil (CSO) that comprises 31–75% of the FA profile is petroselinic (9Z-octadecenoic) acid, which is an uncommon isomer of oleic acid and is found at high levels in a restricted range of seed oils mostly from the Apiaceae family [19]. Other FA of significance in CSO includes linoleic and oleic acids, along with lesser amounts of stearic and palmitic acids, among others [17]. A volatile essential oil fraction composed of terpenoid and phenolic phytochemicals that have antioxidant and other medicinal properties are also found in coriander and is transported at least in part into the lipid phase during extraction [15], [17], [18], [20], [21], [22], [23].

The objective of the current study was to prepare and evaluate C. sativum oil methyl esters (CSME) as a potential biodiesel fuel. Using standard methods, the following fuel properties were determined: low temperature properties, oxidative stability, cetane number, sulfur content, free and total glycerol content, kinematic viscosity, acid value (AV), phosphorous content, lubricity, heat of combustion, Gardner color, iodine value (IV), FA profile, and tocopherol content. Comparison to ASTM D6751 and EN 14214 as well as to soybean oil methyl esters (SME) were further objectives. Soybean oil methyl esters were chosen for comparison as a result of their common use as biodiesel fuel in the United States as well as their typical composition of the five common FA discussed previously.