Biodiesel is an important renewable transportation fuel produced from triglycerides like virgin plant oils and waste cooking oils [13]. It is commercially available and widely used in many countries such as the US, Indonesia, Brazil, Germany and other European countries. Global biodiesel production grew at an average annual rate of 17 % from 2007 to 2012 [4]. In the US, the biodiesel industry recorded a total volume of nearly 5.67 million tonne in 2013, which exceeds the 2.52 million tonne/annum target set by the EPA’s Renewable Fuel Standard [5]. The production of biodiesel in Europe has also increased dramatically in the period 2000–2011 and accounts for 41 % of the global biodiesel output [4]. This increase is driven by the EU objective of a 10 % biofuel share in the transportation sector by 2020 [6].

A wide range of oil-bearing crops have been identified as potential sources for the production of biodiesel. Edible oils such as rapeseed, sunflower oil, palm oil and soybean oil account for more than 95 % of the current feeds used for biodiesel production [2]. However, there are many concerns regarding the use of such plant oils for non-food applications like biodiesel production and this stimulated the search for alternative feeds for the biodiesel industry.

A possible solution is the use of non-edible oils with a high oil content and productivity. Various studies have been performed to investigate the potential of non-edible oils such as jatropha, karanja and rubber seed oil [79] as the feedstock to produce biodiesel. Rubber seed oil (RSO), derived from rubber seeds, is considered a promising source because the seeds are reported to contain a high amount of oil (approximately 40–50 %) [10, 11] and are currently regarded as a waste. The productivity of rubber seeds is reported to be in the range of 100–1200 kg/ha/yr [12, 13]. From a biorefinery perspective, the valorization of rubber seeds by biodiesel production is highly relevant as it increases the economic attractiveness of the rubber plantations.

The conversion of RSO into biodiesel has been reported in the literature [10, 14]. However, the reported high acid value of RSO renders the conversion into biodiesel difficult [10]. Typically, an acid value of 4 mg KOH/g is set as the maximum for plant oils whereas acid value for RSO between 2 and 81.6 mg KOH/g have been reported [15]. These high free fatty acid (FFA) values are not necessarily an intrinsic feature of the RSO, but will be a function of the processing conditions and technology, as well as the storage conditions of the seeds [15].

Literature data on the effect of seed storage on the quality of RSO are scarce and only one study is available [15] (Table 1). In this study, rubber seeds were stored at two different storage conditions viz (i) in a controlled laboratory setting (entry 1 in Table 1) and (ii) in a traditional storehouse (entry 2 in Table 1). The acid value of isolated RSO for entry 1 increased from 2 to 8.6 mg KOH/g, whereas a higher difference was found for condition 2 (from 2 to 30.8 mg KOH/g). In the same report, the effect of storage on the acid value of crude RSO was provided [15]. After 2 months at 27 °C, the acid value increased from 18.1 to 31 mg KOH/g (Table 1).

Table 1

Overview of studies on the influence of storage conditions on the acid value of rubber seeds, rubber seed oil and rapeseed methyl esters

aEstimated room temperature and relative humidity (RH)

bAfter 2 months storage

cAfter 12 months storage

Studies on the influence of storage on relevant product properties of RSO ethyl esters are not available in the open literature. As such, the degradation of rapeseed oil methyl esters under different storage conditions was used as the benchmark [16]. Here, the acid value increased slightly from 0.15 to 0.22 after 1 year of storage at 4 °C (refer to Table 1 for storage conditions). Higher storage temperatures (from 4 to 40 °C) led to an increase in the acid value from 0.22 to 0.75 mg KOH/g.

Therefore, we can conclude that detailed studies on the effect of storage time of rubber seed on relevant properties are not available yet. In addition, the one step trans-esterification of RSO with ethanol has never been reported before. We here report the influence of storage time on (i) the moisture content of rubber seeds and the acid value of the oil within the seeds, (ii) the acidity of isolated RSO and (iii) the acidity of RSO ethyl esters prepared from RSO. For rubber seeds, the effect of storage conditions on the moisture content of two different rubber seed fractions, one with an initial moisture content of 10.7 wt% and another with a moisture content of 3.1 wt%, were determined and modeled. The latter allowed estimation of the diffusion coefficient of water in the rubber seeds at 27 °C, which has not yet been reported in the literature. In addition, the effect of storage on the acid content of the oil in the rubber seeds, isolated RSO and RSO ethyl esters was investigated (27 °C, closed vessels). For this purpose, RSO ethyl esters were synthesized and relevant product properties after synthesis were determined, which is an absolute novelty of this paper.