oil extraction and analysis phần 6

Chapter 6 Oil Content Analysis: Myths and Reality V.J. Barthet and J.K. Daun Canadian Grain Commission, Grain Research Laboratory, Winnipeg, Manitoba R3C 3G8, Canada Abstract The FOSFA (Federation of Oils, Seeds and Fats Associations Limited) extraction method (harmonized as AOCS Am 2-93 or ISO 659) is considered to be the refer-ence method to measure the oil content of oilseeds. The method is based on the extraction of neutral lipids with hexane or petroleum ether, and the extracted com-ponents are estimated gravimetrically and defined as crude fat or oil content. The method requires a triplicate grind/extraction, making it lengthy and very detailed for the analyst. Compared with other official methods (AOAC 996.06), the FOSFA extraction method gave the highest oil recoveries in all oilseeds tested. Accelerating the extraction through the use of new instrumentation, which com-bines solvent extraction with a physical disruption such as pressure or microwave heating, has not been able to give oil recovery equal to the FOSFA extraction. Although the FOSFA method remains the reference method, both ISO and AOCS have identified a need to develop new rapid methods for oil extraction that yield oil equivalent to that from the FOSFA method and that can be used for determina-tion of other factors such as methyl esters or free fatty acids. This study examined the lipid components extracted by each stage of the FOSFA method: First Extraction (FOSFA1), Second Extraction (FOSFA2), and Third Extraction (FOSFA3). In a typical analysis, FOSFA1 accounted for ~86%, FOSFA2 for ~13.5%, and FOSFA3 for as much as 0.5% of the total oil extracted. Although mainly triacylglycerols (TAG) were found, the extracts contained small amounts of other lipid components including nonesterified fatty acids, partial glycerides, fat-soluble vitamins, long-chain alcohols or aldehydes, and sterol or cholesterol esters. More nonpolar material was extracted in the first early stage of the extrac-tion, whereas the later stages contained more polar material. The oil in FOSFA1 and FOSFA2 contained close to 98% TAG based on methyl ester determination. The analyses of the phosphorus content by graphite furnace atomic absorption spectrophotometry showed that almost no phosphorus (below limit of detection) was found in FOSFA1 and FOSFA2 oils but small amounts of phosphorus were present in FOSFA3 oils. This indicates that only ~0.5% of phospholipids were pre-sent in the FOSFA3 oils, suggesting that only very small amounts (<0.005%) of phospholipids are extracted by the total method when the three extracts are com- Copyright © 2004 AOCS Press bined. Moreover, during the extractions of canola by the FOSFA method, the pro-portion of triacylglycerol (TAG) with n-7 fatty acids increased with each step of the process. These fatty acids are associated with the seed coat in canola seeds, and the increase suggests that these structural lipids are the last lipids to be extracted. Introduction The term oil refers to a mixture of lipids that is liquid at room temperature and usu-ally of plant origin. According to Gunstone and Herslöf (1), oil content is the ana-lytical quantity of oil obtained from plant sources (seed or endosperm). It is defined by both the source material and the extraction procedure. Different oil con-tents may be obtained from the same seed sample if the extractions use organic sol-vents of different polarity and/or different conditions of pressure and temperature. For example, if other conditions are the same, using ethyl ether as the extraction solvent will likely give a higher oil content than using petroleum ether because the more polar ethyl ether extracts more polar lipids.1 For an oil processor, there is a direct relation between seed oil content and economic value. For soft oilseeds such as canola, the relative value of oil to meal may be as large as 8:1. If canola oil is sold at $500/tonne, the difference in value obtained between a tonne of seed at 40% oil and a tonne of seed at 41% oil is ~$5. For a nutritional scientist, oil content is related to energy intake and is part of the description of the nutritive value of the product. For a food scientist, oil content is linked to important functional and organoleptic qualities of the food. For a crusher, the oil content gives an estimate of the yield of a given batch; therefore, the oil content is an economic characteristic of the oilseed trade. To satisfy all par-ties involved in the oilseed trade, it is important to have an analytical method that gives the real oil content of the seeds. Moreover, it is important to have this (these) methods standardized. For some researchers, however, it might be acceptable to give up a little bit of accuracy and precision in favor of a very rapid method. For example, plant breeders continually seek rapid methods for estimating oil content. In the 1930s, researchers at the Canadian Grain Commission (2) developed a method using the refractive index of a hexane extract of flax seed. Others used the relationship between seed density and oil content (3) or the amount of oil expressed onto paper by a hydraulic press (4). For rapeseed, the first really useful rapid method for oil content determination was the method developed by Troëng (5), which was used at Svalöv to analyze as many as 60,000 samples/y (6). The method was adopted in Canada as the “Swedish” method. A study carried out by the Associate Committee on Grain Quality in 1962 and 1963 1It is important that the reader be aware of the difference between petroleum ether (also known as com-mercial hexane, or by commercial names such as SkellysolveTM) and diethl ether. Petroleum ether is a mixture of alkanes ranging from C to C and including branched chains defined by a boiling point range (usually between 40 and 60°C). Diethyl ether is a true ether and is considered more polar in nature than petroleum ether. Copyright © 2004 AOCS Press showed that the “Swedish” method recovered at least 1% more oil than the single grind method used at the Grain Research Laboratory (Table 6.1). These rapid extraction methods were replaced later by methods employing spectroscopic techniques such as nuclear magnetic resonance and near infrared spectroscopy. There is continued interest, however, in rapid extraction methods that will give an oil and meal that can be used for further experimentation. A rapid extractor, based on the Svalöv method, for small samples of seed was developed by Agriculture Canada in the 1980s (7). More recent techniques such as supercriti-cal fluid extraction (SFE) and accelerated solvent extraction have been proposed for the measure of oil content. It is necessary for results from these new methods to be compared with results from the reference method for oil content so that the pro-posed new methods may be validated.The FOSFA (Federation of Oils, Seeds and Fats Associations Limited) extraction method, harmonized as AOCS Am 2-93 and ISO 659, is considered the reference method with which to measure oil content in oilseeds by the international oilseed trade. This chapter examines the techniques and apparatus associated with the refer-ence method for oil content extraction. It also provides information on the compo-sition of the oil extracted by the reference method and thus provides a benchmark for studies using alternative methodologies. Some comparisons with other method-ologies are provided, particularly the AOAC method commonly used for food labeling and the AOCS SFE method. The relation between results from the refer-ence method and the efficiency of commercial extraction is also discussed. Soft and Hard Oilseeds. Oilseeds may be divided into groups that include soft oilseeds, such as sunflower, safflower, canola, rapeseed, flax, and sesame, and hard oilseeds, such as soybean and cottonseed. Certain oilseeds are more difficult to extract than others (8). Soybeans, for example, are an example of an oilseed from which it is relatively easy to extract the oil. Within the soft oilseeds, it is easier to TABLE 6.1 Comparison of Ball-Milling and Simple Extraction Procedures for the Determination of Oil Content in Rapeseed. Study by the Associate Committee on Grain Qualitya Method Swedishb Single grind/extractionc Mean SD Mean Difference 1962 1963 (%) 43.45 39.84 0.74 0.30 42.30 38.70 1.15 1.14 aReported in minutes of the 47th and 48th meetings of the Canadian Associate Committee on Grain Quality, Appendices C14 and C11, respectively. bSix laboratories in 1962, five in 1963. cResults from the Grain Research Laboratory. Samples were ground on a flax mill (AOCS Af 3-54) and extracted for 8 h with petroleum ether on a Goldfisch extractor. Copyright © 2004 AOCS Press remove the oil from relatively large seeds such as sunflower and safflower than from smaller seeds such as rapeseed, sesame seeds, or flaxseeds. Particle size dis-tribution has an important effect on the efficiency of the oil extraction and its rate. Regrinding to reduce the particle size is a key process in the FOSFA extraction. Small soft oilseeds may also be difficult to extract because the particle size has to be smaller than their cell size so that the oil present in their cell structure is released. The authors studied SFE to measure oil content and observed an impor-tant matrix effect in soft oilseeds (9). Apparatus for Extraction Methods of Oil Content Analysis Several extractors are used to perform fat or oil extractions. The use of some extractors is recommended by some official methods, whereas others are strictly proscribed. The most common and unavoidable systems used for oil extraction in an analytical laboratory are described in this chapter. Butt Tube. In the Butt tube system, the sample is ground and a weighed portion is placed into a porous thimble or folded into filter paper. The thimble or filter paper is then placed into the Butt tube and the solvent is placed in the flask. The apparatus is assembled as shown in Figure 6.1 and the solvent is boiled; vapors rise to the con-denser where they condense and drip down through the sample back into the boiling solvent below. The extraction process is continuous and can be completed within a few hours, although exhaustive methods call for regrinding of the sample after the ini- Water-cooled condenser Tapered cork stopper Extraction tube Sample thimble sets here Tapered cork stopper 50- or 100-mL Soxhlet flask Fig. 6.1. Left: Butt tube extractor. Right: Twisselman modification. Copyright © 2004 AOCS Press tial oil has been removed and again near the end of the extraction. Once the extraction is finished, the solvent is removed from the extracted oil by distillation followed by vacuum drying. The extracted oil is weighed. The Goldfisch Fat Extractor (Labconco) has long been recognized as a commercial version of the Butt tube system. An improvement on the Butt tube is the Twisselman extractor, which adds a stopcock between the sample container and the condenser. When the stopcock is closed, it is possible to reclaim the solvent from the extracted material. The Twisselman extractor is recommended in the German standard method (DGF B-I.5) (88). Soxhlet Extractor. In a Soxhlet extractor (Fig. 6.2), the solvent is heated in a boil-er; the pure vapor rises up through a by-pass and into the top part of the Soxhlet container where the sample to extract is contained. In the condenser, the vapors are condensed and drip into the sample-containing thimble. When the level of liquid reaches the same level as the top of the siphon, the liquid containing the extracted material is siphoned back into the boiler. Soxhlet extraction is not a continuous procedure, but a batch system with repeated extractions. Usually a minimum of 30–50 cycles is considered necessary to complete the extraction. The use of a Soxhlet extractor is not recommended by the FOSFA method for oil content analysis. The temperature of the solvent in the solvent vessel rises dur-ing the extraction due to the presence of higher concentrations of oil in the solvent. Eventually, the pure solvent siphoning into the hot extract may vaporize very rapidly (“bump”), flooding the condenser and leading to a loss of oil and poor recoveries. Condenser By-pass tube Sample chamber Siphon Receiving flask Fig. 6.2. Soxhlet apparatus. Copyright © 2004 AOCS Press ... - tailieumienphi.vn