Methods of Chemical Analysis

The three caraway breeding research programmes conducted in Wageningen employed the following methods of analysis of oil contents of small samples.

Zijlstra (1916) developed an indirect method based on the lowering the freezing point of the solvent ethylene bromide as a result of the oil extracted from 5g samples of finely ground caraway seeds. Six samples were required for the accurate determination of the oil content of a seed batch. Carvone was determined by refraction of a small amount of oil distilled by high temperature steam.

A steam distillation protocol using Stahl apparatus was operational at the former SVP as from the late 1960s (Krechting 1970). Air-dried samples (c. 10% MC) of 4.5-9g of whole but ruptured seed are extracted in boiling water for 1 hour. The distilled oil is deposited floating on a water column. A sample of the oil is used for a determination of carvone content by refraction.

Analyses by Gas Chromatography (GC) became the standard from about 1980, requiring samples of less than 1g (Oostrom and Van der Kamp 1990). Samples are very finely ground and oven dried, carvone and limonene are individually identified, and total oil content is derived at by adding the two.

Steam distillation does not extract the full oil content from samples so that values are lower than those of the other methods described. Moreover the use of air-dried samples also decreases contents. However, the data are a more accurate reflection of the results of commercial extraction which employs overheated steam. Another advantage is that small amounts of oil may be produced for further studies.

For good comparison, data on contents should not only state analytical methods, but also moisture content of samples.

An accurate indirect and non-destructive method was developed at CPRO-DLO in 1988, employing near-infra red spectral analysis (NIRS) requiring c. 2g samples. Initially a Compscan 3000 with a spinning mirror calibrated with a series of steam distillation data was used during 1988-1992 (Toxopeus and Bouwmeester 1993). Its predicted values of samples analysed with GC correlated well (r=0.85). In subsequent studies at RIKILT-DLO (Tusveld et al. 1991) a NIRS-6500 and a Brann and Luebe Infralyser 500 were used, both equipped with monochromator. Using multilinear regression analysis, the comparative study of calibration lines revealed a line based on 2 wave lengths 1626 and 1736nm, producing data correlating with a coefficient of 0.95 with the independently analysed series (by GC) and with a Root Mean Square error of calibration (RMSC) of 0.19.

At CPRO-DLO an Infralyser 500 calibrated with GC data is in use as from 1993. The calibration line is annually adjusted with values of the new years crop.

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