Chapter 8
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Furan in Thermally Processed Foods Patricia J. Nyman Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration, HFS-706, 5100 Paint Branch Parkway, College Park, Maryland, 20740 Furan is a volatile organic compound that has been classified as a potential human carcinogen. In 2004, furan was unexpectedly found in a broad range of thermally processed foods. Thermal decomposition and rearrangement of organic compounds was proposed for its formation. This chapter will present an overview on furan in thermally processed food and will discuss analysis, occurrence, formation, and exposure. Furan (C4H40) is a colorless volatile organic compound with a boiling point close to room temperature (31.4°C). It is used in the manufacture of agricultural and pharmaceutical products and other organic compounds such as thiophene and tetrahydrofuran and is naturally occurring in certain woods. Furan is an animal carcinogen at high doses and has been identified as a possible or anticipated human carcinogen by the International Agency for Research on Cancer and the U.S. Department of Health and Human Services (1, 2). Until recently, the occurrence of furan in food was not believed to be widespread. Furan had been reported in a few foods such as cooked canned meat and poultry, roasted coffee, roasted filberts, beer, heated soy and rapeseed proteins, fish and milk proteins, wheat bread, and caramel (3, 4). In a review by Maga, carbohydrate thermal decomposition and rearrangement was proposed as the principal formation pathway of furan and furan derivatives in food. In 2004, renewed interest in the occurrence of furan in food was stimulated by a U.S. Food and Drug Administration (U.S. FDA) investigation on furan formation in foods subjected to non-thermal ionizing radiation. During that study, furan was unexpectedly found in a wide range of thermally processed canned and jarred foods. Lower detection limits resulting from improved analytical instrumentation and improved chromatographic techniques were considered important factors contributing to this discovery. In view of the analytical improvements, it is not unreasonable to speculate that low parts per billion (ppb, ng/g) or trace levels of furan (ng/g) have occurred in thermally processed and roasted foods for decades if not longer. U.S. government work. Published 2009 American Chemical Society Al-Taher et al.; Intentional and Unintentional Contaminants in Food and Feed ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
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116 More data on the occurrence of furan in food was needed to evaluate the public health impact from long-term exposure to low ng/g furan levels. The U.S. FDA and European Food Safety Authority (EFSA) began to investigate the occurrence of furan in food, especially thermally processed canned and jarred foods. Requests for additional data from both the U.S. FDA and EFSA resulted in several studies on furan in food (5, 6). These studies were the subject of a review on the analysis, occurrence, and formation of furan in heat-processed foods by Crews and Castle (7). In May 2006, a workshop on ‘Furan in Food’ organized by the European Commission, Director General, Health and Consumer Protection, was held in Brussels. The focus of the workshop was to identify additional data needed to determine a reliable exposure assessment (8). The outcome of the workshop resulted in several publications on the analysis, occurrence, and formation of furan in food (9-13).
Analysis Furan is a gas at room temperature and is ideally suited for headspace analysis (HS). A number of HS methods were developed in response to renewed interest in the occurrence of furan in food. Several of these methods (9, 14-17) are a modification of a U.S. FDA method that used static HS gas chromatography/mass spectrometry (GC/MS) to quantitatively determine furan in canned and jarred foods (18). Other methods used solid phase micro extraction (SPME) with a carboxen/polydimethylsiloxane fiber (75-80 μm film) followed by GC/MS (12, 19-23). These methods generally analyzed between 1 to 10 g test portions. Limits of quantitation (LOQs) were matrix dependent and ranged from 0.5 to 13 ng/g for static HS and from 0.02 to 0.8 ng/g for SPME. All of these methods used deuterated furan (furan-d4) as an internal standard. Most GC separations were conducted using a PLOT (porous layer open tubular) capillary column with a poly-styrene divinyl benzene stationary phase, which retains apolar compounds without cryofocusing. Two studies of model systems with furan precursors were conducted using pyrolysis GC/MS and proton transfer reaction MS (24, 25). Sample Preparation The high volatility of furan at room temperature required certain precautions when samples were handled and prepared for analyses. Most studies followed U.S. FDA specifications for sample preparation with a few modifications (18, 26, 27). In general, samples were chilled (ca. 4°C) for several hours prior to handling and were held on ice during sample preparation. Foods with a viscosity similar to water were transferred directly to HS vials. Non-homogeneous semi-solid and solid foods were homogenized using a food processor, transferred to HS vials, and then diluted to slurry consistency. Test portions were fortified with furan-d4 and immediately sealed. For standard addition analysis, test portions were also fortified with furan. A number of techniques were used to improve furan sensitivity. Hasnip et al. added a few 2-4 mm glass beads to the HS vials to improve mixing (17). Zoller et al. added 0.2 g of amylase to HS vials containing foods that can form a starchy gel (9). Some researchers used sodium chloride or sodium sulfate to
Al-Taher et al.; Intentional and Unintentional Contaminants in Food and Feed ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
117 reduce the solubility of furan in the aqueous phase thereby increasing the concentration of furan in the vapor phase (9, 14, 15, 20).
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HS Thermal Equilibration Headspace sampling often uses elevated temperatures to increase the amount of the analyte in the vapor phase. For furan analysis, excessive temperatures and long thermal equilibration times can lead to furan formation. As a result, most researchers used a lower HS oven temperature (50°C) than specified in the original U.S. FDA method (80°C). The original validation of the U.S. FDA method showed that it performed reliably for canned and jarred foods containing relatively high levels of furan (52 to 118 ng/g) (27). However, additional HS studies using the original conditions (30 min equilibration in a 80°C HS oven) showed that furan formed at low levels (< 3 ng/g) in some fatty foods containing relatively low levels of incurred furan (1 to 6 ng/g) (28). As a result, the HS oven temperature was lowered. Additional oven temperature and thermal equilibration time studies showed that the method performed reliably using the modified conditions (30 min equilibration in a 60°C oven) (26). The U.S. FDA survey data obtained prior to the temperature change was shown to be valid by conducting analyses at both the original (80°C) and modified (60°C) temperatures for several canned and jarred foods previously found to contain low levels. Quantitation by using External Standards and the Method of Standard Additions External standards and the method of standard additions were used to quantify furan in foods. The U.S. FDA used the method of standard additions to avoid matrix effects (18, 26, 27). Matrix effects were characterized by a decrease in the integrated peak areas for furan and furan-d4 and a change in the slope determined from linear regression analyses of the furan/furan-d4 response ratio versus concentration for calibration standards prepared in water and the same curve prepared in the food matrix (data not reported). Altaki et al. compared SPME HS results determined with external standards and standard additions for apple juice, honey, powdered instant coffee, and rice/potato with chicken baby food (19). Comparable results were obtained for all the foods by both methods of quantitation. However, the data are limited in comparison with the hundreds of samples analyzed by various organizations conducting furan analysis. An interlaboratory trial comparing data obtained by static and SPME HS using both external standards and standard additions would be useful to alleviate any uncertainty with respect to the matrix effects and quantitation of furan.
Occurrence Table I summarizes the data from a number of studies and provides the range of furan concentrations reported for various food categories and the corresponding literature citation. Most of the foods analyzed by the U.S. FDA, EFSA, and other studies were found to contain measurable amounts of furan and, in general, comparable furan concentrations were found. Table II reports the number of samples, median furan concentrations, and number of samples
Al-Taher et al.; Intentional and Unintentional Contaminants in Food and Feed ACS Symposium Series; American Chemical Society: Washington, DC, 2009.
118 reported as none detected or as less than the limit of detection (LOD) for some of the food categories including baby foods, infant formula, and adult foods in which higher furan concentrations were reported. Most of the data in Table I were reported as individual values and were used to compile Table II. Data originally reported as a range, average, or median were not included. For example, Hoenicke et al. found
