Food Analysis Gets A Boost

IF YOU ARE WHAT YOU EAT, as the old adage warns, then you really ought to know what's in your food. Analytical chemists have developed a rich assortment of techniques for probing the composition and nutritional value of foods and for determining whether they're free of harmful compounds such as pesticide residues. Many methodologies for analyzing food products have been standardized and are practiced widely. But there's always room for improvement. At a food science symposium earlier this month at Pittcon, researchers described techniques and instruments that simplify, shorten, reduce costs of, and otherwise advance food analysis.

One of the big bottlenecks in many types of food analyses is sample extraction. In that process, samples are treated with solvents to extract and transfer analytes from the food item to the solvent for subsequent identification via chromatography or other methods. Under standard temperature and pressure conditions, the extraction step proceeds relatively slowly. Raising the temperature and pressure increases the analyte diffusion rate and solubility, which in turn speeds up the extraction process. That's the idea behind the accelerated solvent extraction (ASE) procedures described by Bruce E. Richter, a manager at Dionex, in Sunnyvale, Calif.

As Richter pointed out at Pittcon, common extraction methods for quantifying pesticide residues in fruits and vegetables, for example, can take several hours and consume hundreds of milliliters of solvents for standard-sized food samples. In contrast, with automated ASE instrumentation at 200 ºC and 1,500 psi, it takes just 15 mL of solvent and 15 minutes to prepare extracts from the same-size food samples, he said. Richter added that compared with conventional extraction methods, ASE provides similar savings in time, solvent use, and solvent disposal costs in analyses of polychlorinated biphenyls, dioxins, and other types of food contaminants.

Procedures for fat-content measurements can be sped up in much the same way, Richter pointed out. Side-by-side comparisons of standard and ASE-based lipid measurements on vanilla wafers, corn chips, parmesan cheese, and other foods showed that ASE methods provide nearly identical results in a fraction of the time and use less solvent than standard methods require.

Aiming for similar improvements in analytical methods, Stephen D. Wesson of CDS Analytical, in Oxford, Pa., conducted several studies to assess the feasibility of replacing commonly used solvent-based sample-preparation techniques with simpler solvent-free thermal-desorption methods. In the thermal method, heat drives organic compounds from a food product to a sorbent material, where the analytes first accumulate and from where they then stream into a chromatograph for analysis. For some types of analyses, Wesson reported, the simplified sans-solvent method works well.

In one test, Wesson analyzed the skin of a potato from a grocery store and readily detected chlorpropham, a carbanilate herbicide that remained in the potato skin even after multiple washings. He also detected DDE, a chlorinated pesticide, in spiked peaches but not in unspiked peaches. Background interferences from alcohols and other naturally occurring peach compounds complicated that analysis, Wesson acknowledged.

Similarly, in tests using polychlorinated biphenyl standards, Wesson showed that those toxic compounds, which are typically extracted with solvents, are amenable to solvent-free thermal-desorption methods. But when looking for PCBs in salmon, he found that fatty acids and other fatty compounds in the fish significantly interfered with PCB detection.

Motivated by the recent flood of media attention regarding health hazards of bisphenol A, a monomer long used for making water bottles and other types of food containers, Wesson applied the thermal-desorption method to various polymeric food-service products. The tests showed that the technique can easily detect organic compounds evolving from polymers. For example, at just 40 ºC, styrene desorbed from a plastic cup. Likewise, the test revealed that with increasing temperature, silicone baby-bottle nipples evolve siloxanes. "The method is in the infancy stage," Wesson said, "but it has potential."

Whereas many of the presentations at the symposium focused on detecting analytes that can impart a bad taste or odor to foods, Brian Shofran of Leco, in St. Joseph, Mich., discussed an improved way to analyze something that smells and tastes good: chocolate. Specifically, Shofran touted the advantages of liquid chromatography/time-of-flight (TOF) mass spectrometry over quadrupole and other scanning mass spectrometry detection methods in analyzing cocoa flavonoids such as catechin and epicatechin.

Unlike scanning MS instruments, which are more common and require up to one second to record a full mass spectrum, TOF instruments acquire thousands of full spectra per second. That difference endows TOF instruments with higher sensitivity and accuracy over the full mass range-a key advantage in interpreting complex chromatograms typical of cocoa and other natural products, Shofran said.

From chocolate to cheese and potatoes to peaches, food analysis is one scientific task that's here to stay. And no doubt, chemists will continue in their quest to make the analytical methods faster, simpler, and, in a word, better.