Differences in Experimental Outcomes from Thermal Processing: The Case of Poppy Seeds and Opium Alkaloids

FOOD CHEMISTRY AND THERMAL PROCESSING

Numerous reports published in the Journal of Agricultural and Food Chemistry (JAFC) and other journals within the field of food chemistry incorporate some aspect of thermal processing or heat application into their experimental design. Such experiments often test how thermal processing affects retention of bioactive compounds in foods, bioavailability/bioaccessibility of nutrients, formation of process contaminants (e.g., acrylamide and furan), and inactivation of natural toxins. Although thermal processing is often posited to have an effect in these experiments, many of these reports have not measured temperatures on or in the food being processed, making it difficult to form definitive conclusions as to the extent processing affected the outcomes. Indeed, the study of heat transfer in a thermal process requires measurement of both ambient temperature and food temperature.

Heating patterns are important to characterize during processing studies due to physical properties and composition of the food, orientation of the food within the oven, and the type (conventional, convection, or microwave oven), size, and heating properties of the oven that is used.¹ Use of a convection oven for a baking experiment produces a more “even” heat distribution in the food as compared to use of a conventional oven, where a product will be exposed directly to the infrared (IR) radiation from a heating element. The phenomenon of uneven heating is especially apparent during cooking using microwave energy (via a microwave oven), with the potential for considerable uneven heat distributions in a food during cooking. Besides the equipment used for baking, differences in heat transfer are affected by the dimensions, shape, and moisture content of product undergoing baking. Because of these variations, reporting only processing time and temperature parameters can be insufficient information to allow for comparisons across experiments.

THERMAL PROCESSING, POPPY SEEDS, AND OPIUM ALKALOIDS

There is a broader need for researchers to more carefully characterize the experimental conditions by which studies are conducted. The need for such food temperature characterization during experiments can be illustrated by studies on the impact of thermal processing on opium alkaloids in poppy seeds. The presence of opium alkaloids in or on poppy seeds has attracted attention due to high amounts of these alkaloids found in some poppy seed samples intended for use as food.^{2, 3} Specifically, elevated dietary exposure to morphine and codeine from poppy seed samples are generally highlighted as resulting in potential adverse effects. In light of this, some regulatory agencies have proposed permissible limits of opium alkaloids in foods and have suggested mitigation practices for industry, including grinding, washing, and various types of thermal processes.² JAFC has contributed to this body of work by publishing research on how thermal processing can affect levels of opium alkaloids in poppy seeds.^{2, 4} Interestingly, these data and those of others have shown seemingly diverging results, with some processing types resulting in little or no degradation of opium alkaloids, with others showing significant reductions.⁵

Perhaps baking has shown the most conflicting results in terms of its effect of reducing opium alkaloid levels in poppy seeds.⁵ However, these differing results may potentially be explained, in part, by how baking processes were conducted across experiments. This becomes apparent when considering that these experiments assessing the effect of baking on opium alkaloids had various differences, including their baking methods (convection versus conventional oven) and the types of ingredients used to prepare the baked goods. Both of these factors can ultimately affect heat patterns during baking. Regardless of these aspects being plausible explanations underlying the observed differences in alkaloid degradation, several questions are still left unanswered because only Shetge et al.² reported information on the surface and internal temperatures of the baked products in their experiment.

Differences in processing methodologies across studies are to be expected because they are often reflective of how processing varies around the world and even within a country (e.g., among ethnic groups). Some examples of types of foods containing poppy seeds are muffins, cookies, breads, and bagels, all with different shapes, dimensions, and ingredients that influence heat transfer properties. It then follows that because of the variety of processing methods, it is logical that researchers may find that poppy seeds with the same starting concentrations of opium alkaloids could decrease after certain types of processing, but unaffected after other methods.

SUMMARY AND FUTURE OUTLOOK

Lack of sufficient monitoring of the temperature profiles of food during thermal processing experiments presents a barrier to meaningfully compare results across experiments. This is especially applicable to the limited number of studies using thermal processing as a method to mitigate opium alkaloids from poppy seeds. Because mitigation of opium alkaloids from poppy seeds is critical for maintaining a safe food supply, it is imperative that additional studies on this topic be conducted. It is important to note that even though research does indicate that some types of thermal treatments reduce levels of opium alkaloids in poppy seeds, our research suggests that thermal processes may not necessarily eliminate or significantly reduce the these alkaloids.²

To help facilitate monitoring of heat patterns during thermal processing, we have summarized some of the key temperature measuring devices relevant to such experiments (see Table 1). Because there are different heat transfer mechanisms and potential interferences that prevent this transfer, ambient temperature can be a poor indicator of internal or surface temperature of a food. Instruments such as thermocouples, IR cameras, and humidity sensors to monitor experimental conditions are important because they allow for better reproducibility and provide the ability to more easily compare and assess results from published literature. Additionally, these measurements have ramifications on the ability of the U.S. Food and Drug Administration (FDA) and other regulatory agencies to predict a public health risk and to develop “best practices” and control points to suggest as guidance to industry.

Table 1.

Summary of key instrumentation used for characterizing heat transfer during thermal processing experiments.

Instrument	Notes
Thermocouple	Ideal for obtaining the internal temperature at a single point within a product during baking/processing. A K-type probe is normally used for the temperature range incurred during baking, but different types of probes are available depending on the experimental temperatures.
Handheld infrared thermometer	Measures a single average temperature across the food surface in its field of view. Relatively inexpensive and widely available.
Infrared camera	Measures individual temperatures of all the points (determined by its resolution) across the food surface in its field of view. This instrument is the most expensive compared to a thermocouple or handheld thermometer.
Humidity sensor	Measuring humidity is especially relevant if steaming is used during processing.