RESEARCH IN INTRODUCTORY STABLE ISOTOPES: DETERMINING THE SOURCE OF CARBON DIOXIDE AND ORGANIC CARBON IN SELECT CARBONATED BEVERAGES USING STABLE CARBON ISOTOPES

Students enrolled in an introductory course on stable isotopes can benefit from participating in research to enhance learning and understanding the utility of stable isotopes. We report a research problem whereby students conducted an experiment, analyzed the data, and communicated their findings in a poster form. This is a semester long project in which all students participated in all aspects of the research. The project constituted 20 % of the course grade. The problem was to determine the source of carbon dioxide in carbonated beverages. Products of major companies (Coca-Cola group, Pepsi group, Dr. Pepper - 7-Up group) were targeted. The hypotheses tested were: (1) all carbonated beverages made by the same manufacturer (e.g., Coca-Cola group) use the same source of CO₂ and (2) sweeteners dominate the bulk organic carbon composition of carbonated beverages. The objective was to use the stable isotopic composition carbon (δ¹³C) to determine the source of carbon in (1) CO₂and (2) the bulk organic carbon composition.

The sources of CO₂ from the δ¹³C analysis included combustion of methane (δ¹³C = -44 to -41‰), petroleum hydrocarbon/coal (δ¹³C = -35 to -33‰), corn fermentation (δ¹³C = -11 to -7‰) and limestone acidification (δ¹³C = -3 to -1‰). Different beverage manufacturers and even the same manufacturer used more than one source of CO₂ for carbonation. The δ¹³C of organic carbon showed that high fructose corn syrup, cane sugar, aspartame, sucralose and stevia have similar carbon isotopic values (δ¹³C = -12 to -10‰) which corresponded to the δ¹³C of the bulk organic carbon of 72% of the beverages. Beverages with δ¹³C of the bulk organic carbon that were different from that of the sweeteners can be explained by the addition of “secret” ingredients; ingredients not listed in the labeling. The conclusion of the study was that the stable isotopes of carbon is useful in determining the origin of CO₂ used for beverage carbonation and for the bulk composition of carbon in carbonated beverages. Students discussed aspects related to cost of producing/capture of CO₂ and how proximity to source affected ingredients use in regional markets. Students experienced firsthand how to use stable isotopes in problem solving. This type of research can lead to new faculty research and can be combined with other student work into a refereed paper.