thumbnail fketchef Is Sugar from Sugar Beets the Same as Sugar from Sugar Cane? W. Conard Fernelius Kent State University Kent, OH 44242
At the beginning of this century when the beet sugar industry in this country was relatively young, there were those who insisted that cane suear was different from and sunerior to beet sugar. Chemists took no stock in the arguments hecause to them it was obvious that a pure compound was the same regardless of the source. Nevertheless, it actually is possihle to tell whether a given sample of sugar came from cane or from beets by means of a mass spectrometer, an instrument now in common use in most laboratories, since the ratio of isotopes of carbon I3C to 12Cis different in the two kinds of sugar. These two stable isotopes of carbon normally exist in the ratio 1.104 to 98.892. (For present purposes we can forget about the minute amounts of 14C whose weak radioactivity makes carbon dating possible.) In the products of photosynthesis the ratio of I3C t o 12Cis slightly less than their ratio in the C02 of the atmosphere. Such isotopic selectivity is known for many reactions. However, the conversion of C02 to carbohydrates in plants takes place by two routes: the familiar
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Brief descriptions of phenomena, topics, and facts which chemical educators have found to be of interest in their teaching, are presented in a "note type" format throughout the JOURNAL.
path involving the three-carbon intermediate phosphoglycerate ( I ) , and a more recently established path involving a four-carbon dicarhoxylic acid intermediate (2). Plants are divided rather sharply into two classes, designated CQand C4 according to the number of carbon atoms present in the photosynthetic intermediate. The selective rejection of 13C is greater by the C3 pathway than by the C4 pathway (3). If P C equals the difference of the ratio of '3C tO '2C, times 1000, in a sample and the ratio in a reference standard marine carbonate, then 613 for atmospheric carbon dioxide is -7, the average for C3 plants is -26.5, and for C4plants, -12.5. Now, sugar beets are CQplants and sugar cane a C4plant. So a mass spectroscopic examination of the products of combustion will indicate the source of the sugar. Since becs gnther honey from C.1 plants and corn syrup comes trom C4 plants, a measurement of the '3C/12Cratio of asample will show whether it 1s pure honey or onediluted with low-cbst, high-fructose corn syrup (4). ~imilarly,knowledge of the ratio will indicate whether aglass of orange juice is the real thing, which has a large negative shift in W / W , or a synthetic one made from orange pulp, artificial flavoring, and high-fructose corn syrup, with a smaller negative shift in ' W P C (5).Archaeologists are even using P v a l u e s of hone collagen to reconstruct the diets of ancient man ( 3 , 6 ) . Literature Cited (11 Calvin, M.,and Benaon. A , Science, 107,476(19481. (21 Kortshack, H. P.. Hart, C. E., and Burr, C. O.,Plant Physiol., 40,209 (1965). (31 van dor Merwe, N. J., Amen Sci., 70,596 (19821. (41 White, J. W.,snd Dover, L.W., J. Asaor. 011. And. Chem.,61,746 (19781. (51 Johnston, K., Scirnro82, (Oetoherl, 86 119821. 161 Zurer. P. A,, Chrm. Eng. News, Feb. 21,1983, p p 33.36
Volume 61
Number 3 March 1984
249
