Stable carbon isotopic composition indicates large presence of maize in Brazilian soy sauces (shoyu)

Carbon and nitrogen (N) isotopic compositions were used to test the presence of C4 carbon in soy sauces (shoyu) produced in Brazil. Seventy samples of Brazilian shoyu were analyzed and compared with the same product from Japan, China, Singapore, Taiwan, and United States. The average (±standard-deviation) δ13C of the Brazilian shoyu was −15.0 ± 3.4. The average (±standard-deviation) δ13C of five shoyu samples from Japan was −25.2 ± 0.6‰. The δ13C of shoyu from US (−25.3‰) and Singapore (−25.9‰) were close to Japanese samples. Shoyu δ13C values from China were equal to −20‰ and −15.4‰, and from Taiwan equal to −20.2‰. The average δ15N value of Brazilian shoyu was 0.3 ± 1.2‰. The variability of δ15N values among shoyu from other countries was much lower than Brazilian shoyu, and the average was 1.4 ± 0.3‰. Using a simple isotopic mass balance, we estimated that most of the shoyu in Brazil contains <20% soy, and the dominant cereal is maize, probably because in Brazil maize is cheaper than soybean.