Is tempeh a reliable source vitamin B12?Vitamin B12 is required by our body for the production of DNA and red blood cells. It is found in foods that come from animals, such as fish, meat, eggs and dairy products. Tempeh is sometimes reported to contain vitamin B12. But can strict vegetarians rely on tempeh as their single source of vitamin B12?
Types of vitamin B12Vitamin B12 belongs to the group of corrinoids, large molecules with a corrin nucleus that holds an atom of cobalt. Depending on the attachement, different corrinoids exist. Methylcobalamin, hydroxocobalamin and adenosylcobalamin can be converted in our body to active forms, but others cannot be used by our body and are called inactive vitamin B12. The problem is that many analytical methods measure all vitamin B12 analogues, including the inactive ones. Microbial assays for vitamin B12 are unreliable. The gold standard for testing vitamin B12 activity is measuring methylmalonic acid (MMA) in the blood: increased levels of MMA indicate vitamin B12 deficiency. But so far tempeh has not been subjected to this test.
Vitamin B12 production by bacteriaMany bacteria, such as Clostridium freundii and Klebsiella pneumoniae, are know to produce vitamin B12. Studies have demonstrated that vitamin B12 is produced in tempeh when these bacteria are allowed to grow during fermentation . These bacteria can enter the tempeh as a result of contamination, which means that the process is not under control. On the other hand, they cannot be deliberately added as they are not considered food-grade for humans.
Vitamin B12 results of tempehOnly few vitamin B12 values of tempeh can be found in scientific literature. Areekul found between 0.18 and 4.1 mcg vitamin B12 analogues per 100 g tempeh he bought from various markets in Jakarta, Indonesia . A Dutch study detected no vitamin B12  whereas a US study showed only little vitamin B12 analogues (0.05 mcg per 100g) . The USDA National Nutrient Database for Standard Reference (2010) mentions a value of 0.08 mcg/100 g fresh tempeh, which is very low, considering the fact that you need about 3 mcg daily. In conclusion, vegans should not rely on tempeh as their source of vitamin B12 and should consume vitamin B12 supplements.
References S Keuth and B Bisping . Vitamin B12 production by Citrobacter freundii or Klebsiella pneumoniae during tempeh fermentation and proof of enterotoxin absence by PCR. Appl Environ Microbiol. 1994 May; 60(5): 1495–1499.
 Areekul et al. The source and content of vitamin B12 in the tempehs. J Med Assoc Thai 1990 Mar;73(3):152-6.
 Van den Berg et al. Vitamin B12 and Seaweed. Lancet Jan 30, 1988.
 Specker BL et al. Increased urinary methylmalonic acid excretion in breast-fed infants of vegetarian mothers and identification of an acceptable dietary source of vitamin B-12. Am J Clin Nutr 1988 Jan;47(1):89-92.