Though natural proteins are build up by L-amino acids, small proportio
ns of symmetrical D-molecules may occur in organic matter. Relatively
high concentrations of D-amino acids were determined in some marine in
vertebrates. D-amino acids have also been detected in bacterial cell w
alls, vertebrates and even in plants. The conversion of L-amino acids
into their D-forms is regulated either biochemically by enzymatic acti
vity, or induced by heat and alkaline treatment. As a consequence, D-a
mino acids are obviously more widespread in food as would have been ex
pected. D-amino acid levels of 1-10 % of the total amino acid content
are not rarely to be found. Significant values have been detected part
icularly in dairy products like yoghurt and cheese. Expressed in absol
ute figures, consumption of 100g of Emmental cheese could mean an inta
ke of 70 mg of D-amino acids. 100 ml of instant coffee was found to co
ntain 20 mg of D-amino acids. The highest value with nearly 600 mg/100
ml was measured in liquid spice. Whereas roasted coffee and liquid sp
ice may serve as examples for the influence of food processing (roasti
ng, yeast hydrolysis), D-amino acid formation in fermented products is
mainly due to microbial activity. The three predominating amino acids
D-alanine, D-glutamic acid and D-aspartic acid in these products are
integral part of bacterial cell walls. Probably the role of bacteria h
as been underestimated yet. Systematic studies about the impact of D-a
mino acid intake by humans are still lacking. Reports of possible toxi
c effects on liver and kidney were derived from some animal experiment
s. The results are strongly species-dependend. The assumption, that mi
crowave heating of milk causes formation of specific D-amino acids did
not stand experimental verification.