Recent studies have shown that certain D-amino acids are ubiquitously present in plants at higher levels than assumed earlier: D-amino acids were detected in leaves of ginkgo, maple, and sequoia trees. We consume D-amino acids regularly as they are molecular ingredients in fruits such as apple, pineapple, watermelon, papaya, mango, passion fruit, and coconut milk. In the most recent literature, D-amino acids are considered common constituents of plants and food.1
1 d-Amino acids were identified in cactus, grasses, seeds, and seedlings of legumes such as maize, soybeans, runner beans, alfalfa, and also garden cress, and water cress (Brückner and Westhauser
In most of the cases, specific biochemical reaction pathways generate D-amino acids, which are not necessary to present in detail in the context of this book. But it is worth mentioning that the quantity of D-amino acids in foodstuff is often increased by food preparation: alkaline treatment as in the case of potatoes or Mexican tortillas increases the quantity of D-amino acids, acidic treatment as for example with gelatine, and moreover heat, pressure, and also microbial fermented foodstuffs such as cheese, yogurt, fish, beer, wine, and vinegar contain considerable quantities of D-amino acids. D-Amino acid enantiomers are furthermore expected to be created in the course of the Maillard reaction, which is a multistep reaction of reducing sugars with amino compounds leading finally to flavour compounds and melanoidins. The racemization of L-amino acids forming D-amino acids is assumed to occur at all reversible stages of the Maillard reaction. Consequently, D-amino acids in food are not that exotic. The quantity of individual D-amino acids in food can even be used to serve as an indicator for food processing, nutritional quality, bacterial contamination, age, and maturation, as well as authenticity.
Furthermore, in tobacco samples eight amino acids were found in their D-configuration usually in amounts of a few percent with the exception of D-a-alanine which was present in one smokeless tobacco sample at 50% (Kullman et al. 1999). These particularly high values were confirmed by Ali et al. (2006) who detected up to 34% D-a-alanine in different tobacco species. In this case as well, the Maillard reaction is assumed to be involved in the formation of D-amino acids.
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