Determination of nonprotein tryptophan in yoghurts by selective fluorescence and HPLC

The concentrations of total nonprotein (protein-bound and free) and free tryptophan were determined in various kinds of market yoghurts prepared from raw, skimmed or partially skimmed cowʹs milk, with or without fruit, coffee or cereals. The mean values of total nonprotein tryptophan in plain yoghurt made from whole or skimmed milk were 3.45 ± 0.46 and 3.25 ± 0.38 mg/kg, respectively, that is, five to six times higher than in cowʹs milk. Instead, the free fraction of tryptophan was 2.76 ± 0.50 and 2.44 ± 0.36 mg/kg, respectively (seven to eight times higher than in cowʹs milk). In yoghurts with fruit, the mean values were quite similar to those of plain yoghurt, although they varied within each type of yoghurt. The highest contents of both fractions of nonprotein tryptophan were found in yoghurts with coffee or cereal, due to the contribution of these kinds of food. The increase in free tryptophan in yoghurt improves its nutrition value, as more tryptophan is made available for the brain synthesis of the neurotransmitter serotonin, of which tryptophan is the precursor.